Longwave radiative forcing of Saharan dust aerosols estimated from MODIS, MISR, and CERES observations on Terra

Using observations from the Multi-angle Imaging Spectroradiometer (MISR), the Moderate Resolution Imaging Spectroradiometer (MODIS), and the Clouds and the Earth's Radiant Energy System (CERES) instruments onboard the Terra satellite; we present a new technique for studying longwave (LW) radiative forcing of dust aerosols over the Saharan desert for cloud-free conditions. The monthly-mean LW forcing for September 2000 is 7 Wm(-2) and the LW forcing efficiency (LWeff) is 15 Wm(-2) per unit aerosol optical depth. Using radiative transfer calculations, we show that simultaneous measurements of the vertical distribution of aerosols, surface temperature and water vapor are critical to the understanding of dust LW aerosol forcing, and must come from other sources. Using well calibrated, spatially and temporally collocated data sets, we have combined the strengths of three sensors from the same satellite to quantify the LW forcing, and show that dust aerosols have a "warming'' effect over the Saharan desert that will counteract the shortwave "cooling effect'' of other aerosols.