24-10-2012, 05:28 PM
Top-of-Atmosphere Direct Radiative Effect of Aerosols over Global Oceans from Merged CERES and MODIS Observations
ABSTRACT
The direct radiative effect of aerosols (DREA) is defined as the difference between radiative fluxes in the absence and
presence of aerosols. In this study, the direct radiative effect of aerosols is estimated for 46 months (March, 2000 to December,
2003) of merged CERES and MODIS Terra global measurements over ocean. This analysis includes the contribution from
clear regions in both clear and partly cloudy CERES footprints. MODIS-CERES narrow-to-broadband regressions are
developed to convert clear-sky MODIS narrowband radiances to broadband SW radiances, and CERES clear-sky Angular
Distribution Models (ADMs) are used to estimate the corresponding TOA radiative fluxes needed to determine the DREA.
Clear-sky MODIS pixels are identified using two independent cloud masks: (i) the NOAA-NESDIS algorithm used for
inferring aerosol properties from MODIS on the CERES Single Scanner Footprint TOA/Surface Fluxes and Clouds (SSF)
product (NOAA-SSF); and (ii) the standard algorithm used by the MODIS aerosol group to produce the MODO4 product
(MODO4). Over global oceans, direct radiative cooling by aerosols for clear scenes identified from MODO4 is estimated to
be 5.5 W m-2, compared to 3.8 W m-2 for clear scenes from NOAA-SSF. Regionally, differences are largest in areas affected
by dust aerosol, such as oceanic regions adjacent to the Saharan and Saudi Arabian deserts, and in northern Pacific Ocean
regions influenced by dust transported from Asia. The net total-sky (clear and cloudy) DREA is negative (cooling) and is
estimated to be -2.0 W m-2 from MOD04, and -1.6 W m-2 from NOAA-SSF. The DREA is shown to have pronounced
seasonal cycles in the Northern Hemisphere and large year-to-year fluctuations near deserts. However, no systematic trend in
deseasonalized anomalies of the DREA is observed over the 46-month time series considered.