2. MODEL SETUP In this present study, the WRF model has been used for simulating the monsoon precipitation throughout Bangladesh during June-September 2014. The terrain-following hydrostatic pressure is the vertical coordinate and the Arakawa C-grid staggering is the horizontal grid of the model. There are various microphysics and cumulus parameterization (CP) options in WRF model but in this research WSM6-class (Hong and Lim, 2006) graupel and Kain-Fritsch (KF) (Kain and Fritsch, 1990, 1993; Kain, 2004) schemes have been used for simulating the monsoon rainfall. The MoninObukhov similarity theory (Hong and Pan, 1996) for the surface layer and Yonsei University (YSU) scheme (Hong et al., 2006) for planetary boundary layer (PBL) has been used. For short wave radiation (SWR), the Dudhia (1989) scheme and for longwave radiation (LWR) the Rapid Radiative Transfer Model (RRTM) (Mlawer et al., 1997) have been used in the WRF model to simulate the monsoon rainfall. The model has been configured in a single domain of 6 km horizontal grid distance and the number of a grid points in the east-west and north-south directions are 161 and 183, respectively and the vertical levels are 30. The computational stability of the model was maintained by using 3rd order Runge-Kutta time integration scheme and by setting the time step of integration of 36 seconds.
2. MODEL SETUP In this present study, the WRF model has been used for simulating the monsoon precipitation throughout Bangladesh during June-September 2014. The terrain-following hydrostatic pressure is the vertical coordinate and the Arakawa C-grid staggering is the horizontal grid of the model. There are various microphysics and cumulus parameterization (CP) options in the WRF model but in this research WSM6-class (Hong and Lim, 2006) graupel and Kain-Fritsch (KF) (Kain and Fritsch, 1990, 1993; Kain, 2004) schemes have been used for simulating the monsoon rainfall. The MoninObukhov similarity theory (Hong and Pan, 1996) for the surface layer and Yonsei University (YSU) scheme (Hong et al., 2006) for planetary boundary layer (PBL) has been used. For short wave radiation (SWR), the Dudhia (1989) scheme and for longwave radiation (LWR) the Rapid Radiative Transfer Model (RRTM) (Mlawer et al., 1997) have been used in the WRF model to simulate the monsoon rainfall. The model has been configured in a single domain of 6 km horizontal grid distance and the number of grid points in the east-west and north-south directions are 161 and 183, respectively and the vertical levels are 30. The computational stability of the model was maintained by using 3rd order Runge-Kutta time integration scheme and by setting the time step of integration of 36 seconds.
The performances of CP scheme were assessed in accordance with their capability of the simulation of rainfall for the period of the heavy, moderate, and light phases of the event. Among them, the KF scheme was able to account for the mesoscale procedures that make possible improvement of the convective movement (Alam, 2014 and Pattanaik et al., 2011). In the KF scheme, when the total condensate surpasses the threshold value in the updraft, they are transformed into rainfall. In this scheme, the convective available potential energy (CAPE) is consumed by the convection process in a definite time scale. Also, shallow convection is included in the KF scheme except for deep convection. The shallow convection creates non-perceptible condensates and the shallowness of the convection is determined by a vertical extent of the cloud layer that is known by a function of temperature at the Lifting Condensation Level (LCL) of rising air parcel (Kain et al., 1990). In this scheme updraft generates condensate and dumps condensate into the environment downdraft evaporates condensate at a rate that depends on RH and depth of downdraft leftover condensate accumulates at the surface as precipitation. The KF scheme is further efficient to capture the monsoon seasonal mean rainfall pattern with greater spatial correlations in the core rain belts.
3. DATA AND METHODOLOGY Final Reanalysis (FNL) data (1ox1o ) was used as initial and lateral boundary conditions, which was brought from National Centre for Environment Prediction (NCEP). This data is updated at six hours intervals. The model is adjusted with 0000, 0600, 1200 and 1800 UTC initial field of conforming date. The Tropical Rainfall Measuring Mission (TRMM) 3B42RT daily rainfall data sets were downloaded from their website whereas diurnal rain gauge data of 33 stations have been obtained from Bangladesh Meteorological Department (BMD) all over Bangladesh. The model simulated rainfalls have been extracted for 33 BMD rain gauge stations. We have also extracted daily TRMM rainfall data for the above-mentioned 33 meteorological station points during the monsoon season of 2014. During the study period, we made 3 hourly outputs from the WRF model and these 3 hourly rainfall data were then converted into daily and monthly rainfall data of June-September 2014. The WRF model output gives the control (CTL) file and which is converted into text (txt) format data by using the Grid Analysis and Display System (GrADS). These data were transformed into Microsoft Excel and finally plotted with the help of Surfer software. The RMSE and MAE of rainfall have been determined for 33 meteorological stations all over Bangladesh for long time prediction using Microsoft Excel and then plotted with the help of Surfer software. The model simulated rainfalls have been compared with the BMD and TRMM observed rainfall at 33 meteorological stations. BMD observed monsoon seasonal rainfall and model-simulated rainfalls are also used for calculating RMSE, MAE, and CC of rainfall.