We present an assessment of the plane-parallel
bias of the shortwave cloud radiative forcing (SWCRF) of
liquid and ice clouds at 1 deg scales using global MODIS
(Terra and Aqua) cloud optical property retrievals for four
months of the year 2005 representative of the meteorological
seasons. The (negative) bias is estimated as the difference
of SWCRF calculated using the Plane-Parallel Homogeneous
(PPH) approximation and the Independent Column
Approximation (ICA). PPH calculations use MODISderived
gridpoint means while ICA calculations use distributions
of cloud optical thickness and effective radius. Assisted
by a broadband solar radiative transfer algorithm, we
find that the absolute value of global SWCRF bias of liquid
clouds at the top of the atmosphere is about 6Wm−2 for
MODIS overpass times while the SWCRF bias for ice clouds
is smaller in absolute terms by about 0.7Wm−2, but with
stronger spatial variability. If effective radius variability is
neglected and only optical thickness horizontal variations are
accounted for, the absolute SWCRF biases increase by about
0.3–0.4Wm−2 on average. Marine clouds of both phases exhibit
greater (more negative) SWCRF biases than continental
clouds. Finally, morning (Terra)–afternoon (Aqua) differences
in SWCRF bias are much more pronounced for ice
clouds, up to about 15% (Aqua producing stronger negative
bias) on global scales, with virtually all contribution to the
difference coming from land areas. The substantial magnitude
of the global SWCRF bias, which for clouds of both
phases is collectively about 4Wm−2 for diurnal averages,
should be considered a strong motivation for global climate
modelers to accelerate efforts linking cloud schemes capable
of subgrid condensate variability with appropriate radiative
transfer schemes.