Global retrievals of the surface and atmosphere radiation budget and direct aerosol forcing

A new CERES Terra Surface and Atmosphere Radiation Budget (SARB) record for Terra provides radiative fluxes (broadband SW, broadband LW, and window LW ) for the surface, 500 hPa, 200 hPa, 70 hPa, and top of atmosphere (TOA) for ~ 2,000,000 footprints per day. Fluxes are computed using NWP temperature and humidity profiles, MODIS cloud (Minnis et al.) and aerosol (Kaufman et al.) retrievals, and aerosol optical properties from the MATCH (Model for Atmospheric Transport and Chemistry) assimilation. An accurate spectral albedo for the ocean surface is obtained from a Coupled Ocean Atmosphere Radiative Transfer (COART) code. Aerosol forcing at surface and TOA is assessed by a second set of calculations with no aerosols. Fluxes computed for each footprint are compared with those observed by the CERES instrument at TOA; and tuned with the difference. Surface measurements are not used for tuning. A companion presentation (Rutan et al.) describes the rigorous application of surface measurements to validate results systematically at ~40 sites. The CAVE web site (www-cave.larc.nasa.gov/cave/) now gives access to the surface measurements and to “point and click” versions of both the Langley Fu-Liou code used for broadband fluxes and COART. Formal release of the global CERES SARB record as “Terra CRS Edition 2A” is expected during April 2004.

For 822 clear sky footprints collocated with the CAVE surface sites (6 months in 2001), the mean measured insolation was 725 Wm-2; the SARB mean bias for insolation was fortuitously less than 1 Wm-2, giving confidence in the retrieved clear-sky aerosol forcing to insolation of -25 Wm-2. At CAVE the corresponding clear-sky aerosol forcing to downward LW flux at the surface was 2 Wm-2; to clear-sky OLR forcing it was -1 Wm-2; and to reflected SW at TOA it was 4 Wm-2. The single CAVE site (Saudi Solar Village) with very high dust loading yields biases for surface SW insolation that approach those of the estimated aerosol forcing to insolation, showing a need to revise the single scattering albedo for dust; but the retrieved and observed values for thermal IR fluxes at this one site suggest that the large aerosol forcing to downwelling LW (27 Wm-2) is a reasonable estimate.

Global maps of aerosol forcing are diagnosed for cloud and surface albedo effects. For example on 15 July 2001 during daytime, the all-sky forcing to reflected SW at TOA was 2 Wm-2 for the globe and 3 Wm-2 for the ocean only; the corresponding theoretically clear (i.e., as if there were no clouds) aerosol forcing was 5 Wm-2 for the globe and 6 Wm-2 for the ocean only. The global aerosol forcing to SW at TOA for those footprints (8%) that were actually cloud free on that day was –5 Wm-2. On the regional scale, we find that low clouds beneath an aerosol layer containing black carbon and organics off the coast of Angola boost the absorption of SW within the atmosphere by ~100 Wm-2.