Abstract
Weather Research and Forecasting model coupled with chemistry (WRF-Chem) was used to simulate selected severe dust storm events over Egypt in terms of the aerosol optical depth (AOD). Two severe events, which occurred on 22 January 2004 and 31 March 2013, are examined. The analysis includes three dust emission schemes: Goddard Chemistry Aerosol Radiation and Transport (GOCART), GOCART with Air Force Weather Agency (GOCART-AFWA), and GOCART with University of Cologne (GOCART-UOC). Each scheme was tested by adjusting coefficients related to the dust flux. The AOD and Single scattering albedo (SSA) from the model were compared against the same parameters derived from the Moderate-resolution Imaging Spectroradiometer (MODIS). The grid spacing for both of the data sets is 10 km. Results from the March 2013 event were also compared against point measurements from an Aerosol Robotic Network (AERONET) station in Cairo. Using WRF with built-in coefficients, all schemes resulted in underestimating AOD. After tuning the coefficients, it was possible to bring the model results closer to the observations from satellite and AERONET. Each severe event required a different tuning, depending on the origin and composition of the dust storm. Sensitivity analysis for each case is performed to identify the scheme that best simulates the given events based on spatial error distribution. A novel comparison of eigenvalue structures for images of both for AOD and SSA from model and MODIS was used. After tuning, the adjusted coefficient GOCART scheme is found to simulate AOD best across the country in both events. However, the results for the 2004 event from GOCART-UOC were closest to MODIS AOD over Cairo (within 5% bias). On the other hand, GOCART-AFWA produced nearest estimate of AOD for the 2013 event when compared to AERONET measurements (within 7% bias). For both of the events, SSA from GOCART and GOCART-AFWA schemes were found to be comparable to MODIS measurements with accuracy that was close to 98%. The accuracy from GOCART-UOC was around 93%.
Highlights
Sand and dust storms constitute a prime source of aerosols in the atmosphere
Two dynamic solvers are available in the WRF Software Framework (WSF); the Advanced Research Weather Research and Forecasting (WRF) (ARW) and the Non hydrostatic Mesoscale Model (NMM)
The soil texture is categorized into 16 categories that are based on the soil composition and of sand, silt and clay [36,37]
Summary
Sand and dust storms constitute a prime source of aerosols in the atmosphere. Driven mainly by wind, they result from the erosion and transport of mineral sediments from the ground surface. Satellite remote sensing has been used extensively to monitor atmospheric aerosols, including sand/dust storms over the arid region of the Middle East. The motivation behind the current study is the underestimation of the AOD by the WRF-Chem model [8,10] as compared to satellite measurements. This finding has been confirmed in the present study. The objective is to improve the bias of the model in simulating the AOD of severe dust storms over Egypt.
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