An Assessment of Wind Erosion Schemes in Dust Emission Simulations over the Middle East

Extended abstract 1- INTRODUCTION        Atmospheric aerosols, solid and liquid particles in the atmosphere, play a crucial role in the atmospheric radiation equilibrium. These particles have an influence on the scattering and absorption of short wavelength radiation, and on the other hand, affect radiation absorption and emission in long wavelengths. Dust particles are among the important aerosols affecting the air quality and human health. Variation on the amount of dust in the atmosphere results in radiation equilibrium, hence changes the surface temperature. Dust particles can also alter the global climate condition through changing precipitation, wind intensity, regional humidity equilibrium. Wind erosion is a complicated process which leads to the rise of dust particles from the surface. This process is under the influence of different factors such as atmospheric condition (wind, precipitation and temperature), land properties (topography, surface humidity, roughness length and vegetation), soil condition (texture, compound and density), and land use (agriculture). Numerical models using the wind erosion schemes for better understanding of dust impact on the atmosphere are developing. Although their results have improved, they still suffer from uncertainties for the estimation of dust emission. Therefore, it can be anticipated that the estimated values of different models for dust emission with different surface conditions (such as soil texture and vegetation) would be different. In the present study, temporal and spatial distribution of surface flux of aerosols were calculated through coupling three dynamic wind erosion models within the numerical Weather Research and Forecasting model coupled with Chemistry (WFR-Chem). This study addressed a more realistic investigation of temporal-spatial properties of dust emission in the Middle East. To this end, first, the necessary physical condition for the wind erosion event was investigated in the study region. Then, the main structure of the wind erosion models along with the effective parameters was explained. In addition, the wind erosion schemes were introduced and described, and finally, the results were discussed. 2- THEORETICAL FRAMEWORK        Depending on their size, aerosol particles have different moving modes during the wind erosion. According to the field observations in the wind tunnel, particles movement can be classified into three modes: suspension, saltation and creep. In the suspension mode, as dust particles have small final velocity after their entrance into the atmosphere, they can remain suspended. Due to the turbulence of the boundary layer, they can easily diffuse in long distances. As the time of dust particles’ lasting in the atmosphere depends on their final velocity, dust particle suspension can be divided into long-term and short-term suspension modes. In the saltation mode, an intense movement of the sand along the surface happens during the wind erosion. This is the main mechanism for the huge amount of dust transfer in the wind direction which results in the formation of sandy lands and sand hills. In the normal condition, the particles with the diameter of higher than 1000 micron cannot be elevated from the surface; these particles can only move on the surface as a result of the wind or saltation beats which is called creeping. Wind erosion schemes are used to determine the type of aerosol movements, and also to estimate the vertical and horizontal flux. In this study, three schemes of Marticrana-Bergumetti (1995), Lou and Shao (1999), and Shao (2004) are employed. The aim of this study is to investigate the performance of the wind erosion schemes through WRF-Chem model to simulate the dust emission over the Middle East region. 3- METHODOLOGY In this study, first, 3 dust schemes, including Marticorena-Bergametti, Lu-Shao and Shao 2004 were introduced and studied. Then, through WRF-Chem model, the results of dust emission simulation through these three schemes were compared with 4-9 July 2009 dust event in the Middle East. For simulation, new surface data, including topography, soil texture and vegetation were used instead of the available data in WRF-Chem model. The advantage of these data is their higher resolution in comparison with WRF-Chem data, in a way that it can detect the existing dust sources of the region with higher accuracy playing a significant role in the dust emission simulation in the region. 4- RESULTS Results of simulated dust emission with WRF-Chem and Marticrana-Bergumetti (1995); Lou and Shao (1999), and Shao (2004) were investigated for the storm period of 4-9th of July in 2009 in the Middle East. Results show that all the three schemes revealed the beginning of dust emission at 6:00 in July 4th, 2009 in the southeastern deserts of Africa. These three schemes also showed the same emission sources for this event. At the beginning of the storm event, Marticrana-Bergumetti scheme indicated more extensive and intense dust sources, when compared with Lou-Shao and Shao schemes. Source regions continue their activities during the storm but the amount of emissions were different by the application of different schemes. Simulated dust flux show the same spatial distribution for all the three applied schemes, since the parameterization processes of horizontal flux were the same in the three schemes. However, the simulated values of the vertical flux were different. Difference in the vertical dust flux reflects the complexity of the soil texture and friction velocities. Marticrana-Bergumetti estimated larger dust emission for the studied storm in comparison with Shao’s (2004) scheme. At the beginning of the dust storm, the highest difference in the estimated dust emission between Marticrana-Bergametti and Shao (2004) was observed in the southeastern regions of Africa. By the advancement of the storm from west to east, the maximum difference was observed in Saudi Arabia desert followed by the central and eastern regions of Iran. Regions with maximum difference in the estimated values of the two schemes at the beginning of the event had loam or silty-loam soil, whereas the regions with the least difference had clay soil. 5- CONCLUSIONS AND SUGGESTIONS The results showed that all the three wind erosion schemes estimated the same emission resources for these dust events, while, at the beginning of the event, Marticorena-Bergametti scheme showed a wider and more intensified dust emission source in comparison with the two other schemes. Examining the spatial distribution differences of the simulated dust emission revealed that Marticorena-Bergametti and Lu-Shao resulted in spatial distribution ranges several times larger than that of Shao’s (2004) scheme.