Science Plan
East Asian Monsoon Field Experiment (EAMEX): Participation of the
MAHASRI (post-GAME) International Field Experiment
Participating countries:
Indonesia, Japan, Malaysia, Philippine, Taiwan, Thailand,
U.S.A, and Vietnam
Prepared by
The Taiwan EAMEX Committee
Chaired by
Tsing-Chang (Mike) Chen
Iowa State University, Ames, Iowa, U.S.A
and
National Central University, Chung-Li, Taiwan
December 2007
Abstract
During late spring-early summer, Vietnam, Taiwan, and Japan often suffer recurrent flood
disasters caused by rainstorms.
For instance, rainstorms resulted in the “612 flood” in 2005
and the “609 flood” in 2006 that led to tremendous damages in southwest Taiwan.
In terms of
rainfall, the impact of rainstorms on the society is even greater than typhoons. During winter,
snow storms developed from the shallow low pressure systems northeast of Taiwan bring
hazadous weather to southern Japan.
As observed from our pilot study, such a rapidly
growing low pressure system, namely the Taiwan low, plays a crucial role in producing winter
rainfall over northern Taiwan. The Taiwan low often generates prolonged and persistent
precipitation in this area, causing weather hazards to the daily life. In addition, heavy rain
events produced by cold surge-induced weather disturbances also rage over the surrounding
countries of the South China Sea, including Vietnam, Malaysia, Indonesia, and Philippine.
The disaster prevention programs of governments in East/Southeast Asia have always
concentrated on typhoons. Damages and hazards caused by rainstorms and winter rainfall are
as important as by typhoons to the society, but have not received an equal attention. As a
result, forecasts of these weather systems are far from satisfaction. Because of these concerns,
pilot studies have been made for rainstorms and winter weather perturbations in the past few
years. Substantial background knowledge and understanding concerning various aspects of
rainstorms and winter rainfall has been accumulated. Based on these pilot studies, a field
experiment is developed to obtain better observations and resolve a number of issues (shown
below) vital to a better understanding of rainstorms and winter rainfall:
a. Late spring-early summer rainstorms
z genesis, propagation, and development mechanisms,
z relationship with large-scale background circulation,
z interaction with Taiwan topography,
z mutiple-scale interaction/variation.
b. Winter rainfall
z role of Taiwan low in the generation of winter rainfall,
z genesis mechanism and development of Taiwan low,
1
z multiple-scale interactions among weather pertubations in maintaining winter rainfall.
The “East Asian Monsoon Experiment (EAMEX)” is proposed to answer these issues and
improve forecasts in reducing flood damage. The EAMEX is comprised by two components:
“Summer Rainstorm Field Experiment” and “Winter Rainfall Field Experiment”. In order to
conduct the field experiment effectively, the EAMEX develops a close collaboration with the
“Monsoon
Asian
Hydro-Atmosphere
Scientific
Research
and
Prediction
Initiative
(MAHASRI)” led by Japan, which involves eight countries (Japan, Vietnam, Thailand,
Malaysia, Indonesia, Phillipine, China, and the U.S.A) and a number of international field
experiments. Under the collaboration with MAHASRI, the EAMEX will share observations
made by other countries without cost. This arrangement not only expands the envelope of
field observations for the EAMEX, but also enhances the scientific interaction between Taiwan
and the Asian countries.
2
Table of Content
Abstract
1
A. Background and Objectives of the Proposed Experiment
4
1. Pilot studies of the EAMEX
4
2. Summer Rainstorm Experiment
7
2.1) Previous experiments
7
2.2) Background studies and proposed research
8
2.3) Scientific objectives
17
17
3. Winter Rainfall Experiment
3.1) Previous field experiments
17
3.2) Background research for winter rainfall
18
3.3) Scientific objectives
25
B. Experiment Designs and Research Directions
26
26
1. Design of experiments
(1) Summer Rainstorm Experiment
27
(2) Winter Rainfall Experiment
28
(3) Commanding center of field experiment
30
(4) Data management
30
(5) Organization
30
(6) Timetable of operations
31
2. International Link
32
3. Post-experiment research
37
3.1 Diagnostic analyses
37
(1) Summer Rainstorm Experiment
37
(2) Winter Rainfall Experiment
41
3.2 Numerical simulations
44
(1) NCEP GFS
44
(2) WRF
44
4. Expected progress and accomplishments
46
47
References
3
A. Background and Objectives of the Proposed Experiment
The East Asian Monsoon Experiment (EAMEX) consists of two components: Summer
Rainstorm Experiment and Winter Rainfall Experiment. The following sections introduce the
research background, results from pilot studies, scientific objectives, and plans for the field
experiments. Research approaches designed for the EAMEX will focus mainly on dynamical
and hydrological analyses.
1. Pilot studies of the EAMEX
The Iowa State University in the U.S.A and the National Central University in Taiwan
have been collaborating research activity since 1997. During the past decade, these two
institutes have accomplished numerous studies on the weather and climate related to rainfall
over East Asia. Many of these efforts have already appeared in international journals.
research experiences lead us to a well planned and scientifically rich EAMEX.
These
A brief
introduction of these pilot studies is provided as follows.
1.1) Life cycle of East Asian summer monsoon
Using stations distributed over East and Southeast Asia, Ramage (1952) suggested that the
summer monsoon rainfall undergoes a life cycle consisting of the active (May-June), break
(July), and revival (August-September) phases.
Fifty years later, the physical mechanisms of
this unique monsoon life cycle were clearly depicted by Chen et al. (2004a) using global
rainfall and reanalysis datasets.
Rainfall contributions from various East Asian weather
systems during each life cycle were also examined by Wang and Chen (2007). These works
lay the foundation for the rainstorm research.
1.2) Onset of the Meiyu season
Generally, monsoon onset is defined by a sudden and persistent increase of rainfall or a
permanent direction reversal of low-level prevailing winds (Ramage 1972). Over East Asia,
however, such an approach cannot be applied for the monsoon onset in late spring due to
significant midlatitude frontal activity remaining from winter.
The interaction between
midlatitude weather systems and southwest monsoon flow results in the onset of rainfall, but
low-level winds still fluctuate greatly in May. It takes a couple of more weeks (around June)
for the surface northeasterly flow to permanently change to southwesterly flow during summer.
Therefore, the rainfall onset in East Asia is 2-3 weeks earlier than the change in winds (Chen
2005b).
1.3) Diurnal convection
The timing of diurnal rainfall over East Asia generally occurs in the morning over
4
Northeast Asia, afternoon/evening over the East Asian coast and islands, and midnight over
deep inland regions, forming a continental-scale clockwise rotation.
Chen (2005a) found that
such a rotation is supported by the convergence of water vapor flux. Under these regional
diurnal characteristics, the diurnal rainfall in Taiwan occurs mainly over the western mountain
slopes around 1700 LST (Chen et al. 1999).
In the mesoscale perspective, afternoon
thunderstorms form a surface mesohigh with gust fronts pushing outward toward the coast.
This process alters the local circulation from sea breeze to land breeze.
1.4) North Pacific upper-level vortices
The oceanic trough over the North Pacific is accompanied by strong horizontal wind shear,
which may trigger geneses of North Pacific vortices (Chen et al. 2001). These upper-level
cyclonic vortices form a cool, dry downdraft core and often propagate westward after
formation.
About one-third of these vortices migrate through Taiwan and a quarter through
Indochina.
We found that the local diurnal convection would be suppressed during the
passage of these vortices, which results in calm weather.
This phenomenon is important to
daily weather forecasts.
1.5) Initiation of Taiwan tropical depressions
During the week of 9/9-15/2004, three tropical depressions and a tropical storm occurred
consecutively in the vicinity of Taiwan and caused severe floods. The upper-level trough over
the eastern seaboard of East Asia abnormally deepened and extended equatorward.
Several
shortwave troughs embedded in the midlatitude jet were channeled to near Taiwan and
interacted with strong low-level horizontal shear off the southeastern China coast.
Combined
with warm sea surface temperature near Taiwan, these processes generated a number of
organized convective storms which later became tropical disturbances.
1.6) Decay of equatorial waves and their rainfall
The region ahead (west) of an equatorial wave is usually accompanied by convective
clouds. Over the tropical Western Pacific, these waves often turn northward following the
western rim of the Pacific subtropical anticyclone near Taiwan. Before are blocked by the
upper-level anticyclone and dissipate, some of these waves would bring rainfall to Taiwan.
1.7) Taiwan trough and Taiwan lows
One of the prominent features of the wintertime low-level circulation is a shallow trough
extending from the Philippines to southern Japan through the ocean off eastern Taiwan coast,
namely the Taiwan trough. Very little research had been carried out to analyze the formation
mechanism and dynamic functions of this trough. The Taiwan trough often induces cyclonic
perturbations (Taiwan lows) and intensifies the passing frontal systems originally from south
5
China. Applying scale separation, we explored the physics of the Taiwan trough and analyzed
its dynamics and impacts.
1.8) Double cold surges
The East Asian cold surge activity is usually coupled with a series of upper-level,
eastward-propagating synoptic waves. The wave train drives cold airmasses eastward out of
East Asia, forming an aging cold surge over the ocean and a new trailing cold surge over the
continent.
The easterly branch on the southern side of the aging cold surge over the western
Pacific brings warm, moist air into the Taiwan trough and frequently induces perturbations
(Chen et al. 2002). The formation mechanism of Taiwan lows may be linked to this process.
1.9) Cold surges and the 12-24 day mode
The midlatitude shortwave train that drives the cold surge activity features a 12-24 day
life cycle.
Cold surge activity detected by stations over northern Taiwan also responds with
such a frequency, which is reflected by surface pressure and wind fields.
Proper handling of
this mode helps understand the winter weather cycle and forecasts.
1.10) Interannual variation of cold surges
Chen (2002) proposed that extreme ENSO events often excite a midlatitude short-wave
train anomaly spanning across the North Pacific.
Because East Asian cold surges are driven
by the midlatitude cyclone waves, such an anomaly might affect the cold surge activity in the
interannual time scale.
This hypothesis was proven by Chen et al. (2004c) that more (less)
cold surges are induced during warm (cold) years.
1.11) Interannual variations of winter rainfall and the Taiwan low activity
A rainfall center forms over the Taiwan trough off eastern Taiwan coast.
It is found that
rainfall and local sea surface temperatures exhibit a coherent interannual variability. However,
they are not coherent with the ENSO activity, but lag for one year (behind ENSO). This
variability is linked to large-scale sea surface temperature and circulation anomalies that
propagate regularly eastward around the globe.
The most important factor which will break through the scientific barrier and warrant the
success of EAMEX is a correct research direction. Obtaining this correct direction requires
intensive pilot studies on various topics of the Asian monsoon. The series of pilot studies
listed above have provided us sufficient knowledge to establish a proper research direction for
the EAMEX. Scientific objectives and research approaches of both (summer and winter)
components are presented in the next section.
6
2. Summer Rainstorm Experiment
2.1) Previous experiments
During the past two decades, Taiwan engaged in two field experiments, the Taiwan Area
Mesoscale Experiment (TAMEX) in 1987 and the South China Sea Monsoon Experiment
(SCSMEX) in 1998, to understand the active phase of the East Asian summer monsoon and its
related severe weather.
However, formation mechanisms of many severe weather phenomena
were not answered by these two experiments.
The East-Asian monsoon (Mei-Yu) rainstorms
(rainstorms hereafter) cause frequent flooding over the region from northern Vietnam through
Taiwan to southern Japan. To analyze this weather problem, it is necessary to understand the
disadvantage of the previous experiments and develop an experiment that prevents it.
a. TAMEX
The main objectives of TAMEX were to understand the mesoscale processes and
microphysics of convective storms and to improve forecasts of flood-resulting weather systems.
The TAMEX employed upper-air soundings, surface stations, weather radars, and an airborne
platform (P-3) supplied by the U.S.A as the major observing force. A report of TAMEX was
published in BAMS by Kuo and Chen (1990), while the experiment-related studies were made
public in a 1992 issue of MWR. During May and June, on average, 5-7 rainstorms form over
northern Vietnam and the northern South China Sea (SCS). These storms then propagate
east-northeastward through Taiwan, causing costly flooding damages.
