Spatiotemporal changes in precipitation concentration over Iran during 1962–2019

Due to global warming, precipitation concentration is expected to change, and extreme weather events are likely to occur more frequently. This study investigates the spatiotemporal variability of precipitation concentration in Iran. For this purpose, daily precipitation data with a spatial resolution of 0.25° × 0.25° from the Asfazari database are used. Three indices, i.e., the precipitation concentration index (PCI), precipitation concentration period (PCP), and precipitation concentration degree (PCD), are used to examine the variability of precipitation concentration in Iran. The results demonstrate that the central, southeastern, and eastern parts of Iran exhibit maximum temporal precipitation concentration, while the lowest precipitation concentration is observed over the Caspian coasts and the northern half of the country. The year 1998 is selected as the change point due to the considerable difference in the values of the examined indices, and the long-term statistical period is divided into two sub-periods before and after the change. During the sub-period after the change point (1999–2019), precipitation concentration increased in the western, central, eastern, and southeastern parts of Iran, according to PCI and PCD; on the other hand, it decreased in the north, northeast, and northern coastline of the Oman Sea. Furthermore, there are great spatial differences during the occurrence of precipitation along the northern coasts, according to PCP, varying from November (along the Caspian coast) to August (along the northern foothills of the Alborz Mountains). PCP increased during the sub-period after the change point along the northern coastlines of the Persian Gulf and Oman Sea and in the north parts of the country (along the Alborz Mountains), indicating a shift in the period of precipitation from the winter to the warm seasons of spring and summer. Moreover, a decrease in PCP in the northwest and northeast suggests that the period of occurrence of precipitation shifted from the second half of the winter toward the early winter and late fall. After the year of the change point, the frequency of rainy days and precipitation decreased, and PCI and PCD increased.