GACP Projects
Top-of-Atmosphere Clear-Sky Broadband Radiative Flux and Direct Aerosol Radiative Forcing from Satellite Measurements
Norman Loeb, PI
Yongxiang Hu, Co-I
Bing Lin, Co-I
Abstract: The accuracy of satellite-derived estimates of top-of-atmosphere (TOA) aerosol radiative forcing depends 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 fluxes, and hence, aerosol radiative forcing.
In the following, a strategy for quantifying uncertainties in SW and LW clear-sky TOA radiative fluxes and aerosol radiative forcing over ocean is proposed. It involves a detailed comparison between SW and LW TOA fluxes inferred from the Clouds and the Earth's Radiant Energy System (CERES) instrument with fluxes obtained from broadband radiative transfer model calculations. The first part of this investigation will involve development of new angular distribution models (ADMs) for converting observed broadband radiances to fluxes over cloud-free oceans. 16 SW ADMs stratified by aerosol optical depth and wind speed, and 27 LW ADMs stratified by surface emissivity, lapse rate and column precipitable water will be developed. Next, fluxes obtained using the new ADMs will be directly compared with fluxes from broadband radiative transfer model calculations. The calculations will be initialized using coincident aerosol and sea-state parameters inferred from imager and microwave measurements over each CERES footprint. The focus of the comparisons will be to quantify the magnitude of monthly mean regional fluxes and aerosol radiative forcing over ocean and the associated uncertainties. Several months of observations will be considered.
