Convective Cloud-Radiative and Microphysical Impacts from Precipitating Hydrometeors in South Asian Summer Monsoon

In this work the typical assumption that precipitating large hydrometers (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 feedbacks of LHM to both convective systems and their ambient atmosphere in the South Asian summer monsoon system of August 2006. In the first experiment, the LHM were made invisible to the radiative scheme. The results show about 20 W m^2 radiative fluxes differences for surface downward shortwave radiation and OLR. However, the monsoon depressions are not strongly affected by the change of radiative forcing in terms of the genesis location, intensity of sea-level pressure, tracks, and lifetime, neither are mesoscale convective systems. 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 major convection. The cloud ice and cloud water are almost doubled, resulting in a new composition of hydrometeors in cloud systems. Moreover, the monsoon depressions are found to be greatly intensified for prolonged time with the excessive diabetic heating from condensation and adiabatic heating from compensating downward motion as well as reduced evaporative cooling 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.