Wh. Lee et al., CLOUD RADIATION FORCINGS AND FEEDBACKS - GENERAL-CIRCULATION MODEL TESTS AND OBSERVATIONAL VALIDATION, Journal of climate, 10(10), 1997, pp. 2479-2496
Using an atmospheric general circulation model (the National Center fo
r Atmospheric Research Community Climate Model: CCM2), the effects on
climate sensitivity of several different cloud radiation parameterizat
ions have been investigated. In addition to the original cloud radiati
on scheme of CCM2, four parameterizations incorporating prognostic clo
ud water were tested: one version with prescribed cloud radiative prop
erties and three other versions with interactive cloud radiative prope
rties. The authors' numerical experiments employ perpetual July integr
ations driven by globally constant sea surface temperature forcings of
two degrees, both positive and negative. A diagnostic radiation calcu
lation has been applied to investigate the partial contributions of hi
gh, middle, and low cloud to the total cloud radiative forcing, as wel
l as the contributions of water vapor, temperature, and cloud to the n
et climate feedback. The high cloud net radiative forcing is positive,
and the middle and low cloud net radiative forcings are negative. The
total net cloud forcing is negative in all of the model versions. The
effect of interactive cloud radiative properties on global climate se
nsitivity is significant. The net cloud radiative feedbacks consist of
quite different shortwave and longwave components between the schemes
with interactive cloud radiative properties and the schemes with spec
ified properties. The increase in cloud water content in the warmer cl
imate leads to optically thicker middle-and low-level clouds and in tu
rn to negative shortwave feedbacks for the interactive radiative schem
es, while the decrease in cloud amount simply produces a positive shor
twave feedback for the schemes with a specified cloud water path. For
the longwave feedbacks, the decrease in high effective cloudiness for
the schemes without interactive radiative properties leads to a negati
ve feedback, while for the other cases, the longwave feedback is posit
ive. These cloud radiation parameterizations are empirically validated
by using a single-column diagnostic model, together with measurements
from the Atmospheric Radiation Measurement program and from the Tropi
cal Ocean Global Atmosphere Combined Ocean-Atmosphere Response Experim
ent. The inclusion of prognostic cloud water produces a notable improv
ement in the realism of the parameterizations, as judged by these obse
rvations. Furthermore, the observational evidence suggests that derivi
ng cloud radiative properties from cloud water content and microphysic
al characteristics is a promising route to further improvement.