Cloud microphysical and precipitation response to the aerosols during a convective event over Southwestern Iran

A convective precipitation event in southwestern Iran is examined in this study using the aerosol-aware bulk microphysical scheme implemented in the Weather Research and Forecasting (WRF) model. Two simulations were conducted for this event, which included the control and polluted simulations. In the control simulation, the concentration of aerosols in the current climate is considered. In contrast, the aerosol concentration was increased by a factor of 5 at all grid points in the polluted simulation. The aim is to study the effects of aerosols on cloud microphysics and precipitation. The simulated vertical temperature and wind speed profiles are compared with the radiosonde data, and the model well simulates temperature and wind speed. During the convective event, southerly to southwesterly warm and dry winds dominated, causing a substantial transport of aerosols and humidity. The reflection of shortwave radiation by clouds in the innermost domain increases in the polluted experiment, indicating that the first indirect effect of aerosols has a significant impact on the radiative balance of the atmosphere. In contrast to the effect of clouds on shortwave radiation, the effect of clouds on longwave radiation is positive at the top of the atmosphere (TOA) because clouds reflect longwave radiation emitted by the earth's surface.The impact of an increase in the concentration of aerosols on cloud development is substantial in this simulation, which contains a high convergence of vertical moisture flux and strong winds over the region. The convergence of the vertical moisture flux indicates that more water vapor is available to be condensed on aerosols, which increases the cloud water content. Thus, the number density of cloud droplets is higher in the polluted compared to the control simulation. The altitude of the maximum mass density of cloud droplets is between 3 and 6 km; due to higher specific humidity at these altitudes, higher water vapor can be condensed on condensation nuclei. Also, the mass density of rain drops is higher in the polluted compared to the control simulation up to the altitude of 3 km, which is due to a higher collision of cloud droplets in the polluted simulation. An increase in ice and snow, which indicates a higher lifting of droplets to the freezing level, is seen in this simulation with the negative convergence of vertical moisture flux. This indicates that these regions may help the large-scale collection of moisture and its lifting. On the other hand, with a divergence of moisture in the northern and the whole domain, the cloud water content decreases. In addition, with a high moisture difference, there is higher precipitation in the polluted compared to control simulations because, in the humid atmosphere, there is enough water vapor to be condensed on aerosols, which leads to the formation of larger cloud droplets. Thus, the collision of cloud droplets is more efficient, and precipitation increases. In addition, due to a lower cloud base, there is less chance for the evaporation and melting of precipitation.