Aerosol Angular Distribution Models over Cloud Free Oceans

In aerosol studies using satellite data, the difference in top of atmosphere fluxes from broadband instruments such as the Clouds and the Earth's Radiant Energy System (CERES) scanner without and with the presence of aerosols is a measure of the direct radiative forcing of aerosols. However, the CERES instrument does not measure the shortwave (SW) and longwave (LW) fluxes directly, and therefore, angular distribution models (ADMs) are needed to convert the observed radiances to fluxes. Using multiple satellite instruments, we demonstrate a new empirical method for obtaining the shortwave aerosol ADM over cloud-free oceans. We use 10 months (Nov. 2000- Aug. 2001) of multi-spectral MODIS data to obtain aerosol properties within a CERES footprint and we also use SSM/I data to obtain near surface wind speed. The new aerosol ADMs are built as functions of ocean wind speed, and MODIS aerosol optical depth. Among the new features include ADM’s as a function of the ratio of fine to coarse mode aerosol optical depth that can be used to separate anthropogenic from wind-driven aerosols such as dust and sea salt and the use of multiple instruments to obtain aerosol properties over glint regions.