Tuning Dust Schemes in Weather Research Forecast (WRF-Chem) for Simulating Severe Dust Storm Events over Egypt

Weather Research and Forecasting model coupled with chemistry (WRF-Chem) was used to simulate two dust storm events over Egypt in terms of the aerosol optical depth (AOD), single scattering albedo (SSA) and PM load. The events occurred on 22 January 2004 and 31 March 2013. Three dust emission schemes are used in the model: GOCART, GOCART-AFWA and COCART-UOC. Each scheme was tested by adjusting coefficients related to the dust flux in order to reproduce the AOD derived from the MODIS. The grid spacing of both data sets is 10 km. Results from model for March 2013 event were also compared against point measurements from the AERONET station in Cairo. Without tuning the coefficients, all schemes resulted in underestimating AOD. After tuning, it was possible to bring the model results closer to the satellite and AERONET data. Different tuning was required for each severe event, depending on the origin and composition of the dust storm. After tuning the selected coefficient, we found that GOCART-AFWA produced the closest estimate of AOD over Cairo for the 2013 event compared to AERONET measurements (difference within 7%). On the other hand, for the 2004 event, results from the dust scheme COCART-UOC (after tuning) compared best with MODIS data (difference within 5%). For both events, GOCART and GOCART-AFWA schemes were able to reproduce the SSA compared to MODIS measurements with accuracy close to 98% while COCART-UOC underestimated SSA by 7 %.

Sensitivity analysis for the schemes were performed to identify the best scheme that simulates the given events based on spatial error distribution and similarity of eignvalues for AOD and SSA from model and MODIS. The GOCART scheme with adjusted coefficient is found to simulate AOD best in the two events. The maximum dust load for the 2004 and 2013 events is 3.7x106 ug/m3 and 6.5x106 ug/m3 with dominant PM10 of 4x104 ug/m3 and 2x105 ug/m3 for both events respectively.

Comparison of the spatial distribution of AOD and SSA from the model against MODIS over Egypt show significant discrepancies in certain areas, particularly an area in the south west desert. The model could not capture the high value of AOD there because the dust source for this area was not included on the model setup. Explanation of the model behavior between the two examined events in terms of the dominant aerosols microphysical properties