On the Nature of Caspian Clouds

Caspian clouds (CCs) are formed between the southern coast of the Caspian Sea and the Alborz Mountains. The purpose of this study is to identify characteristics of CCs using aerosol, cloud, and meteorological data from Modern-Era Retrospective analysis for Research and Applications version 2 (MERRA-2), Moderate Resolution Imaging Spectroradiometer (MODIS), and ECMWF Reanalysis version 5 (ERA5) during 2000–2020. During this period, we identified and investigated 636 days with CCs. The results indicated that the frequency (%) of these clouds was higher in the summer than in other seasons because synoptic system activity varies between hot and cold periods. The hot season with the beginning of high-pressure subtropical Azores activity and the formation of a stable atmosphere in northern Iran leads to more frequent occurrence of CCs. These clouds are mainly the low- and middle-level clouds in the region, e.g., stratus and altocumulus. CCs resulted in 13.9% of annual rainfall, and 55.9% and 18.7% of the summer and autumn rainfall, respectively, relative to total rainfall from all cloud types in the study region. In the multivariate regression analysis, CC precipitation exhibited a strong positive relationship with the cloud water path (CWP), cloud optical thickness (COT), and cloud effective radius (CER). A comparison of the mean and standard deviation of aerosol optical thickness (AOT) and aerosol index (AI) for CC and non-CC days did not show a significant difference. Examination of the synoptic patterns showed that the main factors in the formation of CCs are the specific environmental conditions of the region and the orographic lift of stable air masses. The Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model indicated that the source of moisture for the formation of CCs was largely the Caspian Sea.