Impact of data assimilation in simulation of thunderstorm (squall line) event over Bangladesh using WRF model, during SAARC–STORM Pilot Field Experiment 2011

Abstract

A widespread thunderstorm (squall line or multicell line) occurred over Bangladesh on 11 May 2011 during 0300 UTC to 1300 UTC. A north–south oriented well-established squall line of 400 km length was noticed to the center of Bangladesh at 0530 UTC and the vertical extend of this system was about 16–18 km. An attempt has been made to study the impact of data assimilation in simulation of the selected thunderstorm event of 11 May 2011 using Weather Research and Forecasting (WRF) model. The WRF model was run in STORM Phase-I domain at 9 km horizontal resolution using six hourly NCEP-FNL datasets from 0000 UTC of 11 May to 0000 UTC of 12 May 2011 as initial and boundary condition for control (CNTL) run. The 0300 UTC Synop, 0000 UTC Temp and 0300 UTC Khepupara DWR reflectivity and redial velocity of 11 May 2011 were assimilated in 3DVar with the first guess of WRF model for data assimilation (DA) run. Model outputs have been analyzed for CNTL and DA to compare and/or asses the model performance. It is found that the DA enhanced in the synoptic and environmental characteristics as compared to that of CNTL. The WRF model outputs with DA help to investigate the synoptic and environmental characteristics responsible for the occurring of this unusual event. The model products showed a cyclonic circulation over Gangetic West Bengal of India at 925 hPa and over Assam and adjoining areas of India at 500 hPa. Strong upward motion was noticed over Bangladesh. Significant moisture incursion was observed over center and southeast parts of Bangladesh at 925 and 850 hPa with southerly/southwesterly flow of 10–20 ms−1 (19–39 knots). Result showed that the WRF model with DA could capture thunderstorm event of 11 May 2011 in reasonably well though there are some spatial and temporal biases in the results.