Monsoon probably is the most prominant weather phenomenon for the people living in the subtropics because monsoonprecipitation, in particular, flood or drought, may have a tremendous effect on agriculture and human lives. For example, the Yangtze River (YR) flood from June through August 1998 destroyed over 30 million acres of farmlands and ruined more than 11 million acres of crops (Lau and Li, 1999).Over Asia there are two well recoganized monsoon systems, the Indian Monsoon and the China Monsoon.
Though Monsoon occurs seasonally resulting from the thermal contrast between the land and the surrounding oceans, its time of onset, area affected, and intensity vary yearly.To accurately predict the monsoon is of vital importance since preventive measures may reduce loss of life and ameliorate economic loss.Meteorologists began forecasting the monsoons a century ago. Mathematical climate models and statistical or empirical models are the main forecasting tools.
Scientists at Goddard Space Flight Center NASA study the 98 devastating natural disaster over Yangtze River region with TRMM data.They use TRMM TMI data-TRMM 2A12, precipitation Radar data-TRMM 2A25, and in-situ observations from South China Sea Monsoon Experiment (SCSMEX).The analysis based on these data reveals the dynamic and thermodynamic conditionsassociated with development of meso-scale convective system that gave rise tothe Yangtze River flood in relation to the evolution of South China Sea Monsoon.
The top panel of the figures below shows the two defined areas, South China Sea (10-24oN, 108-122oE, blue shaded), and Yangtze River area (24-38oN, 116-140oE, red shaded). The bottom one shows the time series of TRMM TMI rain rates (mm day-1) averaged over the South China Sea (blue line) and the Yangtze River area (red line) for the period of May 1 to June 30 1998.
Two lines are nearly out of phase during the whole May denoting thedifferent precipitation variations over the two areas.
High precipitation persisted over the South China Sea for a long period of 25 days (day 15 to day 39, blue line); when it decreased after day 40 (June 9), the precipitation over the Yangtze River area seemed to increase significantly and persisted for more than 10 days.
A question is asked then: "Is there atemporary connection between the evolution of South China Sea Monsoon and Yangtze River area flood?" To answer this question, scientists investigated the evolution of the South China Sea monsoon and Yangtze River area flood.
The lower figure shows the development of monsoon circulation along withthe precipitation field.TRMM TMI rain rate (mm day-1) (shaded area, see intensity scale) are plotted superimposed with 850mb wind (m s-1, arrows) for the period of May 18 - May 23, 1998.It shows a monsoondepression (anti-clockwise strong wind) over the Bay of Bengal developing on May 18, and convection occurring east of China.With the southward shift of the convection, westerly winds associated with the Bay of Bengal depression developed, feeding moisture into the South China Sea around May 20, when the monsoon onset occurred.
The next figure shows the convection condition over the Yangtze River area during the early stage of severe flood.Figures are plotted in the same way as the last one but for the period of June 11-16, 1998.Strong southerlywinds clashed with northerly winds over the Yangtze River area to produce strong horizontal wind shear with strong low level convergence feeding moisture leading to the first stage of severe flood.
TRMM PR data were used to reveal the cloud and precipitation featuresof the convective systems (figures below).The left column is for South China Sea area on May 20, 1998 , and the right column is for Yangtze River area for June 18, 1998. The top panel shows the horizontal distribution of TRMM PR corrected radar reflectivity factor (Z), which represents the respective precipitation intensity, at 2.5 km height. The middle panel shows thevertical structure of radar reflectivity factor, along 16.5oN for South China Sea area (left) and along 27oN for Yangtze River area(right) respectively. There are three isolated convective systems with strong reflectivity(red color, see the intensity scale) along the section.A clear distinct melting level is found around 5 km. The bottom panel shows the vertical distribution of rain cover area percentage corresponding to the measuredZ factor (horizontal axis), where the abundance ofstratiform rain above the melting level is indicated by the characteristicdiagonal structure.
(Courtesy of Dr. Li, GSFC/NASA)
Lau, W. K.M. and X. Li, 1999: Diagnosis of the 1998 Yangtze River flood using TRMM/SCSMEX data, TRMM Global Precipitation Mission Meeting , October 1999.
The precipitation radar (PR) was developed by CRL and NASDA in Japan and is a new instrument.It obtains unique rainfall information by its 215-km cross-track scan through nadir.The instrument is a 128-element active phased array system, operating at 13.8 GHz.The nadir footprint of PR is 4.3 km, with a vertical resolution of 250m.The minimum radar reflectivity factor is about 18 dBZ, corresponding to a rain rate of about 0.5 mm/hour.
TRMM 2A25 contains vertical ranfall rate profiles for one orbit.Also provided are: attenuation corrected Z profiles, parameters of Z-R relation (the relation between Z and rainfall rate), integrated rainfall rate for each ray, range bin numbers of rain layer boundaries, and many intermediate parameters.
A granule of TRMM 2A25 consists of metadata, clutter flags, and swath data. See Readme for TRMM Product 2A25 for information on acquiring and accessing this data product.
The field campaign program of TRMM is designed to provide ground truth for use in algorithm development of TRMM satellite measurement.To meet this goal, TRMM field campaigns employ ground-based radars and rain gauge networks to provide independent estimates of the TRMM variables, which the TRMM satellite also estimates.Also, the campaigns obtain aircraft measurements with instrumentation similar to the TRMM Microwave Imager (TMI) and Precipitation Radar on the TRMM satellite.The NASA DC8 and ER2 aircraft support microwave sensors similar to those aboard the satellite, and the DC8 also supports Airborne Mapping Radar (ARMAR), a prototype of the TRMM satellite radar.
TRMM field campaigns consist of TExas-FLorida UNderflight TEFLUN A (focus on East Texas) and TEFLUN B (focus on East Florida), Large-scale Biosphere-Atmosphere Experiment in Amazonia (TRMM-LBA), Kwajalein Experiment(KWAJEX), South China Sea Monsoon Experiment (SCSMEX), Convection And Moisture EXperiment (CAMEX), and Tropical Ocean Global Atmospheres/Coupled Ocean AtmosphereResponse Experiment (TOGA COARE). GDAAC collects and distributes data from TEFLUN A, TEFLUN B, TRMM-LBA, Kwajalein, and TOGA COARE. For SCSMEX, data users may find them via Colorado State University SCSMEX Site.An overview of these field campaign data with a list of physical parameters is provided by Hydrology TRMM FE Data Overview .
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