Kinematic
Characteristics of mesoscale precipitation systems nearby the
Baiu fronts by Doppler radar observations
1
Kim, Kyung-Eak, 1Joo-Hyung Son, 2Gyu Won Lee
1Department
of Astronomy and Atmospheric Sciences Graduate School, Kyungpook National
University Taegu 702-701, Korea
2Department
of Atmospheric and oceanic Sciences McGill University 805n Sherbrooke St.
W. Montreal, Quebec Canada H3A 2K6
Analysis was made by retrieval of two or three
1. Introduction
dimensional wind fields from a single or dual
The summer monsoon rainfalls over East Asia
Doppler radar observation. Using dual Doppler radar
occur along a quasi-stationary front, called the Baiu
observations, Ishihara et al. (1995) analyzed the
front (Meiyu front in China). The Baiu front is a weak
structure of a Baiu frontal rain band, which was
baroclinic zone that develops in East Asia during the
composed of a convective region forming the
season from May to July and is identifiable more in
leading edge of the rainband and a trailing stratiform
terms of the horizontal moist gradient or changes in
region. They found that the structure of the rainband
wind direction at low levels rather than temperature
was similar to those of mid-latitude squall lines or
gradient.
tropical squall lines. Kanada et al. (2000) studied
Many studies, which were based on the meso-β
that
rainfall
enhancement
of
band-shaped
scale analysis, were made on the Baiu frontal
convective cloud system in the downwind side of
rainfalls by radar observations. Ogura et al. (1985)
Yaku-shima
showed
observations during the Baiu period in July 1996.
that
new
convective
cells,
whose
dimensions were about 10 to 30 km, formed
The
Island
purpose of
by
dual
Doppler
radar
the present study is
to
successively to the west of the slow-moving
investigate how the wind in the vicinity of the front
mesoscale cloud clusters. Using X-band Doppler
affects the development of cloud precipitation on the
radar data, Takeda and Seko (1986) analyzed the
Baiu frontal zone. Two precipitation events around
formation and maintenance of the band-shaped
Yaku-shima Island Japan, which developed during
convective system, whose dimensions were 50 km
the Baiu season, were analyzed using X-band
in length and 20 km in width. Their analysis showed
Doppler radar observation data.
that the convective system was mainly composed of
several
meso-γ
scale
organized
multicellular
2. Data acquisition and analysis
convective clouds, in which new cellular echoes
were successively formed about 5 km apart a pre-
The data employed here are Doppler radar data,
existing cellular echo on its right flank every 20
surface weather maps, GMS IR images, and rainfall
minutes. Recently, Takahashi et al. (1996) analyzed
amounts
both mesoscale and convective scale features of
sounding data. The GMS IR images are used to
Baiu frontal heavy rainfall events, focusing on the
analyze the changes of cloud distribution and types
physical mechanisms of development of heavy
with time. The Doppler radar data are used to
rainfalls. To investigate the detailed kinematic
retrieve mesoscale wind fields, and to analyze their
structure of the Baiu frontal rainfalls, the meso-γ
kinematic properties including the development and
of
AMEDAS
including
atmospheric
structure of precipitation. Fig. 1 shows the domain of
Author’s Corresponding address: Kyung-Eak Kim,
Dept. of Astronomy and atmospheric sciences,
Kyungppok National University, Taegu 702-701
Korea. e-mail: kimke@knu.ac.kr
the
radar
observation
and
the
radiosonde
observation site. The Doppler radar (λ= 3 cm), which
belongs to the Japan Meteorological Research
Institute, was located in Yaku-shima Island, and the
of the radar echo in period 2 is the development of a
radiosonde observations were made at Minamita.
bright band at the melting level (about 4.5 km). This
The radar observations were performed by volume
configuration is also confirmed from the reflectivity in
scanning over 10 elevation angles (0.5°, 1.6°, 2.8°,
Fig. 4a.
In Fig. 3.1(b), there are two prominent
4.2°, 6.1°, 8.9°, 12.9°, 24.0°, 31.0°), with a time
features
in
resolution of about 10 minutes.
convergence at about 2 km and an upper level
the figure:
a strong
lower level
Two mesoscale precipitation systems were
divergence at 5.5 km. Houze(1997) suggested that
chosen for the present study, one developed during
this type of feature occurs in tropical convective
1600 LST 21 June to 0700 LST 22 June 1996 and
clouds. The present analysis shows that the
the other 1900 LST 5 July to 1600 LST 6 July 1996.
convective cloud along the cold-type Baiu front has
The former occurred along the cold-type Baiu front,
a kinematic feature similar to that of tropical
which was a south western part of the Baiu frontal
convective clouds.
wave from the center or crest of the wave. On
Details
of
the
kinematical
features
are
contrast, the latter was occurred in the vicinity of the
examined using the vertical profiles of divergence,
warm-type Baiu front, which was a north eastern
vertical air velocity, reflectivity and fall velocity
part of the Baiu frontal wave from the center of the
averaged over the period from 1800 LST 21, to 1900
wave.
