Newsletter No.4 October-1998

Every year the Asian Monsoon brings about rainy season regularly over the Asian continent and the western Pacific region. However,the interannual variability of monsoon is very large,which sometimes causes hydrological disasters,i.e.,heavy rains,floods and droughts in this region.Highly accurate predictions of seasonal and intra-seasonal monsoon rainfall and water resources are,therefore,very essential and urgent for the people and countries in this region,which occupies more than a half of the global population.In addition,it is becoming clear that the variation of the Asian monsoon is closely linked with the ENSO (El Nino and Southern Oscillation),though the physical processes involved have not fully been understood.The GAME (GEWEX Asian Monsoon Experiment)under GEWEX(Global Energy and Water cycle EXperiment) has been started as part of WCRP(World Climate Research Program)in order to understand the mechanisms and physical processes related to these issues.
To understand the mechanisms of interannual variability of monsoon,it is essential and the first step to fully understand the mechanisms of seasonal cycle itself.The regional differences of the monsoon are likely to be caused by multi-time/space-scale non-linear energy and water cycle processes between land,atmosphere and oceans,under the seasonally-varying solar energy forcing.As a result,convective and precipitation activities also vary complicatedly in time and space,which inturn interact with and are fed back to variations in large-scale atmospheric circulations.
Regional process studies in the four regions of GAME (Chao Phraya river basin;Huai-He river basin;Tibet Plateau;Lana river basin)focus on understanding energy and water cycle processes between land surface and the atmosphere,and modeling these processes.Common scientific targets during the IOP are to investigate mechanisms of seasonal cycles of energy and hydrological processes in the atmosphere-land surface system in each region/river basin.Special attention was paid to understand and model the interactive processes between soil moisture,land surface heat/water budgets,atmospheric boundary layer and convective activity.
To evaluate continental-scale water cycle,data analyses based on objectively-analyzed 4DDA (4-Dimensional Data Assimilation)data.However,existing 4DDA data are not fully accurate enough to evaluate these processes and variability of water cycles in regional river-basin scales.During the IOP from May through September,1998,enhanced radiosonde observations were implemented at about 110 upper-air stations,in cooperation with meteorological agencies of this region (see Figure 1),to obtain highly accurate 4DDA data for hydro-meteorological studies mentioned above.At the same time, intensive observations of energy-water cycle processes and cloud-precipitation processes were in each regional project. On the Tibet plateau,observations of cloud and precipitation systems by a Doppler radar system (Figure 2)were made, which was also part of ground-validation program for TRMM(Tropical Rainfall Measuring Mission)satellite.Very active convection and precipitation systems over the plateau have been found through this observation.The FRSGC is contributing actively to the GAME,by participating in the activities of the GAME-IOP.

photo.1 Doppler radar system on the Tibetan plateau,Gust front is approaching the site (By courtesy of Mr.Hiroyuki Yamada,Hokkaido Univ.).

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