عنوان مقاله [English]
Global warming increases the frequency and severity of extreme weather events. These changes have major social and environmental consequences. The extensive social and economic effects of extreme values in arid and semi-arid regions such as Iran due to having a very vulnerable climate are high, and their sudden changes may lead to devastating events. Its destructive effects, including floods and droughts, have been very large in recent years in Iran. The World Meteorological Organization has defined a joint project between the Commission for Climatology (CCL), Climate Variability and Predictability (CLIVAR) and the World Climate Research Program (WCRP) in order to detect and monitor climate change and its profiles. Calculation methods of different profiles were presented with several softwares, including ClimDex and RClimDex. In this research, we investigate the changes in temperature and precipitation profiles according to CCL/CLIVAR in Kohgiluyeh and Boyer Ahmad province.
Materials and methods
Due to the importance of Kohgiluyeh and Boyer Ahmad province in terms of water, agriculture, industry and human resources, this region was selected for study. In order to ensure that there is no spatial and environmental change in the investigated stations and to have the appropriate statistical quality, the data and information of all meteorological stations of the province were checked since their establishment. Finally, the stations of Yasouj in cold climate and Dogunbadan in warm climate were selected for this study because they had a suitable statistical period (1986-2018) and the least single or consecutive missing data.
Extreme indices based on long-term and homogeneous data express the status of extreme events. These indices show aspects of the climate change event and its effects. The expert team of ETCCDM has classified extreme indices into five groups, which are based on percentiles, absolute extremes, threshold extremes and other profiles.
In order to study the existence of a trend and its significance, the non-parametric Mann-Kendall test was used. This test is developed based on the ranking of data in a time series. The null hypothesis of this test means that there is no trend in the data series and assumption one indicates the existence of a trend in the data series.
Results and Discussion
Humans and the environment often react to changes in maximum and minimum values more than changes in average conditions. Therefore, analyzing the variability and examining the trend of extreme values are more important than average climatic conditions. In this research, extreme indices of temperature and precipitation were calculated and analyzed using the Rclimdex package in R software.
The PRCPTOT index shows the total rainfall on days when the daily rainfall was equal to or more than 1 mm. It has a negative trend in both stations, which is not significant. The index of days with heavy rainfall, which means the number of days with daily rainfall equal to or greater than 10 mm, shows a significant decreasing trend in both stations. The index of consecutive wetter days, which means the maximum number of consecutive days of daily rainfall more than 1 mm, shows a decreasing trend in both stations, and its change trend is significant in Yasouj station, but not significant in Dogonbadan station.
The index of the number of summer days, which is defined as the number of days with a maximum daily temperature of more than 25° C, showed a significant increasing trend in both stations. No ice day observed in Dogonbadan station during the statistical period. However, this index has a negative trend in Yasouj station, which is not significant. The index of warm nights or the number of days with a daily minimum temperature of more than 20° C shows a significant increasing trend in Dogonbadan, and although it has a decreasing trend in Yasuj, the trend of changes wad not significant. Due to the prevailing climate in that region, Dogonbadan station has not experienced zero temperature during the statistical period. In Yasouj, the trend of frost days is increasing, but it is not significant. The Growing season Length index of Dogonbadan has shown a significant increase during the 1986-2018 period. The index of the warmest day in both stations showed a significant increasing trend and the coldest day changes in both stations is positive but not significant. The warmest night in Dogonbadan showed a positive trend and in Yasouj it showed a decreasing trend, which is not significant in any of them. The coldest night showed a decreasing trend in Dogonbadan and an increasing trend in Yasouj, which was not significant in any of them. The average difference between the maximum and minimum daily temperature in Yasouj station is increasing and significant, but this index was decreasing in Dogonbadan, although the trend of its changes was not significant.
Extreme indices of temperature and precipitation and their changes were investigated using the Rclimdex. The results indicate the decreasing trend of all precipitation indices in the statistical period. In the Dogonbadan station, the index of days with heavy rainfall above 10 mm had a significant decreasing trend, and the rest of the changes in other indices were not significant. In the Yasouj station, the profiles of days with heavy rainfall above 10 mm and consecutive wet days had a significant decreasing trend. The frequency of warm events, such as summer days and warm nights, the length of the growing season and the range of day and night temperature changes showed an increasing trend during the study period. In addition, the frequency of cold events, such as cold days and nights and freezing days, was reduced.