The role of clear-sky identification in the study of cloud radiative effects: Combined analysis from ISCCP and the scanner of radiation budget

Two additional narrowband channels of the Scanner of Radiation Budget (ScaRaB) instrument should improve the Earth Radiation Budget Experiment (ERBE) cloud scene identification. Applying the original International Satellite Cloud Climatology Project (ISCCP) algorithms to the ScaRaB narrowband data gives a clear-sky frequency that is about 5% lower than that given by quasi-simultaneous ISCCP data, an indication that the ISCCP cloud detection is very stable. However, one would expect about 10%-20% smaller clear-sky occurrence for the larger ScaRaB pixels. Adapting the ISCCP algorithms to the ScaRaB spatial resolution and to the different time sampling of the ScaRaB data leads to a reduction of residual cloud contamination. A sensitivity study with time-space-collocated ScaRaB and ISCCP data shows that the clear-sky identification method has a greater effect on the clear-sky frequency and therefore on the statistics than on the zonal mean values of the clear-sky fluxes. The zonal outgoing longwave (LW) fluxes corresponding to ERBE clear sky are in general about 2-10 W m^-2 higher than those from the ScaRaB-adapted ISCCP clear-sky identifications. The latter are close to fluxes corresponding to clear-sky regions from ISCCP data, whereas ScaRaB clear-sky LW fluxes obtained with the original ISCCP identification lie about 1-2 W m^-2 below. Especially in the Tropics, where water vapor abundance is high, the ERBE clear-sky LW fluxes seem to be systematically overestimated by about 4 W m^-2, and shortwave (SW) fluxes are lower by about 5-10 W m^-2. However, another source of uncertainty in the monthly mean zonal cloud radiative effects comes from the low frequency of clear-sky occurrence, when averaging over regions that correspond to the spatial resolution of general circulation models. An additional systematic sampling bias in the clear-sky fluxes appears because the clear-sky regions selected by the different algorithms occur in different geographic regions with different cloud properties.