مقایسه و تحلیل پراکنش زمانی و مکانی طوفان‌های گرد و خاک با دیدکمتر از 200 متر در غرب و جنوب غرب ایران

طوفان‌ها پدیده‌های جوی تشدید شده‌ای هستندکه گاه با بارش‌های شدید و گاه با گرد وخاک همراه می‌باشند. طوفان‌های گرد و خاک و بخصوص طوفان‌های با دید افقی کمتر از 200 متر همواره خسارت بار و مخل آسایش ساکنین بوده است. در سالهای اخیر فراوانی این پدیده در مناطق غرب و جنوب غرب ایران بخصوص در دوره سرد سال روند افزایشی نشان می‌دهد. برای بررسی این پدیده، با استفاده از داده‌های روزانه، دید افقی کمتر از 200 متر منطقه غرب و جنوب غرب ایران در یک دوره آماری 33ساله (1987 تا 2019) استخراج شده است. و با توجه به متفاوت بودن دو منطقه مورد نظر پراکنش ماهانه، و سالانه و فصلی طوفان‌های گرد و خاک در این منطقه مورد بررسی قرار گرفت. نتایج این مطالعه نشان داد که تباین اساسی بین دو منطقه مطالعاتی در پراکنش مکانی طوفان‌های گرد و خاک در دو فصل پاییز و زمستان است. در این دو فصل بالاترین فراوانی طوفان‌های گرد و خاک در منطقه غرب در ایستگاه‌های شرقی منطقه بخصوص در ایستگاه‌هایی مانند همدان، کنگاور، قروه و بیجار است و کمترین تعداد طوفان‌های گرد و خاک در غرب منطقه رخ داده است. در صورتی که در منطقه جنوب غرب بالاترین تعداد طوفان‌های گرد و خاک از ایستگاه‌های غربی استان خوزستان گزارش شده است. ایستگاه‌های همانند دزفول و بستان بالاترین فراوانی طوفان‌های گرد و خاک را ثبت کرده اند. به نظر می‌رسد طوفان‌های دوره سرد منطقه غرب ایران از مبادی دورترتولید شده و در لایه‌های میانی جو بر روی منطقه غربی منطقه همانند همدان و کنگاور منتقل شده است. در صورتی که طوفان‌های گرد و خاک منطقه جنوب غرب در شرایط همدیدی متفاوت و از مبادی نزدیک‌تر و در لایه زیرین وردسپهر وارد منطقه شده است.

comparison and analysis of the temporal and spatial distribution of dust storms with visibility of fewer than 200 meters in western and southwestern iran

synoptic analysis of the changes trend of the share of systems due to the sudan low in the cold period of the persian gulf coast during 1976 2017 introduction in the ethiopian sudan range forms the low pressure system without front in the cold and transition seasons that is affecting the climate of the adjacent regions by crossing the red sea. based on the evidence in the context of iran, studying sudan low was first begun by olfat in 1968. olfat refers to low pressures which are formed in northeastern africa and the red sea and then pass saudi arabia and the persian gulf, enter iran, and finally, cause rainfall. the most comprehensive research specifically examining sudan low, was the work carried out by the lashkari in 1996. while he studying the floods that occurred in southwestern of iran, he was identified sudan low by the most important cause of such flooding and he explained how they are formed, and how these low pressure systems were deployed on the southwest of iran. materials and methods the study period with long term variations was considered from 9.5 to 11 years based on solar cycles. precipitation data for 13 synoptic stations are considered above 5 mm in south and southwestern iran. with three criteria were determined for the days of rainfall caused by each type of atmospheric system. the visual analysis of high and low altitude cores and geopotential height at 1000 hpa pressure level (el fandy, 1950a; lashkari, 1996; 2002) were considered based on the aim of the study. accordingly, the approximate locations of activity centers, as well as the range of the formation and displacement of the sudan system were initially identified based on the location of the formation of low and high pressure cores. then, the rainy days due to the sudan system in january were separated from the precipitation of the other atmospheric system. results and discussion according to the selected criteria in the forty year statistical period, 507 precipitation systems were identified with different continuities that led to precipitation in the northern coast of the persian gulf. the pattern of independent sudan low rainfall was responsible for 77% of the precipitation in the persian gulf. decade frequency share of sudan low was lower in the first decade (16%) compared to the next three decades. this system of rainfall was more activated during the second and third decades compared to the first decade. however, rainfall changes were not evident in the mid decade. independent sudan low precipitation provide 25% and 27% of the cold season precipitation of the persian gulf during the second and third decades respectively. in accordance with the 24th solar cycle, at the end of the study period, the sudan low was more effective on the gulf coast than ever before. during this decade, 125 cases of sudan low rainfall was recorded for the persian gulf. thus, the frequency of sudan low during the fourth decade was about 31%, which was higher than in the rest of the decade. overall, the sudan low rainfall was repeated 151 times for 2 days rainfall, during the statistical period studied. this precipitation has increased over the last decades compared to other periods. conclusion the severe variability of rainfall along the timing and location of the permanent persian gulf coasts can have a significant impact on the economic and agricultural behavior of the gulf population in the three provinces of ahwaz, bushehr and hormozgan.the purpose of this study was to evaluate the precipitation changes due to sudan low in the persian gulf coastal region during the cold period. the results of this study showed that the role of integration patterns in influencing the precipitation of the persian gulf coast has decreased with the strengthening and further activation of the sudan low system during the last two decades. that way, about