However, the
community was not aware of that the convective activity over northern Indochina is connected
to the genesis of rainstorm affecting Taiwan.
Therefore, the observing network of TAMEX
was only designed to cover the region between Dong-Sha Island and Taiwan (~115°-122°E).
The majority of TAMEX-related studies assumed that convective rainstorms were spawned
from the “Mei-Yu front”. It was demonstrated by our pilot studies that such a perception is
inaccurate.
b. SCSMEX
Because the basic mechanism of the East Asian monsoon onset and the triggering
mechanisms of rainstorms were not fully explored by the TAMEX, one of the major SCSMEX
tasks was to tackle this problem again.
Based on the lifecycle of the SCS monsoon defined
by the convective activity over the northern SCS (Chen and Chen 1995), the SCSMEX
expanded their research domain to cover the entire SCS. It was pointed out in the SCSMEX
report (Lau et al. 2000) that the midlatitude-tropics interaction during the active monsoon
phase plays a key role in triggering organized convective storms. Once again, the SCSMEX
scientific effort did not pay attention to the linkage of convective activities between northern
7
Indochina and the northern SCS.
c. Conclusion
Despite efforts made by these two previous experiments, the meteorological community
still lacks a full understanding of how these late spring-early summer rainstorms are formed
and maintained. Our lack of knowledge about these storms was reflected by the devastating
floods in Taiwan during early June of 2005 and 2006. Previous conceptual models that
confine rainstorms to be formed only over the northern SCS and southeast China limit our
understanding of the formation and development processes of these storms.
In addition, the
general perception that rainstorms are embedded in fronts is another hurdle to understand the
basic dynamics of rainstorms.
The propagation mechanism of rainstorms by the
midtropospheric westerly jet was absent.
These “conventional wisdoms” regarding
rainstorms should be revised. To reach this goal, the major goal of EAMEX is to explore the
genesis, development, and hydrological processes of rainstorms through a perspective of
multiple-scale interaction.
2.2) Background studies and proposed research
On average, rainstorms produce more than 60% of rainfall over Taiwan during a short
period of time between mid-May and mid-June.
In the upstream region, these storms
contribute up to 50% of late spring/early summer rainfall over northern Vietnam and the Tokin
Bay (Fig. 1). The disastrous impact of these rainstorms over this region may be well known
to the meteorological community, but many aspects of rainstorms, such as genesis mechanism,
propagation dynamics, three-dimensional structure, steering circulation, and interaction with
topography, have not been well explored.
Our in-depth pilot studies form the basic guidance
for the scientific plan of the Summer Rainstorm Experiment.
The scientific objectives of this
experiment are presented as follows:
Fig. 1
8
Percentage of rainstorm-produced rainfall
(PRS) versus the total rainfall (P) during
May-June of 1993-2005.
a. Definition of rainstorm
A series of convective storms occurred during the SCSMEX (Fig. 2; indicated by light
blue arrows) are typical rainstorms. Convection that initiated over northern Vietnam, the
Tokin Bay, and the northern SCS grew rapidly into well-organized rainstorms. These storms
then propagated east-northeastward toward Taiwan and Japan, and eventually merged with the
cold front over the ocean east of Japan.
Because there is not a universally accepted criterion,
a convective system with a rainfall rate of 50 mm in 6 hours was defined by Chen et al. (1998)
as a rainstorm and used in this science plan.
00Z 6/5/1998
Fig. 2
GMS IR image for rainstorms occurring
on 00Z 6/5/1998 (during TAMEX).
Rainstorms are indicated by light blue
arrows.
b. Large-scale background circulation
The major rainstorm activity occurring over the region from northern Indochina to
Taiwan takes place during mid-May and mid-June. How does the environmental flow over
this region develop into such a favorable condition for rainstorm genesis and development
during this particular season?
The midtropospheric westerly jet in South Asia progresses
northward from spring to summer.
Since May the Tibetan plateau separates the westerly jet
into northern and southern branches of strong westerlies (Ye 1981; Luo and Yanai 1983). The
southern branch forms a trough over the Bay of Bengal with a southwesterly jet across
Indochina and the northern SCS (Fig. 3b). This jet meets the southeasterly flow along the
western rim of the western Pacific subtropical anticyclone.
Meanwhile, the upper-level South
Asian anticyclone gradually expands toward the East Asian continent (Fig. 3a). Although this
anticyclone seems to suppress the development of convection, the mid-level southwesterly jet
and the southeasterly flow carry abundant water vapor from the tropics and converge over the
northern SCS.
The surface flow during this period (Fig. 3c) forms a southwest-northeast
oriented trough across the northern SCS. The combination between the mid- and lower-level
flows forms not only a strong shear flow, but also a moisture-rich unstable zone.
environment becomes a favorable region for the development of deep convection.
This
After this
period, the mid-level westerly flow moves northward across the Tibetan Plateau (Ye 1981) and
the convective activity shifts to 30°N (Yoshikane and Kimura 2003；Chen et al. 2004c).
9
The surface trough over the northern SCS during mid-May and mid-June has not been
documented in the literature. It is shown in Fig. 4 that this shallow trough extending from
northern Indochina to southern Japan across the northern SCS forms a region favorable for the
rainstorm development (Fig. 2).
Let us name this trough the East-Asian trough.
The
rainbelt (Fig. 4a) coincides with the surface low pressure (Fig. 4b) along this shallow East
Asian trough. The latitude-height cross-sections of the local Hadley circulation along 115°E
indicate that moisture is converged toward this trough. Within this trough, static stability is
low (Fig. 4c) and upward motion is strong (Fig. 4d). Such a structure suggests that, once the
atmosphere in this trough is perturbed, deep moist convection is likely to be induced. The
formation mechanisms of this shallow trough and the midtropospheric southwesterly jet will be
the main focus of our research effort.
Fig. 3 Long term mean streamlines at (a) 200
mb, (b) 700 mb, and (c) surface during
May-June
superimposed
with
windspeed (shadings). Red dash lines
indicate major troughs.
Fig. 4 Surface streamline superimposed (a) precipitation
and (b) surface pressure, and N-S cross sections of
vertical divergent circulation superimposed with
(c) potential temperature and mixing ratio and (d)
eddy geopotential height across 115°E.
c. Genesis mechanism(s) of rainstorms
Analyzing rainstorm cases during the 2005 and 2006 active monsoon seasons with the
NCEP (National Center for Environmental Prediction) GFS (Global Forecast System) initial
analyses (Kanamitsu 1989), we found that rainstorms often couple with midtropospheric
cyclonic perturbations.
These shortwave perturbations, with a horizontal scale smaller than
1000 km, were not identified before the high resolution GFS analyses were available.
Embedded in the mid-level westerly/southwesterly jet, these perturbations initiate near 600 mb
over northern Vietnam and the northern SCS, trigger convection ahead of their trough, and
propagate eastward with convective clouds. The African easterly wave (AEW) is also a type
10
of perturbation embedded in the midtropospheric flow.
The Charney-Stern instability
(Charney and Stern 1962) along the southern flank of the African easterly jet at 600-700 mb
was identified by Burpee (1974) as the genesis mechanism of AEWs.
The Charney-Stern
criteria for instability is met when the meridional gradient of potential vorticity ( ∂q
∂y
)
changes its sign. Using the GFS analysis fields, we found that the midtroposphere over
northern Vietnam (104°E) and the northern SCS (114°E) exhibits a clear sign change in ∂q
especially in the southern flank of the mid-level westerly jet (Fig. 5).
∂y
,
This result suggests that
the wave perturbations coupled with rainstorms like the AEW may be generated by the
Charney-Stern instability of the midtropospheric flow across this region. Because of this
possible genesis mechanism of rainstorms, we will explore the dynamic role of the mid-level
westerly jet and the associated instability in the rainstorm activity.
Fig. 5 North-south cross sections of zonal
wind (blue dots) and meridional gradient
of potential vorticity along 104°E (left)
and 114°E (right) using the May-June
climatology of 2005-2006.
By tracing cloud-top temperatures of rainstorms during the past two decades, we found
two regions where these storms initiate most frequently: one is over an area covering northern
Indochina and southwest China (the eastern slopes of the Tibetan Plateau) and the other one is
over the ocean from Tokin Bay to northern SCS. Previous studies have found that, during the
East Asian monsoon onset, nocturnal convective activity thrives over the northern SCS and
southern coast of China (e.g. Chen and Takahashi 1995; Ciesielski and Johnson 2006). In
contrast, the hilly areas over northern Indochina and southwest China exhibit regular
afternoon/evening thunderstorm activity (Satomura 2000; Okumura et al. 2003). However,
none of the previous studies ever connected these diurnal convective activities with the
rainstorm occurrence.
As shown in Fig. 6, the onset time of rainstorms over these areas
11
coincides with that of the local diurnal convection—late evening over land and early morning
over ocean.
Therefore, the diurnal forcing may be part of the genesis mechanisms of
rainstorms.
To confirm this theory, we computed the diurnal variation of velocity potential (χ)
for this region.
A well-organized dipole structure of χ appears over Indochina and the SCS
(Fig. 6). Positive low-level χ forms over the SCS during the nigh/early morning when the
surface temperature is lower over land and warmer over ocean (00Z; left column). The
polarity of the χ dipole structure reverses in the afternoon/evening (12Z; right column). The
upper-level χ shows an opposite sign of the low-level dipole, indicating a regional-scale
divergent circulation (central row; across 20°N).
Upward motion induced over the SCS
(Indochina) in the early morning (evening) is conducive to developing convection.
However,
how this diurnal mode interacts with the dynamic instability in the midtropospheric flow needs
to be explored. Possible diurnal variation of the mid-level jet and instability will also be
investigated.
Fig. 6 Departure of velocity potential (χ)
from daily mean superimposed with
divergent
winds
and
genesis
locations of rainstorms at 300 mb
(top row) and 925 mb (bottom row).
Early morning (Evening) geneses are
marked by numbers in blue (red).
The east-west cross section of
divergent circulation induced by
these χ anomalies are shown in the
middle row. The departure of 00Z
(12Z) is shown in the left (right)
column.
d. Structure and life cycle of rainstorms
Before profiling the detailed structure of individual rainstorms, the synoptic-scale flow
pattern associated with rainstorm occurrence should be examined.
is used for this purpose.
A case during May, 2006
On May 28, a pair of rainstorms formed over the Tokin Bay and the
northern SCS, respectively, and propagated northeastward toward Taiwan. At the same time,
a midlatitude cyclone developing into its occluded stage moved to the Japan Sea.
This
cyclone formed a trailing boundary in which the cold, dry northwesterly flow met with the
warm, moist southwesterly flow around the northern SCS (Fig. 7a).
A stationary front
marked on the JMA (Japan Meteorological Agency) weather map outlines this boundary (Fig.
12
7b).
Actually, the frontal cloudband is only recognizable east of Taiwan, while cellular
convective clouds dominate the west. By checking the north-south cross-section across the
rainstorm (115°N), we found that the associated atmospheric environment consists of 1) a
mid-level (600mb) jet core over about 23°N (not shown), 2) equivalent potential temperature
(θe) decreasing with height at lower levels, 3) strong instability revealed from the meridional
gradient of θe, and 4) a strong updraft which forms a vertical moisture tongue over the storm
area (Fig. 7c).
These features indicate that the synoptic and mesoscale environments
surrounding rainstorms is characterized by a mid-level jet, dynamic instability, and thermal
instability, like those responsible for the genesis of AEWs over North Africa.
With moisture
provided by the low-level southwesterly flow and the mid-level westerly jet, deep moist
convection can be easily maintained.
Such an environment enables these rainstorms to
produce so much rainfall.
The research community has been used to consider rainstorms as a type of midlatitude
mesoscale convective system (MCSs; e.g. Gallus and Johnson 1992) because of their frequent
coexistence with “fronts”. In fact, midlatitude MCSs are characterized by a tilted updraft and
rear downdraft across the stratiform precipitation region.
Their propagation direction is
usually perpendicular to the surface front. It is revealed from Fig. 7c that vertical updrafts
with the “hot tower” type of tropical deep convection propagate along the surface frontal
boundary. In addition, the midtropospheric westerly jet provides a different dynamical setting
for these convective storms.
applicable to rainstorms.
