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Simulation of Heavy Rainfall Event Over the South and Southeastern Part of Bangladesh Due to Monsoon Depression Using Wrf Model

Saifullah*
Department of Physics, Khulna University of Engineering & Technology, Khulna, Bangladesh

M. A. K. Mallik
Bangladesh Meteorological Department, Agargaon, Dhaka, Bangladesh

Abstract/ Executive Summary:

In our present study, the rainfall event of 13-14 September 2004 was nominated to simulate over the south and southeastern part of Bangladesh due to monsoon depression using the WRF model. WRF Single-moment 6-class (WSM6) microphysics scheme, Kain-Fritsch cumulus parameterization scheme, and Yonsei University (YSU) planetary boundary layer scheme were used to run the model. A domain was used with a horizontal resolution of 9 km for capturing the area of heavy rainfall events (HRE). On the basis of the initial condition 0000 UTC on 13 September 2004, the model was run for 48 hours. The mean sea level pressure, wind pattern, relative humidity, low-level vorticity, vertical wind shear, low-level divergence, and rainfall distribution have been examined on behalf of the evaluation of model performance. The model-derived rainfall was compared with the rain gauge rainfall of the Bangladesh Meteorological Department (BMD). The WRF model suggested that a cyclonic flow appears in between 850 and 500 hPa levels, which remains over the southwestern part of Bangladesh also the adjacent area. Also, the model suggested that from the enormous area of the Bay of Bengal a large amount of moisture carried by a strong southwesterly flow towards the southeastern part of Bangladesh and the adjoining area and so the HRE over these regions might be characterized by the positive vorticity and strong vertical wind shear between 850 to 200 hPa level.

Keywords: Cumulus parameterization scheme, Heavy rainfall event, Monsoon depression, Microphysics, Planetary boundary layer scheme, WRF Model

Location: In Bangladesh

Start Date:

End Date:

Program Area: Risk Management in Agriculture

Commodity: Rainfall, Modeling

Objectives:

The main objectives of our study are to simulate the HRE of 13-14 September 2004 which was observed over the south and southeastern parts of Bangladesh due to monsoon depression using the WRF model and to identify which of the parameters (i.e., mean sea level pressure, wind pattern, relative humidity, low-level vorticity, vertical wind shear, low-level divergence, etc.) are responsible for the occurrence of this HRE.

Methodology:

For the initial and lateral boundary condition, the National Centre for Environmental Prediction (NCEP) high-resolution Global Final (FNL) analysis data was used in this present study. This FNL data cover the whole world every six hours with 1.0°×1.0° grids. To simulate the terrain or topography the 30 second United States of Geological Survey (USGS) data was used. As the vegetation or land use coverage the 25 categories of USGS data was used. The heavy rainfall that occurs all over the country or in different parts of the country has been recorded by BMD. This BMD observed rainfall data was used for comparison with the model simulated data. The heavy rainfall event is a severe weather phenomenon that was selected for simulation over the south and southeastern part of Bangladesh on 13-14 September 2004. There are several mesoscale models for simulation but in this study, the Weather Research and Forecasting (WRF) model was used. The WRF is consists of fully compressible non-hydrostatic equations and different prognostic variables. The terrain-following hydrostatic pressure is used as the model vertical coordinate. The Arakawa C-grid staggering is the horizontal grid of the model. In the model, the 3rdorder Runge-Kutta time integration is used. A domain was used with a horizontal resolution of 9 km for capturing the area of the heavy rainfall events. The domain is consists of 295 grid points in the east-west direction and 304 grid points in the north-south direction. The domain was configured to have the same vertical structure of 38 unequally spaced sigma (non-dimensional pressure) levels. The center (23.5⁰ N, 90⁰ E) of the domain was taken over in Bangladesh. There are different physical schemes are used in this study. As a microphysics scheme the WRF Single Moment 6-class (WSM6) scheme (Hong and Lim, 2006), like a cumulus parameterization scheme the Kain-Fritsch (new Eta) scheme (Kain, 2004), and as a planetary boundary layer scheme, the Yonsei University (YSU) scheme (Hong et al., 2006) was used. The prognostic equations for cloud water, rainwater, cloud ice, snow, and graupel mixing ratio are consistent in the WSM6 microphysics scheme. Alam (2014), Hong and Lim (2006) suggested that among all the microphysics schemes, the WSM6 scheme is the most suitable for cloud-resolving grids, considering the efficiency and theoretical backgrounds.

The performances of cumulus parameterization schemes were evaluated in terms of their ability to simulate the amount of rainfall during the heavy, moderate, and light phases of the event. Among them, the KF scheme was able to account for the mesoscale processes that lead to the development of convection (Pattanaik et al., 2011). On the basis of the initial condition 0000 UTC on 13 September 2004, the model was run for 48 hours. The mean sea level pressure, wind pattern, relative humidity, low-level vorticity, vertical wind shear, low-level divergence, and rainfall distribution have been examined on behalf of the evaluation of model performance. The synoptic situations valid for 0600 UTC, 1200 UTC, and 1800 UTC of 13-14 September 2004 are analyzed in the present paper.

Source of Information: Journal of Engineering Science 09(2), 2018, 23-32

Article Link (Online Journal):

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

According to our current research work, the subsequent conclusions can be made as follows: The lowermost central pressure of the depression is about 999 hPa at 1200 UTC on 14 September 2004 in accordance with the model simulation. As a consequence of the high pressure over the Tibetan plateau, the depression over the southwestern part of Bangladesh and the contiguous region remains static for a long spell. The depression didn’t travel on the way to further east or southeast by reason of extremely high pressure dominant over these areas. A cyclonic flow appears at 850 hPa and 500 hPa levels, which remains over the southwestern part of Bangladesh also the adjacent area. The convergence of strong southwesterly flow transports a high amount of moisture from the vast area of the Bay of Bengal towards the eastern and southeastern part of Bangladesh and the neighborhood. The humidity sustained up to 400 hPa level that enriches the convective movement along 22.50⁰ N (Position of M. Court) which we observed from the vertical profile of humidity. The WRF model recommends that for the formation of depression and moist air updrafts the positive vorticity, resilient vertical wind shear, strong convergence at 850hPa, and prominent divergence at 200 hPa level was so much advantageous. The heavy rainfall event was observed over the area of Sitakunda, Sandwip, Hatiya, Kutubdia, M. Court, Bhola, and the neighborhood.

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