LST 21, June( Fig. 4).
The reflectivity profile for
period 1 in Fig. 4(a) shows that without showing any
structure of bright band, the radar echo intensity had
3. Analysis results
almost uniform value of 31 dBz below the melting
3.1 Cold-type Baiu front
level. Fig. 4(b) shows a typical profile of divergence
structure as found in tropical convective clouds
A synoptic surface weather map is presented in
(Houze, 1997).
The vertical velocity of air is shown
Fig. 2, where the cold-type Baiu front is developed in
in Fig. 4(c), where the maximum updraft speed is
the south west direction, and the radar is just
Fig. 3 shows the
about 3.5 ms-1. Fig. 4(d) represents the average fall
velocity of precipitation particles, obtained from the
time-height cross sections of radar reflectivity and
integration of horizontal divergence. According to the
divergence. The cross sections are divided into four
figure, the fall velocity increases with decreasing
periods according to the geometrical configurations
height roughly over the layer from 1.0 km to 3.5 km,
of radar reflectivity in Fig. 3.1(a). The four periods
where the vertical air velocity significantly decreases.
are the followings: (1) 1630 LST 21, June to 2100
Fig. 4(d) shows that the highest increase in the fall
LST 21, June, (2) 2100 LST 21, June to 0115 LST
velocity occurs in the region between 4.8 km and 5.3
22, June, (3) 0115 LST 22, June to 0500 LST 22,
km.
located at the rear of the front.
June, (4) 0500 LST 23, June to 0645 LST 23, June.
Fig. 5 shows the vertical profiles of radar
The description on our analysis here is made only
reflectivity, divergence, vertical air velocity, and fall
for period 1 and 2. The kinematical characteristics of
velocity averaged over period 2, 2100 LST 21 to
wind field during precipitation development are
0115 LST 22, June. The radar reflectivity profile (Fig.
analyzed by Volume Velocity Processing (VVP)
5a) indicates again the cloud being a typical
method (Waldteufel and Corbin , 1979).
stratiform precipitation, showing a bright band at 4.5
km. According to the divergence profile ( Fig. 5b),
(1). Period 1 (1630 to 2100 LST)
The prominent configuration of the radar echo
there were two convergence layers; the lower one
ranging from 1 km to 2.2 km and
the other one at
in period 1 in Fig. 3.1(a) is the development of a
the elevation higher than 4.2km. This shows a
continuous radar echo greater than 30 dBZ,
contrast to the case of typical tropical stratiform
extending from near-surface up to 5 km. This type of
precipitation, where a convergence layer between
radar echo configuration is usually found in typical
two divergence layers above and below the
convective clouds in tropical regions, and also in a
convergence
dissipation stage of convective clouds in mid-latitude
downward vertical air motion (Fig. 5c) is less than
regions (Houze, 1997). The prominent configuration
0.1 ms-1, and has the maximum at 4.5-km elevation.
layer
exists
(Houze,1997).
The
The fall velocity profile (Fig. 5d) shows some
the front, affected the formation and precipitation
different characteristics, compared with
structure of clouds developed along the front.
the profile
in Fig. 4d. The maximum fall velocities are found at
4.5 km and about 4.0 km, respectively. The fall
(2). Period 2 (2100 to 0115LST)
speed in Fig. 4d has the maximum increase
The characteristic configuration of the radar
between 4.7 km and 5.3 km while the speed in Fig.
echo in this period is the bright band in Figs. 3a and
5d has the maximum between 4 km and 5 km.
5b, a typical feature of stratiform precipitation.
