The northern region of the Bay of Bengal is known to receive the maximum mean precipitation in south Asia during the summer monsoon. In the active phase of the monsoon, diurnal variability is a prominent mode: deep convection forms over the northern part of the Bay and reaches maximum intensity at around 0600 local time, adopting the shape of NE-SW elongated roll, with dynamic features resembling propagating gravity waves (Zuidema 2003). GCMs have considerable difficulty in getting the correct phase of such diurnal variation or, in general, all diurnal variations in the Tropics (Yang and Slingo 2001). During the Joint Air-Sea Monsoon Interaction Experiment (JASMINE, Webster et al. 2001), a multi-day, vigorous, diurnally repeating, southward moving disturbances was observed in the Bay. The system initiated over land, then emanated into the Bay with a fast propagating speed of 50 km/hr (14 m/s). Inspection of the satellite rainrate products (3B4XRT) from the Tropical Rainfall Measuring Mission (TRMM) suggests that similar systems occurred during the Indian summer monsoon in 2003.

We seek to numerically realize, using MM5, the convective activity observed by TRMM ranging from diurnal to multi-day scales over the Bay of Bengal during August 2003. A three-domain interactively-nested technique is used, with horizontal grid resolution telescoping from 81 to 9 km. Our goal is to recreate the dynamical and thermodynamical features associated with convective systems, in order to detail possible mechanisms for their genesis and maintenance. Preliminary results with the 81-km large domain experiment show that the model captures convective events over the Bay and further south in the Indian Ocean reasonably well. Sensitivity to convective parameterization schemes will be addressed.