Wh. Lee et Rcj. Somerville, EFFECTS OF ALTERNATIVE CLOUD RADIATION PARAMETERIZATIONS IN A GENERAL-CIRCULATION MODEL, Annales geophysicae, 14(1), 1996, pp. 107-114
Using the National Center for Atmospheric Research (MCAR) general circ
ulation model (CCM2), a suite of alternative cloud radiation parameter
izations has been tested. Our methodology relies on perpetual July int
egrations driven by +/-2 K sea surface temperature forcing. The tested
parameterizations include relative humidity based clouds and versions
of schemes involving a prognostic cloud water budget. We are especial
ly interested in testing the effect of cloud optical thickness feedbac
ks on global climate sensitivity. All schemes exhibit negative cloud r
adiation feedbacks, i.e., cloud moderates the global warming. However,
these negative net cloud radiation feedbacks consist of quite differe
nt shortwave and longwave components between a scheme with interactive
cloud radiative properties and several schemes with specified cloud w
ater paths. An increase in cloud water content in the warmer climate l
eads to optically thicker middle- and low-level clouds and in turn neg
ative shortwave feedbacks for the interactive radiative scheme, while
a decrease in cloud amount leads to a positive shortwave feedback for
the other schemes. For the longwave feedbacks, a decrease in high effe
ctive cloudiness for the schemes without interactive radiative propert
ies leads to a negative feedback, while no distinct changes in effecti
ve high cloudiness and the resulting feedback are exhibited for the sc
heme with interactive radiative properties. The resulting magnitude of
negative net cloud radiation feed-back is largest for the scheme with
interactive radiative properties. Even though the simulated values of
cloud radiative forcing for the present climate using this method dif
fer most from the observational data, the approach shows great promise
for the future.