An Estimation of Atmospheric Moisture Transport Over Indian Sub-continent by Using 4D-VAR Data Assimilation System

By using an atmospheric general circulation model and a 4-dimensional variational (4D-VAR) data assimilation system, origin and transport processes of water over Indian sub-continent in summer and autumn are examined. Koster et al.(2004) pointed out that Indian sub-continent is one of the specific locations for which soil moisture anomalies have a substantial impact on precipitation in boreal summer (JJA) through a multimodel experiment of land-atmosphere coupling strength. On the other hand, the diagnostic estimation of recycling ratio by Trenberth (1999), which is defined as the relative contribution of the locally evaporated water to the precipitating water in the same region, shows that the value of recycling ratio over Indian sub-continent in autumn is higher than that in summer. Then we investigate the seasonal march of atmospheric moisture transport of precipitating water over Indian sub-continent, especially focus on the differences between boreal summer and autumn season. The global distribution of recycling ratio in our model has been well-reproduced in comparison with Trenberth_fs results. The moisture source distribution of precipitating water over India can be detected through sensitivity experiments using our 4D-VAR data assimilation system. These calculations are conducted on the Earth Simulator. In summer monsoon season, two typical cases can be seen in our model. One is that the atmospheric moisture which is a source of rainfall over Indian subcontinent is mainly supplied from Arabian Sea transported by Somali jet, which results in the typical summer Indian monsoon circulation. In addition, some other cases show that the atmospheric moisture is mainly supplied by the evapotranspiration from Indian land area, that is, locally recycled water. These results indicate that the amount of water supply over India is mainly determined by the combination of these two cases. And the local recycling of water is considered to be important to decide the summer precipitation over India as well as the transport of water vapor from the outside of Indian sub-continent. In autumn, the contribution of atmospheric moisture transport from the outside becomes relatively smaller as accompanied by summer monsoon withdrawal. As the result, water recycling in this area is more important than that in summer. This implies that over India the condition of soil moisture through summer monsoon season could strongly affect the rainfall activity in autumn. We are now developing the 4D-VAR data assimilation system using a coupled atmosphere-ocean-land global circulation model, and this method will be applied to the interannual cases in the near future.