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Factors Controlling ERBE Longwave Clear Sky and Cloud Forcing Fluxes

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  • 1 Atmospheric Science Program, University of California at Davis, Davis, California
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Abstract

The factors controlling the Earth Radiation Budget satellite (ERBS) longwave clear sky and cloud-forcing fluxes are investigated using statistical analyses of the ERBS fluxes with International Satellite Cloud Climatology Project (ISCCP) cloud and ECMWF thermodynamic variables. For both land and ocean between 60°S and 60°N statistically significant models exist relating interannual variations of clear sky fluxes and surface temperature, precipitable water, tropospheric temperature, total cloud amount, and cloud-top pressure. An analysis of mean clear sky fluxes suggests that mean ERBS longwave clear sky fluxes are overestimates of the “true” values by between +2 and +10 W m−2 over the area investigated. The biases appear to be most related to errors in the detection or exclusion of low clouds in the Earth Radiation Budget Experiment clear sky determination algorithm. Other statistical models show that variations in total cloud amount and cloud-top pressure control cloud forcing over oceans, but that total cloud amount and clear sky flux are most important over land. Thus over land, especially at higher latitudes, care must be taken not to interpret changes in cloud forcing solely in terms of variations in cloud parameters.

Abstract

The factors controlling the Earth Radiation Budget satellite (ERBS) longwave clear sky and cloud-forcing fluxes are investigated using statistical analyses of the ERBS fluxes with International Satellite Cloud Climatology Project (ISCCP) cloud and ECMWF thermodynamic variables. For both land and ocean between 60°S and 60°N statistically significant models exist relating interannual variations of clear sky fluxes and surface temperature, precipitable water, tropospheric temperature, total cloud amount, and cloud-top pressure. An analysis of mean clear sky fluxes suggests that mean ERBS longwave clear sky fluxes are overestimates of the “true” values by between +2 and +10 W m−2 over the area investigated. The biases appear to be most related to errors in the detection or exclusion of low clouds in the Earth Radiation Budget Experiment clear sky determination algorithm. Other statistical models show that variations in total cloud amount and cloud-top pressure control cloud forcing over oceans, but that total cloud amount and clear sky flux are most important over land. Thus over land, especially at higher latitudes, care must be taken not to interpret changes in cloud forcing solely in terms of variations in cloud parameters.

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