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Predictability of Orographic Rainfall Features Over Sylhet Using Nwp Model

Md. Omar Faruq*
Bangladesh Meteorological Department, Agargaon, Sher-E-Bangla Nagar, Dhaka 1207, Bangladesh

M. A. K. Mallik
Bangladesh Meteorological Department, Agargaon, Sher-E-Bangla Nagar, Dhaka 1207, Bangladesh

M. A. M. Chowdhury
Department of Physics, Jahangirnagar University, Savar, Dhaka, Bangladesh

M. A. E. Akhter
Department of Physics, Khulna University of Engineering & Technology, Khulna - 9203, Bangladesh

M. Arif Hossain
Bangladesh Meteorological Department, Agargaon, Sher-E-Bangla Nagar, Dhaka 1207, Bangladesh

Abstract/ Executive Summary:

Topography and orography are two physical factors that produce high-impact rainfall over the Northeastern part of Bangladesh. To predict the orographic rainfall of 29 March 2017 over Sylhet, Bangladesh an attempt has been performed using Weather Research and Forecasting (WRF) model. The model has been run in a single domain of 10 km horizontal resolution for 48-h and 72-h using six-hourly global final datasets from 0000 UTC of each initial day of the event as initial and lateral boundary conditions with NSSL 2-moment microphysics scheme, Kain–Fritsch cumulous scheme and Yonsei University Planetary Boundary Layer (PBL) scheme. The model outputs such as sea level pressure, wind flow, vorticity, wind shear, humidity, Convective Available Potential Energy (CAPE), Convective Inhibition, Lifted Index, K-index, Total Total Index and rainfall have been analyzed. The model predicted weather parameters were visualized by Grid Analysis and Display System (GrADS) and Geographic Information System (GIS) software and validated with observed data of Bangladesh Meteorological Department (BMD), Tropical Rainfall Measuring Mission (TRMM) and European Centre for Medium-Range Weather Forecasts (ECMRWF) data. The analysis determines that the CAPE of magnitude 800- 1000 JKg-1, positive vorticity of (6-10)×10-5s -1 and relative humidity of 80-100% up to 500-400 hPa levels are accountable for the happening of the orographic extreme rainfall and other parameters are compatible with the observed or theoretical values. This study indicates that the model with an appropriate model setup is capable to predict the orographic precipitation realistically well and can be used for upcoming events.

Keywords: WRF-ARW; Vorticity; CAPE; CIN; LI; TT.

Location: Sylhet, Bangladesh

Start Date: 29-03-2017

End Date:

Program Area: Knowledge Management

Commodity: Rainfall

Objectives:

An effort has been made to forecast the high impact rainfall of 119 mm recorded over Sylhet, Bangladesh on 29 March 2017 by WRF model and an endeavor is also taken to find out the meteorological features of that event.

Methodology:

2. Experimental Setup, Data Used and Methodology The Advanced Research WRF model, developed at the National Center for Atmospheric Research (NCAR), is one of the two different dynamical hubs of the WRF model. The other core version, the Non-hydrostatic Mesoscale Model (NMM), was developed at the Environmental Modeling Center of the National Centers for Environmental Prediction (NCEP). The Numerical Weather Prediction (NWP) model used in this study is the ARW model of version 3.9.1. The WRF model provides a flexible and portable open-source community model for both atmospheric research and operational forecasting (Skamarock et al., 2008). It is a limited-area, non-hydrostatic basic equation model with multiple options for numerous parameterization schemes for different physical methods.

The "1" resolution FNL data covering the entire globe every 6-h were taken as the initial and lateral boundary conditions. 30 arcsec United States Geological Survey (USGS) data GTOPO30 was used as topography and 25 categories of USGS data were taken as vegetation/land use (Modis and Hi-Def Lakes) coverage. The observed rainfall data of BMD were used to compare or authenticate the model simulated rainfall. The simulation was done on a single domain of 10 km horizontal resolution and the domain (251×251 grid points with 38 unequally spaced sigma levels) of the NWP study.

Source of Information: Journal of Engineering Science 11(2), 2020, 61-73

Article Link (Online Journal): DOI: https://doi.org/10.3329/jes.v11i2.50898

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

On the basis of the present study, the following conclusions can be drawn: i. The NSSL 2-moment microphysics coupling to the Kain–Fritsch cumulus scheme and YSU PBL scheme options of WRF model produces representative results in both spatial and quantitative evaluations. Therefore, these schemes have been considered as the prediction of the thunderstorm which passes over NE part of Bangladesh. ii. The model predicted lowest MSLP of the thunderstorm is about 1006-1008 hPa for 48-h, and 72-h model run. The model captured the south-westerly wind flow at 10-m height and 850 hPa level which transports of moisture from the enormous area of the BoB towards the NE part of Bangladesh and neighborhood, this south-westerly wind changes to easterly by the confrontation of hills. One of its components is uplifted and conjugate with the west-northwesterly dry and cold wind at 500 hPa which is very close to the observation. iii. The model predicted vorticity over NE part of Bangladesh at 850 hPa level is positive of magnitude (06- 10)×10-5 s-1 and negative of magnitude (6-10)×10-5 s-1 for 48-h and 72-h model run which directs updraft and downdraft. At 500 hPa level, positive vorticity is dominant, is the indication of further updrafts of the system. iv. The model predicted positive vertical wind shear of the order of (0-20) knots is governing and it is the indication of the further updraft of the system. v. The RH is found 75-100% over Sylhet and adjoining area which is very close to the observation and 60- 100% moisture is extended up to 600-350 hPa levels. vi. The model simulated CAPE is found 400 – 800 JKg-1 at developing stage and 800-1000 JKg-1 or more at mature stage over Sylhet and adjoining area which is moderately liable for the unstable condition of the atmosphere. The model simulated LI is -2 to -1.2⁰C, K-index lies between 31 to 35⁰C and TT is found 43 to 45⁰C which are close to the observation and responsible for the marginal unstable to large instability of the atmospheric condition. vii. The model captured the rainfall event reasonably well enough though some spatial and computational error exits. The bias correction may increase the competency of the model for prediction of orographic rainfall and may be forecasted more precisely and accurately.

Knowledge Management: Journal