It is evident that the midlatitude MCS perspective is not
The Summer Rainstorm Experiment will provide better
observations to explore further details of the rainstorm structure. To accomplish this research
task, a composite analysis will be performed to summarize these details.
(c)
(a)
(b)
Fig. 7 (a) 700-mb streamline superimposed
with IR image, (b) JMA surface weather
map, and (c) N-S cross sections of
divergent circulation superimposed with
equivalent temperature on 09Z 5/28/2006.
13
e. Scale and propagation mechanism of rainstorms
The ambiguous definition of MCSs and their high fluctuation of convective clouds make
it difficult to define a proper horizontal scale for rainstorms.
search for propagation dynamics will be challenging.
Without knowing the scale, the
For any organized, long-lived
convective system with a regular track, there usually is a perturbation embedded in the ambient
flow that drives it.
The AEW and the Indian monsoon depression (Sikka 1978; Chen et al.
2005) are good examples.
Analyzing the GFS initial analyses, we found that a typical
rainstorm has an east-west dimension of about 700 km. To prevent the white noise nature of
vorticity, let us use streamfunction (Ψ) to depict the rotational flow. The perturbation leading
to rainstorms can be isolated by the Fourier spatially filtered Ψ field at 600 mb (Fig. 8). Thus,
the streamfunction budget analysis (Chen and Chen 1990) can be used to illustrate the
propagation mechanism.
After a scale separation of the streamfunction budget, the important
dynamic processes in the rainstorm propagation can be highlighted.
Based on our preliminary
analyses, positive vorticity tendency exists ahead (east) of the cyclonic perturbation throughout
its journey across the northern SCS.
propagation of rainstorms.
Such a tendency maintains the steady eastward
The horizontal advection of relative vorticity at mid-levels
appears to contribute the most to the vorticity tendency (not shown). Based on this finding, it
is possible to make better forecasts for the propagation and development of these storms. To
reach this goal, the vorticity dynamics of these terms will be analyzed.
Fig. 8 Total (left) and short-wave
regime (right) streamfunction at
600 mb of a rainstorm on
5/28/06.
The perturbation
trough is indicated by a blue
line in the left panel and “L” in
the right panel.
f. Interannual variation of the rainstorm activity
Sea surface temperature (SST) over the western Pacific fluctuates out of phase with the
SST anomalies over the eastern tropical Pacific and modifies the atmospheric circulation (e.g.
Nitta 1987；Chen and Weng 1998). This circulation anomaly affects tropical cyclone activity
over the western North Pacific (e.g. Chen and Weng 1998; Chen et al. 2004a), as well as
summer monsoon onset over Southeast Asia (Zhang et al. 2002). A possible modulation on
the rainstorm population by such circulation anomalies is investigated in our pilot study.
According to our preliminary analysis of the rainstorm population from 1979 to 2005 (Fig. 9),
a positive correlation exists between the number of rainstorms and SST anomalies over the
14
NINO3.4 region (tropical central Pacific).
Rainstorm frequency during warm years is almost
twice as high as during cold years. It was shown previously that the mid-level westerly jet
ahead of a stationary trough over the Bay of Bengal is closely related to the rainstorm activity.
We will focus on the impact of circulation anomalies in different scales (i.e. global, regional,
and local) on the interannual variation of rainstorm activity.
Fig. 9 Top: Histogram of the rainstorm population
during May-June from 1979 to 2005, with
statistics shown to the right. Bottom: Time
series of SST anomalies over the NOAA
NINO3.4 region.
g. Hydrological cycle associated with rainstorms
The most significant impact of rainstorms on human life is rainfall.
hydrological process associated with rainstorms cannot be neglected.
Therefore, the
The proposed study
investigates the hydrological cycle of rainstorms during different phases in their life cycle:
genesis, mature, and decay. A case study of two 2005 rainstorms formed over northern
Vietnam provides a good example (Fig. 10). Moisture converged toward Storm a after it
propagated out of Indochina (12Z 6/8), and most of the moisture was released in the form of
rain when the storm approached Taiwan (12Z 6/9).
Such a process is realized by an apparent
decrease in moisture content (W) and increase in water vapor convergence (QD) and rainfall (P).
Storm b exhibited similar behavior but was lack of the moisture supply over land (12Z 6/8).
However, the convective storms that develop over northern Vietnam can quickly gather
moisture when they move over the ocean.
The mid-level westerly jet seems to play an
important role in securing a high moisture belt along the northern SCS by transporting water
vapor (ΨQ) from the moisture-laden Bay of Bengal.
15
Fig. 10 Streamfunction of water vapor flux superimposed with precipitable water (left) and
potential function of water vapor flux superimposed with rainfall and divergent water
vapor flux (right) on 12Z 6/18/05 and 12Z 6/9/05. Two rainstorm perturbations are
marked by red (a) and orange (b) lines.
When rainstorms encounter topography, its low-level flow should interact with the terrain
to generate vortex stretching and orographic uplifting.
This effect may be the cause of
abnormally large amounts of precipitation produced by rainstorms.
Using Taiwan as an
example, we found that these storms produce higher rainfall amounts across the island than
over the ocean.
This observation indicates that the topographic modulation of rainfall
generated by rainstorms indeed exists. The time evolution of the hydrological cycle of Storm
a through Taiwan is shown in Fig. 11. Water vapor convergence and precipitation increased
rapidly before the storm made landfall in Taiwan (blue arrow). A surge in precipitation
amounts took place when Storm a crossed Taiwan (red arrow), followed by an adjustment of
the hydrological state with increasing water vapor divergence and decreasing precipitable
water contents. We will analyze all rainstorm cases during the experiment period, as well as
historical events, using the similar analysis procedure as shown here.
will be presented to summarize the results.
Fig. 11 Time series showing different terms of the
water vapor budget equation in a 5 deg. x 5
deg. domain following Rainstorm a.
Precipitation is represented by the histogram.
16
A composite analysis
2.3) Scientific objectives
The following scientific objectives are developed from the aforementioned pilot studies.
In other words, all scientific issues proposed in this science plan are derived from positive
preliminary results.
Observations obtained in the field experiment will be used to verify and
expand these analyses.
z
Large-scale background flow: Circulation that maintains the development of rainstorms.
z
Late spring-early summer rainstorms: Their genesis and propagation mechanisms,
structure, dynamics, and climatology.
z
Hydrological process: Moisture source, hydrological cycle, diurnal cycle, topographic
effect, and impact on regional hydrology.
z
Multiple-scale processes of rainstorms: Interannual variation, intraseasonal oscillation (i.e.
MJO, 12-24 day mode, etc.), synoptic-scale disturbances, and diurnal cycle.
z
Effect of midlatitude-tropics interaction on the rainstorm activity: Life cycle of the East
Asian summer monsoon, rainstorm genesis and development, and impact on the monsoon
onset.
3. Winter Rainfall Experiment
3.1) Previous field experiments
a. AMTEX
The Air Mass Transformation Experiment (AMTEX; Lenschow 1972)) was conducted in
the winter of 1975/76 by Japan, U.S.A, former U.S.S.R, and other Asian countries. The
AMTEX targeted the modification process between the air mass of cold surges and ocean
surface.
Although the interaction of multiple-scale weather disturbances was one of the
scientific objectives, the main focus was the air mass transformation processes.
Coincidentally, an explosive cyclone that originated near eastern Taiwan swiped southern
Japan and caused damages (known in the community as the AMTEX storm). Although a
promise was made by the AMTEX community to explore this storm in a further depth, this
promise was never fulfilled.
Chen et al. (1983, 1985) later pointed out that the AMTEX storm
was developed from a Taiwan low. However, the genesis and development mechanisms of
Taiwan lows into midlatitude cyclones remain unanswered issues to the synoptic disturbances
induced by cold surges during the winter monsoon season.
b. WMONEX
17
The Winter Monsoon Experiment (WMONEX; Greenfield and Krishnamurti 1979)
conducted in the winter of 1978/79 is an extension of the international Monsoon Experiment.
Its observation network covers South Asia, equatorial regions, and Australia. Cold surges,
winter monsoon circulations, and equatorial waves are on the list of the scientific objectives of
the WMONEX.
The most well-known theory developed from the WMONEX is the
tropics-subtropics interaction coupled with the single cold surge model—after cold surges
intruding the tropics induce cumulus convection, the mass flux is redistributed “back” to the
midlatitudes through upward motion in the tropics, which forms a local Hadley circulation.
However, the attention of the meteorological community to this theory blurred the importance
of winter weather in East Asia.
The research related to East/Southeast Asian weather
disturbances and their synoptic and intraseasonal variations was not established as the focus of
the WMONEX.
3.2) Background research for winter rainfall
a. Wintertime large-scale circulation
The large-scale winter circulation over the Asian-Pacific region is characterized by a
meridional stratification of flow regimes consisting of the ITCZ (Intertropical Convective
Zone), the subtropical anticyclone extending from the East Asian continent, and the cyclone
track over the midlatitudes (Fig. 12, left). Rainfall, which tends to occur within cyclonic flow,
forms along the ITCZ (A) and the storm track (C).
On the other hand, a weak precipitation
center appears between Taiwan and Japan (dashed line B). This rainfall center is apparently
connected to the persistent winter rainfall in northern Taiwan, but has not been documented.
One of the most profound theories proposed in the WMONEX period is a local Hadley
circulation induced by the equatorward intrusion of cold surges (Chang et al. 1979; Chang and
Lau 1980), which generates upward motion in the tropics and downward motion in the
midlatitudes (Fig. 12, right).
Using modern reanalysis datasets, we found a stationary
perturbation embedded in this Hadley cell near 25°N corresponding with the weak
precipitation area (B). Under the relatively dry and cold air mass in East Asia during winter,
how this secondary circulation is formed and how precipitation is maintained remain unknown.
The proposed study and the experiment attempt to disclose this unique regional feature,
including its impact on weather systems and regional climate.
18
Fig. 12
Left: Long term mean 925-mb streamline
superimposed with rainfall during Dec-Feb. Right:
North-south cross section of local Hadley circulation
superimposed with vertical velocity and rainfall
across 120°E.
b. Formation mechanism of the Taiwan trough
A significant distinction of low-level winds between winter and summer over East Asia is
the winter easterlies.
This easterly flow generates perturbations over the landmass, forming
stationary trough-ridge systems. According to the mean 925-mb streamfunction and vertical
velocity fields in Fig. 13a, the major perturbation areas are distributed over eastern Indian, the
Indochina Peninsula, and along the windward side of the western Pacific island chain.
Persistent upward motion occurs within these perturbation areas but is very shallow (Fig. 13b).
Using the Fourier scale analysis, it is shown in Fig. 13c that these three perturbation areas are
all restricted to below 600 mb, with the shallowest one (under 850 mb) over the Luzon Strait.
This trough is regarded as the Taiwan trough, because it stretches from eastern Taiwan to
southern Japan.
Winter weather disturbances are very likely to be induced around this
shallow trough by its interaction with the
easterly flow of the East Asian anticyclone.
The formation mechanism and dynamics of
the Taiwan trough are unknown and will be
the main research topics of winter circulation.
Fig. 13 Long term winter mean streamfunction
superimposed with vertical velocity at (a)
925 mb and (b) east-west cross section
along 20°N. (c) Same as (b) except for
the short-wave regime of streamfunction
by the Fourier analysis.
19
c. Taiwan low
Cyclonic perturbations often develop along the Taiwan trough during winter. In fact,
these perturbations may sometimes grow into explosive midlatitude cyclones and affect the
weather systems in Taiwan and Japan. The double-surge pattern consisting of an aging high
pressure system over the ocean and a newly formed surge over land was proposed by Chen et
al. (2002). The saddle area between these two surges is prone to weather disturbances.
When this region passes through the Taiwan trough, it is very likely to interact with the surface
cyclonic flows and generate perturbations. Clouds and rain forming by such processes often
affect northern Taiwan, hence these perturbed low pressure systems are called Taiwan lows.
The Taiwan low is capable of developing its own frontal system and becoming a major winter
storm.
