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Simulation of Radiation Fog Events Over Dhaka, Bangladesh Using WRF Model and Validation with METAR and Radiosonde Data

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Simulation of Radiation Fog Events Over Dhaka, Bangladesh Using WRF Model and Validation with METAR and Radiosonde Data The Dhaka University Journal of Earth and Environmental Sciences, Vol. 9 (1): 2020 Simulation of Radiation Fog Events over Dhaka, Bangladesh Using WRF Model and Validation with METAR and Radiosonde Data Sariya Mehrin1, Md. Mahbub Jahan Khan2, Dr. Md. Abdul Mannan3 and Khan Md. Golam Rabbani1 1Department of Meteorology, University of Dhaka, Dhaka 1000, Bangladesh 2Biman Bangladesh Airlines, Head Office, Kurmitola, Dhaka 1229, Bangladesh 3Bangladesh Meteorological Department, Agargaon, Dhaka 1207, Bangladesh Manuscript received: 08 December 2020; accepted for publication: 20 February 2021 ABSTRACT: Fog causes severe hazards in the fields of aviation, transportation, agriculture and public health over Dhaka, Bangladesh during the winter season every year. The characterization of fog occurrences, its onset, duration and dissipation time over Hazrat Shahjalal International Airport, Dhaka are the topics of interest in the present study. Attempts have therefore been made to investigate the climatological perspectives of fog over Dhaka, Bangladesh by conducting two selected dense fog events occurred during 24-25 December 2019 and 14-15 January 2020 using WRF- ARW model. The model performance is evaluated by analyzing different meteorological parameters namely visibility, relative humidity, temperature, and wind. The model outputs have been compared with METAR data from Dhaka Airport, Sounding data and INSAT 3D satellite images for validation purpose. Considering RMSE, the model underestimates of relative humidity. Model simulations are good for other meteorological parameters. Thermodynamic analysis reveals that calm wind persists at surface level during fog formation, southwesterly dry wind was over Dhaka and inversion layer is found to persist in the lower troposphere over Dhaka during the event dates. It is observed in the satellite images that fog/low-level cloud was present over Dhaka during the fog events. Keywords: Fog, INSAT 3D, METAR, Visibility, WRF Model INTRODUCTION microphysics, radiated transfer, horizontal and vertical diffusions in the planetary boundary layer (PBL) that Dhaka, the capital city of Bangladesh, is facing are forced by the prevailing synoptic background as frequent fog occurrences and hazards associated with low visibility during the winter season (December – well as a mesoscale disturbance (Faruq et al., 2018). February) every year. Fog causes severe hazards in the Combinations of these factors make fog forecasting fields of aviation, transportation sector, agriculture, over any region quite challenging. and public health. Fog is formed by radiation cooling Fog formation over different parts of Bangladesh of land surface after sunset by infrared thermal is a common phenomenon in the winter season radiation in calm conditions with a clear sky. The (December-February) every year. Most of the places cooling ground then cools adjacent air by conduction, observe dense fog during peak winter months of causing the air temperature to fall and reach the dew December and January. Both radiation and advection point, forming fog (https://en.wikipedia). Fog will first fog frequently occur over relatively flat areas like form at or near the surface, thickening as the air Bangladesh. Radiation fog also forms in the rear continues to cool. The layer of fog will also deepen sector of a western disturbance while advection fog overnight as the air above the initial fog layer also develops in the forward sector of the western cools. As this air cools, the fog will extend upward. disturbance (Dimir et al., 2015). The persistence and Wind would disrupt the formation of radiation fog. intensification of foggy conditions are also driven by Radiation fog is usually patchy, tends to stay in one the high concentration of pollutants and abundant place and goes away the next day under the sun’s rays. moisture supply. The fog phenomenon is associated with the It is well known that Fog limits visibility and thus comprehensive physical processes such as cloud affects human activities that rely on good visibility condition. These activities are part of the core activities of modern societies, most notably aircraft Corresponding author: Sariya Mehrin Email: [email protected] operations, shipping (Fu et al., 2006), and road traffic (Bartok et al., 2012). As we talk about aviation, fog formation accidents compose 24% of all aviation DOI: https://doi.org/10.3329/dujees.v9i1.54860 accidents. The monetary impact of accidents, injuries, delays and unexpected costs from fog is about $720 40 Mehrin et al. million annually (https://www.westcostweather.com skill (Pattanayak and Mohanty, 2008). It is a limited- /weather-for-pilots/fog). An understanding of fog area, non-hydrostatic primitive equation model with climatology over the region of study will provide a multiple options for various parameterization schemes foundation for future studies aiming to improve and for different