Simulation study of heavy rainfall episodes over the southern Indian peninsula

Southern peninsular India experiences widespread thunderstorm activities during the pre-monsoon season due to the presence of a north-south trough, an easterly trough, and low pressure areas over the surrounding Indian Seas. In March 2008, southern peninsular India received unusual heavy rains because of the interaction between a large amplitude easterly trough and a mid-latitude westerly trough during 12 to 14 March, and because of the formation of a low pressure system over the Arabian Sea during 21 to 24 March. Simulation of these two heavy rainfall events is attempted using the Weather Research and Forecasting (WRF) model. The model's performance is studied in terms of the wind, pressure, movement of these synoptic scale systems, moisture and rainfall fields. Results indicate that this model has the capability to predict the movement of both easterly and westerly troughs and to simulate low pressure very well. The model is able to capture the areas of precipitation maxima in both the cases. The bias score is found to be more than 1 in precipitation thresholds up to 35mm and less than 1 in precipitation thresholds above 35 mm. The maximum values of critical success index (CSI) and equitable threat score (ETS) are 0.49, 0.32 and 0.42, 0.13 in 24 and 48h forecasts, respectively, in Case 1 and 0.69, 0.23 and 0.71, 0.19 in Case 2. The values of CSI and ETS drop considerably above the 30mm rainfall threshold in both 24 and 48h forecasts showing that the model performs better in lower rainfall thresholds.