Comparison of best track parameters of RSMC, New Delhi with satellite estimates over north Indian Ocean

Abstract

India Meteorological Department (IMD), as a Regional Specialized Meteorological Centre (RSMC) has the responsibility of monitoring and prediction of cyclonic disturbances (CDs) including depressions and tropical cyclone (TC); collection, processing and archival of all data pertaining to CDs and preparation of best track data over the north Indian Ocean (NIO). The process of post-season analysis of CDs to determine the best estimate of a CD's position and intensity along with other characteristics during its lifetime is described as "best tracking". The best tracking procedure has undergone several changes world-over including NIO due to change in monitoring and analysis tools & procedure. However, the geostationary satellite remains the main tool for monitoring of location and intensity of CDs. There have been a few attempts to document the role and extent of satellite estimates in determining the best track location and intensity of CDs over the NIO. Hence, a study has been undertaken to compare the location and intensity of CDs based on best track parameters prepared by RSMC, New Delhi with those estimated by satellite division of IMD based on INSAT and Kalpana satellites. The average difference in location of CDs over the NIO, BOB and AS is about 39, 40 and 37 km respectively. The difference in location is 50 km or less in about 65% of the total cases and about 6% of the cases have a difference of 100 km or more over the NIO as a whole. It is about 62% and 6% over the BOB and about 70% and 5% over the AS respectively. Difference in location gradually decreases with increase in T number. It is about 30 km when the intensity of the system is T4.0 or more (very severe cyclonic storms and above intensity) over the BOB, AS and NIO. Considering the spatial distribution of difference in location of CDs, it is higher near the coast and decreases as we move away from the coast. The intensity in the best track agrees with the satellite estimates in about 85.5% of the cases. While the satellite based intensity is underestimated (Satellite division estimated T number is less than best track T number) in 9.5% cases, it is overestimated (Satellite division estimated T number is greater than best track derived T number) in about 5% cases over the NIO. Considering the spatial distribution, the difference occurs mostly when the CD lies near the coast or the islands in the NIO like the difference in location.