GCMでシミュレートされたインドモンスーンとその変動に対する積雲対流ダウンドラフトの効果

Abstract

The sensitivity of mean monsoon and its variability simulated by Hadley Centre Climate Model to changes in convection scheme, has been examined by integrating the model with and without parameterisation of convective downdrafts. Summer monsoon seasons during 7 years 1982, 1983, 1984, 1987, 1988, 1991 and 1994 have been simulated. These years have been selected for the study as the monsoon rainfall over India showed large interannual variability in these years. Results are compared with NCEP reanalysis circulation and Xie-Arkin precipitation data.Analysis of the model output shows that some of the systematic errors in the model in the monsoon simulation are reduced with incorporation of downdrafts. The systematic error by which the monsoon low level westerlies extend deep into the west Pacific ocean is substantially reduced by the inclusion of downdrafts. The lower tropospheric monsoon flow over the north Indian ocean is strengthened with downdrafts. TEJ simulated with downdrafts is closer to NCEP reanalysis. Inclusion of downdraft improves the precipitation simulation over land areas of India, and also simulates the correct position of the monsoon trough. When downdrafts are not included there is excessive precipitation over the west equatorial Indian ocean and less precipitation over India. Surface latent heat flux increases with the downdrafts. The systematic error of the model which simulates stronger low level circulation in El Niño years than in La Niña years is reduced, though not completely eliminated. The interannual variability of simulated monsoon precipitation is more realistic with downdrafts. Spells of active and ‘break, ’ or weak monsoon situations, are more prominently simulated when downdrafts are included. Larger number of strong synoptic disturbances are simulated over India with the inclusion of downdrafts.