The Shortwave Radiative Forcing Bias of Homogeneous Liquid and Ice Clouds Observed by MODIS

We analyze the plane-parallel bias of the shortwave cloud radiative forcing (SWCRF) of liquid and ice Clouds at I degree scales using global MODIS (Terra and Aqua) cloud optical property retrievals for four months of 2005 representative of the meteorological seasons. The (negative) bias is estimated as the difference of the SWCRF calculated using the Plane-Parallel Homogeneous (PPH) method and the Independent Column Approximation (ICA). These calculations require MODIS-derived means (for PPH Calculations) and distributions (for ICA calculations) of cloud optical thickness and effective radius as well as ancillary surface albedo and atmospheric information. that are inserted into a broadband solar radiative transfer code. The absolute value of global SWCRF bias of liquid Clouds at the top of the atmosphere is similar to 6 Wm(-2) for MODIS overpass times while the SWCRF bias, for ice clouds is smaller in absolute terms by similar to 0.7 Wm(-2), but with stronger spatial variability. Marine clouds of both phases are characterized by larger (more negative) SWCRF biases than continental clouds. For clouds of both phases the SWCRF bias is collectively about 4 Wm(-2) for diurnal averages.