The Coupling of Convection and Circulation of Monsoon Depressions during the South Asian Summer Monsoon

This work is inspired by Professor Yanai's studies of the apparent heat source and moisture sink (Q1 and Q2) for the convection-coupled atmospheric phenomena. Conceptually, convective systems interact with the environmental atmosphere through compensating subsidence and detrainment. The former results in heating and drying effects; the latter the cooling and moistening effects. It is hypothesized that, coupled with the monsoon circulation, convective heating may serve as an energy source for the maintenance of a monsoon depression (MD). This study is therefore designed to investigate the coupling behaviors following the evolution and propagation of MDs by case analyses with the ERA-Interim reanalysis and cloud-system-resolving simulations using the Advanced Research WRF (WRF-ARW) model.

A strong MD in the well-established monsoon flow, also known as the monsoon depression (MD), during August 1 to 5 2006 has been previously simulated with ensembles of the WRF-ARW V.3.0.1 by Wang and Tung (2010). They reported that high-resolution (< O(10 km)) horizontal mesh is necessary in order to obtain realistic genesis and propagation of an MD. Beyond that, the simulation is obviously sensitive to the model moist convective processes including cumulus parameterization and, in particular in the cloud-system-resolving modeling (CSRM), the microphysical assumptions. Based upon their CSRM ensemble, further analyses to reveal how various microphysical assumptions lead to different radiative and convective heating and influence the convection-environment coupling are in progress. Preliminary kinematic analysis of the MD anomalous circulations and temperature has been performed for this ensemble, and validated with the anomalous MD structure in the reanalysis. The model and the reanalysis both show that, despite its vertically tilted structure in wind and temperature, the maximum warm anomaly within the MD is collocated with the maximum updraft over the surface precipitation maximum. This preliminarily supports the hypothesis that convective heating contributes as an energy source for the MD. The roles of convection in the MD system-wide heat and moisture balances will be evaluated by applying Q1 and Q2 analysis to both the reanalysis and the spatially-filtered CSRM simulations.