Sensitivity Study for the Simulation of Heavy Rainfall Events During Monsoon Season over Bangladesh Using High Resolution WRF-ARW Model

Abstract

In the present study, the Advanced Research WRF (ARW) v3.5.1 model have been used to simulate the heavy rainfall events in the southeastern region of Bangladesh during 23-26 June 2015, 23-26 July 2015 and 30 August – 1 September 2015. The initial and boundary conditions are drawn from the global operational analysis and forecast products of National Center for Environmental Prediction (NCEP-FNL) available at 1o ×1o resolution. The model was configured in single domain, 6 km horizontal grid spacing with 161×183 grids in the east-west and north-south directions and 30 vertical levels. Time step of integration is set to 30 seconds for maintaining computational stability as the model uses third-order Runge-utta time integration scheme. For the sensitivity study of heavy rainfall events, different 12 microphysics (MP) schemes coupled with Kain-Fritsch cumulus parameterization scheme were used. The microphysics schemes are Kessler, Lin et al., WSM3, Ferrier, WSM6, Thomson graupel, MYDM, MDM, CAM V5.12-Moment 5 class, SBU, WDM6 and NSSL2 schemes. The amount of rainfall observed at Cox’s Bazar, Teknaf, Kutubdia, Barisal, Chittagong and Sitakunda stations are 1039, 607, 602, 457, 424 and 400 mm respectively during 23-26 June 2015, at Chittagong, Kutubdia, Teknaf, Sandwip and Cox’s Bazar stations are 867, 740, 690, 535 and 518 mm respectively during 23-26 July 2015 and at Sylhet, Teknaf, Chittagong, Hatiya and Sitakunda are 396, 330, 213, 200 and 198 mm respectively during 30 August to 1 September 2015. All Bangladesh station averaged rainfall suggests that among 12 MPs, TH, SBU, WSM6 and WDM6 during 23-26 June, MDM, Lin, SBU, KS and TH schemes during 23-26 July and WSM3, WDM6, CAM, MDM and MYDM schemes during 30 August to 1 September have simulated almost similar amount of rainfall as observed during the study period of 2015. The model has also simulated similar amounts of rainfall in the heavy rainfall area of 5 stations in the SE region by SBU, WDM6, Lin, TH and WSM6 schemes on 23-26 June, KS, Lin, WDM6, MDM and SBU schemes on 23-26 July and CAM, NSSL-2, MYDM and WSM3 schemes during 30 August to 1 September 2015. The maximum updraft has been identified for different MP schemes every day during 23-26 June, 23-26 July and 30 August to 1 September 2015 when heavy rainfall is observed in the SE region of Bangladesh. The maximum updraft is found as high as 20 ms-1 in the upper troposphere by using Lin et al., WSM3, MYDM, MDM and WDM6 schemes at around 600- 200, 400-100 and 600-200 hPa for the heavy rainfall events of 23-26 June, 23-26 July and 30 August to 1 September 2015 respectively.