واکاوی فضایی وردایی توزیع درون‌سالانۀ بارش ایران‌زمین طی دهه‌های اخیر (1394-1355)

یکی از انواع رفتارهای ظاهراً آشفتۀ بارش، وردایی ماه‌به‌ماه بارش در مقیاس دهه‌ای است که نمایه‌ای از ساختار اقلیم بارشی بشمار می‌آید. در پژوهش حاضر توزیع ماه‌به‌ماه بارش ایران‌زمین و تغییرات دهه‌ای آن با بکارگیری نمایۀ ضریب تغییرات برای دورۀ آماری 1394-1349 و با بهره‌گیری از داده‌های شبکه‌ای اسفزاری نسخۀ سوم بررسی شد. روابط این متغیر با متغیرهای مکانی توپوگرافیک با استفاده از داده‌های مدل رقومی ارتفاع و براساس شگردهای رگرسیونی برای چهار دورۀ ده‌ساله (13641355، 13741365، 13841375 و 13941385) بررسی شد و نیز برای دستیابی به الگوی فضایی از شگرد تحلیل خوشه‌ای استفاده شد. یافته‌های پژوهش حاضر تنوع تباین ماهانۀ بارش در کشور را تایید نمود، امّا برخلاف انتظار روند بلند مدت تغییرپذیری ماه‌به‌ماه بارش عموماً در کشور فاقد معنی آماری است. همچنین طی دهه‌های مختلف تغییرات قابل توجهی بین نواحی با کمینه، بیشینه و در نتیجه دامنۀ تغییرات مکانی و نیز میانگین پهنه‌ای ضریب تغییرات ماه‌به‌ماه بارش مشاهده نشد. با این وجود، نواحی توام با بارش نسبتاً یکنواخت کشور برای چهاردهۀ متوالی رفتار تناوبی داشته‌‌اند. همچنین گسترۀ پهنه‌های توام با مقادیر مختلف تغییرپذیری نیز تغییر می‌یافته‌است. اگرچه در هر دهه و نیز در کل دوره این گستره‌ها با عوامل مکانی توپوگرافیک در ارتباط معنی‌داری بوده‌است، اما این رابطه نسبتاً کم بوده است. تحلیل فضایی مبتنی بر تحلیل خوشه‌ای، 5 ناحیۀ تغییرپذیری ماه‌به‌ماه بارش به‌دست داد. این نواحی از عرض جغرافیایی حدود 32 درجه به سمت شمال، به طور تقریبی از مدارات تبعیت می‌کنند؛ ولی در نیمۀ جنوبی کشور عموماً با نصف‌النهارات ارتباط بیشتری به‌نمایش می‌گذارند.

Spatial Analysis of variation of inter-annual distribution of precipitation in Iran over recent decades (1970-2016)

IntroductionGeographical situation of Iran is a place for interacting many physical and human processes which lead to specific precipitation climatology in the country. The month to month variation of precipitation is one of the features which the precipitation climatology may reflect due to tempo spatial characteristics. In fact, monthly distribution of precipitation is one of precipitation normal features building up the climate structure. In order to recognize this fundamental characteristic three following questions have been raised:1) Have the month to month distribution of precipitation changed over recent four decades?2) How is the pattern of relationship of month to month distribution of precipitation and spatio topographical variables?3) Is it possible to find a spatial pattern for decadal changes of precipitation of month to month distribution?Data and MethodsIn order to find a responses for the abovementioned questions the distribution of month to month precipitation and its decadal changes was considered by adopting coefficients of variations (CV) for 46 years (19702016) and using the third version of Asfazari dataset. The relationship of precipitation data and spatiotopographical variables calculated based on regression techniques. Moreover, the spatial pattern considered by using cluster analysis. The CV calculated as follow:here ،، are ith raw #39;s and jth column #39;s CV, standard deviation, and monthly mean, respectively. CV and its relationships with spatiotopographical variables were calculated in two temporal scale, for whole the under investigation period (19702016) and in decadal period for four decades (19771986, 19871996, 19972006, 20072016). Discussion The results of current study proved that the month to month different in precipitation amounts have had spatial variations, whilst the temporal trends is not statistically significant. In addition, the minimum, maximum, and consequently, the range of values also the averages have not experienced significantly changes. However, the region experiencing the same values of precipitation illustrated oscillatory behavior. Accordingly, the decadal variations have happened in different areas. Although the there have been statistically significant relationships between monthly CV and spatio topographical factors, the correlations were low. Based on cluster analysis, we found 5 regions according to CV and its anomalies in compares with normal CV for all under investigation period. These regions generally follow the latitudes from 32 N toward northern latitudes, whilst the region in the south of 32 N generally follow the longitude patterns.ResultsPrecipitation is known a chiastic and complicated climate element. One of chiastic behaviors which precipitation shows in its different time scale behavior is its month to month distribution among a given year. In current research the decadal variation of abovementioned behavior among recent four decades and the variation of its relationships and the spatio topographical features , as parts of climate structure of the country, have investigated in details. Our finding illustrated that the month to month different in precipitation amounts have had tempo spatial variations, whilst the temporal long term trends is not statistically significant. Moreover, the values of minimum, maximum, and consequently, the range of month to month CV also the decadal averages have not experienced significantly changes over four under study decades. However, the region experiencing the same values of precipitation depicted oscillatory behavior. consequently, the decadal variations have happened in different areas. Although there have been statistically significant relationships between monthly CV and spatio topographical variables, the correlations were not considerably high. Based on cluster analysis technique, we found 5 regions according to CV and its anomalies in compares to normal CV for all under study decades. These regions generally follow the latitudes from 32 N toward northern latitudes, whilst the region in the south of 32 N generally follow the longitude patterns.KeyWords: Iran precipitation, Month to month changes in precipitation, Inter annual variation of precipitation, Precipitation anomaly, Spatial analysis of precipitation