Long-Term Trends in Extreme Precipitation Events in Northwest Iran: A Comprehensive Analysis Using ERA5 Reanalysis Data

This paper aims to evaluate the precipitation extremes over Northwest of Iran based on 5th generation reanalysis precipitation data (ERA5) for long-term historical period from 1941 to 2020. The trend of 10 selected extremes precipitation indices (EPI) were evaluated in monthly, seasonal, annual and decadal time scales. The nonparametric Mann–Kendall test were applied to 960 grid (600,000 km2) with a high spatial resolution of 0.25° × 0.25° to detect the possible temporal trends in all EPI on the significance levels of 1% and 5%, include frequency and intensity indices. Notably, anomalies in precipitation for the recent reference period (1991-2020) compared to a different subperiods (e.g. 1961-1990 and 1941-2020) seeking to distinguish shifts in precipitation patterns. 

Our findings have uncovered a significant increase in the frequency of daily heavy precipitation events over the studied period, particularly in the latter decades. Trends showing mostly positive patterns, many of which are statistically significant. Moreover, in some areas over the past 30 years, the total rainfall has increased by approximately 25%. Although most events occur during the winter (DJF) and spring (MAM) seasons, the highest number of events belongs to March. The largest positive trend in terms of intensity and frequency is associated with the fall season (SON) and the month of October and November in this region. Also the highest number of grid points with significant positive trends is related to fall in seasonal timescale and November in monthly timescale. Although the results of the annual study indicate that 56.5% of the respected territory have a positive trend which statistically significant in at least one of the indices, this percentage increases to 80.3% on a seasonal scale for (SON). In some cases, negative trends were observed for few number of grids, none of which were statistically significant in annual scale. Also, periodic behavior observed, characterized by oscillations spanning on interdecadal scales.

Overall, the combination of the results of the EPI trends, illustrate a considerable changes towards more intense and more frequent precipitation on interannual scales. Consequently, this region can be considered as a high risk area in terms of extreme events and flash floods.