ارزیابی عملکرد داده‎های بازتحلیل ERA5 در تخمین بارش ایران و واکاوی فضایی رژیم بارشی کشور

نوع مقاله : مقاله پژوهشی

نویسندگان

گروه جغرافیایی طبیعی، دانشکده جغرافیا، دانشگاه تهران، تهران، ایران

چکیده

مقدمه
بارش از مهم­ترین عناصر اقلیمی محسوب می‎شود که در تعیین نقش و پراکندگی دیگر متغیرهای اقلیمی می‎تواند مؤثر باشد. این متغیر ناپایسته‎ترین متغیر اقلیمی در مقیاس زمانی و مکانی است. انجمن هواشناسی آمریکا خصایص توزیع فصلی بارش در یک مکان خاص را رژیم بارش تعریف می‎کند. هدف این مطالعه ارزیابی داده‏های بارش ماهانه پایگاه داده  ERA5نسبت به بارش اندازه‎گیری شده در ایستگاه‏های همدید کشور است. سپس توزیع فضایی رژیم بارش و آماره‌های ضریب تغییرات و چولگی آن به منظور شناسایی رفتار بارش ایران در مقیاس زمانی سالانه، فصلی و نیز ماهانه مورد واکاوی قرار می‎گیرد.  
مواد و روش­ها
در پژوهش حاضر از داده‎ بارش شبکه‌ای پایگاه داده ERA5 با قدرت تفکیک 25/0×25/0 در سال­های 2021-1979 برای پهنه ایران استفاده شده است. برای ارزیابی دقت داده‎های بازتحلیل بارش ماهانه ERA5 نسبت به داده‎های بارش اندازه‎گیری شده در ایستگاه‎های همدید کشور، 4 آماره شاخص ضریب همبستگی (Correlation Coefficient)، ریشه دوم میانگین مربعات خطای نرمال شده (NRMSE)، شاخص توافق ویلموت و شاخص اریبی استاندارد شده (Standard BIAS) به کارگرفته شد.
نتایج و بحث
ضریب همبستگی نشان داد که استفاده از بارش برآوردی این پایگاه در ماه‎های بارشی قابلیت اعتماد بالایی دارد. شاخص ریشه دوم میانگین مربعات خطای نرمال شده (NRMSE) نیز کمترین میزان(توافق عالی بین داده‎ها) را داشته است. همچنین شاخص توافق ویلموت در ماه‎های اکتبر تا آوریل کارایی بالای این پایگاه را در برآورد بارش ماهانه اغلب ایستگاه‎های همدید مورد بررسی نشان می‎دهد. بررسی شاخص اریبی استاندارد نشان داد بیشترین کم برآوردی این پایگاه، به ترتیب در ماه‎های آگوست تا اکتبر و جولای بوده است. بارش با آغاز دوره سرد از در ایران از ماه سپتامبر شروع و می به طول می‎انجامد. سری زمانی بارش ماهانه بیان‌گر رفتار نامنظم بارش کشور در هر ماه می‎باشد. عمده مناطق کشور بیشترین درصد بارشی خود را در فصل زمستان و فصل بهار دریافت می‎کنند. پربارش‌ترین و کم‌بارش‌ترین فصل‌ ایران به ترتیب زمستان و تابستان هستند. همچنین بررسی سری‎زمانی 42 ساله بارش ایران نوسانات دریافت بارش در پهنه کشور در سال‎های مختلف را روشن نمود.
نتیجه­ گیری
مقایسه سری‎زمانی بارش ماهانه در پایگاه داده بازتحلیل ERA5 نسبت به بارش اندازه‎گیری شده ایستگاه‎های همدید، نشان از الگوی مشابه این نوع داده در طولانی مدت داشت. بارش با آغاز دوره سرد از سپتامبر در کشور شروع می­شود و تا می به طول می‎انجامد، که اوج میانگین بارش در مناطق سواحل دریای خزر و سپس در نواحی رشته‌کوه‌های زاگرس دریافت می‌شود. سهم بارش زمستانه کشور 5/44 درصد کل بارش سال است، به عبارتی رژیم اصلی بارش عمدتاً زمستانه است. نتایج تا حدودی بیانگیر تغییر تدریجی رژیم بارش کشور می‎باشد، چرا که از سهم بارش بهاره (2/8 میلی‎متر کاهش در 42 سال) و زمستانه (26/28 میلی‎متر کاهش در 42 سال) کشور کاسته شده و به سهم بارش پاییزه کشور اضافه گردیده است.