Being in its downstream area, Japan suffers the most from impacts caused by
transformative Taiwan lows. Shown in Fig. 14 is the evolution of a typical Taiwan low
developing into a frontal system toward southern Japan.
Fig. 14 925-mb streamlines superimposed
wind speed on 00Z
(left) and 06Z
(right) 12/10/05.
The cyclonic perturbbation north of
Taiwan is a Taiwan
low.
The well-known AMTEX storm that struck Japan in 1975 developed from a Taiwan low
(Chen et al. 1983, 1985). However, the population of Taiwan lows was never documented.
Based on our statistics (Fig. 15), Taiwan lows transforming into cyclone waves usually appear
around 200-300 km off the eastern Taiwan coast. Those that do not form surface fronts are
scattered around the Luzon Strait and the open ocean east of Taiwan.
Superimposing their
occurrence frequency with the 925-mb streamfunction, we found that Taiwan lows with the
potential of growing into cyclones form most frequently over the Taiwan trough.
The
proposal aims to examine the genesis and growth of Taiwan lows in this shallow trough.
Possible mechanisms include boundary layer processes involving heat exchange between warm
sea surface and low-level air, dynamical processes relating vortex stretching to vorticity
tendency, and transient activities in multiple time-scales.
20
Fig. 15
H
Long term mean streamline at 925 mb
during winter (Dec-Feb) superimposed with
the genesis frequency of Taiwan lows. Red
dots mean Taiwan lows turning into
cyclones, while light blue dots represent
cyclonic perturbations without growing into
frontal systems.
South china fronts
intensified by the Taiwan trough are marked
by dark purple dots.
d. Intraseasonal variation
Using stations around northern Taiwan, we found that a regular 12-24 day oscillation
stands out in the winter weather system. This intraseasonal mode is highly correlated with the
mid- to high-latitude cyclone wave activity that drives cold air outbreaks in East Asia, but is
different from the typical frontal system featuring a 6-8 day signal. These frequencies coexist
in the spectrum analysis of surface pressure over an island 60 km northeast of Taipei (Fig. 16).
A clear 12-24 day signal appears in almost every year from 1979-2005.
Rainfall over the
Taiwan trough also shows a distinct 12-24 day signal in correspondence with the surface
pressure.
The impact of these disturbances in multiple time scales on the weather around Taiwan
can be realized by the y-t diagram of rainfall across 25°N in Fig. 17.
A number of features
emerge:
zRainfall usually begins from east of 120°E, indicating an intensification when weather
systems move across this region.
zHigh and low frequencies coexist in the rainfall variations, which is consistent with Fig. 16.
zRainfall patterns with regular eastward propagation are caused mostly by frontal systems,
including South China fronts (purple triangles) and Taiwan low (red dots). Precipitation not
related to these frontal systems is usually scattered and weak.
Detailed analysis from one of these cases (Fig. 16, right) shows that geneses of Taiwan lows or
intensification of South China fronts often occur before or after the maximum intensity of cold
surges (indicated by pressure time series). These results imply that Taiwan lows are induced
by the interaction between cold surges and the Taiwan trough. Following the 12-24 day
signal of the cold surge activity, the Taiwan low frequency also exhibits such an intraseasonal
oscillation.
Research topics for these intraseasonal modes include their structure, interaction
with large-scale circulation, and impact on local rainfall.
will be performed.
21
Comprehensive synoptic analyses
Fig. 16 (a) Winter mean precipitation (contour) and
its variance (shadings) and the year-by-year power
spectral analyses of (b) surface pressure at
Penchiayu [yellow dot in (a)] and precipitation
over the maximum variance region [red box in (a)].
e. Interannual variation
Fig. 17 Y-T diagram of rainfall across 25N
from 11/1/2005 to 3/31/2006. Surface
pressure over Penchaiyu, both real and
bandpass filtered, is plotted the left.
Taiwan lows and South China fronts are
marked as red dots and purple triangles.
The East Asian cold surge activity exhibits a distinct interannual variation coupled with
the El Nino/La Nina cycle (Chen et al. 2004c).
Strong SST(NINO3.4) anomalies generate a
midlatitude short-wave train across the North Pacific (Chen 2002) and, in turn, alter the cold
surge frequency through modulating the cyclonic wave activity. Our pilot study indicates that
the rainfall center off eastern Taiwan shows a pronounced interannual variation, but is not
coherent with the SST(NINO3.4) anomalies (Fig. 18, lower panel).
Instead, the rainfall
variation is in phase with the SST anomalies over the western Pacific (from the Philippine Sea
to south of Japan) (Fig. 18, upper panel). The correlation coefficient between these two
variables reaches 0.84.
A closer examination reveals that there is a one-year lag between the
variations of rainfall and the SST(NINO3.4) anomalies (a correlation coefficient of 0.68).
Such characteristics reflect an eastward-propagating SST anomalous pattern (not shown)
leading to a lag of the SST anomalies over the western North Pacific behind those over the
central-eastern Pacific.
A coupling between local rainfall variations with global-scale
atmosphere/ocean anomalous patterns is evident. Profiling large-scale circulation anomalies
will help disclose the interannual variation of rainfall over the Taiwan trough.
22
Fig. 18 Top: Histogram of winter precipitation over
ocean east of Taiwan (red box in Fig. 16a) from
1979 to 2005 superimposed with SST anomalies
over the same area (time series). Bottom: Same
as the top panel except for superimposing SST
anomalies over the NINO3.4 region. Their
correlation coefficients are shown to the right.
σ=0.47
0.68
(SST+1yr)
f. Water vapor budget analysis for Taiwan lows
Although winter rainfall produced by Taiwan lows is generally not as intense as that
generated by summer convective storms, it often lasts a long period of time. As a result, local
hydrological factors such as air and soil moisture, surface runoff, and water supply are greatly
affected.
Over the Midwest of the United States, for example, the amount of snowfall in the
previous winter significantly affects the soil moisture in spring, which then affects the
agricultural activity in the following summer.
The alternation of cold surge and Taiwan low
activities would modulate the local hydrological cycle through rainfall amounts.
The
agriculture industry over northern and northeastern Taiwan would be impacted by such
variations.
Because the Taiwan trough plays a key role in forming the rainfall center off eastern
Taiwan, it is necessary to study the formation mechanism(s) of this shallow trough before
looking into the perturbations it generates (i.e. Taiwan trough perturbations or Taiwan lows).
For this purpose, the water vapor budget analyses for the Taiwan trough from a climatological
perspective should be carried out first. Due to the shallow structure of the Taiwan trough, it is
expected that the outcome will be very different from a similar analysis for the summer
monsoon. Proposed hydrological analyses include:
23
z
Taiwan trough (climatology);
z
Taiwan low cases (statistics, composite, etc.);
z
Regional hydrological impact by Taiwan lows;
z
Effect of intraseasonal modes;
z
Effect of interannual variation.
g. Heavy rainfall events in central Vietnam
Rainfall in the Southeast Asian monsoon mainly occurs in summer and winter (Fig. 19).
However, heavy rainfall events in central Vietnam, which often cause disastrous damages
during late fall-early winter and are generally considered to be triggered by northwesterly
monsoon flows, have not attracted much research attention.
The research attention of the East
Asian winter monsoon is often focused on the cold surge activity, its impact on the tropical
convection, and the tropics-midlatitude interaction (e.g. Lau and Chang 1988). However, the
impact of the central Vietnam heavy rainfall on the society cannot be neglected anymore.
Based on our recent observations and analyses with the ERA-40 reanalysis data (Källberg et al.
2004), cold-surge vortices often form over the Philippine Sea and the SCS and propagate
westward toward Vietnam (Fig. 20), causing cold-season heavy rainfall events in central
Vietnam.
To disclose its importance, several aspects of the fall-early winter heavy rainfall
phenomenon in central Vietnam are proposed:
z
Genesis and propagation mechanisms and structure of cold-surge vortices over the
Philippine Seas and the SCS.
z
Water vapor budget of cold-surge vortices.
z
Impact of the ENSO cycle on the interannual variation in the occurrence frequency of
cold-surge vortices, and their propagation tracks.
Fig. 19 Monthly station rainfall along
the coastal Vietnam.
A y-t
diagram of GPCP rainfall along
108°E summarizes that the rainy
season of central Vietnam is
October and November.
24
Fig. 20 Winter mean 925-mb streamlines in October,
November, and December superimposed with
rainfall and the occurrence frequency of coldsurge vortices (orange lines). The genesis
locations of these vortices are marked by red
triangles.
3.3) Scientific objectives
Based on the aforementioned pilot studies, the scientific objectives of the Winter Rainfall
Experiment consist of:
z
Taiwan lows: formation mechanism(s), structure, and propagation.
z
AMTEX storm: the development process from Taiwan lows, and the transformation from
low-pressure disturbances into midlatitude cyclones.
z
Impact of multiple-scale processes on Taiwan lows: including interannual variation, MJO,
12-24 day mode, and synoptic disturbances.
z
Midlatitude-tropics interaction: including cold surges, Taiwan lows, South China fronts,
Southeast Asian cold-surge vortices, and heavy rainfall event in central Vietnam.
z
Regional hydrological cycle: contribution of rainfall by Taiwan lows to the surrounding
area.
25
B. Experiment Designs and Research Directions
1. Design of experiments
The EAMEX will obtain observations from the following six sources: 1) surface stations
(traditional and mobile), 2) upper-air soundings (radiosonde, GPS-sonde, and wind profiler), 3)
radar stations (weather, lidar, ocean current, etc.), 4) remote sensing (orbital and geostationary),
5) research vessels, and 6) the GEOSS (Global Earth Observing System of Systems) real-time
numerical assimilation data server.
observation sources.
Table 1 lists the characteristics and number of these
The experiment designs between the summer rainstorm and winter
rainfall components are different with respect to their phenomenal characteristics.
Therefore,
their observations will be incorporated with different international projects accordingly.
Table 1
Observing facilities for EAMEX
1. Surface stations and number
a. Taiwan
• traditional stations：26
• automatic stations：15
• ARMTS：97
• automatic rain gauges：348 (including ARMTS)
• tower：NCU（1）
、NAVY（1）
• mobile stations：NCU（3）及 CCIT（2）
b. other countries
Stations belonging to Vietnam, Thailand, Japan (island chain), Philippine, Indonesia, Malaysia,
and Hong Kong.
2. Upper-air soundings
• Taiwan：Taipei、Hualian、Makon、Pingdong、Tainan、Dong-Sha island、NCU (ISS)、Navy(1)
• Japan：Sakishima、Okinawa、Amami、Daitõ-jima、Tibetan Plataue
• Vietnam：Hanoi、Vinh、Dian-Bian-Fu、Bach Long Vi Island
• Tibetan Plateau：JICA project (China-Japan Cooperative Project on Weather Disaster Reduction)
3. Radar stations
• Taiwan：Taipei、Hualian、Tainan、KengDing、Green Island、Taichung、Int. Airport、mobile
• Japan：Sakishima、Okinawa、Amami、mobile (offered by JAMSTEC)
• Operational weather radars in Vietnam、Thailand、Philippine
4. Unconventional observations
• wind profiler：NCU ISS (Taiwan)、Okinawa、Tibetan Plateau (JICA)
• Lidar：Ishigaki Island
• ocean current radar：Ishigaki Island
5. Remote sensing
• NOAA-15/17 (USA)：SST、precipitation、IR/VIS（1 km）
• QSCAT (USA; Hoffman and Leidner 2005)：surface winds（~25 km）
• MTSAT (Japan; JMA 1999)： VIS, IR, WV（1 km / 4 km）
• FS-3 (Taiwan)：soundings of temperature, water vapor, and geopotential height（2.5 degree）
• TRMM：precipitation profile（0.25 degree）
6. Research vessels
• Taiwan（3）
26
(1) Summer Rainstorm Experiment
a. Observing network
The main scientific objectives of the Summer Rainstorm Experiment include the physical
nature of rainstorms and their impact on the regional weather. A detailed array of observing
facilities for this experiment is displayed in Fig. 21. The experiment focuses on two major
areas: 1) genesis zones in the upstream region (red network; Fig. 21a) and 2)
development/impact in the downstream region (blue network; Fig. 21b).
