Sc. Sherwood et al., RESPONSE OF AN ATMOSPHERIC GENERAL-CIRCULATION MODEL TO RADIATIVE FORCING OF TROPICAL CLOUDS, JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 99(D10), 1994, pp. 20829-20845
The effects of upper tropospheric cloud radiative forcing (CRF) on the
atmosphere have been examined using a recent version of the atmospher
ic general circulation model (AGCM) developed by the Max Planck Instit
ute of Meteorology and the University of Hamburg. This model reproduce
s satellite-observed radiative forcing of clouds well overall, except
that model maxima somewhat exceed those of observations. Three simulat
ions have been performed where the clouds above 600 mbar have been ren
dered transparent to all radiation: first, throughout the tropics in t
he ''NC'' experiment; then only over oceans warmer than 25 degrees C i
n the ''NCW'' experiment; and finally, only over the western Pacific w
arm oceans in the ''NCWP'' experiment. The local radiative effects of
these clouds when they are present in the model are radiative heating
of the middle to upper troposphere due to convergence of longwave and
solar radiation; radiative cooling of the tropical atmosphere near and
above the tropopause; a large reduction of solar radiation (50 to 100
W/m(2)) reaching the surface; and a slight increase (5 to 20 W/m(2))
in the downward longwave radiation at the surface. The removal of clou
d radiative forcing significantly alters the circulation of the model
atmosphere, as in previous AGCM studies, showing that a seemingly mode
rate heat source such as CRF is nonetheless capable of widespread infl
uence over the global circulation and precipitation. The experiment re
sponses include a significant weakening (in NCW) or rearrangement (in
NCWP) of the Walker circulation. Zonal mean cloud cover, rainfall, and
low-level convergence change modestly in the experiments, while zonal
departures of these from their tropical means shift considerably. Reg
ions over the warmest oceans which lose CRF become much less cloudy, i
ndicating a positive local feedback to convection. The experiment circ
ulation changes are diagnosed in terms of simple energy budget argumen
ts, which suggest that the importance of CRF is enabled by the small m
agnitude of the atmospheric moist energy transport in the tropics. The
y also suggest that the response of the zonal mean atmosphere may be s
trongly dependent on the response of zonal eddies and on interactions
between surface fluxes and tropospheric lapse rates. The response of t
he zonal eddies itself should be relatively independent of these inter
actions.