تحلیل اثر تغییر اقلیم بر ناایستایی بارش های حدی ایران

Introduction

Climate change will probably change the occurrence of extreme events, According to the sixth report of the IPCC, it is predicted that the intensity of the maximum daily rainfall will probably increase by a percentage for every 1-degree increase in the global temperature. The intensification of hydrological cycles due to climate change will most likely affect the intensity and frequency of extreme rainfall. Therefore, the temperature can be considered as an important covariate because the warmer atmosphere is likely to hold more moisture, thereby generating a favorable condition to increase extreme rainfall. The frequency of extremes has been changing and is likely to continue changing in the future. So, Classical stationary frequency analysis techniques used in climate studies are no longer reliable. In the stationary model, the statistical parameters of the probability distribution function do not change over time or other covariates, while in the non-stationary model, the parameters of the probability distribution function are not constant, and can change with time or other covariates. Therefore, models and concepts that can account for non-stationary probability distribution analysis of climatic and hydrologic are needed.In climate studies, classical and static frequency analysis techniques do not provide accurate results from climate realities due to the probability of precipitation occurrence and its return period. In static modeling, statistical parameters of the probability distribution function do not change over time or in relation to other auxiliary variables, whereas in the non-static model, the parameters of the probability distribution function are not constant, and can vary relative to time or other covariables. Therefore, the models of inertial probability distribution should be considered. Reviewing and evaluating the background of the research shows that despite the importance and efficiency of the instability methods compared to static method in the investigation of extreme rainfall, there has been less attention in the studies conducted in the country. Our aim in this study is to analyze the instability of extreme precipitation affected by increasing temperature. For this purpose, trend and return period of extreme precipitation were calculated using nonstatic GEV distribution and considering the maximum temperature and mean annual temperature as variables affecting the instability of extreme precipitation.

Data and Methods

This study was conducted using daily rainfall data from 36 stations from 1960-2021. We employ a non-parametric Mann-Kendall trend test to examine whether the annual extreme rainfall intensity is influenced by climate change and temperature increase. Nonstationary frequency analysis for extreme rainfall was conducted using non-stationary GEV distribution by incorporating the annual average daily temperature and maximum daily temperature recorded at the time the extreme rainfall occurred. We considered the annual average daily temperature and maximum daily temperature as a function of location and scale parameters.

Result

The research findings indicate that rising temperature has led to a non-stationary trend in the intensity and frequency of extreme rainfall in the country. However, this non-stationarity does not follow a consistent pattern throughout the country. The intensity and frequency of extreme rainfall have generally decreased due to the increase in temperature. The intensity of extreme rainfall was significantly decreased in 6 stations including Hamedan, Shiraz, Shahr-e Kord, Zahedan, Iranshahr, and Shahroud. On the other hand, the trend of nonstationarity in Kerman was significantly increasing with increasing maximum temperature. Only two stations, Zahedan and Arak, were found to be significantly non-stationary concerning the annual mean temperature, both stations showed a decreasing trend. The southern and southeastern regions of the country have shown a more consistent and intense decrease in the intensity and frequency of extreme rainfall. Conversely, the northern regions, including stations along the Caspian Sea, Tehran and Qazvin, and the northeast of the country, have experienced an increase in the intensity of extreme rainfall. The probability of extreme rainfall about the average annual temperature has shown a different pattern, with the greatest increase in the intensity of rainfall observed in the stations of the northern, western, and southwestern regions. Overall, the central regions as well as the eastern, northeastern, and southeastern regions of the country have experienced a higher percentage of decrease in rainfall intensity with an increase in average temperature. However, Kerman station in the east has shown a completely different pattern

Conclusion

It can be inferred that the increase in temperature due to climate change has caused non-stationarity and changes in the intensity and frequency of rainfall in Iran, but the pattern of change is not uniform throughout the country. Generally, there has been a decrease in the intensity and frequency of extreme rainfall due to the increase in temperature.