عنوان مقاله [English]
Dust storm is a meteorological phenomenon common in arid and semi-arid regions. Iran and particularly its west and south-west parts, periodically faces with the dust phenomenon and its problems. Westerlies are dominating current input to Iran. In regions that located western Iran, there are some deserts areas such as southern Iraq and Badiatosham, Robhgkhali, and Sahara. The geographical position of half west Iran and its proximity to the deserts has been caused frequent occurrence of dust with different intensities during a year. It seems the frequency of occurence of this phenomena and apatial expansion is more intense in recent years. The main aim of this study is Analysis of spatial and temporal variations and how the phenomenon of dust formation and identifies the origins and pathways of dusts entered in west of Iran. According to importance of this phenomenon and the problems that have been established in different areas, in this study, we attempted to first, to be done statistical analysis of the dust phenomenon, frequency of its occurrence in the past and present. And also recognized regions of source and the weather conditions, as well as through the combined methods diffusion patterns and dust storms routing have been analyzed.
2-Materials and Methods
The data used covered a 30- year's period (1979-2008) and include: three hourly dust data in synoptic stations (8times in 24 hours), brightness temperature, MODIS 11-12Âµm bands, GDAS data, view field, velocity and direction of wind, temperature, geopotential height and omega in the atmosphere, are different in various levels. Software such as GrADS were used for prepared weather charts that were received from NCEP-NCAR with netCDF format. GDAS data for tracking the wind path in the HYSPLIT model and MODIS satellite images and using ENVI4.5 via NDDI equation was extracted dust phenomena over study area.
3-Results and discussion
Dust storms have great significance for the physical environment and the world's human inhabitants. The results indicate that Bushehr and Dezful stations have known as dust crisis centers in south-west Iran. Seasonally spring has the most frequency of dust occurrence and July, May and June presented the monthly most frequency. In other hand December has the lowest occurrence. During the day, the most occurrence of dust occurred between 9 am to 6 pm. in the warm period The Border regions between Syria and Iraq and in the cold period the West and South-West Iraq with East and Northern Saudi Arabia are two main dust sources for study area. Also, the main pathways for entered dust in study area are northwest - southeast and south â northern paths. According to the results of satellite images processing and model outputs, border region between Syria and Iraq and the northwest â southeast path considered as main source and pathways of dust for west region of Iran. In the warm period of year because of strengthening low pressure formed on Iraq and its integration with the low pressure moved to south and southwest Iran and finally place against high pressure of Mediterranean caused to the severe pressure gradient and high speed wind on Iraq and Syria. Due to the low humidity and dry characteristics of the region, the dust core is formed. Location of low pressure leads to severe suction of the air of adjacent deserts and moving this phenomenon with entrancing winds to Iran. At the end of cold period, dynamic processes are main factor of formation and transformation of this phenomenon. Formatting trough in middle level and following it formatting center of surface convergence and activity between two center of upper divergence and surface convergence caused extreme instability and lifted the air onto Iraq and Saudi Arabia. The Result of this mechanism formatting speed wind streams and if humidity be low, with respect to characteristics of these areas, dust core is formed.