نوع مقاله : مقاله پژوهشی
نویسندگان
گروه جغرافیای طبیعی، دانشکده علوم زمین، دانشگاه شهید بهشتی، تهران، ایران
چکیده
کلیدواژهها
موضوعات
عنوان مقاله [English]
نویسندگان [English]
Introduction: Dust storms with visibility of less than 200 meters are dangerous storms for human life and activity. By knowing the behavioral characteristics and how these storms form and spread over the sources of dust production, it is possible to reduce its many damages.
Materials and methods: According to the purpose of the research, to investigate the historical trend of the occurrence of severe storms and the synoptic patterns that cause such storms, a statistical period of 33 years was selected. Based on this statistical basis, 16 synoptic stations were selected as sample stations. The days in which code was reported in at least two observatories with visibility less than 200 meters were selected as a day with a storm. To select synoptic systems, the days when this phenomenon was reported in at least one third of the sample stations were selected as a dust system. In this way, 68 study samples were extracted.
Results and discussion: The highest number of dust storms with visibility less than 200 meters occurs in two turbulent seasons in terms of weather, i. e. winter and autumn. In monthly terms, occurred in January (352 cases) and December. In terms of historical trends, from the beginning of the period to 2007, the number of storms has been almost constant. Since 2008, the number of such storms has increased significantly and this process continued until 2012. In terms of spatial distribution, the highest number of storms occurred in the cold period of the year, in the eastern stations of the region, on the contrary, in the warm period of the year, occurred in the western. Two recurring patterns are the main cause of dust storms in the cold period of the year. In the African gyre pattern, the significant northward expansion of this anticyclone ridge and the cold advection of the subpolar latitudes over the western region of Asia creates a severe temperature and pressure gradient in the entire lower and middle layer of the troposphere. In the Sudan low pattern, with the significant expansion of the Sudan low pressure tongue over the western region of Asia and the formation of a deep trough in the middle layers, it causes the instabilities to intensify over the dust sources of Syria and Iraq. Most summer dust storms have followed a general pattern. In these patterns, three low pressure systems, Saudi Arabia, Pakistan and sometimes Lut desert, are the main cause of dust storms in the lower layer (up to the level of 850hpa).
Conclusion: The result of this research showed that storms with visibility of less than 200 meters are increasing in the western region of Iran. In terms of spatial distribution, the highest number of such storms occurs in the cold period of the year in the eastern part of the region. From a synoptic point of view, dust particles rise from dust sources in Syria and western Iraq and enter the eastern part of the study area along the atmospheric currents of the middle layer of the troposphere. Meanwhile, the western part of the study area is affected by the highest number of dust storms in the hot period of the year.
کلیدواژهها [English]
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