Rainfall estimation of landfalling tropical cyclones over Indian coasts through satellite imagery

One of the most significant impacts of landfalling tropical cyclones is caused by the copious rainfall associated with it. The main emphasis of present study is to provide some guidance to the operational forecasters for indicating the possible rainfall over the areas likely to be affected by the cyclones after landfall. Study of 14 past landfalling cyclones reveals that the maximum rainfall occurred in the first forward quadrant of tropical cyclone movement, followed by the second quadrant and the areas near the track of the cyclones. Isohyetal analysis of 24 hours rainfall for each cyclone reveals that occurrence of heavy rainfall is generally confined up to 150 kms radius from the storm centre and rainfall is found to generally extend up to 300 kms with gradual decrease in amount. The rainfall receiving areas are mostly covered with convective clouds with cloud top temperatures of -80 to -60 degrees C, prior to and after the landfall of the systems. In 93% of tropical cyclones out of the 14 cases studied, 70% convection lay to the right of the track. To examine the rainfall asymmetry due to asymmetry in distribution of convection, cloud top temperatures derived from satellite infrared imagery data have been taken as the proxy of strong convection. It is also revealed in the study that the slow moving tropical cyclones cause heavy rain rather than fast moving tropical cyclones. The Bay of Bengal cyclones which crossed coast as cyclonic storm and very severe cyclonic storm caused 71.4% rainfall within the range 010 cm, 22.8% rainfall in the range 11-20 cm and 4.3% rainfall within the range 21-30 cm in the area of radius of 300 kms from the centre of the cyclonic storms. For the Arabian Sea tropical cyclones, in general, about 70% rainfall occurred within the range 16-25 cm in 24 hours.