پديد آورندگان :
عساكره، حسين دانشگاه زنجان - دانشكده ادبيات و علوم انساني , مسعوديان، ابوالفضل دانشگاه اصفهان , تركاراني، فاطمه دانشگاه زنجان
كليدواژه :
بارش ايران , تغييرات ماه به ماه بارش , وردايي درون سالانۀ بارش , ناهنجاري بارش , واكاوي فضايي بارش
چكيده فارسي :
يكي از انواع رفتارهاي ظاهراً آشفتۀ بارش، وردايي ماه‌به‌ماه بارش در مقياس دهه‌اي است كه نمايه‌اي از ساختار اقليم بارشي بشمار مي‌آيد. در پژوهش حاضر توزيع ماه‌به‌ماه بارش ايران‌زمين و تغييرات دهه‌اي آن با بكارگيري نمايۀ ضريب تغييرات براي دورۀ آماري 1394-1349 و با بهره‌گيري از داده‌هاي شبكه‌اي اسفزاري نسخۀ سوم بررسي شد. روابط اين متغير با متغيرهاي مكاني - توپوگرافيك با استفاده از داده‌هاي مدل رقومي ارتفاع و براساس شگردهاي رگرسيوني براي چهار دورۀ ده‌ساله (1364-1355، 1374-1365، 1384-1375 و 1394-1385) بررسي شد و نيز براي دستيابي به الگوي فضايي از شگرد تحليل خوشه‌اي استفاده شد. يافته‌هاي پژوهش حاضر تنوع تباين ماهانۀ بارش در كشور را تاييد نمود، امّا برخلاف انتظار روند بلند مدت تغييرپذيري ماه‌به‌ماه بارش عموماً در كشور فاقد معني آماري است. همچنين طي دهه‌هاي مختلف تغييرات قابل توجهي بين نواحي با كمينه، بيشينه و در نتيجه دامنۀ تغييرات مكاني و نيز ميانگين پهنه‌اي ضريب تغييرات ماه‌به‌ماه بارش مشاهده نشد. با اين وجود، نواحي توأم با بارش نسبتاً يكنواخت كشور براي چهاردهۀ متوالي رفتار تناوبي داشته‌‌اند. همچنين گسترۀ پهنه‌هاي توأم با مقادير مختلف تغييرپذيري نيز تغيير مي‌يافته‌است. اگرچه در هر دهه و نيز در كل دوره اين گستره‌ها با عوامل مكاني - توپوگرافيك در ارتباط معني‌داري بوده‌است، اما اين رابطه نسبتاً كم بوده است. تحليل فضايي مبتني بر تحليل خوشه‌اي، 5 ناحيۀ تغييرپذيري ماه‌به‌ماه بارش به‌دست داد. اين نواحي از عرض جغرافيايي حدود 32 درجه به سمت شمال، به طور تقريبي از مدارات تبعيت مي‌كنند؛ ولي در نيمۀ جنوبي كشور عموماً با نصف‌النهارات ارتباط بيشتري به‌نمايش مي‌گذارند.
چكيده لاتين :
Introduction
Geographical 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 Methods
In 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 (1970-2016) and using the third version of Asfazari dataset. The relationship of precipitation data and spatio-topographical variables calculated based on regression techniques. Moreover, the spatial pattern considered by using cluster analysis. The CV calculated as follow:
here ،، are ith raw's and jth column's CV, standard deviation, and monthly mean, respectively.
CV and its relationships with spatio-topographical variables were calculated in two temporal scale, for whole the under investigation period (1970-2016) and in decadal period for four decades (1977-1986, 1987-1996, 1997-2006, 2007-2016).
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.
Results
Precipitation 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 above-mentioned 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.
