Abstract:
Heavy rain events have become a major source of meteorological disasters over Bangladesh, responsible for deadly floods and landslide in recent years. Such events caused severe property damage and loss of life. Therefore, rainfall forecasting is essential for the socioeconomic development of Bangladesh. Especially, forecast of heavy rainfall is crucial for flood warning, disaster management and crop production. Forecasting heavy rainfall events are quite challenging due to low forecasting efficiency over the monsoon region including Bangladesh. Numerical Weather Prediction (NWP) models are the great tool for the rainfall forecasting which are widely used all over the globe. Weather Research and Forecasting (WRF) model is a next generation NWP model, which can be used in variety of scientific applications including operational weather forecast. There are several physical options in WRF, which are responsible for generating rainfall in the model. With a selection of suitable physical schemes, the forecast skill of heavy rainfall events can be increased effectively. Therefore, this study is conducted to evaluate the high impact rainfall events over Bangladesh using WRF model.
The rainfall event during 11th June, 2007 and 24th-27th June, 2012 are the two severest and deadliest rainfall events over Bangladesh in recent decades. In present study, these two events are selected to evaluate performance of WRF model over the country. There are twelve cumulus schemes and ten microphysics schemes in the latest version of WRF. Based on the review of previous literature, Kessler, Lin et al., WRF Single–moment 6–class and Stony–Brook University schemes are chosen as microphysics schemes and Kain-Fritsch, Betts-Miller-Janjic, New Grell 3D, Tiedtke and New Arakawa–Schubert schemes are chosen as cumulus schemes to assess the heavy rainfall events over the region. Considering these cumulus and microphysics schemes, nineteen combinations of physical schemes are simulated in the study for a heavy rainfall event using WRF model. Several evaluation indices are calculated for four days of the event which include Root Mean Square, percentage bias, and false alarm, hit score, proportion correct etc. Using these indices, accuracy of rainfall events with different thresholds are evaluated as well as false forecast ratio are also measured for each physical schemes of WRF. After verifying several aspects of rainfall forecast for different rainfall threshold value, a suitable physical scheme is found that can produce effective forecast in heavy rainfall events over the country.
From the analysis, Stoony Brook University scheme along with Tiedtkel scheme has been found as the most suitable scheme over the eastern hilly region of the country. In generating high impact rainfall event, cumulus physical schemes play greater role than microphysics physical schemes. WRF Single–moment 6–class microphysics scheme and New Grell 3D cumulus schemes also showed reasonable performance in capturing heavy rainfall events. It is also found that, the existing default physical scheme (Kessler and Kain-Fritsch (new) scheme) that is commonly used by Bangladesh Meteorological Department (BMD) has poorly performed in capturing high impact rainfall over the Chittagong Division.
