Off shore trough and very heavy rainfall events along the West Coast of India during ARMEX-2002

Abstract

Arabian Sea Monsoon Experiment (ARMEX) 2O02 was carried out from mid June to mid August to study the presence of off-shore trough (OST) and embedded vortices. Four cases of heavy rainfall along the west coast (rainfall exceeding 12 cm in 24 hour) of India were recorded on 14-16 June, 20-22 June, 26-28 June and 7-10 August 2002. The heavy rainfal1 event of 26-28 June was due to a low pressure system that moved from Bay of Bengal across Madhya Pradesh to Gujarat and adjoining Rajasthan. The other three heavy rainfall events were associated with the off shore trough and /or off shore vortices.

               Of the various sea buoys deployed in the Arabian Sea, only one buoy located off Goa, did give hint of an OST both in strong and weak wind conditions. However, surface wind data from other buoys and QSCAT surface wind did not always support the presence of OST. The indications were very subtle and it was found difficult to observe them on the NCMRWF (National Centre for Medium Range Weather Forecasting) analysis or forecast charts. In the present study it has been observed that off shore trough may be observed in weak as well as strong monsoon conditions. However, heavy rainfall events were noticed only when the monsoon current is strong both in the Arabian Sea as well as Bay of Bengal in association with some synoptic systems.  In addition, an east-west shear line in wind flow pattern extending from lower to middle or upper tropospheric levels and joining the circulation features in the Arabian Sea and Bay of Bengal has been noticed in all cases of heavy rainfall events. Meso-scale vortices/organized convection systems were also identified during heavy rainfall events on the basis of cloud features noticed in the satellite pictures and TRMM rain rate patterns.