WORLD METEOROLOGICAL ORGANIZATION
COMMISSION FOR ATMOSPHERIC SCIENCES
THORPEX ICSC
PDP Working Group
Third Meeting
CAS/THORPEX ICSC/
PDP-3/Doc.4.1
(04.VII.2010)
Item: 4.1
Original: ENGLISH
ETH, Zurich
5-7 July 2010
T-PARC and TCS08
(Submitted by Patrick Harr)
(For information and discussion)
1.
Introduction
The THORPEX Pacific Asian Regional Campaign (T-PARC) is a multi-national field campaign that
addresses the shorter-range dynamics and forecast skill of high-impact weather events in one region
(Eastern Asian and the western North Pacific) and the downstream impact on the medium-range
dynamics and forecast skill of another region (in particular, the eastern North Pacific and North
America). Although many significant weather events occur over eastern Asia and the western North
Pacific, the focus of T-PARC is on various aspects of typhoon activity, which include formation,
intensification, structure change, motion, and extra-tropical transition. Because of the significant
impact of typhoon activity on the region of eastern Asia and the western North Pacific, T-PARC is
comprised of several affiliated programs. These programs and their national sponsor include:
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Tropical Cyclone Structure-2008 (TCS-08) [United States]
Typhoon Hunter-2008 (TH-08) [Japan]
Predictability and Observation Experiment (PROBEX) [South Korea]
Tibetan Plateau Experiment [China]
The South China Sea Experiment [China]
Dropwindsonde Observations for Typhoon Surveillance near the Taiwan Region
(DOTSTAR) [Taiwan]
The combination of observational platforms and collaborative experiments is set such that the
experimental design for T-PARC addressed three primary components.
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A tropical measurement strategy was designed to examine circulations of the tropical western
North Pacific monsoon environment as they related to tropical cyclone formation, tropical
cyclone intensification, and tropical cyclone structure change.
The measurement strategy for the extra-tropical transition (ET) and downstream impacts was
based on the poleward movement of a decaying tropical cyclone and the resulting intense
cyclogenesis that often results from its interaction with the mid-latitude circulation.
The third measurement strategy focused on identification of regions in which extra
observations may reduce numerical forecast error growth. In T-PARC, the targeted
observations were aimed primarily at reducing errors associated with forecasts of tropical
cyclone track over the western North Pacific.
The report submitted to the ICSC 8 meeting in November 2009 highlighted the field program
accomplishments. In this report, brief summaries of early results are presented based on analyses of
data obtained during the field phase of T-PARC. For consistency, results are provided under the
same section titles as defined in the ICSC 8 report on the field program activities.
2.
Early Stage Tropical Cyclone Objectives
During the first two weeks of TPARC/TCS-08, the formation of TY Nuri was examined over a period of
successive days with combined WC-130J and P-3 aircraft missions. Numerical analyses and
observational data show that the surrounding base state flow was an easterly trade wind flow and the
precursor disturbance to Typhoon Nuri was an easterly wave that originated in the ITCZ in the Central
Pacific that could be tracked more than 10 days prior to tropical storm formation (Lussier 2010).
However, forecasts from a suite of operational numerical forecast centers did not predict the formation
of the tropical cyclone at lead time greater than 48 h. The combination of dropsonde, flight-level and
Doppler radar data provided a unique data set that defined the characteristics and organization of
deep convection within a developing synoptic-scale tropical wave. Careful analysis of Doppler radar
data identified a line of strong convection in the pre-Nuri disturbance that consisted of intense vortices
concentrated in small regions with a strong local circulation, and with downdrafts located along its
sides (Cisneros et al. 2010). The convective feature develops in a high vorticity monsoon environment.
Rapid development from a tropical depression into a tropical storm was documented on the second
set of aircraft missions. During this period spinup due to vorticity convergence far exceeded the
spindown tendency of surface friction (Raymond and Lopez-Carillo 2010). This convergence was
aided by the presence of ambient vorticity of the monsoon environment. The vorticity pattern at low
levels had a great deal of fine-scale structure, much of which appeared to be associated with deep
convective towers. Spinup associated with tilting was moderately strong, but variable in sign and less
important than the convergence of vorticity.
Observations collected during two consecutive aircraft missions into pre-depression Hagupit revealed
a developing low-level circulation (LLC) four days prior to the issuance of a tropical cyclone formation
alert (Bell and Montgomery 2010). Model analyses and satellite imagery suggested that the early
circulation was part of a westward propagating disturbance at 18 N latitude, well-displaced from the
ITCZ and any southwesterly monsoonal flow. The two-plane mission on the next day revealed that the
LLC was persistent but with a complex vertical shear signature was evident in the convective
structures. Hagupit later developed into a major typhoon causing over 1 billion dollars in damage and
67 deaths. ELDORA radar data were collected in both stratiform and deep convective regions to
identify the structure and evolution of the pre-depression disturbance.
