1 GACP 3rd YEAR PROGRESS REPORT Proposal Title: Top-of-Atmosphere Clear-Sky Broadband Radiative Flux and Direct Aerosol Radiative Forcing from Satellite Measurements

The accuracy of satellite-derived estimates of top-of-atmosphere (TOA) aerosol radiative forcing (natural + anthropogenic)also known as the direct radiative effect of aerosolsdepends critically on the accuracy of shortwave (SW) and longwave (LW) TOA radiative fluxes. Since satellites cannot directly measure flux instantaneously, assumptions are needed to account for the angular and spectral dependence of the radiation field to convert a radiance measurement to a flux estimate. If these assumptions are incorrect, they will lead to errors in TOA radiative flux, and hence, aerosol radiative forcing. In this investigation, measurements from the Clouds and the Earth's Radiant Energy System (CERES) instrument are used to determine the direct radiative effect of aerosols over ocean. A unique advantage of CERES is that the measurements are broadband, thereby eliminating the need for assumptions on the spectral dependence of the radiance field. However, models are needed to convert the broadband radiances to TOA radiative fluxes. The first part of this investigation involves the development of empirical angular distribution models (ADMs) for converting observed broadband radiances to fluxes over cloud-free oceans. Fluxes based on the new ADMs are then combined with imager retrievals of aerosol optical depth to determine the spatial and temporal dependence of the aerosol direct effect over the tropical oceans. To determine aerosol radiative forcing from CERES, an approach similar to that outlined in Haywood et al. (1999) is used. The aerosol direct radiative effect is obtained from the difference between a no-aerosol flux inferred from the measurements and the diurnally averaged regional flux from CERES