Enhanced CERES LW broadband diurnally averaged fluxes using Geostationary imager radiances

The Clouds and the Earth's Radiant Energy System (CERES) instruments are onboard the Terra and Aqua satellites and are measuring top of atmosphere (TOA) reflected shortwave radiance and TOA outgoing longwave (LW) radiance. CERES provides broadband fluxes based on these radiance measurements. CERES incorporates Geostationary (GEO) satellite fluxes to complement the lack of diurnal sampling due to the fixed Terra (10:30 AM local equator crossing time) and Aqua (1:30PM) overpass times. The GEO enhanced CERES SYN1deg data product provides monthly, monthly-3-hourly, and 3-hourly synoptic TOA fluxes that can be used by modelers for more detailed diagnostic and process studies.

All GEO imagers contain IR window (11µm) and water vapor (6.7µm) channels. These imager radiances need to be converted into LW broadband fluxes. Due to the lack of simultaneous GEO and CERES radiances, Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) IR measurements, with similar wavelengths as the GEO imagers, are convolved spatially to match the CERES footprints. The matched MODIS and CERES radiances are used to derive the LW narrowband to broadband coefficients as a function of satellite view angle, surface type, PW, IR radiance, and surface type. To take into account the GEO and MODIS spectral channel differences the GEO IR radiances are first inter-calibrated against MODIS. The GEO LW fluxes are then normalized with CERES fluxes in order to radiometrically scale the GEO fluxes to CERES. The derived GEO LW fluxes are validated against the CERES and GERB observed fluxes. This LW narrowband to broadband approach is an improvement over earlier CERES GEO enhanced flux products.