7B.1 Cloud-radiative and microphysical impacts from precipitating hydrometeors in simulations of South Asian Summer Monsoon

Wu, Wanchen, and Wen-wen Tung, Purdue University

In this work the typical assumption that precipitating large hydrometors (LHM) resulted from convective clouds fall out instantaneously in the GCM is examined using the WRF-ARW Version 3.3. A series of experiments were performed to study the radiative and microphysical feedback of LHM to both convective systems and their ambient atmosphere in the South Asian summer monsoon system of August 2006. For the control run, the WRF model was set up with two one-way-nested domains with horizontal grid lengths of 27 and 9 km, respectively. The inner 9-km domain covered the Indian subcontinent, the Bay of Bengal, and part of Indochina. The WRF Single-moment 6-class Microphysics Scheme (WSM6), new Goddard radiative transfer scheme, University of Washington shallow convection and TKE schemes were used in both domains. The Kain-Fritsch convective parameterization scheme was used in the outer domain but turned off in the inner domain so that the convective cloud processes were only represented and featured by WSM6 in the latter.

In the first experiment, the LHM were made invisible to the radiative scheme. The results show an average gain of incoming shortwave flux of 20—30 W/m2 at surface and outgoing longwave radiation flux (OLR) of 5—20 W/m2 at the top of atmosphere over the Bay of Bengal. However, the monsoon depressions are not strongly affected by the change of radiative forcing in terms of composite wind fields, intensity of sea-level pressure, tracks, and lifetime. In the second experiment, the LHM were prohibited from suspending within the model grids from one model time step to the next. The results show a significant northward shift of monsoon depressions with low level convergence over land, resulting in more than 30 W/m2 local reduction in both downward shortwave flux at surface and the OLR. Moreover, the monsoon depressions are greatly intensified for prolonged time when coupled with the excessive diabetic heating from condensation in the rapid precipitating process. In conclusion, the study may set an upper bound to the GCM bias resulted from the instantaneous fall-out assumption of LHM; the bias are likely due to miscounted microphysical effects of the cloud particles and the ensuing change of radiative forcing. The necessity of correct parameterization of microphysical processes in convective clouds in GCM is thus revealed.