These differences could be attributed to the
band developed at 4.5 km from 2100 LST to 2320
difference between the precipitation types of two
LST 21(Fig. 3a).
clouds.
sounding at 2100LST 21, June, the atmosphere is
The
According to the atmospheric
The influence of the Baiu front on the
stable and has a low wind shear above 2.5 km,
precipitation development is analyzed by using the
showing a favorable condition for the formation of
parallel
of
stratiform cloud. The divergence profile for period 2
divergence relative to the front. In this case, the x
in Fig. 3(b) shows two convergence layers centered
and y axes are
taken so as to be parallel and
at 1.5 km and at 5 km, respectively and a
perpendicular to the front, respectively. Fig. 6
divergence layer in between the two convergence
represents the time-height cross sections of the two
layers. Comparing the profiles of radar reflectivity
divergence components; one is parallel to the front
and divergence obtained in this study with the typical
and the other is perpendicular.
and
perpendicular
components
Comparing the
profiles of those in tropical startiform precipitation
divergence profiles of period 1 in Fig. 3(b) and Fig. 6,
(Houze, 1997), the reflectivity pattern is very similar
the profile of period 1 in Fig. 3(b) is very similar to
to that in tropical case. However, the divergence
the profile in Fig. 6(a), especially from 1600 to 1900
profile obtained here is very different from the one in
LST, rather than the profile in Fig. 6(b). The similarity
tropical stratiform precipitation. This is quite evident
of the two profiles suggests that the air motion
by comparison of the divergence profile in Fig. 5b
parallel to the front significantly affects the development
and a characteristic profile of divergence in tropical
of convective precipitation over period 1.
stratiform precipitation (Houze, 1997). The typical
Fig. 7 shows the time height cross section of
divergence
structure
in
tropical
stratiform
two components of vertical wind shear: the shear
precipitation has two divergence layers, one at lower
parallel to the front (a) and the shear perpendicular
level near the surface and one at upper levels, along
to the front (b). The magnitudes of the wind shear
with a convergence layer between the two layers.
below 2 km in Fig. 7a are much less than those in
However, the divergence structure in Fig. 5 has two
Fig. 7b; the magnitude of wind shears in Fig. 7b are
intensive convergence layers (at about 1.5 and 5
roughly a factor of ten larger than those in 7a.
km) and two divergence layers, one below 1.0 km
Comparisons between Fig. 6 and Fig. 7 reveals that
and one above 4.2 km. Our analysis suggests that
during the period 1, the strong divergence at 2 km in
the lower level convergence is attributable to the
Fig. 6a had a close link to the vertical shear of the
Baiu front. This is because the wind shear analysis
wind component perpendicular to the front given in
(Fig. 7) indicates that the lower level convergence
Fig.7b. Further, During period 2, as can be seen
region is found to be the strong wind shear region
from comparisons, the region of strong divergence
near the surface.
at 1.5 km in Fig. 6b is almost identical to the region
Further, this suggestion can be supported by
of strong vertical shear of the wind component
comparison of the divergence profiles of period 2 in
perpendicular to the front given in Fig.7b. As far as
Fig. 3b and Fig. 6. As can be seen from it, the
the divergence and vertical wind shear below 2 km
divergence field below 3 km in Fig. 3b. is more
are concerned, the greatest vertical wind shears for
similar to the divergence field perpendicular to the
the periods 1 and 2 are two orders of magnitude
Baiu front (Fig. 6b) while the divergence filed above
greater than the greatest divergence below 2 km.
3 km in Fig. 3b, similar to the divergence field
The dominance of the vertical wind shears over the
perpendicular to the front( Fig. 6a). The effect of
divergences strongly suggests that the strong wind
wind shear on the precipitation development
shear at the lower level, which was perpendicular to
analyzed for period 2, based on the comparison of
is
Figs. 6 and 7. The comparison indicates that the
lower
divergence
can
be
explained
by
the
region of strong wind shear, which is perpendicular
divergence of wind component perpendicular to the
to the front, coincided with the region of strong
front. The analysis of the time–height cross sections
convergence below 3 km.
of wind shear components (Fig. 13) shows that for
period 1 the wind shear perpendicular to the front
3.2 Warm-type Baiu front
are stronger than the shear parallel to the front.
However, it appears that for the period there is no
Fig. 8 shows a surface weather map at 1200
close link between Figs. 12 and 13.
UTC 6 July 1996, where the warm-type Baiu front is
developed in the north east direction, and the radar
is
located ahead of the front. The time-height cross
(2). Period 3 (1145LST to 1600LST)
Fig. 9 shows that the configuration of the radar
section of radar reflectivity is divided into three
reflectivity
periods according to geometrical configurations of
characteristics, compared with those of period 1 and
radar reflectivity distribution, as done in cold-type
2. The isolines from 22 dBZ to 28 dBZ above about
Baiu front.
