شبيه‌سازي گسيل غبار با مدل پيش‌بيني عددي وضع هوا WRF-Chem و با استفاده از داده‌هاي جديد سطح در منطقه خاورميانه

Dust emission simulation with the WRF-Chem model using new surface data in the Middle East region

هدف از اين پژوهش بررسي ميزان گسيل غبار برآوردشده با مدل منطقه‌اي پيش‌بيني عددي و تحقيقاتي WRF-Chem در منطقه خاور‌ميانه با استفاده از سري داده‌هاي بهبود يافته پارامترهاي سطح است. اين داده‌ها، كه در برگيرنده پوشش گياهي، بافت خاك و توپوگرافي‌اند، به‌‌كمك داده‌هاي ماهواره‌اي حس‌گر ماديس و داده‌هاي USGS تهيه شده‌اند، و با تلفيق آنها، مناطق مستعد گسيل غبار منطقه مشخص شده است. از آنجا كه داده‌هاي سطح موجود در مدل WRF-Chem شامل داده‌هاي ميانگين پنج‌‌ساله و با تفكيك افقي كم است و همچنين به‌‌دليل اهميت شرايط سطحي مناطق چشمه غبار در تعيين ميزان گسيل غبار و نحوه توزيع آن در منطقه، مقايسه‌اي بين نتايج داده‌هاي موجود در مدل WRF-Chem با داده‌هاي جديد صورت گرفت كه نشان‌دهنده ناتواني داده‌هاي موجود در مدل براي نمايان كردن جزييات خصوصيات بافت خاك و پوشش گياهي منطقه بود. هرچند كه داده‌هاي جديد، ميزان خطا را در شناسايي چشمه‌هاي غبار و به‌‌دنبال آن تعيين ميزان گسيل و نحوه توزيع غبار كاهش مي‌دهد. علاوه بر آن، گسيل غبار با داده‌هاي سطح موجود در مدل و داده‌هاي ورودي جديد براي توفان غبار دوره 13 تا 18 تيرماه 1388 شبيه‌سازي و چشمه‌هاي غبار موثر و ميزان گسيل از هريك از آنها و نحوه توزيع غبار در منطقه بررسي شد. نتايج شبيه‌سازي‌ها، چهار چشمه عمده غبار را در منطقه خاورميانه نشان مي‌دهد كه شامل سودان، عربستان سعودي و بخشي از عراق، ايران و افغانستان، و پاكستان هستند كه هريك از اين چشمه‌ها در طي رويداد غبار، نقش‌هاي متفاوتي دارند. همچنين اين توفان غبار بر فراز چشمه سودان شكل مي‌گيرد و با جريان‌هاي جنوب‌غربي به سمت شمال‌شرق تا ايران و تركمنستان كشيده شده است. در پايان دوره، چشمه ضعيف‌تري در جنوب شرق ايران در بار غبار موجود در منطقه مشاركت كرده است. همچنين نتايج حاصل از شبيه‌سازي گسيل غبار، نشان مي‌دهد كه الگوي گسيل غبار حاصل از داده‌هاي جديد، گستردگي بيشتري در اطراف چشمه‌هاي اصلي غبار تعيين شده در منطقه خاورميانه دارند.

Airborne dust particles can affect the radiation balance of the surface and the atmosphere both directly through scattering and absorbing radiation and indirectly by changing the nucleation, optical properties and the life time of clouds by acting as cloud condensation nuclei. Mineral dust also significantly affects air quality and human health. The results of study on sources and the temporal characteristics of dust storms in the Middle East presented the Middle East is as one of the region most affected by dust in the world, next to Africa. In this paper, we have studied the rates of dust emission simulated by WRF-Chem model by using new datasets of surface parameters. These data are vegetation cover, soil texture and topography, derived from MODIS sensor and USGS. We have defined the potential sources of dust by combining the derived datasets. The surface condition of the dust source regions can be determined with higher resolution, and hence the emission of dust and its distribution in the study area can be simulated more precisely. The default surface data available in WRF-Chem model are low resolution and are based on 5-year averages. Because of the crucial role surface conditions play in dust emission and loading, we compared simulations of WRF-Chem model by using the default surface data and those by using the new data. Results show the inadequacy of the default data in presenting details of soil texture and vegetation cover over the region. The new static data improve the identification of dust sources and the determination of dust emission and dust distribution. There is no dust sources input data in WRF-Chem model. We have determined and entered the potential dust sources as input data into the WRF-Chem model and found improved estimations of dust emission rates in the region. To determine the potential dust sources in the Middle East, topography, vegetation and soil texture data were used. By using MODIS sensor dataset for leaf area index, the amount of 0.2 for leaf area index is estimated over the region. So Dasht-e Kavir and Dasht-e Lut as a basement height in the study area, we estimated 1000 m above the see level as the threshold height, below which the soil erosion and dust emission from the surface may occur. We simulated dust emission using the new static data for the 4-9 July 2009 dust event over the Middle East and identified the contributing sources of dust in this event. Now, available the dust emission scheme in the WRF-Chem model is GOCART. In the scheme dust particles with a radius of 0.1 to 6 µm are examined. In this scheme particle size distribution is done in seven categories and mass distribution of particles varies linearly depending on radius within each category. In this paper the GOCART emission scheme is used to estimation the rate of dust emission and loading. Based on the simulation results four major dust sources are found in this region. These regions are Sudan, Saudi Arabia and Iraq, Iran and Afghanistan, and Pakistan, each playing different roles during the dust event. According to the results, the 4-9 July 2009 dust storms originated over Sudan and were transported to Iran and Turkmenistan by southwesterly flows. At the end of the period, a weak source in the southeast of Iran contributed in dust loading over the region. We found that the pattern of dust emission in the region was highly consistent with the geographical distribution of potential dust sources as defined by the new datasets of surface parameters. Dust emission simulation results by using the new data and the default data in the model show that they have the same pattern for dust sources in the Middle East. However, the emission pattern resulted from the new data was more accurate than the one from the default data. The pattern from the new data shows that the dust emission contributing areas extend to the surrounding areas of the main sources of dust emission in the Middle East. Therefore, the dust emission pattern from the new data is more widespread than that from using the default data in the model. The amounts of the model simulated dust emission using the new data and the default data were compared for the 4-9 July 2009. Results show that the estimated dust emission amount by using the new data is consistently greater than that by using the default data over the region. These areas include a small area in East Iran (Lut Desert), the eastern edge of the desert of Saudi Arabia, Afghanistan and Pakistan border, the eastern border of Iraq and sporadic areas in sub-Saharan Africa. These areas are part of the marginal areas of dust sources in the Middle East that the new surface data have entered in WRF-Chem model. The extent of dust emission patterns around the main dust sources in the Middle East, which can be achieved by new data.