Rainstorm geneses (red/blue dots in Fig. 21a) are distributed mainly over the northern
Indochina-southwest China, the Tokin Bay, and the northern SCS. The upstream network
over northern Vietnam and southwest China is specifically designed for observing the
over-land rainstorm genesis process.
Upper-air observations over Vietnam, Thailand, and the
Tibetan Plateau (by the China-Japan Cooperative Project on Weather Disaster Reduction) are
incorporated in this network.
The network covering the second genesis region over the
northern SCS, which consists of three research vessels and observations at Dong-Sha Island
and Hong Kong, is particularly useful for the nocturnal genesis and development of rainstorms
over ocean.
The propagation mechanism of rainstorms initiating in Vietnam can also be
diagnosed.
The downstream site of this experiment (Fig. 21b) employs various observing
facilities available in Taiwan, including stations operated by different disciplines, round-island
radar networks, and additional observations by Japan islands and a research vessel.
Four to
eight upper-air soundings will be launched during the intensive observing periods (IOPs)
depending on the case evolution.
(a)
Fig. 21
The array of the Summer Rainstorm Experiment, including (a) all available facilities and three
observing networks covering the upstream, midstream, and downstream of rainstorm occurrence
frequency, and (b) details of the downstream network over Taiwan. The upstream observing sites
consist of upper-air observations by the China-Japan Weather Disaster Prevention Project and
several Vietnamese radiosonde stations.
27
The observing networks shown in Fig. 21 also consist of several foreign field experiments
(details listed in International Links).
Other than surface observations, dropsondes and
remote sensing data (Formosat-3, QSCAT, MTSAT, NOAA-15/17, TRMM, CloudSat, etc.) will
be merged into the entire observational dataset. These data serve to cover regions where
traditional facilities cannot observe.
b. Weather forecast during IOPs
Improving forecasts of rainstorms is one of the major goals of the Summer Rainstorm
Experiment.
ahead.
However, the planning for IOPs requires accurate weather forecasts 24-72 hours
In view of the lead time needed in such a requirement, we have tested the forecast
skill for rainstorms by the NCEP GFS model in one of our pilot studies.
Studying all cases of
rainstorm during the May-June seasons of 2005 and 2006, we found that the GFS is capable of
capturing the mid-level perturbation that leads to the development of rainstorms and
maintaining accuracy up to 72 hours.
For the rainstorm movement, the GFS exhibits a
position error within 50 km up to 94 hours after the geneses (Fig. 22; upper panel) and a speed
error as low as 2 ms-1 (Fig. 22; lower panel).
Such performance greatly enhances our
confidence in planning the operation for the experiment. It also increases the chance for the
observing networks to properly capture rainstorms.
Fig. 22 Position error (top panel) and speed error (bottom
panel) of rainstorm perturbations by the GFS for the
2005-06 active monsoon season. The range of one
standard deviation from the mean values is shaded.
(2) Winter Rainfall Experiment
Taiwan lows are the major rain producer over northern Taiwan during winter. Therefore,
the Winter Rainfall Experiment focuses on the region with most frequent Taiwan low activity
which is about 250 km off the northeastern Taiwan coast. Over this area, only Taiwan and
Japan can offer land-based observations. The cooperation between EAMEX and Japan
warrants all available observations along the southwest island chain of Japan established by the
JMA. The Cold Surge Experiment led by Japan has set a laser-radar (lidar) and two ocean
current radars in Ishigaki and a wind profiler in Okinawa. These facilities, which are very
useful in obtaining boundary layer observations, will be made available for the EAMEX. The
28
principle observing network is shown in Fig. 23a encircling the highest frequency region of
Taiwan lows. For those that tend to develop into midlatitude cyclones, an intensive network
is designed. It consists of the upper-air sounding stations over Taiwan and the Japan islands
and two research vessels deployed to the north and south, respectively (Fig. 23b). In addition,
the round-island radar and sounding networks in Taiwan will be used for observing the
terrain-flow interaction during Taiwan low occurrences.
It was pointed out in Section 2.2g that heavy rainfall events in central Vietnam are caused
by the westward-propagating cold-surge vortices.
Genesis of these vortices generally occurs
over the Philippine Sea and the SCS. Thus, upper-air observations at the southern Philippines
(Fig. 23c), including GPS-sondes (blue) and conventional radiosondes (purple), would help
strengthen the initialization for the NCEP GFS to capture these vortices. Three radiosonde
stations in the Philippines are proposed to be activated in conjunction with the Winter Rainfall
Experiment, as shown in Fig. 23c.
(a)
(b)
Fig. 23 Array of the Winter Rainfall
Experiment, including (a) available
facilities and the observing network
designed for Taiwan lows, (b) the
cooperative network between Taiwan
and Japan for Taiwan-low geneses, and
(c) an extended network for cold-surge
vortices causing heavy rainfall events in
central Vietnam.
Trajectories of
Taiwan lows and cold-surge vortices are
marked by dots in (a) and light red lines
in (c).
(c)
29
(3) Commanding center of field experiments
The role of commanding center is to coordinate and direct the operation of observations,
communicate each observing element, collect data, and issue the intensive observing period
(IOP) based on all possible forecast information.
The EAMEX proposes two sets of the
commanding center design:
z Iowa State University (ISU) /NCEP Environmental Modeling Center (EMC) (U.S.A) –The
Atmospheric Science Program in Iowa State University owns basic communication
facilities and good data storage capacity. Supported by the high-speed internet system, the
university can handle the requirement as the experiment commanding center.
The EMC
will be another data center for observations made in the EAMEX. Observational data will
be fed into the GFS model to produce real-time forecasts, as well as to assimilate these
observations into a gridded EAMEX dataset for later analyses.
z National Central University/Central Weather Bureau (CWB, Taiwan) – The Atmospheric
Science Department at National Central University in Taiwan can function as the ISU
Atmospheric Science Program with the assistance of Taiwan CWB due to its advantage as a
national weather operational center in manpower, facility, and the ability of handling large
volumes of data.
(4) Data management
In accord with the designs of the commanding center, the observational data collected
during the IOPs will be stored at either one of the following three institutes:
z Iowa State University (U.S.A)
z National Central University (Taiwan)
z National Center for Environmental Prediction (U.S.A)
The mission of the data management center is to manage the large volume of observational
data into readable, convenient formats for access, and to store the data for public use. A
website will be built to share raw and post-processed EAMEX data.
(5) Organization
As many as eight countries (Japan, Vietnam, Thailand, Malaysia, Indonesia, Philippine,
Taiwan, and the U.S.A) will participate in the EAMEX. Other than the routine observations
made by each country, several institutes are involved in the EAMEX:
a. Taiwan
z
Academic institutes:
National Central University, Chinese Culture University, National
30
Taiwan Normal University, National Defense University, Nanjuan Institute of Techniques.
z
Operational institutes: Central Weather Bureau, Civil Aviation Bureau, Air Force Weather
Wing, Naval Atmosphere and Ocean Bureau, National Typhoon and Disaster Prevention
Center.
b. International
z
Academic institutes:
Iowa State University (U.S.A), Tokyo Metropolitan University
(Japan), Ryukyu University (Japan), Hokkaido University (Japan), and MRI (Japan).
z
Operation institutes: NCEP (U.S.A), JMA (Japan), Cold Surge Experiment (Japan),
Vietnam National Hydro-Meteorological Service (NHMS), and weather services of
Thailand, Indonesia, and Malaysia.
Details of each institute will be given in International Links.
The PI, Prof. Tsing-Chang Chen, has reached an agreement with Dr. Jordan Alpert, a
senior scientist of the NCEP EMC, that the NCEP GFS will be the official numerical prediction
and assimilation system for the EAMEX. It was shown that the forecast ability of the GFS
for rainstorm perturbations is satisfactory.
The NCEP EMC is happy about this finding and
promises to cooperate with the EAMEX for future testing on the GFS.
(6) Timetable of operations
The Summer Rainstorm Experiment will coordinate the observing networks in 2007,
launch IOPs in May 15 and June 30 2008, and conduct post-experiment data processing,
analyses, and research as soon as IOPs end. The Winter Rainfall Experiment will launch
IOPs in the fall (October—November) of 2007 and the winter of 2007/08 (December
2007—February 2008), and start post-experiment data processing/analyses and research
immediately after IOPs finish. Under the coordination of Prof. T.-C. Chen during the past
two years, the EAMEX will operate in parallel with several international field experiments,
including CEOP (Coordinated Enhanced Observing Period) second phase, IPY (International
Polar Year), China-Japan Cooperative Project on Weather Disaster Reduction (over the Tibetan
Plateau), and AMY (Asian Monsoon Year). After launching the Formosa-series satellites,
Taiwan was invited by the U.S.A to participate in the IPY. The timetable of EAMEX along
with each field experiment is listed in Fig. 24.
The corporation between the EAMEX and
these experiments will maximize the data availability throughout the operational periods.
31
Fig. 24
Time table of EAMEX
with respect to other
international projects.
Intensive observing
periods (IOPs) are
marked by yellow strips.
2. International Link
The international cooperation between the EAMEX and Asian countries not only
increases the quantity of observational data, but also lowers the cost for experiment operations.
The field experiments cooperating with the EAMEX are introduced as follows:
a. MAHASRI (Monsoon Asian Hydro-Atmosphere Scientific Research and Prediction
Initiative)
After the decade-long international experiment of GAME (GEWEX Asian Monsoon
Experiment), Japan proposed a new field experiment targeting the interaction between ocean,
atmosphere, and water vapor and improvements for intraseasonal and interannual predictions.
The original plan for MAHASRI involves four major Asian regions: East Asia, Northeast Asia,
the Tibetan Plateau, and the tropics.
Most of the Asian countries were invited. The PI, Prof.
T.-C. Chen who served the International Science Panel of GAME, has participated in the
development and planning of MAHASRI in 2005 and 2006. Convinced by the scientifically
well-prepared plan, the EAMEX was accepted by the panel of MAHASRI to be a parallel
experiment as the East Asia component of MAHASRI (Fig. 25).
32
Fig. 25
The official logo of MAHASRI. The
EXMAE represents the East Asia
component of the MAHASRI.
b. Participating countries/field experiments
After the coordination in several GAME/MAHASRI meetings, seven East/ Southeast Asian
countries (Japan, Vietnam, Thailand, Indonesia, Malaysia, Philippine, and U.S.A) have agreed
to participate in the EAMEX. In addition to providing their routine observations, a number of
field experiments will be conducted in parallel with the EAMEX. The participating countries,
their parallel field experiments, and the domain covered by these experiments are shown in Fig.
26. Observational data will be exchanged freely between these countries/experiments. A
brief introduction of each experiment is provided in Section 2d. Table 2 lists the countries
and their representatives who participated in the EAMEX meeting on September 29-30, 2006
at NCU in Taiwan.
Fig. 26
33
International field experiments
over Asia in parallel with the
EAMEX.
Table 2
Representatives and institutes of field experiments participating in the EAMEX
Country
Japan
Experiments or Institutes
Representative/Title
Prof. Koike/CEOP chairman
• China-Japan Cooperative Project on Weather
Disaster Reduction over the Tibetan Plateau
• Remote sensing facilities in Okinawa by JMA
Dr. Satoh/JMA NICT director
• MAHASRI Cold Surge Field Experiment
Prof. Fujiyoshi/JAMSTEC director
• JMA weather radar network
Dr. Nakazawa/THOPEX chair in
Asia/JMA MRI
Dr. Tan Thanh
Dir. Syamsudin/NASED
Dr. Yamanaka/JAMSTEC chief
scientist
Khovadhana, Director
Moten, Director
Malano, Deputy Director
Prof. Ming-Cheng Yen
Prof. Mike Chen
Dr. Jordan Alpert, senior scientist
Vietnam
Indonesia
National Hydro-Meteorological Service
Hydrometeorological Array for Isv-Monsoon
AUtomonitoring (HARIMAU; with JAMSTEC)
Thailand
Malaysia
Philippine
Taiwan
U.S.A
GEOSS and MAHASRI tropics (GaME-T)
Weather Service
Atmosphere, Geology, and Astronomy Administration
National Central University
Iowa State University
NCEP Environmental Modeling Center
c. Observing facilities under the international coordination
The collaboration between EAMEX and a total of eight countries helps obtain the
maximum availability of observing facilities.
