Effect of aerosols on different regime of clouds and their precipitation over Tehran city

In this study we investigate the impact of aerosols on different cloud regimes and amount of their precipitation over Tehran for the period of 2003-2014, utilizing data from Moderate-Resolution Imaging Spectroradiometer (MODIS), Tropical Rainfall Measuring Mission (TRMM) and surface data from local synoptic stations. The regimes were determined using a k-means clustering method on retrieved cloud properties. The results indicate that in mixed clouds, increase in aerosols has led to increase in the mean cloud effective radius (CER), cloud height as well as lighting and precipitation amounts of weak and moderate convective cells which may be attributed to more freezing of cloud droplets above the 0 °C isotherm and its associated latent heat releases. Vice versa, warm clouds intensity in polluted air condition are weeker than clean air condition. The results also showed that, nimbostratus clouds thickness and their precipitation increase under high aerosol index (AI). Both TRMM and rain gauges data, however, showed that the average amount of precipitation has decreased during polluted episodes in comparison with clean episodes during the period of study. Elaborating on the observed overall changes in the amount of precipitation, it may be concluded that over Seeding was a critical factor in reducing the mean precipitation over Tehran. Also, observed differences on clouds microphysics over different parts of the city may be related to local pollutions, type of ambient air aerosols and topography conditions.