مقايسه طرحواره‌هاي گسيل گردوخاك در برآورد شار قايم آن در نواحي چشمه‌اي ايران

A comparison of dust emission schemes in estimation of vertical dust flux in dust source regions of Iran

در اين پژوهش شبيه‌سازي‌هاي آهنگ گسيل گردوخاك در ايران توسط دو طرحواره مارتيكورنا- برگامتي- آلفارو (MBA) و شايو در 24 مي 2012، كه پديده گردوخاك در بسياري از نقاط كشور گزارش شده بود، باهم مقايسه شده است. داده‌هاي لازم براي اجراي طرحواره‌ها از شيبه‌سازي مدل WRF بدست آمده است. بررسي‌ها نشان داد كه تعداد چشمه‌هاي گردوخاك حاصل از طرحواره MBA كمتر از طرحواره شايو است. شار قايم گردوخاك (VDF) طرحواره MBA در بعضي نواحي چشمه‌اي بزرگ‌تر و در بعضي ديگر كوچك‌تر از طرحواره شايو است. در مناطقي كه طرحواره شايو مقدار صفر را براي VDF برآورد كند، طرحواره MBA نيز همين مقدار را برآورد مي‌كند، اما برعكس آن لزوماً درست نيست. در تاريخ مورد بررسي هر دو طرحواره بيشينه VDF را در ساعت 11 نشان داده اند. بيشينه VDFتوسط طرحواره شايو 8/86 در نقطه 27/30 و 35/57 و توسط طرحواره MBA برابر 45/18 در نقطه 24/33 و 35/51 رخ داده است. طول زبري و فشار موم‌ساني خاك به‌ترتيب تاثيرگذارترين پارامتر‌ها در برآورد VDF با طرحواره‌هاي MBA و شايو هستند. سرعت اصطكاكي و پارامتر‌هاي سطح و خاك در برآورد شار قايم گردوخاك آن‌قدر پيچيده عمل مي‌كنند كه در مقايسه برآوردهاي طرحواره‌ها بايد تاثير هم‌زمان آنها را در نظر گرفت. بيشينه VDF در زمان‌هايي از شبانه‌روز است كه رطوبت خاك كمينه و سرعت اصطكاكي ( ) بيشينه مقدار را داشته باشد. همچنين، در هر ناحيه‌چشمه‌اي به دليل تغييرات ناچيز رطوبت خاك در يك شبانه روز، تغييرات شبانه‌روزي شار قايم گردوخاك بيشتر متاثر از تغييرات شبانه‌روزي سرعت اصطكاكي است. ارزيابي اين طرحواره‌هاي گسيل، نيازمند برآورد VDF به روش‌هاي تجربي و يا سنجش از دور است.

The first step in studying dust events is the estimation of dust emission from sources. In this study, the simulations of the vertical dust flux (VDF) over Iran by the MBA (Marticorena, Bergametti, and Alfaro) and Shao schemes are compared. The MBA scheme is energy-based and derived on the basis of energy balance between the kinetic energy of saltating particles and binding energy of the surface during the bombardment. The Shao scheme, on the other hand, is a volume removal- based approach, in which dust emission rate is estimated on the basis of the volume removed by saltating particles when they impact the surface. Because of the difficulty in measuring interparticle binding forces (or energy), Shao scheme uses the plastic pressure. This quantity is directly related to interparticle binding forces (or energy). The plastic pressure, that shows the surface resistance against the penetration of impacting particles, varies between Pa for light spray fine soil and Pa for deep wetted soil. Another difference between the Shao and MBA schemes is in their parameterizations of the threshold friction velocity, at which the wind erosion is initiated. The surface and soil- related factors, which are introduced as roughness and moisture correction functions, are different in Shao and MBA schemes. Final vertical dust flux estimation is different among various versions of the Shao scheme. In this study, we use the version introduced by Shao (2004). The required data to estimate vertical dust flux by the dust emission schemes, including friction velocity, soil moisture, soil texture, surface cover fraction and roughness length were obtained from WRF model. Soil particle size distribution and plastic pressure for different grid squares are estimated based the soil texture data by reviewing the literature. In this study, dust source regions and emission rates estimated by different parameterization schemes are compared for the 24th May 2012, when dust event was reported in many stations of Iran. According to results, compared to the Shao scheme, the MBA predicted smaller number of source regions in Iran. The vertical dust flux simulated by MBA is higher than that by the Shao scheme in some common source regions, but lower in some others. After applying roughness and moisture correction functions to the threshold friction velocity, the VDF simulated by the MBA will be zero wherever the Shao schemes’ VDF is zero, but this is not necessarily true the other way. Both of the schemes simulated the maximum VDF at 11am. The highest value of the estimated VDF was (for the grid centered at , ) by the Shao scheme, while it was (or the grid centered at , ) for the MBA scheme. The most influential parameters in the estimation of VDF are the roughness length and plastic pressure in MBA and Shao, respectively. Impacts of the friction velocity, and surface and soil- related factors on the estimated vertical dust flux, though complicated, should be considered simultaneously. In every source region, VDF increases when the soil moisture decreases and the friction velocity increases. Furthermore, the VDF variation during a day is mainly affected by daily changes of the friction velocity, because of negligible changes in the soil moisture during the day. Thorough evaluation of these schemes requires experimental and remote sensing data.