The available facilities in each country and
observing networks of individual field experiments are illustrated in Fig. 27.
In addition, the
Global Telecommunication System (GTS) which routinely makes surface and upper-air
observations can be incorporated with these facilities to cover regions outside the experimental
domains.
The facilities shown in Fig. 27 provide the following advantages to the EAMEX:
z Summer Rainstorm Experiment: The primary observation area for this experiment is the
rainstorm genesis region over northern Indochina and southwest China where elevated
terrain dominates. Fortunately, the China-Japan Cooperative Project on Weather Disaster
Reduction led by Japan has established an upper-air observing network over the Tibetan
Plateau and southwest China consisting of many GPS-sondes, three wind profilers, and
three surface towers. Vietnam will add two radiosonde stations at Dien Bien Phu and
Vinh and prepare for another one at Bach Long Vi Island in the Tokin Bay. These
upper-air observations will be used to disclose the flow structure associated with rainstorms
geneses.
Because the mid-level westerly flow around the southern periphery of the
Tibetan Plateau is important to the rainstorm development, a high-density observing
network in northern Thailand is also included.
For the second genesis region and
rainstorm development over the northern SCS, the AIPO experiment conducted by China
34
provides ship observations (green lines in Fig. 27) and upper-air soundings at Xi-Sha Island
in addition to the existing observations in Hong Kong, Dong-Sha Island, and research
vessels.
z Winter Rainfall Experiment: The genesis region of Taiwan lows is covered by the observing
network between Taiwan, Japan island chains, and research vessels.
Additional facilities
provided by the JMA NICT, including lidar, ocean current radar, and wind profilers,
strengthen the observing capability of this network.
The interaction between synoptic and
meso-/micro-scales during Taiwan low occurrences is expected to be well detected by these
facilities.
For late fall heavy rainfall events in Vietnam, the activity of cold-surge vortices
across the Philippine Sea and the SCS can be monitored by observations over the
surrounding countries of the South China Sea, including Philippines, Indonesia, Malaysia,
and Vietnam.
Observations provided by these countries also help extend the research to
winter weather in the Maritime Continent.
Fig. 27 Available routine observing facilities over Asia,
including operational radar sites and GTS stations.
d. International projects and field experiments operating parallel with the EAMEX
z CEOP (Coordinated Enhanced Observing Period project)
Being part of the GEWEX under the World Climate Research Program, CEOP was
established to combine surface stations, upper-air soundings, and remote sensing into an
integrated observational dataset to feed into assimilation systems.
The goal is to provide
a well integrated global atmospheric dataset for various purposes.
For example, the
CEOP data has been used to calculate water vapor and energy budgets.
Results indicated
35
that global numerical models show great deficiencies in simulating diurnal and seasonal
cycles of atmospheric moisture and energy (CEOP 2005). So far, there are 35 stations
being chosen to participate in the CEOP monitoring system.
The NCU station in Taiwan
managed by Prof. T.-C. Mike Chen and Prof. M.-C. Yen is one of these CEOP sites. Due
to the success of CEOP, a second phase has been planned, namely the CEOP-II. The
NCU station is again invited to be part of the CEOP monitoring sites around the globe.
Observations provided by the CEOP can also be incorporated into the EAMEX.
z IPY (International Polar Year)
The Formosat-series orbital satellites launched by Taiwan provide high-resolution
atmospheric temperature, moisture, and geopotential height soundings. All observations
made by Formosats will be integrated into the IPY database. The parallel operations of
EAMEX and IPY should help expand the available data sources in a large-scale
perspective.
z THORPEX (THe Observing system Research and Predictability Experiment)
THOPREX is a research branch of the World Weather Research Programme under the
World Meteorological Organization. Its ultimate goal is to improve the accuracy for
severe weather forecasts to within 14 days.
z HARIMAU (Hydrometeorological ARray for Isv-Monsoon AUtomonitorning)
HARIMAU is an ongoing long-term observational experiment sponsored by JAMSTEC
and Indonesia. Nearly ten radar stations and several wind profilers have been built over
the Sumatra Islands to observe monsoon hydrology and tropical weather disturbances.
z China-Japan Cooperative Project on Weather Disaster Reduction
Under the cooperation between Japan and China, a number of advanced GPS-sounding
stations are built over the Tibetan Plateau and its east/southeast slopes.
This project has
agreed to exchange data with the EAMEX.
z Ocean-Atmosphere Interaction over the Joining Area of Asia and Indian-Pacific Ocean
(AIPO)
Funded by the Academic Sinica of China, the AIPO set an observational tower in Xi-Sha
Island along with its own upper-air sounding station. A research vessel will be deployed
to perform observation around the northern SCS.
z GaME-T (GAME-MAHASRI Tropics)
Thailand was to undertake the tropic component of MAHASRI. A dense station network
and upper-air soundings over northern Thailand will assist in the upstream observation for
rainstorms.
36
3. Post-experiment research
During the past three decades, a number of field experiments had been conducted for
weather and climate in Asia, such as TAMEX and SCSMEX in summer and AMTEX and
WMONEX in winter.
However, some important weather and climate phenomena were not
solved by these previous experiments. These topics become part of the EAMEX scientific
direction.
a. Impact of multiple-scale processes on rainfall
z Interannual variation
z Intraseasonal oscillations (MJO, 12-24 day mode, etc.)
z Synoptic-scale disturbances
b. Tropics-midlatitude interaction
z East Asian monsoon life cycle
z Rainstorm genesis and development mechanisms
z Monsoon onset
c. Regional hydrological cycle
z Hydrological cycle of rainstorms
z Impact of rainstorms on the hydrological cycle over Taiwan, Vietnam, and their vicinity
To achieve these goals, the EAMEX will perform comprehensive 1) diagnostic analyses and 2)
numerical simulations on these topics.
3.1 Diagnostic analyses
(1) Summer Rainstorm Experiment
In the followings, Items 1.1-1.7 are diagnostic analyses on dynamical issues, while Item
1.8 and part of Items 1.5-1.7 are hydrological cycle analyses.
Item 1.9 concentrates on
numerical simulations which will be carried out by the NCEP EMC science team.
a. Background circulation
Large-scale circulation structure
The regional flow pattern is embedded in the large-scale, even global-scale circulation
system.
We adopt Chen’s (2006) approach for wintertime stationary waves to investigate
the basic dynamics governing the late spring-early summer circulation in a large-scale
perspective.
The velocity potential maintenance equation (Chen and Yen 1991) will be
used to reveal the maintenance mechanism of the streamfunction.
Regional mid-level trough over the southern flank of the Tibetan Plateau, the westerly jet
37
ahead of this trough, and a shallow trough over the northern SCS
The evolution of the Asian summer monsoon will be explored using streamlines, vorticity,
and vertical velocity.
Hovmöller diagrams and vertical cross-sections of winds and
streamfunction will be made to portray the dynamical process coupling with the East-Asian
trough and the midtropospheric westerly jet.
The formation mechanism of this jet is
another main focus.
Rainstorm activity in response to the midtropospheric trough and jet
The streamfunction budget (Chen and Chen 1990) and vorticity budget analyses will be
applied to disclose how the regional circulation is maintained. The decomposition of
streamfunction and vorticity tendencies into different dynamic processes help us identify
the major vorticity source(s) contributing to rainstorm genesis.
b. Development of rainstorms
The midtropospheric perturbations associated with rainstorms exhibit a dynamical genesis
process similar to African easterly waves (AEWs).
Burpee (1974) proposed that the
southern track of AEWs is induced by the Charney-Stern instability in the midtroposphere
associated with the African easterly jet.
Based on our pilot studies, three hypotheses of
rainstorm genesis mechanisms are proposed:
The baroclinic instability in the midtropospheric westerly flow triggers perturbations
The meridional gradient of potential vorticity will be used to analyze such instability, while
the north-south gradient of potential temperature can be calculated for static instability.
Their evolution will be illustrated in Hovmöller diagrams.
The baroclinic instability in the midtroposphere may be influenced by diurnal variation
In view of the pronounced diurnal variation reflecting the land-sea contrast (Chen 2005a),
we will extract the possible diurnal component of the baroclinic instability from the
mid-level flow to compare with the diurnal distribution of rainstorms.
The coupling between the low-level thermal instability and the mid-level baroclinic
instability
By examining the lapse rate of equivalent potential temperature, one can obtain the thermal
instability in the lower troposphere.
The Richardson number will be used for evaluating
the low-level instability caused by boundary-layer process.
how rainstorms may be induced by different instabilities.
c. Structure of rainstorm perturbations
38
These approaches will reveal
The scale of perturbations that form rainstorms is only about 700 km. It is very difficult to
use conventional synoptic observations to understand their structure in detail.
Field
experimental datasets are ideal for verifying the structure of rainstorm perturbations
simulated by the GFS. Research approaches for this issue include: synoptic analyses of
different variables, stratification of rainstorms into a genesis-mature-decay life cycle,
composite analyses to highlight their common characteristics, and comparison between
typical monsoon rainstorms and midlatitude MCSs.
The internal structure of rainstorms can be profiled by radar observations. Using dual
Doppler radar analysis, it is possible to construct a three-dimensional circulation within these
storms. Model simulations by the GFS and Weather Research and Forecasting (WRF)
models will be compared with the radar observation.
d. Development and propagation mechanism of rainstorms
Although the mid- to lower-level ambient flow during rainstorm occurrences is generally
westerly or southwesterly, the upper-level flow is constantly northeasterly.
A three-
dimensional vorticity budget analysis will be applied to all cases to understand why these
disturbances can propagate eastward against the upper-level northeasterly winds.
After
identifying the scale of rainstorms perturbations (~700 km), it is possible to apply the
streamfunction analysis to a proper wave regime to highlight the dynamics between the
perturbation and the environmental flow.
Chen et al.’s (2005) approach in studying the
Indian monsoon depression is ideal for this task.
After the dynamical processes are
understood, numerical experiments will be performed to test which model physics may be
crucial to the rainstorm propagation.
The NCEP EMC will undertake the numerical part of
this research.
e. Interaction between rainstorms and the intraseasonal oscillation
Because the triggering mechanism for the SCS monsoon onset comes from the northward
displacement of the 30-60 day monsoon trough over this area (Chen and Chen 1995), the
intraseasonal oscillation may play a key role in enhancing the rainstorm activity.
In
addition, the 12-24 day monsoon mode driven by midlatitude short waves remains important
in East Asia during the active monsoon season. How these monsoon modes interact to
modulate the rainstorm activity will be examined.
We will adopt analysis approaches
proposed by Chen and Chen (1995) and Chen et al. (2000) to isolate each monsoon mode and
assess its impact.
Correlation maps and composite analyses for the life cycle of
39
intraseasonal modes against the rainstorm population are the main techniques that will be
used to explore this issue.
f. Interannual variation of rainstorms
Because of the seesaw oscillation of SST anomalies between the eastern and western Pacific,
cyclonic (anticyclonic) anomalous circulation tends to form during cold (warm) years
(defined by eastern tropical Pacific SST) (Nitta 1987).
Such an variability can affect
regional weather, such as the typhoon activity (e.g. Chen and Weng 1998; Chen et al. 2004).
It is shown in our pilot studies that the year-to-year rainstorm activity exhibits significant
variability.
For this issue, we will adopt Chen et al.’s (2004) synoptic analyses of
circulation and SST to identify the anomalies favorable for high/low rainstorm frequency,
and then link these anomalies to possible interannual modes (such as ENSO).
The
anomalous patterns with respect to the rainstorm activity and interannual modes will be
reconfirmed using the Empirical Orthogonal Function (EOF), composite analyses, and
correlation maps.
g. Interaction between rainstorms and terrain
It is observed that the behavior of rainstorms over land is different from those over ocean.
To understand possible flow-terrain effects over Vietnam or Taiwan, the following modified
vorticity equation will be applied to rainstorm cases:
1
h
ζ t ≈ −V ⋅ ∇ζ − vβ − (ζ + f ) V ⋅ ∇h .
