Numerical investigation of aerosol indirect effects on shortwave and longwave radiation: A case study

Through modifying the number concentration and size of cloud droplets, aerosols have complex impacts on radiative properties of clouds, which consequently change the radiation balance of the Earth, and modify the atmospheric air temperature. By conducting numerical experiments for a mid-latitude cloud system in April, the indirect effects of aerosols on shortwave and longwave radiation, and subsequent impacts on the near-surface air temperature are investigated over Tehran. To this end, three numerical experiments (control, clean and polluted) with initial identical dynamical and thermodynamic conditions, but different cloud-nucleating aerosol concentrations were conducted using the Weather Research and Forecasting (WRF) model. Simulations were conducted over three nested domains with two-way interactions (nesting ratios: 1:3:3; horizontal resolutions: 21, 7 and 2.333 km). A two-moment aerosol-aware bulk microphysical scheme, recently developed, discussed and tested by Thompson and Eidhammer (2014), was used. In the control experiment that represents conditions of the current era in terms of the aerosol number concentrations, concentrations of atmospheric aerosols were derived from 7-yr global simulations of the Goddard Chemistry Aerosol Radiation and Transport (GOCART) model which include mass mixing ratios of sulfate, dust, black carbon (BC), organic carbon (OC), and sea salt. Hygroscopic aerosol number concentrations were reduced to one-fifth in the clean experiment, and increased by a factor of 5 in the polluted experiment. The meteorological initial and lateral boundary conditions in the three experiments were derived from the National Center for Environmental Prediction final analysis (NCEP/FNL) data at 1˚ horizontal resolution and 6 h temporal intervals. Results indicate that increasing (decreasing) cloud-nucleating aerosol concentrations in the polluted (clean) experiment is associated with more (less) numerous cloud droplets of overall smaller (larger) size. Indeed, mean cloud droplet number concentrations (effective radius of cloud droplets) in cloudy grid points averaged over the innermost domain and during the simulation period were found to be approximately 46, 158 and 417 cm-3 (8.5, 6.1 and 5.2 μm) in the clean, control and polluted experiments, respectively. Thus, the total droplet cross-sectional area of cloud droplets increases in the polluted experiment, leading to an enhancement in the shortwave cloud radiative forcing (or cloud albedo), such that less shortwave radiation reaches to the Earth surface. In contrast, the total droplet cross-sectional area of cloud droplets decreases in the clean experiment, leading to a reduction in shortwave cloud radiative forcing (or cloud albedo). In contrast to the significant changes in the shortwave cloud radiative forcing by aerosols, results indicate that changing the number and size of cloud condensation nuclei in the polluted and clean experiments has little impact on longwave cloud radiative forcing. Values of shortwave and longwave cloud radiative forcing indicate that as the positive longwave cloud radiative forcing in all experiments are nearly half of the negative shortwave cloud radiative forcing, clouds have an overall cooling effect on the climate system, counteracting the warming caused by increases in concentrations of the atmospheric greenhouse gases. Comparing the near-surface air temperature of the three experiments reveals that the enhancement of cloud albedo in the polluted experiment leads to a reduction in the near-surface air temperature, while reduction of cloud albedo in the clean experiment leads to the enhancement of the near-surface air temperature.