Simulating the Role of Caspian Sea on the Occurrence of Regional Scale Precipitation Over Southern Coast of Caspian Sea

The main purpose of this research is to study the role of Caspian Sea on the rainfall occurrence in the southern coast of Caspian Sea. The ICTP Regional Climate Model (RegCM4) coupled with a Lake model was used to investigate the role of Caspian Sea at regional scale. The NCEP/NCAR reanalysis dataset were used as the initial and lateral boundary conditions and the eliminated Caspian Sea condition was compared with the control and was made for 2003-2005. The result indicate that changing the rainfall amount in southern coast of Caspian Sea is generally related to the intensity of northerly wind blowing over the Caspian Sea. The simulation indicates that by eliminating Caspian Sea, most of the rainfall changes would occur in the southern coast of Caspian Sea. The role of Caspian Sea is different according to season and the most regional effect of Caspian Sea is observed in autumn and winter and lower effects is shown in spring. However, by eliminating the Caspian Sea, the most rainfall reduction occurs in southwestern to southern parts of the Caspian Sea. The findings also indicate that Caspian Sea will play a more effective and widespread role when the wind blows more strongly compared with the time when it blows with a less intensity. Eliminating the Caspian Sea, ascending air significantly decreases by increasing the surface drag and decreasing the wind velocity in the southern coastlines. This mechanism would accompany with the evaporation amount reduction, decreasing moisture transport from sea surface and decrease of rainfall in the farthermost south regions of the sea. However, RegCM4 model faced with a challenge in enhancing the amount and spatial distribution of spring precipitation over the area, even though; the results are acceptable and reliable for all other seasons.