Atmospheric Chemistry and Physics Discussions
www.atmos-chem-phys-discuss.net
1680-7367
1680-7375
9
2
2009
10.5194/acpd-9-10337-2009
http://www.atmos-chem-phys-discuss.net/9/10337/2009/
http://www.atmos-chem-phys-discuss.net/9/10337/2009/acpd-9-10337-2009.html
http://www.atmos-chem-phys-discuss.net/9/10337/2009/acpd-9-10337-2009.pdf
10337
10366
2009-04-28
The shortwave radiative forcing bias of liquid and ice clouds from MODIS observations
L. Oreopoulos
lazaros.oreopoulos@nasa.gov
S. E. Platnick
G. Hong
P. Yang
R. F. Cahalan
Laboratory for Atmospheres, NASA-GSFC, Greenbelt, MD, USA
Dept. of Atmospheric Sciences, Texas A&M University, College Station, TX, USA
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 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
require MODIS-derived gridpoint means while ICA calculations require
distributions of cloud optical thickness and effective radius as well
as ancillary surface albedo and atmospheric information consistent
with the MODIS retrievals. With the aid of 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
6 W m<sup>−2</sup> for MODIS overpass times while the SWCRF bias for
ice clouds is smaller in absolute terms by about
0.7 W m<sup>−2</sup>, 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 0.3–0.4 W m<sup>−2</sup> 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 than liquid
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 SWCRF bias,
which for clouds of both phases is collectively about
4 W m<sup>−2</sup> for diurnal averages, should be a strong
motivation for global climate modelers to accelerate efforts linking
cloud schemes capable of subgrid condensate variability with
appropriate radiative transfer schemes.
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