Toward Improving the Derivation of SW Broadband Fluxes from Geostationary Narrowband Radiances for CERES

The Clouds and the Earth’s Radiant Energy System (CERES) project provides top of atmosphere (TOA) broadband fluxes for the modeling community for climate studies and model validation. CERES fluxes observed by Aqua and Terra platforms need to be diurnally averaged to be meaningful. CERES provides daily averaged fluxes that assume constant meteorology between CERES measurements in the SSF1deg product, as well as using geostationary (GEO) 3-hourly derived broadband fluxes to resolve the diurnal cycle between CERES measurements in the SYN1deg product. The GEO derived fluxes are based on narrowband GEO visible channel to broadband coefficients and are a function of viewing and solar geometry, surface type, cloud optical depth, and cloud phase. The coefficients are derived from radiative transfer computations based on DISORT and from empirical models based on coincident visible imager and CERES radiances. After the radiance conversion, the GEO derived radiances are converted to broadband fluxes using the same Angular Directional Models (ADM) that CERES uses. To ensure that CERES instrument calibration is preserved the GEO shortwave derived fluxes are normalized to the CERES observed fluxes.

Incorporating GEO visible channel radiances has its challenges. The GEO satellites do not have any onboard calibration and must be monitored against MODIS radiances. Additionally, each GEO satellite visible channel has its own spectral response function. Due to the lack of coincident, collocated, and co-angled GEO and CERES radiance pairs radiative transfer computations must be employed. The narrowband to broadband approach can be validated with coincident CERES and MODIS radiances. The shortwave narrowband to broadband process is being reexamined and improved for CERES Edition 4 products and will be presented at the meeting.