Tornadic Storms study over Bangladesh: Observed by TRMM, Radar and Simulated by using WRF-ARW ModelMohan Kumar Das In the present paper the tornadic storms are studied based on field survey, ground and radar observations. Low level moisture influx by southerly flow from the Bay of Bengal coupled with upper level westerly jet stream causing intense instability and shear in the wind fields triggered a series of storms for two weeks during 30 August to 14 September in 2008 . The exact time and locations of the storms are investigated by using the hourly precipitation data retrieved from a S-band radar of BMD located at Dhaka. Subsequently, the storms are simulated by using the WRF-ARW model at 2 km horizontal resolution based on 6 hourly NCEP-FNL data. The maximum intensity of surface wind speed simulated by the model was only 9 m sec -1 (32.4 km hr-1 for the 3rd September 2008 case) among the 3 cases, placing it as F0 on the Fujita scale. This seems to be highly underestimated. The highest vertical velocity (updraft) simulated by the model was 320 m sec-1 (1152 km hr-1) around 700-850 hpa, which is overestimated. The updraft reached up to 250 hPa (~10 km above the ground). It seems that the funnel vortex did not reach the ground, and might have passed a few hundred meters above the surface. The maximum values of SREH simulated by the model was 1088 m2 sec-2. The maximum and minimum values of the BRNSHR simulated by the model were 224 and 64 m2 sec-2, which were generally large enough to produce rotating storms according to the prescribed range. Results shows that while there are difference of 2-7 hours between the observed and simulated time of the storms, the distances between observed and simulated locations of the storms are several tens of kilometers. GrADS and RIP software were used to diagnose various parameters. |
