اندركنش گردش جو و پوشش سطح زمين در سازوكار تشكيل و گسترش توفانهاي گردوغبار تابستانه‎ي خاورميانه (توفان گردوغبار تيرماه 1388)

Interaction between Atmospheric Circulation and Land Cover in the Mechanism of Creation of Summertime Dust Storms in Middle East (Case Study, July 2009)

توفان هاي گردوغبار يكي از انواع مخاطرات جوّي هستند كه در منطقه‎ي خاورميانه، در طيِّ سال، بارها امكان وقوع آنها وجود دارد. در دهه‎ي پيشين، رخداد اين توفان ها فزوني يافته كه بررسي علل اين افزايش وقوع و سازوكارهاي تشكيل آن، توجّه پژوهشگران را به‎خود جلب كرده است. هدف مقاله‎ي پيش رو نيز، دست‎يابي به‎شيوه‎ي سازوكار تشكيل و فعّاليّت توفان‎هاي گردوغبار مذكور است. در اين مقاله، فراواني توفان هاي دهه‎ي 80 و گردوغبار تيرماه 1388 را با رويكردي اقليم‎شناسي، همراه با در نظر گرفتن شرايط سطح زمين با استفاده از داده‎هاي شبكه‎بندي‎شده، شامل ارتفاع ژيوپتانسيل، سمت و سرعت باد، اُمگا و فشار سطح دريا از مركز ملّي پيش‎بيني هاي محيطي (NCEP)، داده هاي پوشش سطح زمين و خاك مورد استفاده در مدل اقليمي منطقه اي (RegCM) مطالعه و بررسي شده است. نتايج اين بررسي نشان داد كه تشكيل كم‎فشار روي خليج فارس و پيشروي آن تا جنوب تركيه و مساعدت جريان‎هاي سطوح مياني و بالايي وردسپهر با استقرار ناوه روي اين سطوح، در ساحل شرقي مديترانه، سبب ايجاد گردش چرخندي در جنوب شرق سوريه، در سطح زمين روي زمين هاي بدون پوشش سطحي (پوشش گياهي، سنگ‎فرش بياباني) و داراي خاك سطحي ريزدانه و گچي شده است. قرارگيري شرايط ناپايدار جوّي روي چنين خاك هايي، زمينه را براي تشكيل گردوغبار فراهم كرده است. در طيِّ روزهاي بعد با حركت به سمت شرق ناوه و عميق شدن شرايط ناپايداري، گردوغبار با حركت شرق سوي خود از روي ارتفاعات زاگرس عبور كرده و تمام كشور، به‎استثناي جنوب‎شرق را تحت تاثير قرار داده است. آن چيزي كه در اين توفان گردوغبار مشهود است، همراهي شرايط گردش جو (ناپايداري با عمق مناسب) و شرايط سطح زمين (زمين بدون پوشش و خاك خشك و نرم) در ايجاد و فعّاليّت آن است.

Introduction In some regions, particularly the Middle East, the dust storms occur with high frequency. Since the past decade, the occurrence of dust storms has increased in this region (Kutiel & Furman, 2003: 419). Several factors are involved in the creation of the dust storms, but the main factors are circulation of the atmosphere and earth surface. Synoptic studies have shown that low and high pressure (especially dynamic), and the vertical motion of air are the main causes of dust storms in the Middle East (Omidvar, 2006; Lashkari, et.al., 2008; Mehrshahi, 2009; Tavousi, 2010). Daily mean of dust storms in Iran, especially Khuzestan in the warm season is more than other seasons, and the highest frequency has been observed in the Zabol (Raeespour, 2010; Farajzadeh, 2011). The frequency of these storms will be increased after droughts, because the ground is dry and has less vegetation (Miri, et.al., 2009). To investigate the mechanism of dust storms in Middle East, dust storm occurrence in first days of July 2009, that is the biggest storms, has been elected, in this study interaction between atmospheric circulation and earth surface properties including soil moisture, land cover and soil type has been studied by a synoptic approach. Methodology Dust storm classified based on the origin. The Storm of Syrian origin, has been selected for this study. The Satellite images and Synoptic maps showed that period of this storm is 30 June to 9 July and south of Syria, Iraq and Iran influenced by dust storm. In this research, several types of data are used including zonal and Meridian wind, Geopotential height, Omega, Precipitation, sea level pressure, soil moisture from NCEP, Land use from RegCM/3 Model and satellite images (MODIS). The method of the research is the synoptic approach. Results and Discussion This dust storms caused by the development of Persian Gulf low pressure and extension of a trough to south of Turkey that create cyclonic circulation in the surface of southeastern Syria. These mentioned Conditions and also wind speed increasing in surface, low level of troposphere with location of the trough in the middle and upper level of the troposphere alongside dry and Gypsiferous soil have effects in the creation of dust storm. After creation of the dust storm by surface and low level instability over desert of Syria on the 1 July, then it spread over Iraq due to the development and extension of instability caused by the trough in middle and upper level and northwest wind. Through shifting to the east and changing the wind direction and also more important instability through troposphere, dust storm on the 4 July passed from Zagros Mountains and spread over Iran. In the next days, dust storms moved to east and northeast and influenced almost of Iran. This condition Remained to July 9 over Iran. Conclusion Maximum radiation and minimum precipitation in warm season caused the negligible soil moisture in this region by. The soil of dust storm area, except a very small of margin River in southern Syria and northwest of Iraq is soft, fine grain, Gypsiferous, and without any cover. Middle East at the warm season is dominated by subtropical high pressure and subtropical jets stream, but the circulation of the atmosphere during dust storm, shows that a low pressure extends over Persian Gulf and south of turkey. This condition causes the formation of cyclonic circulation in the East of Syria on 1 July. Wind speed Increasing, cyclonic circulation, dry soil and lack of coverage, provided the conditions for removing the soil particles. Due to the extended ground surfaceʹs low pressure and instability of low level of troposphere, dust storm enhances and moves to south-southeast over Iraq and Persian Gulf. But unstable layer is shallow, and then storm has not passed from the heights of West Iran. Dust storm passed from Zagros Mountains and entered to Iran due to moving trough from west to east of Iran and development of instability on July 4. In the next day (5 July) a cyclonic circulation has been created in central Iran, this circulation and extended instability caused the spread of dust over almost of Iran exactly southeastern regions. This study shows that both of atmospheric circulation and condition of ground surface are important in creation of dust storm in the Middle East.