Observational and numerical studies of coherent warm-season rainfall over the Bay of Bengal

An observational analysis of the precipitation coherence and diurnal variations over the Bay of Bengal and the adjacent coastal region is conducted using the TRMM Real-Time Multi-Satellite Precipitation Analysis (MPA-RT) products for three warm seasons (i.e, May through September for 2002-2004). The time- distance plots (Hovmoller diagrams) of rainfall episodes are constructed for the diagnosis of the precipitation coherency. The averaging windows for the latitude-time and longitude-time diagrams are 94E-91E in the zonal direction and 10N-22N in the meridional direction. Results show frequent long-lived precipitation episodes exceeding the typical lifetime of individual convective systems. Most of the coherent events translate southward and do not have a steering level, unlike those previously documented in several midlatitude and tropical locations. On average, the coherent signature has a meridional distance of approximately 5 degrees and a 1-day duration and propagates southward at about 8 m/s. Episodes are mostly initiated over the off-shore inland region during the middle daytime and over the coastal ocean during the night.

We have performed cloud-system-resolving simulations of two multi-day coherent precipitation episodes, corresponding to a 5-day period during 15-19 June 2003 and a 5-day period during 18-22 July 2004. The nonhydrostatic version of the Pennsylvania State University/National Center for Atmospheric Research mesoscale model (MM5) is used. In all simulations, triple interacting domains are applied, consisting of a 22.5-km grid, a 7.5-km grid, and a 2.5-km grid. As demonstrated by the latitude-time distributions of the zonally-averaged rain rate, the 2.5-km-resolution model is very encouraging in qualitatively reproducing the observed coherent rainfall patterns. Analyses are underway to investigate the structures of the simulated convective events and the physical processes responsible for their formation and propagation.