Thursday, 13 February 2003
Clear sky angular dependence models over oceans from CERES
Measurements from the Earth Radiation Budget Experiment (ERBE) and the recent Clouds and the Earth's Radiant Energy System (CERES) have been successfully used in monitoring the outgoing energy from the earth-atmosphere system for more than three decades. These measurements include the broad band shortwave (SW) and longwave (LW) radiances at the top of the atmosphere, and angular distribution models (ADMs) are used to invert measured radiances to fluxes. Current ADMs, however, do not account for aerosols and surface effects such as wind speed. In this study, aerosol angular distribution models are constructed for biomass burning and dust regions for September 2000 over cloud free oceans, using the cross track and rotational azimuth plane scan mode CERES data. The Moderate-Resolution Imaging Spectroradiometer (MODIS) is collocated with CERES data, and used to discriminate aerosol scenes from totally clear and cloudy scenes within the CERES pixels. As indicated in this study, lack of dust aerosol ADMs could induce an increase in the derived SW aerosol direct forcing (SWARF) on the order of 20%. This study shows that at certain observation geometries, if ocean surface wind speeds are not considered when building ADMs, it could lead to maximum errors in shortwave fluxes on the order of 30-60%. This study also shows that ocean wind effect could be reduced to less than 5% if observations are averaged over full angular spectrum.