Aerosol impact on precipitation under different relative humidities: A case study

Although cloud properties and precipitation formation are primarily affected by atmospheric dynamics, cloud microphysical features also play key roles. The aerosol number concentration strongly influences cloud microphysics and precipitation formation, mainly through affecting the formation of cloud droplets and ice crystals.
In the current research, using the Thompson aerosol-aware microphysics scheme implemented on the Weather Research and Forecasting (WRF) model, the effects of aerosol number concentration was investigated on the precipitation formation of a heavy rainfall in Tehran. The aerosol number concentrations were obtained from the Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model, while the National Center for Environmental Prediction Final Analysis (NCEP/FNL) dataset was used for the initial and lateral boundary conditions. Two numerical simulations were conducted, referred to as the clean and polluted experiments. The initial hygroscopic aerosol number concentrations, compared to the values obtained from the GOCART model, were reduced to one-fifth and increased by a factor of 5 in the clean and polluted experiments, respectively. The model simulations were run with three nested domains, with horizontal resolutions of 21, 7 and 2.3333 km, and 45 levels in the vertical position, reaching up to the 50 hPa level. Simulations were conducted for 30 hours, starting from 18:00 UTC April 13, 2012, from which, the first 6 hours were considered as the model spin-up. The Rapid Radiative Transfer Model (RRTM; Mlawer et al., 1997) was used for the shortwave and longwave radiation, respectively. The land surface scheme and surface layer scheme were based on the five-layer thermal diffusion and the revised MM5 similarity theory, respectively (Zhang and Anthes, 1982). The non-local Yonsei University (YSU) scheme was employed for the parameterizations of the boundary layer processes (Hong et al., 2006). The Kain-Fritsch scheme (Kain, 2004) was used to parameterize moist convection in the mother and first nested domains, while it was explicitly modelled in the innermost domain.
Results indicated that changes in the aerosol number concentration are associated with changes in the spatial distribution of precipitation. Stronger updraft cores were found in the polluted experiment, entailing higher precipitation, longer growth times, and larger sizes of hydrometeor; accordingly, more raindrops survived from the evaporation after falling from the cloud base, increasing the surface precipitation. On the other hand, surface precipitation decreased in the downstream, primarily due to the decrease in the effective radii of ice crystals, reducing the riming processes and the amounts of graupels. Results further indicated that the increase in the aerosol number concentration is associated with the increase in the rate of precipitation under high relative humidities, while the reverse is true when the available water vapour is relatively low.