بررسی وردایی دهه‌ای بارش سالانۀ ایران‌زمین طی چهار دهۀ اخیر (1394-1355)

طی سدۀ گذشته اقلیم کرۀ زمین دگرگونی را در مقیاس‌های مکانی و زمانی مختلفی تجربه کرده‌است. این روند دگرگونی احتمالاً برای آینده نیز ادامه خواهد داشت. یکی‌از جلوه‌های دگرگونی اقلیم که تغییر(پذیری) آن را نشان می‌دهد، در وردایی رفتار دهه‌ای بارش قابل ردیابی است. به‌منظور ردیابی تغییرپذیری بارش ایران، به‌عنوان نمایه‌ای از تغییر اقلیم این سرزمین، طی چهار دهۀ اخیر (1394-1355) از پایگاه داده‌ای اسفزاری نسخۀ سوم (حاصل میان‌یابی داده‌های بارش روزانه 2188 ایستگاه همدید، اقلیمی و باران‌سنجی سازمان هواشناسی به مدت 46 سال از 1349 تا 1394 و مجموعاً 16801 روز و با تفکیک مکانی 10 کیلومتر و درنتیجه با ابعاد 16801×205×167) استفاده شده‌است. به منظور بررسی وردایی دهه‌ای بارش، میانگین بارش دهه‌های منتهی به دهۀ 1394-1385 محاسبه و ویژگی‌های مکانی بارش ایران طی چهار دورۀ ده‌ساله به روش توصیفی ترسیمی بررسی و مطالعه شد. دراین راستا میانگین بارش هردهه با کل دورۀ آماری و دهۀ پیش از آن مقایسه شد. نقشه‌های بارش هر دهه، نقشه‌های ناهنجاری آن در قیاس با میانگین کل و نقشه‌های ناهنجاری آن در قیاس با دهۀ پیشین ترسیم و بررسی شد. میانگین بارش در انتقال از دهۀ اول به دوم افزایشی و پس از آن کاهشی بوده است. از آن‌پس رفتار عمومی کاهشی بارش، هم در نواحی کم‌بارش و هم نواحی پربارش در طی دهه‌های متوالی دیده می‌شود. بارش عموماً در نواحی توام با بارش زیاد (درکرانه‌های دریاهای خزر، عمان و خلیج فارس، دامنه‌های غربی رشته کوه‌های زاگرس و دامنه‌های شمالی رشته کوه‌های البرز)، تغییرات بیشتری را نسبت به بخش‌های کم‌بارش کشور (نظیر نواحی مرکزی، شرقی و شمال شرقی) متحمل شده‌اند؛ به تبع آن بسیاری آماره‌های مکانی بارش، نظیر ضریب‌تغییرات، چولگی، کشیدگی، آستانه‌های بارش (آستانه‌های کم و زیاد)، تغییر یافته‌است. این‌امر در جابه‌جایی خط همبارش میانگین هر دهه و کاهش سرزمین‌های توام با بارشی بیش‌از میانگینِ دهه‌ای نیز منعکس شده‌است.

An investigation of Decadal variation of Iran precipitation over four decades (1976-2016)

IntroductionAccording to previous investigation and examining climatic elements, the hypotheses of global warming and consequently, global climate change is confirmed by majority of climatologists society around the world. The global changes probably continue for the next decades. The changes in climatic elements, by and large, categorized into two types; trends and variation. The trends refer to long term changes, whiles variations indicate vary time interval changes including oscillation, phase, jump (sift), and persistence.Precipitation is one of climatic elements which can properly reflect chaotic behavior of climate system, and illustrate the nature of changes in the system. Trends, Oscillation, and persistence in this element are investigated in national and international scale, whilst the decadal variations as an index of climate variation can contribute to the current literature. In current study we attempted to illustrate an objective feature of precipitation characteristics and its anomalies over four recent decades by using Asfezari National Dataset (AND).          Data and MethodsIn the present study, the gridded precipitation data of the third version of AND with spatial resolution of 10×10 km during the time period of 1970/3/21 to 2016/3/19 (46 years including 16801 days) is used. This dataset adopted from 2188 synoptic, climatology, and rain gauge stations and subjected to interpolation by using Kriging interpolation method. The dataset covers an area from  N  and E. Accordingly, a pixels cover the area for 16203 days. Consequently, the dataset includes  dimensions.General spatial features of Iran precipitation for the whole under investigation period was studied based on climatological annual precipitation. Next, the same characteristics calculated for four decades ending up to 2016/3/19. Finally, for every decade the anomalies of precipitation in compare with the whole understudy period and its previous decades calculated in order to discover the spatial pattern of decadal fluctuation in precipitation.   DiscussionGeneral characteristics of annual precipitationAnnual mean of precipitation over Iran is 250.5 mm. Due to decline in temperature contrast and strength of fronts in the Mediterranean cyclones, as a main source of precipitation in Iran, the annual precipitation over Iran decreases from west to east, and from north to south.The annual precipitation in 63.2% of Iran is lower than the climatic annual mean. The annual mean of precipitation in this area which generally located in east and south of the country is approximately 150.5 mm. Thus, the total precipitation in this area is equal to the total precipitation in the rest 36.8% of the country which its annual precipitation is more than the annual precipitation in the country, 422 mm.  The spatial variation of precipitation is confirm by other statistics, for instance, skewness, kurtosis, the extreme threshold indices. For instance, a large part of Iran (26.73%) includes 100150 mm annual precipitation, whiles the precipitation in 15.8% of the country reaches to 150200 mm. Parts of northeast of Iran, and the coast  of Persian Gulf  and Oman Sea in the south, in addition to southern slops of Alborz mountain chain experience a precipitation amount of lower than 100 mm. In contrast to the abovementioned dry regions, the (approximately) wet regions include limited areas for each precipitation class. For example, only 9.1% of the country characterized with 500 mm of precipitation, while the classes of 200300, 300400, and 400500 comprise 20.62, 12.64, and 6.11 percents of the country, respectively.Decadal variation of precipitation            In current section the spatial distribution and statistical features of precipitation in each decades was illustrated. The following list includes our finding of statistical graphical analysis of precipitation in four successive decades:1) The difference between spatial mean and median of annual precipitation increased from the first to the last decades. The increasing in this characteristic refers to increase in spatial asymmetrical distribution of precipitation over the country.2) A comparison between spatial distribution of precipitation maps showed that generally, the areas experienced precipitation above the decadal and whole period average are decreased from the first and last decades.3) The increase in spatial skewness from the first decade to the last decade is another evidence of increasing in precipitation spatial differences.4) The last but not the least finding is the changes in the extreme threshold indices. From the first to the last decade, the range of 10th and 90th percentiles have increased.           ConclusionPrevious studies depicted that the amount of Iran precipitation has decreased over recent decades. In order to investigate the role of each decade in the decreasing values, the gridded precipitation data of the third version of AND with spatial resolution of 10×10 km during the time period of 1970/3/21 to 2016/3/19 (16801 days) is used. General spatial features of Iran precipitation for the whole under investigation period was investigated based on climatological annual precipitation. Next, the same characteristics calculated for four decades ending up to 2016/3/19. Finally, anomalies of precipitation in compare with the whole understudy period and previous decades calculated in order to discover the spatial pattern of decadal fluctuation in precipitation.  Our finding showed that by and large, precipitation has decreased over recent decades. The changes has been more pronounced in southern and northern coastal area, western slopes of Zagros mountain chain, and northern slopes of Alborz mountain chains. Previous researchers attribute these changes to changes in humidity advections in recent years.