The direct observations of large aerosol radiative forcing in the Himalayan region

Abstract

We show here that absorbing aerosols have led to a large reduction of surface solar radiation during winter over the Himalayan region. Our results are based on radiometric, aerosol and Lidar observations made at three sites in Nepal during winter 2003. The monthly mean aerosol optical depth (AOD) ranged from 0.2 to 0.34 and the TERRA satellite MODIS data reveal that AODS measured over these sites were typical of the entire Himalayan region. The near-surface aerosol single scattering albedo was in the range from 0.7 to 0.9. The presence of strongly absorbing aerosols resulted in a relatively large diurnal mean aerosol surface radiative forcing efficiency of -73 Wm-2 (per unit optical depth). The seasonal mean reduction in solar flux was as high as 25 Wm-2 and aerosol heating as much as 1 K per day within the first two kilometers. Copyright 2004 by the American Geophysical Union.