ارزيابي آهنگ رفتار زماني‌- مكاني بارش در دو دهه اخير در ايران

Assessment track of spatiotemporal precipitation the past two decades in Iran

هدف از پژوهش حاضر ارزيابي تغييرات زماني- مكاني بارش در ايران در دو دهه اخير است. نخست، داده‏هاي پايگاه داده- بارش ياخته‏اي APHRODITE با تفكيك مكاني 25/0 × 25/0 درجه قوسي در بازه زماني 1/1/1951 تا 31/12/2007 استخراج شد. براي دست‏يابي به تغييرات درون‌دهه‏اي بارش، از روش‏هاي نوين آمار فضايي مانند خودهمبستگي فضايي موران جهاني، شاخص انسلين محلي موران، و لكه‏هاي داغ با استفاده از قابليت‏هاي برنامه‏نويسي در دو محيط و بهره گرفته شد. نتايج نشان داد تغييرات درون‌دهه‏اي بارش در ايران داراي الگوي خوشه‏اي بالاست. در اين ميان، بر اساس شاخص محلي موران و لكه‏هاي داغ، بارش در كرانه‏هاي ساحلي درياي خزر و بخش‏هاي غرب و جنوب غرب كشور (عمدتاً زاگرس) داراي خودهمبستگي فضايي مثبت (خوشه‏هاي بارش با ارزش بالا) است و در بخش‏هايي از نواحي مركزي و همچنين بخش‏هايي از جنوب شرق كشور داراي خودهمبستگي فضايي منفي (خوشه‏هاي بارش با ارزش پايين) است. در ساير مناطق، بارش هيچ‏گونه الگوي معني‏دار يا خودهمبستگي فضايي نداشته است. به ‏طور كلي، بيشترين تغيير الگوهاي بارشي متعلق به فصل پاييز است، سپس فصل تابستان. همچنين، كمترين تغييرات نيز مربوط به فصول بهار و زمستان است. برون‌دادهاي آماره‏هاي مورد مطالعه بيانگر آن است كه در دهه‏هاي اخير تغيير اقليم نمودي آشكارتر يافته و الگوهاي پُربارش در مناطق جنوبي در حال عقب‏نشيني است و فقط در حال محدودشدن به كانون‏هاي عمده در زاگرس و كرانه‏هاي درياي خزر است.

Introduction Rainfall is a fundamental meteorological element, and directly or indirectly affected human life. In many climatic studies, reviews and analysis of suitable temporal and spatial resolution data in atmosphere based on detailed monitoring is necessary, and the need for resources and authoritative database also raised here. Today, with the emergence of the phenomenon of climate change and its consequences and the need to study all aspects of climate, these kind of data especially in recent decades has been more needed. On the other hand, since spatial patterns of rainfall in short-term time scale was often too heterogeneous then achieving a suitable method for estimating large regional rainfall in large areas is essential. Research Methodology To assess the temporal and spatial behavior of precipitation in Iran during last two decades, the Aphrodite pixel database was used. The base period of spatial data with spatial resolution of the Middle East and the period from 01/01/1988 to 31/12/2007 AD by pixel size value 25/0 × 25/0 ° arc were taken. To assess spatial autocorrelation data during last two decades the spatial autocorrelation of Global Moran Method was applied. In this study, cluster analysis and outlier analysis (Anselin Local Moranʹs I) and also applied hot spot analysis (Getis-OrdGi*) to study the temporal and spatial changes in precipitation patterns. Results and discussion The global Moran index for all four seasons of two periods of study is more than 0.75. Based on global Moran index, rainfall in the country in two decades of study, indicate a cluster pattern of up to 95 and 99 percent confidence level. Due to the high value of Z and low value of Probability the hypothesis of no spatial autocorrelation between data during two periods did not verified. In most parts of the country (43.78%) there was not any type of patterns. Then the lack of spatial autocorrelation during second period caused increasing the amount of area equal 5.88 percent to 49.66. Areas with no spatial autocorrelation in the summer reached maximum value, and it was in the first and second period equal 90.88 and 92.36 percent consequently. In other seasons, spring and autumn there were also areas with no spatial autocorrelation pattern and is allocated relatively half of the country. The amounts of spatial autocorrelation for rainfall data in spring were negative during the first period and it decreased relative to the second period of 1.53 percent. This decrease is mostly found in Northern Khorasan and Central Zagras. There was a low-LL cluster at any of a two periods of study in the summer. The cluster of low (LL) in the autumn of the second period (25.30%) compared to the first period (28.01 %) changed small value (2.71 however, it changed significantly in terms of location and the center of low precipitation patterns displaced toward east and southeast regions of Iran. The patterns of negative spatial autocorrelation in winter, autumn and spring, were changes in spatial and temporal dimension, Then the most of the decline in the two decades of study is associated with the second period in winter season with a numerical reduction (6.62%) compared to the first period (31.30 %).It is worth mentioning that most of the reduction is allocated to Zagros region, South East of Iran and Northern Khorasan. In general it can be concluded that local factors and general atmospheric circulation systems in the first and second stage affected on distribution of precipitation patterns in Iran. Conclusion Results showed that long-term rainfall patterns over a period of interaction of local factors and elements of atmospheric circulation are formed. The geographical feature of precipitation patterns are based on local factors, especially latitude and topography; however, we should not ignore the role of external factors in the formation of rainfall patterns because the external factors like general atmospheric circulation play a significant role in the precipitation regime and spatial and temporal changes of precipitation. If we pay attention to cluster rainfall map of Iran we can conclude that the cluster of high rainfall and low precipitation are not similar to each other due to the effect of the general atmospheric circulation patterns. In general we can conclude that the precipitation patterns are affected and controlled by two main factors, which include: (1) local factors control the location (geographic feature of precipitation patterns) and (2) external factors controlling time (regime of precipitation patterns). Finally this study could be a model for other studies of climatic parameters by a general comparison of precipitation patterns of pixel base data of Aphrodite compared with measured values in climatology and synoptic stations. Then in all studies in general and in other fields of study for example ecology and environmental sciences in particular which require updated and accurate climatic data in terms of time and space, we can use Aphrodite data.