Abstract: Significant systematic clouds, radiation and
moist biases are found in the CMIP3/CMIP5 models ensemble average in
sub- and tropical Pacific regions in conjunctions with biases of upward
motion, eastward low-level wind with moist convergence north of SPCZ
and south of ITCZ – referred to as the V-Shaped regions.
We
characterize the impacts of precipitating snow and radiation effects,
which are not included in most of the CMIP3/CMIP5 models, by using the
National Center for Atmospheric Research (NCAR)-coupled GCM that the
model includes diagnostic snow for radiation calculation. A
number of differences associated with snow-radiation effect off are
found consistent with the biases in CMIP3/CMIP5 simulations. With
snow-radiation off, there excessive longwave radiative cooling at the
top of atmosphere while shortwave radiative warming at the surface
resulting in net radiative cooling near the cloud top, triggering
compensating upward motion in the heavily precipitating regions of the
tropics such as ITCZ, SPCZ and warm pool. This leads to local
subsidence at the north edge of SPCZ and south edge of ITCZ, generating
low-level eastward/southeastward wind (i.e., weakening mean
northwestward trade wind) and moist convergence from the warm pool,
SPCZ and ITCZ and resulting increase total precipitable water and
column of moisture. This further leads to the model’s biases of
local and remote areas of sea surface temperatures (SSTs), sea surface
height, and surface wind stress and large-scale circulations.