processes. WRF model is widely used in make accurate predictions of fog and its impact on almost every meteorological application and produces aviation applications. mathematical simulation based on actual observational The Advanced Weather Research and Forecasting or idealized conditions (Mmm.ucar.edu, 2019). (WRF – ARW) Model is a next-generation NWP Model used by researcher and forecasters all over the The WRF-ARW model is run on a triple nested world. Monitoring the spatial and temporal extent of domain at 9 km, 3 km and 1 km horizontal resolution fog over large areas becomes difficult with ground- and the domain size is taken 62x85 km, 52x70 km and based observations alone. The efficiency of WRF 40x58 km respectively (Figure 1). Out of the three model to predict the fog is studied by many domains, first domain (d01) covers full Bangladesh, researchers (Roman-Cascon et al. 2012. along with portions of Kolkata, Bhutan, Agartala and Shilong. The second domain (d02) covers of Dhaka, Fog Hours have been identified on BMD criteria Pabna, Tangail, Cumilla. Domain 3 which focuses on as those hours during which fog reduces visibility to Dhaka is having a model resolution of 1 km. The less than 1000 m (Faruq et al., 2018). Two fog events vertical levels are increased to 50 eta level. Output namely 24-25 December 2019 and 14-15 January history intervals are 180 minutes, 180 minutes and 60 2020 are selected for the present study. Details of fog minutes respectively with a time-step of 27 seconds. events selected for this study is given in Table 1. Table 1: Details of Fog Events Simulated in this Study Total Fog Event Date Visibility < 1000 Visibility < 200 m m (light) (dense) Onset- Total Onset- Total Dissipation fog Dissipation fog hours hours 24-25 2000-0600 10 2000 - 07 Event- December 0300 1 2019 14-15 1900-0500 10 2100 - 06 Event- January 0300 2 2020 The objectives of this study are to simulate two dense radiation fog events using WRF-ARW model and validate with METAR and Radiosonde Data, and Figure 1: Model Domains Used in this Study to compare simulated Temperature, Dew point The domain is centered over Bangladesh to Temperature, Wind speed, Relative Humidity with the represent the regional-scale circulations and to solve observation during the fog situation. the complex flows of this region. The model is run using the Kain-Fritsch (new Eta) scheme for cumulus MODEL EXPERIMENTAL SET-UP parameterization (Kain , 2004), Yonsei University The Weather Research and Forecasting Model (YSU) scheme for the boundary layer Version 4.0.3 (WRFv4.0.3) model has been used in parameterization (Hong, et al., 2006), WSM 6 class this study. Advanced Research Weather Research graupel schemes for microphysics (Hong and Lim, Forecasting (WRF – ARW) model is a meso-scale 2006), and Rapid Radiative Transfer Model (RRTM) model developed at the National Center for for longwave (Mlawer et al. 1997), and for short Atmospheric Research (NCAR) because of its wave radiation scheme (Dudhia et al. 1989) for the superior performance in generating fine-scale selected case. atmospheric structures as well as its better forecast Simulation of Radiation Fog Events over Dhaka, Bangladesh 41 DATA USED AND METHODOLOGY for visualization of the graphics. INSAT 3D images from MOSDAC gallery are used for verification. Data used The climatology of the radiation fog is studied using The Final (FNL) Global Data Assimilation System BMD observed data of Dhaka station during November – (GDAS) data of National Centre for Environmental February (winter season) from 2010 – 2019. Time series Prediction (NCEP) with the 0.25 ֠ °×0.25° horizontal and diagrams are plotted for visibility (km), relative humidity 6-h temporal resolution were used as the initial and (%), air temperature (C), dew point temperature (C), dew lateral boundary condition. The United States Geological point depression (C) and wind speed (m/s) using Survey (USGS) Global datasets with 30sec horizontal METAR data and studied. The analysis of model resolution were used to create terrain/topography and simulated air temperature, relative humidity, wind speed vegetation/land use field. METAR data at an interval of and dew point temperature etc., are studied. INSAT – 3D 30 minutes from Dhaka Hazrat Shahjalal International images of MOSDAC is used to validate the model Airport (HSIA) located at 23.8434 N, 90.4029 E is used outputs. Model simulated visibility is calculated using in this study. In addition, Synoptic 24hour fog data Kunkel (1984) formula using RIP4.7. The onset, duration (visibility in m, onset-dissipation time, the meridional and dissipation of dense fog events occurred on 24-25 and zonal component of wind, temperature, dew point December 2019 and 14-15 January 2020 of Dhaka are depression and relative humidity was collected from analyzed using the time series of visibility and compared. BMD (Bangladesh Meteorological Department). The The thermodynamic characteristics of fog events atmospheric sounding data is used for thermodynamic occurred on 24-25 December 2019 and 14-15 January analysis of the events from University of Wyoming 2020 of Dhaka is also analyzed using skew-T diagram to (http://weather.uwyo.edu/upperair/sounding.html).
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