A case study of the impact of aerosols on properties of clouds and precipitation under different relative humidity

Aerosols can act as cloud condensation nuclei in inhomogeneous nucleation. Thus, changes in the concentration of aerosols can affect cloud microphysics and precipitation by changing the number and concentration of cloud droplets. Using the two-moment Thompson cloud microphysics for two nested domains in the weather and research forecasting (WRF) model, the impacts of aerosols on cloud microphysics and precipitation are investigated for a convective system in northern Iran on 17-23 April 2019, in which the first 24 hours are considered as spin-up of the model. Aerosols are obtained from the Goddard chemistry aerosol radiation and transport (GOCART) model and used in the WRF model. Mass mixing ratios are of sulfate, dust, black carbon, organic carbon, and sea salt, although black carbon is ignored in the cloud condensation nuclei activation. Dust aerosols larger than 0.5 μm in diameter are accumulated into the ice-nucleating mode, while all other aerosol types mentioned above are combined into the cloud-droplet-nucleating mode. Three numerical experiments have been conducted. In the control experiment, the extracted aerosols from the GOCART model are used in the WRF model. In the polluted experiment, the number of hygroscopic aerosols in all model grid points that contain aerosols is increased by a factor of five. The third experiment is similar to the second experiment in terms of the number of hygroscopic aerosols. However, relative humidity is increased by 10 percent in all grid points with relative humidity between 0 and 90 percent. It is changed to 100 percent in all grid points with relative humidity higher than 90 percent. This experiment is referred to as the polluted-humid experiment. The maximum number of ice crystals is found in the polluted-humid experiment. The increase of relative humidity in the polluted-humid experiment also leads to the formation of more cloud droplets, and thus more release of latent heat of condensation, which results in higher cloud tops and the formation of more ice crystals. The lifted condensation level (LCL) is also shifted to lower heights in the polluted-humid experiment. In the polluted experiment, the accumulated precipitation in the innermost domain is decreased, which can be due to a decrease in the size of cloud droplets, associated with which is less collision of cloud droplets, which results in a delay in warm precipitation. Nevertheless, higher relative humidity in the polluted-humid experiment and associated larger cloud droplets are accompanied by higher accumulated precipitation.