The analyses of in situ data obtained during the formations of TY Nuri and TY Hagupit have been
generalized to a broad set of convective episodes using satellite data to examine 16 ring-like
mesoscale convective events (Elsberry and Chollet 2010). Three of these systems led to formation of
tropical depression. The most dramatic of these events (termed mesoscale convective blowouts) were
ring-like curved bands of more than 5° latitude diameter with radial outflow in all directions around a
cloud-free area. A second unique aspect was the re-formation(s) of a convective cluster within the
cloud-free area that eventually became the locus of the tropical tepression. Comparison with 25 km
resolution ECMWF analyses from the Year of the Tropical Convection (YOTC) archive has revealed
the three-dimensional structure of the synoptic environment of the mesoscale convective events.
The performance of the ECMWF, UKMO, GFS, and NOGAPS models in predicting tropical cyclone
formation has been evaluated (Elsberry et al. 2009). It was found that an experienced analyst was
able to define individual model characteristics and tendencies related to the pre-tropical cyclone
seedling to tropical cyclone transition. When all four global model forecasts were in agreement as to
position and evolution, high confidence can be given to the prediction scenario with few false alarms.
The consensus technique was most successful for those seedlings that will later become tropical
storms or typhoons, but was not successful for weaker systems.
An early TPARC/TCS08 result is the validation of two subjective and four objective satellite-based
tropical cyclone estimation methods with in situ aircraft observations in four typhoons (Berger et al.
2010). Significant spread was found in the subjective estimates among the three operational agencies
and five independent analysts. Objective techniques based on microwave and on infrared imagery
were very competitive with the subjective techniques. Therefore, the satellite consensus (SATCON)
improved the accuracy over all methods
The above studies are a sample of the investigations being done with aircraft and remotely-sensed
data in conjunction with comparison to numerically-produced analyses and forecasts. These types of
studies will provide guidance for diagnostic analyses of numerical forecasts that will lead to increased
utilization of numerical forecasts of tropical cyclone formation.
4.
Targeted observations during T-PARC and TCS08
As part of the T-PARC/TCS08 objective to investigate the entire life cycle of a tropical cyclone,
improvements to the forecasts of tropical cyclone motion were sought by obtaining targeted dropsonde,
driftsonde, ship, wind lidar and water vapour lidar observations in regions with high sensitivity. Each
targeted observation case was selected two days prior to the targeted observation time, and the final
flight paths were chosen one day earlier, based on the targeted observation guidance. Several
common verification regions, Guam, Taiwan, and Japanese areas, had been agreed prior to the
experiment among the nine targeting groups. During the field phase of T-PARC/TCS08, a targeted
observation team identified potential opportunities to collect targeted observations. In addition to fixed
verification regions, sensitive area calculations were performed for individually selected verification
regions through the ECMWF/MetOffice PREVIEW Data Targeting System. JMA also conducted
moving sensitive area calculations in the vicinity of typhoon centers (Komori et al. 2010, JMA Tech.
Review 12). The entire team (members not in Monterey participated via a web-based
telecommunication system) then met to discuss these potential targeting opportunities, timelines, and
flight tracks.
The influence of these targeted observations on the typhoon track forecast skill was investigated by
data denial experiments using the global models of ECMWF, JMA and NCEP and the limited area
Weather Research and Forecasting (WRF) model nested in the Korean global model. All models show
an improving tendency of typhoon track forecasts, but the degree of improvement varied from about
20-40% in NCEP and WRF to a comparably low influence in ECMWF and JMA. This is likely related to
lower track forecast errors without dropsondes in the latter two models, presumably caused by a more
extensive use of satellite data and 4D-Var assimilation at ECMWF and JMA compared to 3D-Var of
NCEP and WRF. The different behaviour of the models emphasizes that the benefit gained strongly
depends on the quality of the first-guess field and the assimilation system (Weissmann et al. 2010,
MWR in press).
Given the tremendous damages by tropical cyclones, even small track forecast improvements can
justify the expenses of airborne surveillance missions. However, it seems the potential for forecast
improvements decreases in more complex data assimilation systems using a large amount of satellite
observations. As the use of satellite observations in data assimilation is likely to increase even further
in the future, alternative ways of observation targeting, e.g. the adaptive use of satellite observations
or the use of airborne remote sensing observations with a larger data coverage may have a larger
potential for tropical cyclone track improvements in the longer term.
In addition to the data denial experiments mentioned above, experiments are ongoing to investigate
the influence of targeted dropsondes and wind lidar observations in the NRL NOGAPS model and of
wind and water vapour lidar observations in the ECMWF model. These studies also use the adjoint
forecasts sensitive to observations to address the benefit of observations. One case study using NRL
P3 wind lidar data showed a beneficial influence of these observations on the WRF typhoon track
forecast skill (Pu et al. 2010, GRL).
