Aerosol-cloud-precipitation interaction during some convective events over southwestern Iran using the WRF model

Abstract

Four convective precipitation events in southwestern Iran are 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 each of the events, which included the control and polluted simulations. In the control simulation, the concentration of aerosols in the current climate is considered, while the concentration of aerosols was increased by a factor of 5 at all grid points in the polluted simulation. The aim of this research is to evaluate the effects of aerosols on cloud microphysics and precipitation. During the convective events, southerly to southwesterly warm and dry winds dominated, causing a substantial transport of aerosols and humidity. Relatively high values of planetary boundary-layer height (PBLH) are simulated in the polluted simulations, which indicate a higher convective activity and more efficient transport of aerosols and moisture in the vertical. Also, the convective available potential energy (CAPE) increases in polluted simulations, implying that an increase in the concentration of aerosols is associated with more favorable thermodynamic conditions for convection. In all simulations, the reflection of shortwave radiation by clouds increases, indicating that the first indirect effect of aerosols plays a key role in the energy budget of the Earth. On the other hand, an increase in the concentration of aerosols only slightly influences longwave radiation. Altitudes of the lifting condensation level (LCL) and the level of free convection (LFC) increase in all polluted compared to control simulations. This indicates that the air parcel needs to rise to a higher altitude to get saturated. The impact of an increase in the concentration of aerosols on cloud development is substantial in a simulation that contains a higher convergence of vertical moisture flux and stronger winds over the region. The convergence of the vertical moisture flux in the three simulations indicates that more water vapor is available in these simulations 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 control simulations. The altitude of the maximum mass density of cloud droplets is nearly the same in the simulation (between 3 and 6 km). Due to higher specific humidity in these altitudes, higher water vapor can be condensed on condensation nuclei. An increase in ice and snow, which indicates a higher lifting of droplets to the freezing level, is seen in the simulations with the negative convergence of vertical moisture flux. This indicates that these regions may help the large-scale collection of moisture and its lifting. In simulations with a divergence of moisture in the northern and the whole domain, the cloud water content decreases. In simulations with a higher 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.