Intense Rainfall Events over the West Coast of India

Abstract

West coast of the Indian peninsula is one of the regions which receive very heavy rainfall during the summer monsoon season. Average June-September rainfall over the parts of coastal Karnataka, Konkan and Goa exceed 250 cm. Rainfall over the coast shows variability within the season; with active spells of heavy rainfall and weak spells with little or no rainfall. On many occasions, at some stations, rainfall exceeds 15 cm dayð€€€1 and occasionally 30 or even 40 cm dayð€€€1. These heavy rainfall events cause extensive damage along the coast. In the present work, the characteristics of intense rainfall events are studied. Using daily rainfall data obtained from India Meteorological Department, the most favourable locations and time for the occurrence of these events are identified. The characteristics of cloud systems over the coast and eastern Arabian sea during intense rainfall events are studied using INSAT-IB brightness temperature data with high spatial resolution of 0.25 x 0.25 . The synoptic and large scale systems associated with intense rainfall events are identified from daily averaged outgoing longwave radiation (OLR) data with spatial resolution of 2.5 x 2.5 . The probability of getting an intense rainfall event (above 15 or 20 cm dayð€€€1) is found to be highest between 10 June and 25 August. Within this period, the probability is maximum around mid June and last two weeks of July. The probability of getting intense rainfall also varies from station to station. Maximum probability is observed near 15 N. In fact the Western Ghats are closest to the coast in this region. Another peak in the probability is observed near Mumbai. These intense rainfall events, which occur on less than 2.5% of total rainfall days, contribute more than 10% of total rainfall at many stations. But the contribution to total rainfall from the intense events is not very high in the southern most parts of the coast. Cloudiness over the west coast shows variability in the intraseasonal scale, which is in close correspondence with the variability in rainfall. Generally, during the very active spells, a large fraction of the west coast is covered by clouds with top temperature below 235 K. During these spells, deep cloud systems are more frequent near 15 N and 19 N. During such spells, there are also deep clouds over the Bay of Bengal. Arabian sea is almost free from convection during the very weak spells. Convection over the Bay of Bengal was also suppressed during the very weak spells. The variability in the cloud coverage off the coast is also clearly seen in the coarse resolution, daily averaged OLR data. Cloud systems are identified by connecting contiguous grids with brightness temperature below a threshold of 255 K. It was found that, as the size of the cloud system increases, the height of the clouds also increases. However, for the smaller scale cloud systems (with size below 50 km), the cloud top temperature is always above 235 K. During the very active spells of intraseasonal variability, there are large number of deeper and larger cloud systems as compared to the very weak spells. Most of the intense rainfall events identified, which occured between 1985-1988, are associated with very large (size 300 km) and very deep (cloud top temperature below 235 K) cloud systems. But on a few occasions, intense rainfall events are associated with small and shallow cloud systems also. It has been believed for a long time that, the intense rainfall events over the west coast are generally associated with the off-shore convective systems. The analysis of OLR patterns during 100 such intense rainfall events (with rainfall above 20 cm dayð€€€1) shows that, 62% of these events are associated with large scale systems, viz tropical convergence zones (TCZ). On 15 occasions, the intense events were associated with off-shore convective systems. On 7 occasions mid tropospheric cyclones and one occasion off-shore vortex are found to be responsible for these events. On 13 occasions, TCZ was found to be also present over the Arabian Sea with either MTC or off-shore convective system. A large number of intense rainfall events occurred during the northward propagations of TCZ. Thus some of these events could be predicted if northward propagation of the TCZ from the equatorial Indian Ocean can be predicted.