Identifying the Precipitation Regime of the Iranian Coast of Caspian Sea

Precipitation is one of the most important climatic variables playing a decisive role for different purposes. Temporal changes in precipitation affect many climatic and environmental phenomena (such as runoff, floods, air temperature, and humidity) as well as many human activities (such as agriculture and housing). The precipitation regime includes all characteristics and statistics of precipitation in relation to its distribution throughout the year, and the temporal distribution of precipitation according to the months or seasons of the year is called the "Precipitation Regime".

The daily data of 385 stations were obtained from Iran Meteorological Organization and the Ministry of Energy for the period of 2016-1966 (51 years). The hidden aspects of precipitation and precipitation regime of the Iranian coast of Caspian Sea were studied. At first, these stations were used in order to create maps with a spatial resolution of 3 × 3 km, and the general specifications of the monthly and annual precipitation were presented. Sinusoidal behaviors of monthly precipitation in each pixels were then investigated. Accordingly, first to sixth harmonics were extracted. Finally, the cluster analysis method was used based on the Euclidean distance and the "Ward" method of linkage to identify the spatial patterns of precipitation based on the contribution of different periodic and its zoning. Then, the homogeneity and seasonal index of precipitation was estimated.

The results show that the mean annual precipitation is higher on the coastline, especially in the southwest of the Caspian Sea, and decreases as it passes from the coast. In the southwest parts of the Caspian Sea, maximum precipitation occurs in the autumn. At the Alborz highlands, the maximum and minimum precipitation fall during winter and summer, respectively. The monthly precipitation coefficient of variation indicates that with seasonal changes from winter to spring, precipitation changes in the Caspian region are declining, and with changes in seasonal precipitation from summer to autumn, precipitation changes are in the ascendant. The largest variability coefficient of the month to month of the precipitation (60 to 70 percent) was calculated at the coastline of the Caspian Sea. This shows notable month to month precipitation changes and seasonal instability in these areas. The coefficient of variation is gradually reduced by distance from the coastline. The lowest coefficient of variation was obtained in the southern parts of the Caspian Sea (the Alborz altitudes) between 15% and 30%. This suggests a small difference in rainfall over the course of the months. In other words, they indicate the activity of various rainy systems, or at least the continuity of rainy systems in these areas, and the stability of the precipitation season. The homogeneity index indicates that the precipitation distribution is more concentrated in the coastal areas of the Caspian Sea, and it becomes more uniform with the advance towards the southern parts of the area (part of the Alborz heights). The seasonal precipitation index of the Caspian Sea region indicates three types of precipitation regime. The lowest spatial extent (6.28%) is related to the uniform precipitation regime, found in small parts of the Alborz heights. The most abundant regime has a wetter season. This precipitation regime, which includes 76.13% of the Iranian coast of Caspian Sea, is observed in the eastern and western regions of the Caspian Sea. The third regime (13.25% of the study area), which is mainly seasonal with a short dry season, covers the Caspian Sea coastline, parts of the Talesh heights, and a small part of the eastern region.

The results revealed that the precipitation classes obtained based on the seasonal index were closer to the reality due to the similarity of these classes with the average monthly and annual precipitation. Therefore, this index seems to be the most optimal tool for determining precipitation regimes in the Caspian region. According to this precipitation regime classification, there are three classes of precipitation regime in the Caspian region. The existence of these three classes indicates the presence and activity of different synoptic and local systems in the Caspian region.