Examining Direct and Indirect Radiative Forcings of Smoke Aerosol during the 2002 Yakutsk Wildfire Season

Extreme wildfire events have occurred in the Yakutsk region of Russia during the summer of 2002. The wildfires released large amounts of smoke aerosol which was transported over the large region. Using a fully coupled mesoscale WRF-Chem-SMOKE model in conjunction with satellite observations, we examine how smoke aerosol perturbed the surface and TOA radiative energy balance in the region. Both the shortwave (SW) and longwave (LW) components of the radiative budget are considered, examining the smoke direct radiative forcing in cloud-free conditions and the indirect smoke (via clouds) radiative forcing. To facilitate assessment of the smoke impact, WRF-Chem-SMOKE runs initialized by FNL reanalysis data were performed with on-line smoke emission and for the smoke-free case. Hourly size- and composition-resolved smoke emission was computed from MODIS fire radiative power (FRP) data. Model results are compared with SW and LW fluxes derived from satellite retrievals, including the NASA/GEWEX Surface Radiation Budget (SRB) data and the Clouds and the Earth's Radiant Energy System (CERES) data. There is general agreement between modeled and satellite-derived radiative parameters. We found a significant decrease in surface downward SW fluxes caused by heavy smoke plumes, from 100 W/m2 to 150 W/m2 in daily mean SW fluxes. We also examine smoke-induced changes in cloud properties and coverage, and assess implications for the radiation budget.