The three periods are the followings: (1)
5 km are almost parallel to the isohypses during the
1900 LST 5 July to 2200 LST 5 July, (2) 2200 LST 5
periods 1 and 2, for which stratiform precipitation is
July to 0945 LST, (3) 1145 LST 6 July to 1600 LST 6
developed. However, the convex shape of reflectivity
July. The description on our analysis here is made
isolines for period 3, which extends above 5km, is
only for period 1 and 3 in this paper.
explicable by a non-uniform vertical air motion in
cumuliform
(1). Period 1 (1900 to 2200 LST)
for
period
3
precipitation
has
cloud.
some
The
different
nature of
cumuliform precipitation for period 3 is quite evident
During period 1, there was a stratiform
in Fig. 11a since there is no signature of a
precipitation, which was evident from the radar
development of bright band. Fig. 9b displays a time
reflectivity patterns in Figs. 9a and 10a, showing a
series plot of the divergence profile for period 3.
typical bright band at the melting level. The vertical
However, it is not easy to get an overall picture of
profiles of the radar reflectivity distribution show a
the profile. The overall divergence profile is given in
bright
band
at
4.5-km
elevation,
the
Fig. 11b, where there is divergence below a height
The divergence
of 2.3 km and also a weak convergence. It is
distribution in Fig. 8b shows that there is a
remarkable that the divergence profile in Fig. 11b is
divergence at 1900 LST just below 5 km while a
opposite to that in Fig. 4b, where there exists
convergence exists above 5 km. The divergence
divergence in upper layer and convergence in lower
region extends to 2-km elevation at 2200UTC.
layer. Further, there is
reflectivity is greater than 34 dBZ.
where
Fig.
a contrast between the
10 represents the average vertical profiles of the
profiles of vertical air speed of Fig. 4c and 11c. The
radar reflectivity, the divergence, the vertical velocity
vertical air speed in Fig. 11c is
of air and the fall velocity of precipitation particles for
3, indicating a predominance of downward air
period 1. The divergence profile in Fig. 10b indicates
motion over the period. Fig. 11d displays a fall
that the divergence is very dominant except for only
velocity profile averaged for period 3.
a shallow, weak convergence layer at around 1-km
shows that the profile has a maximum fall speed at
elevation. Fig. 10c shows that there is
negative for period
The profile
downdraft
5.5 km and then a steady decrease in fall velocity
during the stratiform precipitation as in the case of
down to a height of 4.5 km. Below 4.5 km, the
the cold-type front. However, the downdraft in the
velocity maintains an uniform velocity (7 ms-1) down
warm-type font is stronger than that in the cold-type-
to a height of about 1.3 km.
front. According to Fig. 10d, there is an increase in
According
to
the
analysis
of
the
two
fall speed of precipitation particle from 5 km to 4km,
divergence components in Fig. 12, the overall
as the same feature can be seen in Fig. 5d.
divergence feature for period 3 in Fig. 9b is more
According to the analysis of the two divergence
similar to that in Fig. 11b. The analysis of the time–
components in Fig. 12, as done in Fig.6, the upper
height cross sections
convergence for period 1 is explicable by the
that below 2 km,
convergence of wind component parallel while the
is stronger than the shear perpendicular to it
for period 3 (Fig. 13) shows
the wind shear parallel to the front
However, it appears that for the period there is no
and a large horizontal dimension. A stratiform cloud
close link between Figs. 12 and 13, as discussed
without the band had a small size, and developed
earlier for period 1 in 3.2 .
along a short shear line just below the large
stratiform cloud.
4. Summary and conclusion
References
The present study shows that the precipitation
structure and kinematic characteristics along the
Houze Jr., R. A., 1997: Stratiform precipitation
Baiu front highly depend on its type of Baiu, that is,
regions of convection: a meteorological
cold-type or warm-type.
paradox? Bull. Amer. Meteor. Soc., 78,
2179-2196.
Regardless of the type of
Baiu front,
the frontal systems were found to be
composed
of
systems:
Ishihara, M., Y. Fujiyoshi, A. Tabata, H. Sakakibara,
convective system whose top higher than the
three
different
cloud
K. Akaeda and H. Okamura, 1995: Dual
melting level, stratiform cloud with bright band, and
Doppler
clouds developed along the vertical shear line of
mesoscale rainband
horizontal wind. It is found from the present study
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of wind along the frontal surface plays an important
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role in organizing an unique precipitation structure in
radar
analysis
of
an
intense
generated along the
Kanada, S., H. Minda, B. Geng and T. Takeda, 2000:
the Baiu front, which is not found in tropical squall
Rainfall
lines.
convective cloud system in the downwind
Schematic models of precipitation structure (not
shown) were constructed for both cold-type Baiu
front and warm-type Baiu front, based on the vertical
enhancement
of
band-shaped
side of an isolated land. J. Meteor. Soc.
