Effect of Cloud Types on the Earth Radiation Budget Calculated with the ISCCP Cl Dataset: Methodology and Initial Results

Abstract

A method is introduced to derive cloud effects on the earth radiation budget. The ISCCP Cl cloud data for daylight cases are used in combination with a radiative transfer model to estimate the outgoing broadband radiative fluxes at the top of the atmosphere. Two tests are performed: the modeled narrowband filtered radiances are verified against the ISCCP satellite observation data (internal tests) and the modeled broadband fluxes are compared against ERBE data (external tests). After successful completion of the tests, the reflected solar (OSR) and the outgoing longwave (OLR) radiation for each of the 35 ISCCP cloud classes are determined. Results are shown for a reduced set of nine cloud classes (three bins for cloud top height, three bins for the optical thickness). The global monthly and annual means of albedo and greenhouse effect are given for one month of each season (April, July, October 1985; January 1986). For the global annual mean cloud radiative cooling of the earth in the shortwave spectral range dominates over cloud radiative warming in the longwave spectral range by a factor of 2. Seasonal variations of the net effect are mainly due to changes in the shortwave cloud-forcing component [?43.9 to ?52.9 W m?2 (shortwave), 22.5 to 25.1 W m?2 (longwave)]. The mean sensitivity of the net radiation budget to changes in cloud amount attains its minimum of ?29.4 W m?2 per 100% change in cloud cover in April and its maximum of ?47.7 W m?2 per 100% change in cloud cover in January. This is also mainly due to the solar component. A comparison with results of other studies are shown. The effects of the nine cloud types are analyzed in detail. Cooling, warming, and radiatively neutral cloud types could be distinguished. Low stratus and midlevel nimbostratus clouds contribute about 80% to the total cooling of all clouds. Their effect on the OLR, however, is small (less than 20%) but they contribute almost 50% to the OSR. The deep convective clouds produce the largest single effect for both longwave (8.8 W m?2) and shortwave (?1 3 W m?2) forcing but their net effect is small. Only cirrus clouds have net warming effects.