Sensitivity studies revealed that it is still unclear how to properly use the information provided by
dropsondes in the tropical cyclone core and eyewall region (Harnisch and Weissmann 2010, MWR, in
press; Yamashita et. al 2010, JMA Tech. Review 12).
Several issues about the characteristics of sensitive areas based on the ETKF have been analyzed
(Chen et al. 2010) with respect to:
 sensitivity to observations of winds, temperature and specific humidity targeted at different
vertical levels;
 sensitivity to different global model ensembles or their combination;
 sensitivity to the forecast lead time, and comparison with the ensemble variance;
 sensitivity to the structure through various stages of the TC life cycle.
5.
Extratropical Transition and Downstream Impacts
During the field phase of T-PARC, several tropical disturbances moved poleward to undergo a
transition into the mid-latitudes. The character of these disturbances included a weak circulation
associated with widespread deep convection, a midget tropical cyclone, a typhoon, and a super
typhoon. Corresponding to the variety of tropical disturbances was a wide range of forecast and actual
structural changes and downstream developments, which provide a broad spectrum of forcing and
downstream impacts to be investigated.
A three-aircraft mission was conducted into what was thought to be a weakening TY Sinlaku.
However, during the mission convection increased rapidly and the ELDORA observations identify
some of the deepest convection observed during the T-PARC/TCS-08 period. Sinlaku re-intensified to
a typhoon and then began ET 24 h later. This period represents a source of a large amount of
variability in deterministic and ensemble forecast fields from a variety of operational forecast centers
(Harr et al. 2010). The measurement strategy associated with the case of TY Sinlaku was defined to
observe the important physical mechanisms associated with the ET process that influences
predictability downstream. Through the combination of ELDORA observations, dropsonde data, and
ECMWF model fields, the character of the interaction between the convection and the tilted TC vortex
has been examined (Sanabia and Harr 2010). In particular, the role(s) of the vertical distribution of
vorticity contained in the deep convection was found to be critical to the re-organization of the tilted
vortex to an upright circulation that intensified to typhoon strength. The overall combination of vertical
wind shear, TC vortex, and deep convective activity provided for a unique interaction among synopticscale, storm-scale, and mesoscale factors. The structural changes of Sinlaku as a source of eddy
kinetic energy for downstream development was found to have significant impact on the forecasts of
synoptic-scale features across the North Pacific and North America (Harr et al. 2010).
TY Jangmi also provided a source of reduced predictability, but this was related to modification of the
mid-latitude jet stream due to outflow from Jangmi rather than increased convection and downstream
ridge building (Grams 2010). Shortly after recurvature an enhanced poleward outflow occurred from
the decaying tropical cyclone. As the typhoon approached the midlatitudes the jet expanded further
south and the wind speed in the jet core increased. Numerical simulations with the mesoscale
COSMO model combined with potential vorticity (PV) inversion were used by Grams (2010) to
investigate the physical mechanisms responsible for the interaction between TY Jangmi and the
midlatitude flow. The analysis of the PV structure along with trajectory calculations showed that most
of the lifting of tropical air to the jet level occurs in a small band at the baroclinic zone. This results in
an enhanced outflow and may explain the acceleration and deflection of the jet. Furthermore, a new
PV anomaly evolves associated with latent heat release at the baroclinic zone. This low level PV
anomaly shows characteristics of a diabatic Rossby wave that propagated along the baroclinic zone,
and developed into an extratropical cyclone. Therefore, physical processes that occurred at the
interface between the subtropical and mid-latitudes were found to be crucial for the direct impact of ET
on the midlatitude flow.
Examination of the evolution in other numerical models indicates that
downstream midlatitude development is sensitive to the ouflow-jet interaction and the latent heat
release at the baroclinic zone. Variability in these features appears to be related to variability in
forecast accuracy.
The above examples represent a sampling of investigations being conducted on the variety of data
obtained during the T-PARC/TCS-08 field campaigns. As indicated in these studies, results are
directed to eventually increase predictability of tropical cyclone-related weather. Comparisons between
numerical analyzed features, forecast representations, and in situ observations, are beginning to prove
useful in terms of furthering diagnostic analysis of operationally-produced analyses and forecasts.
Comparisons among operational models in which the observations are utilized with different methods
should provide additional guidance to the interpretation of numerically-generated forecast products
related to tropical cyclones and related weather phenomenon.
References
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TCS-08. 29th Conference on Hurricanes and Tropical Meteorology, American
Meteorological
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NWP impact studies during T-PARC. 29th Conference on Hurricanes and Tropical
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