Operational nowcasting of thunderstorms in India and its verification

Abstract

Thunderstorm is a severe weather phenomenon, which develops mainly due to intense convection and is accompanied by heavy rainfall, thunder, lightning, hail and often with the passage of a squall line. Usually, these thunderstorms have the spatial extent of a few kilometres and life span less than an hour. IMD implemented nowcasting of thunderstorm and associated weather for major cities of the country that come under their coverage of Doppler weather Radar network in December 2012. A total of 120 cities were covered for issue of three hourly thunderstorm nowcast. This paper discusses the nowcasting techniques and monthly and seasonal verification of the thunderstorm nowcast issued by various Meteorological centres and Regional Meteorological Centres of IMD for the Pre-Monsoon and Monsoon Period, 2013. The performance results for occurrence/non-occurrence of thunderstorm/squall/hail Nowcast are expressed in terms of Forecast accuracy (ACC),False alarm ratio (FAR), Probability of detection (POD), Critical Success Index or the threat score (CSI) and Equitable Threat Score (ETS). The results indicated that the average POD for all months remained above 0.6 and average FAR was below 0.5. Similarly ETS and CSI both were between 0.5 and 0.9 for all months. The convective scale events in pre-monsoon season had a higher probability of detection and lower False alarm ratio as compared to monsoon season. The Skill scores varied from one region to another depending upon the experience of the forecaster and the frequency of the event.