نوع مقاله : مقاله پژوهشی

نویسنده

ژئومورفولوژی، دانشکده جغرافیای دانشگاه تهران

چکیده

دشت کویر با شرایط اقلیمی بسیار خشک، قلمرو وسیعی را در ایران مرکزی پوشش می‌دهد. توپوگرافی نسبتاً هموار و پست و خشکی محیط موجب تسلط فرسایش و نقل‌وانتقال بادی در این قلمرو شده است. انباشت توده‌های بزرگی از ماسه‌های بادی شامل ریگ‌های جن، سرگردان، خارتوران و چاه جم مشخص‌ترین اشکال ناهمواری در این منطقه است. این توده‌های ماسه‌ای الگوی حلقوی ناپیوسته‌ای را به دور دشت کویر تشکیل داده‌اند. به نظر می‌رسد حلقوی بودن آن‌ها نتیجه راستای چرخشی وزش بادها در این قلمروست. به‌عنوان فرضیه، این چرخش حلقوی تسلط سامانه‌ای حرارتی را در مرکز دشت کویر بازسازی می‌کند. هدف بررسی علل استقرار این ریگ‌ها و مورفولوژی خاص آن‌هاست. تکنیک پژوهش مقایسه مورفومتری تپه‌های ماسه‌ای به‌عنوان شاخصی برای تعیین جهت وزش باد غالب از طریق ابزارهای مشاهده غیرمستقیم بوده است. نتایج حاصل از تجزیه‌وتحلیل تصاویر ماهواره‌ای و گلبادهای منطقه، نشانگر وجود راستای همگرای وزش باد نسبت به مرکز دشت کویر است. همچنین، تصویر فضایی مورفولوژی این ریگ‌ها الگویی را ارائه می‌کند که از طریق آن می‌توان موقعیت سامانه کم‌فشار همرفتی گسترده را بر بخش مرکزی دشت کویر بازسازی کرد. انطباق این سامانه کم‌فشار محلی موجب مکان‌گزینی این توده‌های ماسه‌ای به‌صورت هاله‌ای به دور این دشت شده است. 

کلیدواژه‌ها

موضوعات

عنوان مقاله [English]

The effects of conversion winds on morphology and location of ergs around of the Dasht-e-Kavir (Iran Central Desert)

نویسنده [English]

  • Mojtaba Yamani

Geomorphology, Faculty of Geography, University of Tehran

چکیده [English]

In desert areas high temperature and low precipitation limit growth and development of plants. Under this circumstances wind becomes the most important erosion and transportation factor in these areas. There are four great sand masses in the central part of Dasht-e-Kavir namely Rig-e-Jen, Rig-e-Sargardan (Shotori or Halvan), Rig-e-Kharturan, and Rig-e- Chah-e-Jam. These great sandy masses show an especial pattern, so that all of them are located almost on an irregular circle around Dasht-e-Kavir. Their circular alignment can be resembled to a circulation around Dasht-e-Kavir. So far, the placement of these sandy masses was interpreted under the influence of topography and stoss-side of current features around Dasht-e-Kavir. However, the elongation and combination of these sandy masses don't show a good correlation with their relative location. As a result, the main question is that: circular arrangement of sand and gravel masses around Dasht-e-Kavir was the consequence of which factors and what is the relationship between wind direction pattern on Dasht-e-Kavir and situation of these sand and gravel masses? Distribution and morphology of these sand and gravel masses around Dasht-e-Kavir is the result of currents of convergent winds under dominance of influence of thermal low pressure system during hot season. The latter issue is the central hypothesis of this research.
 
Study area
Daht-e-Kavir is located in central Iran Tectonic Zone. This plain is covered with thick clastics and evaporitic deposits in the second part of Tertiary. As a result, the topography of this plain is almost relatively smooth and vast. Among all morphological features development of four vast sand dunes is outstanding. They are known as: Rig-e-Sargardan, Rig-e-Shotori (Halvan), Rig-e-Kharturan and Rig-e-Chah-e-Jam.
 
Materials and Methods
Statistical data from meteorological stations were used to interpret wind and pressure patterns on the earth surface. Aerial photos and satellite image data were used to determine the location of sand dunes and ergs and morphology of sand dunes. Additionally, some data were gathered through field works. For investigating and analyzing climatic conditions, wind rose diagrams of the area for different time periods were prepared. According to the role of pressure as the most important climatic factor and its direct effect on generation, intensity and change in wind direction, studying the climatic systems and pressure patterns on synoptic maps is important. As a result, the data for wind and pressure patterns in three levels of 850, 1000 and HP were gathered and the maps were drawn by Grads software. According to the type of climatic patterns, the type of climatic systems and their role in distribution and morphology of sand dunes were identified.
 
Result and Discussion
Generally, speaking, general circulation of atmospheric currents in the region in the summer is under the influence of subtropical high pressure system. So, the precipitation is very scarce. In this region most of the precipitation is influenced by passage of dynamic cold low-pressure systems which pass from northern part of Iran. Passage of these currents from the northern Alborz slopes is sometimes spread over the divides of the studied basin and causes precipitation in the northern parts of this region. Sometimes warm and wet monsoonal currents from Indian Ocean affect the high lands up to 2 -3 kilometers high while confronted with low-pressure systems of mid-latitudes causing instability and showers. In the cold periods of the year, when the subtropical high-pressure system in Iran moves towards lower latitudes (lower than 20 N), passage of mid-latitudes low and high-pressure Mediterranean currents is facilitated. The result of these changes is precipitation which takes place during winter.
Intensity of sunlight, scarce precipitation and low humidity result in severe dryness in the region. This is especially in the summer when it's very hot during the days and almost cool at night. Consequently, day and night temperature difference is very high and in the dry lands up to 95% of sun rays affects the ground. Under these conditions, the high temperature difference between central parts of the Dasht-e-Kavir and mountainous regions around it results in pressure difference between these two regions and consequently causes severe convectional instability and generation of severe winds between these two regions.
 
Interpretation of rose diagrams represents that prevailing wind direction during warm periods in the northern half of Dasht-e-Kavir is often eastern. In the west of Dasht-e-Kavir the winds are northward. Then in the southern half of Dasht-e-Kavir the trend of prevailing winds is also westward that change course to southward towards the eastern part of Dasht-e-Kavir. The most important point is that morphology of the sandy features is in complete correspondence with the above mentioned directions. Comparison of seasonal rose diagrams and morphology of ergs in Dasht-e-Kavir show that summer rose diagrams have the best correspondence with the morphology of sandy features on Dasht-e-Kavir. As a result, morphology of ergs in Dasht-e-Kavir is mainly controlled by the regime of summer winds especially in July and August months. On the other hand, in cold periods, wind current system dominantly changes. The winds in this period have lower speed and are much more irregular and show more instability. As a result, they don’t have a major role in sand transportation in this period and can’t have a significant influence on the morphology of the ergs.

کلیدواژه‌ها [English]

  • Sand mass
  • desert plain
  • sand
  • convective system
  • wind erosion