تحلیلِ ماهواره‌ای ـ همدیدی پدیده‎ی‌ گردوغبار (گردوغبار تیرماه 1388)

Introduction
Dust originates from arid and dry regions where high velocity winds are able to remove mostly silt-sized materials, deflating susceptible surfaces. Dust storms as the dominant phenomena in arid and semi arid areas have the greatest impacts on air quality. One-third of the Earth’s land area is covered by dust-producing surfaces, made up of hyper-arid regions like the Sahara that covers 0.9 billion hectares, and dry lands which occupy 5.2 billion hectares (Jickells, et al., 2005). Dust consists of particles in the atmosphere that come from various sources such as soil dust lifted by wind (an Aeolian process), volcanic eruptions, and pollution. This airborne dust is considered as an aerosol and once in the atmosphere, it can produce strong local radiative forcing. Saharan dust in particular can be transported and deposited as far as the Caribbean and Amazonia, and may affect air temperatures, cause ocean cooling, and alter rainfall amounts (Middleton, Goudie, 2001). Dust storms are important parts of air pollution resources, Phenomenon that occurs in large areas of Iran in recent years. Conditions in neighboring countries. Iran is located on the arid belt. More than half of the country's land area is arid and semiarid climate. 18 provinces and 82 cities have been identified as a critical focus of wind erosion. The west and southwest regions of Iran periodically were affected by dust storms, which in recent years its frequency and spatial scope have been expanded. One of the most severe dust storms occurred in July 2009 that overed 17 provinces in the western half of Iran. Its impacts can be investigated beyond the closure of schools and agencies, flight cancellation, loss of vision, increased respiratory diseases, etc. Recognition of resources and centers originating particles, patterns and the formation and dispersion of dust are the main objectives of this study.

Methodology
The area covers approximately half –west of Iran spanning longitude 46 to 50oE and latitude 28 to 36oN. The area is covered by the Zagros Mountain Ranges and western foothills, including agricultural lands, sparse oak forest. MODIS images were used to identify the main sources of dust. MODIS (or Moderate Resolution Imaging Spectro-radiometer) is a key instrument aboard the Terra (EOS AM) and Aqua (EOS PM)satellites. Terra's orbit around the Earth passes from north to south across the equator in the morning, while Aqua passes south to north over the equator in the afternoon. Terra MODIS and Aqua MODIS are viewing the entire Earth's surface every 1 to 2 days, acquiring data in 36 spectral bands, or groups of wavelengths. MODIS is playing a vital role in the development of validated, global, interactive Earth system models able to predict global change accurately enough to assist policy makers in making sound decisions concerning the protection of our environment. Brightness temperature difference between suspended particles matters in the 8.5, 11 and 12 micrometers wavelengths; providing the possibility of identifying dust phenomenon. Thus, with increasing amounts of dust, the difference (BT8.5-BT11) and (BT11-BT12) is increased and amounts (BT11-BT12) decreased. Also for studying and analyzing of atmospheric synoptic conditions during the dust phenomena, U(Zonal) and V (Meridinal) components of wind, average surface pressure and geo-potential height in 500hpa were used.

Results and Discussion
After the detection of dust and creation of false color images and synoptic situation of desert regions that located in the north and south of Al-raqqeh city, north side of the Euphrates River in Syria and deserts of Iraq were identified as main sources of dust formation. Research results show that Trough axis location, region of upper divergence in 500hpa and formation of thermal low pressure center at ground surface have major role in formation, dispersion and transmission of dust. Interesting point is that the spatial spreads of dust occur in clash with Zagros Mountains Range, so Zagros elevations actas a cause of spatial distribution which spreads dust storms in the north and south side. Trough axis placement in Syria and Mesopotamia in 500hpa level in related with surface low pressure expanded over Persian Gulf and Arabian Peninsula (surface unstable condition) in the absence of moisture result dust storm.

Conclusion
Using two sources of remote sensing and weather maps data and information more accurate results from the transfer and distribution pattern of the suspended particles were provided. So that the surface pressure pattern, geo- potential height and wind vectors drawn on maps of surface and 500hpalevel are transmission and dispersion pattern obtained from satellite imagery detection. The most important factor in spatial distribution pattern of dust storms is Zagros

Mountains as a barrier that in addition to the accumulation of suspended particles in the western slopes, leads transmission and dispersion to the north and south directions.