توزیع مکانی روند تغییر فراسنج‌های فرین بارش شمال‌غرب ایران و ارتباط آن با گردش عمومی جو منطقه

گرمایش جهانی موجب افزایش میزان بخار آب اتمسفر و تغییر در چرخه آب ‌شناسی و افزایش بارش ‌های فرین شده است. هدف از پژوهش جاری بررسی روند تغییرات فراسنج‌ های فرین بارش شمال‌ غرب ایران و ارتباط آنها با گردش عمومی جو است. بدین منظور داده ‌های بارش 20 ایستگاه همدید شمال ‌غرب ایران در مقیاس روزانه برای سال ‌های 1365 تا 1390 استخراج و شاخص‌ های بارش محاسبه و نقشه‌ های توزیع مکانی این شاخص ‌ها ترسیم شد. برای بررسی تغییرات گردش عمومی جو و مطالعه تاثیر آن بر وقایع حدی نیز، داده‌ های ترکیبی میانگین گردش اتمسفری سالانه دوره 1364-1340 که نمایشگر اقلیم گذشته و مقطع زمانی1395-1365 به عنوان دوره نوین بر پایه داده ‌های بازتحلیل شده ncep/ncar دریافت شد. سپس نقشه ‌های تفاضل با استفاده از متغیرهای ارتفاع ژئوپتانسیل سطح hp500، امگای سطح زمین، تاوائی نسبی سطح hp500، آب بارش‌ پذیر و میزان بارش برای آشکارسازی تغییرات گردش اتمسفری در دو دوره فوق محاسبه و تولید شد و نتایج حاصل با برون‌داد فراسنج ‌های حدی دما مقایسه شد. بررسی تغییرات فراسنج ‌ها حاکی از این بود که تمام شاخص‌ های بارش به جز شاخص بیشینه طول دوره رشد (cdd) دارای روند کاهشی هستند. مطالعه گردش عمومی جو نیز حاکی از افزایش ارتفاع تراز hp500 و در نتیجه پایداری جو منطقه می ‌باشد. مطالعه امگا و چرخندگی نسبی نیز حاکی از تضعیف جریانات صعودی و چرخندگی مثبت در منطقه است. نقشه‌ های تفاضل آب قابل بارش و نرخ بارش نیز کمبود بار رطوبتی جو و گرایش منطقه به سمت اقلیم خشک را نشان می ‌دهد.

spatial distribution of changes in the trend of extreme precipitation indices in northwest iran and its relationship with the general circulation of the atmosphere in the region

introductionextreme precipitation has a significant impact on the frequency, severity, and duration of natural hazards, such as floods, droughts, and landslides. this has a significant impact on human life, the economy, natural ecosystems, and agriculture (song et al, 2015: 34). between 1880 and 2012, there was a 0.85 °c increase in the average global temperature, with a general increase in precipitation in the mid-latitudes of the northern hemisphere (ipcc, 2013: 2; lio et al, 2017: 822). in addition, there is a possibility of a rise in extreme precipitation in the future (klein tank et al, 2006: 1), and so far, the reason for these changes and their relationship with the general circulation of the atmosphere have not been considered. the aim of this study is to analyze the trend of changes in extreme precipitation indices in northwestern iran and its association with the general circulation of the atmosphere. material and methodsin order to analyze the changes in extreme precipitation events in northwestern iran, daily precipitation data was collected from 20 synoptic stations in the region between 1986 and 2010. the region that is being studied encompasses west azerbaijan, east azerbaijan, ardabil, zanjan, and kurdistan. in assessing limit events, high quality and reliable long-term climate data with daily (or higher) resolution is required (clintanak et al., 2009: 9). the first step was to examine the quality control and homogeneity of data. the rclimdex software package, introduced as a standard tool by etccdi, was used to perform quality control and evaluate data homogeneity in this research. the expert team on climate change detection, monitoring and indices (etccdmi) introduced 11 indexes to examine changes in precipitation level indices in northwest iran. rclimdex software calculates these indicators with a significance level of 0.05. this process seeks to establish a standard set of indicators to examine and compare the characteristics of different regions. the software was used to calculate precipitation indices and display the trend and rate of change on a map. results and discussionthe extreme precipitation indices were calculated to determine the regional trend and percentage of stations with positive and negative trends for the studied stations in northwestern iran. afterward, a map was created showing the spatial distribution of the slope for each of the indices. all precipitation indexes, except for the maximum growth period index (cdd), are declining according to the results. the probability of precipitation has decreased due to the more stable winter atmosphere in the region from the point of view of general atmospheric circulation. the region’s spring atmosphere, similar to that of winter, shows an increase in stability, which will result in less rainfall. in summer, except for the coastal provinces of the caspian sea and the coasts of the oman sea, the rest of the country has recorded a decrease in rainfall of 1 mm per day. most parts of the country experienced an increase in atmosphere thickness to 6 meters in autumn in the study area. autumn in the region is typically stable and barotropic, but the study area is experiencing less rainfall. this study examines the trend of changes in extreme precipitation indices in northwestern iran and its relation to a large-scale general circulation of the atmosphere. according to the results, 75% of stations in the region are experiencing a decrease in the maximum daily rainfall (rx1day) and 80% are experiencing a decrease in the maximum five-day rainfall (rx5day).