The topographic effect of terrain on rainstorms can be expressed as
1
h
ζ t ≈ −(ζ + f ) V ⋅ ∇h
where
1
V ⋅ ∇h ≈ ∇ ⋅ V .
h
Precipitation can be estimated in the form of water vapor flux convergence
P ≈ −∇ ⋅ Q,
where Q =
1 p0
∫ Vq dp .
g 0
We will apply these equations on the experimental data and focus
on 1) rainfall enhancement by the flow-terrain interaction, 2) modulation of terrain on the
lower structure of rainstorms, 3) the water vapor source of rainstorms over land, and 4)
whether the enhancement of rainfall eventually dries out the storm.
40
h. Hydrological cycle of rainstorms
Before evaluating the impact of rainstorms on the large-scale and regional hydrological
cycles, the water vapor budget analysis will be used to understand the hydrological process
of rainstorms. Each case collected in the IOPs will be thoroughly analyzed.
A composite
analysis of these storms should outline the common features of their hydrological cycle.
Research directions in this issue include: 1) source of water vapor supply, 2) diurnal variation
of the hydrological cycle, 3) topographic modulation of the hydrological structure, 4) role of
rainstorms in the regional hydrology, and 5) multiple-scale interactions of hydrological
processes.
Climatological and case analyses will be performed for this issue.
Yoon and Chen’s
(2005) hydrological analysis for Indian monsoon depressions will be adopted to explore the
water budget of all selected cases.
Yoon and Chen (2005) used a scale-filtered water vapor
budget equation on individual cases:
∂W
+ ∇2 χQ = E − P ,
∂t
where ∇2χQ≡∇⋅Q (Chen 1985), to emphasize the short-wave features of perturbations.
Combining the dynamics of rainstorms revealed from Section 1.3, we will obtain a complete
kinematic structure of these storms.
i. Forecast and numerical simulation
It has been tested that the GFS can properly handle short-term forecasts of rainstorm
perturbations. Impact of the experimental data on forecasts of rainstorm will be assessed by
comparing model performance with and without the EAMEX data in the initial conditions.
The NCEP EMC will perform this research task.
In order to understand the possible impact
of model physics on rainstorm forecasts, model intercomparison between the WRF and the
NCEP GFS will be made.
(2) Winter Rainfall Experiment
a. Background circulation
The shallow Taiwan trough stationary off the eastern coast of Taiwan is the primary objective.
The formation mechanism, structure, and dynamics of this north-south elongated trough and
its relation with the large-scale circulation will be analyzed.
Chen’s (2006) wintertime
stationary wave model will be extended to understand the dynamics of the large-scale
circulation affecting this shallow trough. Topics of interest include:
41
Structure of the wintertime Asian circulation
Using the streamfunction budget equation incorporated with the velocity potential
maintenance equation, it is possible to delineate the divergent circulation in maintaining the
streamfunction of the large-scale flow pattern.
Regional circulation over East Asia-western North Pacific
After depicting the three dimensional structure in terms of geopotential height, winds, and
vorticity, a Fourier scale-separation scheme will be applied to isolate necessary components
in the circulation that sustains/forms the Taiwan trough.
The roles of western North Pacific island chain and the Kuroshio Current
In addition to the atmospheric circulation, planetary boundary layer processes may also be
important in the formation of the Taiwan trough. This direction will be pursued through
two approaches: dynamics by using the vorticity budget analysis and thermodynamics by
applying the heat budget analysis (Chen and Baker 1986). The role of surface heat flux
exchange will also be assessed.
b. Scale and structure of the Taiwan trough
The low-level vortex stretching and the warm Kuroshio Current may be the formation
mechanism for the Taiwan trough.
However, due to its very narrow east-west dimension,
the dynamics and thermodynamics associated with this trough can easily be interfered by the
surrounding environment. A scale separation technique is necessary. Preliminary results
of the rainstorm scale are shown in Fig. 8 and will be extended for further analyses.
c. Formation and development of Taiwan lows
Once the perturbations induced by the Taiwan trough develop into Taiwan lows, they tend to
grow rapidly and eventually become midlatitude cyclones. This process remains unknown
today.
Comprehensive case studies will be conducted in terms of three dimensional
analyses of synoptic patterns, vorticity budget, streamfunction budget after scale separation,
and water vapor budget.
Statistical analysis will be applied to summarize the characteristics
of these cases.
d. Interaction between Taiwan lows and terrain
It is known that eastern and northeastern Taiwan receives the most winter rainfall. The
topographic modulation on Taiwan lows will be analyzed using the relation between
1
V ⋅ ∇h and ∇ ⋅ V .
h
42
e. Interaction between Taiwan lows and intraseasonal oscillations
The East Asian cold surge activity exhibits a pronounced 12-24 day cycle. The EAMEX
will explore the synoptic-scale, intraseasonal, and interannual variations of Taiwan lows.
Applying the power spectrum and bandpass filtering techniques, we can isolate each
important mode and examine its spatial structure.
Our pilot study indicated that the
prominent modes include 3-5 day, 6-8 day, and 12-24 day oscillations.
The following
approaches will be applied on these modes: streamline charts and weather maps, correlation
maps, water vapor budget analysis, and reconstruction of individual modes to evaluate their
contributions.
f. Interannual variation of Taiwan lows
Although the East Asian cold surge activity undergoes consistent variation following the
anomalous midlatitude short-wave train (Chen 2002; Chen et al. 2004), rainfall variation over
the Taiwan trough region shows otherwise. The time series of rainfall, which is coherent
with the SST anomaly there, shows a one-year lag behind major warm/cold years. We
found that this lag is connected with a global-scale, eastward-propagating SST anomalous
pattern.
This unique phenomenon has not been documented in the literature.
We will
pursue the following approaches: 1) analyze the global-scale circulation (streamlines,
streamfunction, velocity potential, water vapor flux potential, etc.) using the EOF, correlation
maps, composite, and statistics, and 2) examine the dynamics and thermodynamics of
anomalous circulation in terms of the streamfunction budget and the heat budget analyses,
respectively.
We will pay particular attention to the down-scaling process linking the
large-scale circulation anomalies to regional flow patterns.
g. Water vapor source for Taiwan trough perturbations
How is rainfall along the Taiwan trough maintained by water vapor under the cold, dry East
Asian winter monsoon?
The hydrological cycle for this region during winter has not been
analyzed by previous studies. Using the modified water vapor budget equation (Chen et al.
1996),
∂W
+ ∇ ⋅ QD = E − P ,
∂t
where QD is the divergent component of water vapor flux obtained from χ Q [= ∇ −2 (∇ ⋅ Q)]
(Chen 1985), the regional hydrological cycle of the Taiwan trough can be examined.
Emphases of the hydrological cycle analysis are made on the climatology, individual cases
43
(including case composite), impact on the surrounding region, and intraseasonal and
interannual variations.
3.2 Numerical simulations
Numerical simulation is the most efficient way to verify theories and demonstrate the
development in the three- dimensional structure of weather systems. It is also a powerful tool
in assimilating observational data from different sources.
The goal of this section is to
evaluate the impact of experimental data on numerical simulations.
Comparisons of model
simulations with and without the EAMEX data will reveal whether any significant
improvement can be gained in the forecasts. For forecasts, various forecast scores will be
applied to objectively determine the impact.
Depending on these results, the numerical study
will be pursued along two directions:
If experimental data has a significant impact: the EAMEX data provides a significant
improvement in model simulations.
Therefore, it can be used to study many aspects
involved with the dynamics, thermodynamics, and moisture structure of weather systems
(rainstorms or Taiwan lows). By tuning the parameters in the model, one will be able to
understand the physical process of the weather disturbance, as well as to improve the
model performance.
If experimental data does not have a significant impact: intensively observed data do not
change the model performance.
Thus, other tests need to be performed. For example,
evaluating the quality of observational data, adjusting model resolution, comparing with
different models, etc.
The EAMEX will employ the simulation and assimilation capabilities of numerical models,
including the global NCEP GFS and the regional WRF models. Because the NCEP GFS
provides medium range forecasts, it is the model of choice for the EAMEX.
(1) NCEP GFS
The original design for the NCEP GFS model was to perform a 10-day medium range
forecast (Sela 1980, 1982). After years of improvement, the GFS has a T254 resolution (finer
than 0.5 degree), 64 vertical levels, and 3-hr cycle capability. In addition, the GFS employs
an advanced statistical spectral interpolation method so that it does not require a complicated
initialization process, and hence reduces initial errors. All observational data collected during
the EAMEX will be fed into the GFS for forecasts. The outstanding performance in capturing
44
rainstorm perturbations has been illustrated in early sections.
Eventually, all EAMEX
observations will be assimilated by the GFS into a four-dimensional, gridded reanalysis dataset
and provided to the public. Overall, the mission of the GFS is to provide real-time forecasts
and assimilate observational data from the EAMEX. It is also a chance for the NCEP EMC to
evaluate the performance of its own global forecast model over the East/Southeast Asian
monsoon region. Drs. J. Alpert and K. Kumar of the NCEP EMC were very interested in
pursuing the aforementioned testing. They believed that the EAMEX will definitely help
improve the performance of the GFS.
(2) WRF
The WRF (Weather Research and Forecasting) model was developed by six different
institutes in the U.S.A to be a next-generation weather forecast system.
It features multiple
cores with a 3DVAR data assimilation system (Michalakes et al. 2001).
Due to its ability to
run at 4-km grid spacing, we can examine the multiple-scale interaction involved in each type
of weather disturbance.
The PI has been involved with two published studies (Clark et al.
2006a, b) testing the capability of the WRF model. The design for numerical simulations is
categorized into the following three items:
a. With / Without observational dataset
To evaluate the impact of EAMEX observations on the WRF model.
b. Dry / Wet runs
Water vapor is a vital element for rain-producing weather disturbances.
The parameter
settings in the model will help understand how moisture supply affects the weather system.
Evaluation of cumulus parameterization schemes with the EAMEX data will also bring
new information to the modeling community.
c. Include / Exclude terrains
The topography of Taiwan is believed to be an important component in enhancing
precipitation of rainstorms and Taiwan lows.
Terrains over northern Indochina and
southwest China may also be the triggering mechanism for rainstorm perturbations.
To
test the role of terrains, we should design a control run (with realistic terrain information),
an aqua run (without the Taiwan island), and an island run (with the Taiwan island but
without terrains).
The same procedure can be applied to the mountains over northern
Indochina and southwest China. For example, a flat run without the Tibetan Plateau could
be conducted.
45
4. Expected progress and accomplishments
The EAMEX is designed for three years which represent three phases of the experiment:
z First year (2007/08) – Preparation and pilot experiments.
Using existing data sources (e.g. ERA-40 and NCEP reanalyses) to
conduct pilot studies for the experiment.
z Second year (2008/09) – Field observation.
Collect possible events and follow the pilot studies to analyze these
cases.
z Third year (2009/10) – Post-experiment research.
Comprehensively analyze cases collected during the IOPs to
explore new mechanisms, in addition to confirming mechanisms
suggested by the pilot studies.
z The following three years after 2010 will be an extensive analysis period for
EAMEX-related research.
The EAMEX project consists of field observations, data collection, data processing,
diagnostic analysis, and numerical simulations, which cover all aspects of meteorological
research.
Expected accomplishments are:
z Understand weather systems that cause flooding in Taiwan, Japan, Vietnam, and other
countries in the vicinity of the South China Sea – including late spring-early summer
rainstorms and late fall-winter weather disturbances.
z Improve forecasts of related weather systems in East/Southeast Asia through these
understandings.
z Provide field experiment datasets to the meteorological community for future research.
z Hold the EAMEX Implementation Workshop in 2007 (in Taiwan).
z Hold the Post EAMEX Summer Rainstorm Experiment Workshop in June 2009 (in
Taiwan).
z Hold the post EAMEX Winter Rainfall Experiment Workshop in June 2010 (in other
countries).
z Publish new findings derived from the EAMEX in SCI journals annually.
Supplemented by extensive pilot studies on all proposed issues, the EAMEX is confident that it
can accomplish all scientific issues during and after the experiment.