Japan, 78, 47-67.
Ogura , Y., T. Asai and K. Dohi, 1985:
A case study
profiles of radar reflectivity and divergence, and
of heavy precipitation event along the Baiu
shear lines. In the case of cold-type Baiu front, both
front
small convective clouds and stratiform clouds
Nagasaki heavy rainfall. J. Meteor. Soc.
developed along the shear line. The vertical profile
Japan, 63, 883-900.
in northern Kyushu, 23 July 1982:
of divergence in the convective system, which was
Takahashi, N., H. Hiroshi, K. Kikuchi and K. Iwanami, 1996:
formed ahead of the front, had a similar structure as
Mesoscale and convective scale features of heavy
found in tropical convective precipitations. The
rainfall events in late period of the Baiu season in July
vertical
1988, Nagasaki Prefecture. J. Meteor. Soc. Japan,
profile
of
divergence
in
stratiform
precipitation, which was developed at the level
higher than the shear line, had a more complicated
structure,
compared
to
the
typical
profile
of
divergence in tropical stratiform precipitation.
Takeda, T. and K. Seko, 1986: Formation and
maintenance
of
band-shaped
convective
radar echoes. J. Meteor. Soc. Japan, 86, 941-
In the case of the warm-type front, two types of
stratiform system were developed: one with the
bright band and the other without the band.
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954.
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developed: one behind the wind shear line and the
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Acknowledgement
developed after shear line showed the same
features of the dissipation stage of convective
This study was supported by the Korea Science
clouds in mid-latitude, based on the vertical profiles
and Engineering Foundation (KOSEF 985-400-004-
of
of
2). The authors greatly appreciate Dr. Hitoshi
precipitation particles and the divergence. The
Sakakibara, Meteorological Research Institute of
convective clouds along the shear line developed
Japan, for his kind supply of the radar observation
just below the stratiform cloud, which had a bright
data for the present study.
the
radar
reflectivity,
the
fall
velocity
Fig.1. The location of Doppler radar (Yakushima) and radiosonde observation sites.
Fig. 2. The surface weather maps at 1200 UTC
(2100 LST) 21 June, 1996.
Fig. 4. The vertical profile of (a) radar reflectivity
(dBZ), (b) divergence (10-4 s-1), (d) vertical
velocity (ms-1) and (c) fall velocity of air (ms -1)
averaged from 1800 LST 21 June to 1900 LST
21 June, 1996.
Fig. 3. Time-height cross sections of (a)
reflectivity (dBZ) and (b) divergence (1.0  10-4
s-1) from 1600 LST 21 to 0700 LST 22 June,
1996.
Fig. 5. Same as Fig. 4 except for from 2100 LST
21 June to 0115 LST 22 June, 1996.
Fig. 8. The surface weather maps at (a) 1200
UTC(2100 LST) 5 July, 1996. The arrow in the
figure indicates the radar observation site.
Fig. 6. Time-height cross sections of divergence
components (10-4 s-1) parallel (a) and perpendicular (b) to the front of Case 1, respectively.
Fig. 9. Same as Fig. 3 except for from 1900 LST
Fig. 7. Time height cross sections of vertical
wind shear components (10-3 s-1) parallel (a)
5 to 1600 LST 6 July, 1996. Time-height cross
and perpendicular (b) to the front of Case 1,
section of
(a) reflectivity (dBZ) and (b) diverge-
respectively.
nce (1.0  10-4 s-1).
Fig. 10. The vertical profiles (a) radar reflectivity
Fig. 12. Time-height cross sections of divergen-
(dBZ), (b) divergence (10-4 s-1), (c) vertical
ce components (10-4 s-1) parallel (a) and
velocity of air (ms-1), and (d) fall velocity (ms-1)
perpendicular (b) to the front of Case 2,
averaged from 1900 LST to 2200 LST 6 July,
respectively.
1996.
Fig. 13. Time height cross sections of vertical
Fig. 11. The same as Fig. 10 except for from
wind shear components (10-3 s-1) parallel (a) and
1145 LST 6 to 1600 LST 7 July 1996.
perpendicular (b) to the front of Case 2,
respectively.