کلیدواژه‌ها

موضوعات


عنوان مقاله [English]

Evaluation the performance of ERA5 reanalysis data in Iran's rainfall estimation and spatial analysis of the country's precipitation regime

نویسندگان [English]

  • Susan Heidari
  • Mostafa Karimi
  • Azar Beyranvand
Department of Physical Geography, Faculty of Geography, University Tehran Tehran, Iran
چکیده [English]

Introduction
Precipitation is one of the most important climatic elements that can be effective in determining the role and distribution of other climatic variables. This variable is the most unstable climatic variable in the temporal and spatial scale. The American Meteorological Society defines the characteristics of the seasonal distribution of precipitation in a particular place as a precipitation regime. The aim of this study is to evaluate the monthly rainfall data of ERA5 database compared to the rainfall measured in the synoptic stations of the country. Then, the spatial distribution of the rainfall regime and its coefficient of variation and skewness are analyzed in order to identify Iran's rainfall behavior in annual, seasonal and monthly time scales.
 
Materials and Methods
In the present study, the reanalyzed gridded precipitation data (ERA5) with a resolution of 0.25 x 0.25 was used from the years 1979 to 2021. According to the findings of Hassler and Lauer (2011), ERA5 data has made a clear improvement compared to ERA-Interim data and has shown small biases compared to other reanalysis data. To evaluate the monthly data of this database compared to the monthly rainfall data in synoptic stations of the country, the correlation coefficient, Normalized Root Mean Square Error (NRMSE), the Wilmot agreement index and the standardized Bias index were used.
 
Results and Discussion
The correlation coefficient showed that using the estimated precipitation of this database in rainy months has high reliability. Indicators NRMSE and Wilmot's agreement show the high efficiency of this database. Standard BIAS showed that the most underestimation of this database was in the months of August to October and July, respectively.
Rainfall begins in September and lasts until May in Iran. The monthly rainfall time series shows the irregular behavior of rainfall in each month. Most regions of the country receive their highest percentage of precipitation in winter and spring. 
The rainiest and least rainy seasons of Iran are winter and summer, respectively. Also, the 42-year time series analysis of Iran's rainfall clarified the fluctuations of rainfall in the country in different years.
The coefficient of monthly changes in precipitation is indicative of the intensity of month-to-month changes in precipitation. This index can provide a relative pattern of rainfall variability. In fact, the map of the coefficient of monthly changes in precipitation is reminiscent of extreme fluctuations in monthly precipitation; So that the range of monthly rainfall changes is less than 20% from December to March in the northwest, north and west of the country and reaches more than 450% in July to September in the south and southeast.
Most regions of the country receive their highest percentage of precipitation in winter and spring. The rainiest and least rainy seasons of Iran are winter and summer, respectively. The most important advantage of winter rainfall is that it is scattered in all regions of the country. But summer rain can be seen only in the northern parts and sometimes parts of the east and southeast of the country.
In the autumn season, there is an increase of about 17.5 mm of precipitation in the studied period. But in other seasons, there is a decreasing trend, and the maximum of this negative trend is related to the winter season, which shows a decrease of 28 mm in rainfall. The above changes are statistically significant at the 99% confidence level. ،hese changes show the shift of Iran's seasonal rainfall regime from winter and spring to autumn over time.
Conclusion
A similar rainfall pattern was seen between the two types of data in Iran. Rainfall begins from September and lasts until May. Peak of the average rainfall is received in the coastal areas of the Caspian Sea and then in the areas of the Zagros mountain. The share of winter precipitation in the country is 44.5% of the total precipitation of the year, in other words, the main regime of precipitation is mainly winter. The results to some extent indicate the gradual change of the country's rainfall regime. Because the precipitation of spring (8.2 mm decrease in 42 years) and winter (28.26 mm decrease in 42 years) in Iran has been reduced but autumn rainfall in Iran has been added.
 

کلیدواژه‌ها [English]

  • Data validation of ERA5
  • Comparative data analysis
  • Spatial Variability of Precipitation
  • Precipitation Trends
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