The formation
mechanisms of rainstorms and Taiwan lows, as well as their developments, will bring new
discoveries to the meteorological community. The design of EAMEX is based on mutual
cooperation among countries to increase data exchange and reduce costs. Taiwan, as well as
all participating countries, will be benefited by the success of EAMEX.
46
References
Burpee, R. W., 1974: Characteristics of the North African easterly waves during the summers of 1968
and 1969. J. Atmos. Sci., 31, 1556–1570.
CEOP 2005: CEOP News Letter No.8, Coordinated Enhanced Observing Period project, 8pages.
Chang, C.-P., J. E. Erickson, and K.-M. Lau, 1979: Northeasterly cold surges and near-equatorial
disturbances over the Winter MONEX area during December 1974. Part I: Synoptic aspects.
Mon. Wea. Rev., 107, 812–829.
Chang, C.-P., and K.-M. Lau, 1980: Northeasterly cold surges and near-equatorial disturbances over the
Winter MONEX area during December 1974. Part II: Planetary-scale aspects. Mon. Wea. Rev.,
108, 298–312.
Charney, J. G., M. Stern, 1962: On the stability of internal baroclinic jets in a rotating atmosphere. J.
Atmos. Sci., 19, 159-172.
Chen, S. J., Y.-H. Kuo, W. Wang, Z.-Y. Tao, and B. Cui, 1998: A modeling case study of heavy
rainstorms along the Mei-Yu front. Mon. Wea. Rev., 126, 2330–2351.
Chen, T.-C., 1985: Global Water Vapor Flux and Maintenance during FGGE. Mon. Wea. Rev., 113,
1801-1819.
Chen, T.-C., 2002: A North-Pacific short-wave train during the extreme phases of ENSO. J. Climate,
15, 2359-2376.
Chen, T.-C., 2003: Maintenance of Summer Monsoon Circulations: A Planetary-Scale Perspective. J.
Climate, 16, 2022-2037.
Chen, T.-C., 2005a: Variation of the Asian Monsoon Water Vapor Budget: Interaction with the
Global-Scale Modes. The Asian Monsoon, B. Wang, ed., Spring Verlag, 417-458.
Chen, T.-C., 2005b: The structure and maintenance of stationary waves in the winter Northern
Hemisphere. J. Atmos. Sci., 62, 3637-3660.
Chen, T.-C., 2005c: Life cycle and onset of the East-Asian summer monsoon. International Asian
Monsoon Symposium (IAMS), University of Hawaii, Honolulu, HI, 18-20 February 2004.
Chen, T.-C., 2006: Characteristics of African easterly waves depicted by ERA-40 reanalyses. Mon. Wea.
Rev., 134, 3539-3566.
Chen, T.-C. and W. E. Baker, 1986: Global diabatic heating during FGGE SOP-1 and SOP-2. Mon. Wea.
Rev., 114, 2578-2589.
Chen, T.-C., C.-B. Chang and D. J. Perkey, 1983: Numerical study of an AMTEX'75 oceanic cyclone.
Mon. Wea. Rev., 111, 1818-1829.
Chen, T.-C., C.-B. Chang and D. J. Perkey, 1985: Synoptic study of a medium-scale oceanic cyclone
during AMTEX '75. Mon. Wea. Rev., 113, 349-361.
Chen, T.-C. and J.-M. Chen, 1990: On the maintenance of stationary eddies in terms of the
streamfunction budget analysis. J. Atmos. Sci., 47, 2818-2824.
Chen, T.-C. and J.-M. Chen. 1995: An observational study of the South China Sea monsoon during the
1979 summer: Onset and life cycle. Mon. Wea. Rev., 123, 2295–2318.
Chen. T.-C., M.-C. Yen, W.-R. Huang, and W. A. Gallus, 2002: An East-Asian cold surge: Case study.
Mon. Wea. Rev., 130, 2271-2290.
Chen, T.-C., W.-R. Huang, and J.-h. Yoon, 2004c: Interannual variation of the East-Asian cold surge
activity. J. Climate, 17, 401-413.
Chen, T.-C. and K. Takahashi, 1995: Diurnal variation of outgoing longwave radiation in the vicinity of
the South China Sea: Effect of intraseasonal oscillation. Mon. Wea. Rev., 123, 566-577.
Chen, T.-C., S.-Y. Wang, W.-R. Huang and M.-C. Yen. 2004a: Variation of the East Asian summer
monsoon rainfall. J. Climate. 17, 744–762.
Chen, T.-C., S.-Y. Wang, M.-C. Yen and W. A. Gallus Jr., 2004b: Role of the monsoon gyre in the
interannual variation of tropical cyclone formation over the western North Pacific. Wea.
Forcasting, 19, 776-785.
Chen, T.-C., S.-Y. Wang and M.-C. Yen, 2007: Enhancement of afternoon thunderstorm activity by
urbanization in a valley: Taipei. J. Appl. Meteor. (in press).
Chen, T.-C. and M.-C. Yen, 1991: A study for the diabatic heating associated with the planetary scale
Madden-Julian Oscillation. J. Geophys. Res., 96, No. D7, 13, 163-13, 177.
Chen, T.-C., M.-C. Yen, J.-C. Hsieh and R. W. Arritt, 1999: Diurnal and Seasonal Variations of the
47
Rainfall Measured by the Automatic Rainfall and Meteorological Telemetry System in Taiwan.
Bull. Amer. Meteor. Soc., 80, 2299-2312.
Chen, T.-C., M.-C. Yen, W.-R. Huang and W. A. Gallus Jr.. 2002: An East Asian cold surge: Case study.
Mon. Wea. Rev., 130, 2271–2290.
Chen, T.-C., M.-C. Yen, G.-R. Liu, and S.-Y. Wang, 2001: Summer upper-level vortex over the North
Pacific. Bull. Amer. Meteor. Soc., 82, 1991-2006.
Chen, T.-C., M.-C. Yen and S. Schubert, 1996: Hydrologic Processes Associated with Cyclone Systems
over the United States. Bull. Amer. Meteor. Soc., 77, 1557-1567.
Chen, T.-C., J.-H. Yoon, S.-Y. Wang, 2005: Westward propagation of the Indian monsoon depression.
Tellus, 57A, 758-769.
Ciesielski, P. E. and R. H. Johnson, 2006: Contrasting characteristics of convection over the northern
and southern South China Sea during SCSMEX. Mon. Wea. Rev., 134, 1041-1062.
Clark, A. J., T.-C. Chen, and W. A Gallus Jr., 2006a: Contributions of mixed physics and perturbed
lateral boundary conditions to the skill and spread precipitation forecasts from a WRF
ensemble. Mon. Wea. Rev. (in press)
Clark, A. J., W. A Gallus, Jr., and T.-C. Chen, 2006b: Comparison of the diurnal cycle in
convection-resolving and non-convection-resolving mesoscale models. Mon. Wea. Rev.
(accepted with revision)
Gallus Jr., W. A. and R. H. Johnson, 1992: The momentum budget of an intense midlatitude squall line.
J. Atmos. Sci., 49, 422-450.
Greenfield, R. S., and T. N. Krishnamurti, 1979: The Winter Monsoon Experiment – Report of
December 1978 field phase. Bull. Amer. Meteor. Soc., 60, 439–444.
Huffman, G. J., and coauthors, 1997: The Global Precipitation Climatology Project (GPCP) combined
precipitation dataset. Bull. Amer. Meteor. Soc., 78, 5-20.
Hoffman R. N. and S. M. Leinder, 2005: An introduction to the near-real-time QuickSCAT data. Wea.
Forecasting, 20, 476-493.
Kalnay, E., and coauthors, 1996: The NCEP/NCAR 40-Year Reanalysis Project. Bull. Amer. Meteor.
Soc., 77, 437–471.
Kuo, Y.-H. and G. T.-J. Chen. 1990: The Taiwan Area Mesoscale Experiment (TAMEX): An overview.
Bull. Amer. Meteor. Soc., 71, 488–503.
Lau, K.-M., Y. Ding, J.-T. Wang, R. Johnson, T. Keenan, R. Cifelli, J. Gerlach, O. Thiele, T. Rickenbach,
S.-C. Tsay and P.-H. Lin. 2000: A Report of the Field Operations and Early Results of the South
China Sea Monsoon Experiment (SCSMEX). Bull. Amer. Meteor. Soc.. 81, 1261–1270.
Lau, K.M., G. Yang, and S. Shen, 1988: Seasonal and Intraseasonal Climatology of Summer Monsoon
Rainfall over Eeat Asia. Mon. Wea. Rev., 116, 18–37.
Lenschow, D. H., 1972: The Air Mass Transformation Experiment (AMTEX). A report of the AMTEX
Study Conference, 10-13 November 1971, Tokyo, Japan. Bull. Amer. Meteor. Soc., 53, 353–357.
Luo, Y. and M. Yanai, 1983: The large-scale circulation and heat sources over the Tibetan Plateau and
surrounding areas during the early summer of 1979. Part I: Precipitation and kinematic analyses.
Mon. Wea. Rev., 111, 922-944.
Michalakes, J., S. and coauthors (2001): Development of a Next Generation Regional Weather Research
and Forecast Model. Developments in Teracomputing: Proceedings of the Ninth ECMWF
Workshop on the Use of High Performance Computing in Meteorology. Eds. Walter Zwieflhofer
and Norbert Kreitz. World Scientific, Singapore. pp. 269-276.
Nitta, T., 1987: Convective activities in the tropical western Pacific and their impacts on the Northern
Hemisphere summer circulation. J. Meteorol. Soc. Jpn, 65, 165–171.
Okumura, K., T. Satomura, T. Oki, and W. Khantiyanan, 2003: Diurnal variation of precipitation by
moving mesoscale systems: Radar observations in northern Thailand. Geo. Res. Let., 30, 2073.
Ramage, C. S., 1952: Variation of rainfall over south China through the wet season. Bull. Amer. Meteor.
Soc., 33, 308-311.
Ramage, C. S., 1972: Monsoon Meteorology. Academic Press, 296 pp.
Satomura, T., 2000: Diurnal variation of precipitation over the Indo-China Peninsula: Two-dimensional
numerical simulation, J. Meteorol. Soc. Japan, 78, 461–475.
Schumacher, R. S. and R. H. Johnson, 2005: Organization and environmental properties of
extreme-rain-producing Mesoscale Convective Systems. Mon. Wea. Rev., 133, 961-976.
48
Sela, J. G., 1980: Spectral modeling at the National Meteorological Center, Mon. Wea. Rev., 108,
1279-1292.
Sela, J. G., 1982: The NMC Spectral Model, NOAA Technical Report NWS-30, 36 pp.
Simpson, J. S., C. Kummerow, W.-K. Tao, and R. F. Adler, 1996: On the Tropical Rainfall Measuring
Mission （TRMM）. Meteorol. Atmos. Phys. 60, 19-36.
Tao, S.Y., and L.X. Chen, 1987: A review of recent research on the East Asian summer monsoon in
China. Monsoon Meteorology, C.-P. Chang and T. N. Krishnamurti, Eds., Oxford University
Press, 60-92.
Wang, S.-Y. and T.-C. Chen, 2007: East Asian summer monsoon rainfall contributed by different
weather systems measured over an East Asian island. J. Appl. Meteor. Clim. (pending review)
Ye, D., 1981: Some characteristics of the summer circulation over the Qinghai-Xizang (Tibet) Plateau
and its neighborhood. Bull. Amer. Meteor. Soc., 62, 14-19.
Yeh, T.-C., Dao, S.-Y., and Li, M.-T., 1959: The abrupt change of circulation over the Northern
Hemisphere during June and October. In The Atmosphere and the Sea in Motion, Ed. by B.
Bolin, Rockefeller Inst. Press, New York, p. 249-267.
Yoon, J.-H., T.-C. Chen, 2005: Water vapor budget of the Indian monsoon depression. Tellus, 57A,
770-782.
Yoshikane, T. and F. Kimura, 2003: Formation mechanism of the simulated SPCZ and Baiu Front using
a regional climate model. J. Atmos. Sci., 60, 2612–2632.
Zhang, Y., T. Li, B. Wang, and G. Wu, 2002: Onset of the summer monsoon over the Indochina
Peninsula: Climatology and interannual variations. J. Climate, 15, 3206–3221.
49