Kozo Ninomiya

AGCM experiment of the effect of cumulus suppression on convection center formation over the Bay of Bengal

[1] We examined the effect of cumulus triggering on the distribution of precipitation during the Indian summer monsoon using the CCSR/NIES/FRCGC atmospheric general circulation model (AGCM) with different cumulus parameterizations. To investigate the effect, we carried out two kinds of simulation. The no-cumulus-suppression (NOCS) run used the original AGCM in which we implemented the prognostic Arakawa-Schubert (AS) scheme. In the other run, we introduced cumulus suppression (CS) as an additional condition of the AS scheme; the CS permitted cumulus convection only when the environmental relative humidity averaged in the modeled cumulus cloud region of the AS scheme exceeded 80%. Special attention was given to the formation of the convection center over the Bay of Bengal (CCBB). The NOCS run could not reproduce the CCBB because the original AS scheme was controlled solely by the convective available potential energy (CAPE). In the CS run, the CS suppressed the unrealistic precipitation simulated in the NOCS run east of the Indian subcontinent (around Sri Lanka) through large-scale topographical effects and well reproduced the westward propagating disturbances coming from the South China Sea and Indochina Peninsula. The westward propagating disturbances created heavy rain in the Bay of Bengal. As a result, the CS run reproduced the realistic distribution of precipitation during the Indian summer monsoon. The AGCM experiment showed that the triggering of cumulus convection played a key role in the distribution of precipitation in the Indian summer monsoon. For instance, the CS suppressed the unrealistic precipitation east of the Indian subcontinent. In addition, in the CS run, ascending motion orographically produced by the large-scale horizontal flow triggered cumulus convection around the northeast coast of the Bay of Bengal, and the propagating disturbances brought heavy rainfall to the Bay of Bengal.