Wd. Collins, Parameterization of generalized cloud overlap for radiative calculations in general circulation models, J ATMOS SCI, 58(21), 2001, pp. 3224-3242
New radiative parameterizations have been developed for the National Center
for Atmospheric Research (NCAR) Community Atmospheric Model (CAM). The CAM
is the next version of the NCAR Community Climate Model (CCM). This paper
describes the generalized treatment of vertical cloud overlap in the radiat
ive calculations. The new parameterizations compute the shortwave and longw
ave fluxes and heating rates for random overlap, maximum overlap, or an arb
itrary combination of maximum and random overlap. The specification of the
type of overlap is identical for the two bands, and it is completely separa
ted from the radiative parameterizations. In the prototype of CAM (CAM 0.1)
, adjacent cloud layers are maximally overlapped and groups of clouds separ
ated by cloud-free layers are randomly overlapped. The introduction of the
generalized overlap assumptions permits more realistic treatments of cloud-
radiative interactions. The parameterizations are based upon representation
s of the radiative transfer equations that are more accurate than previous
approximations. These techniques increase the computational cost of the rad
iative calculations by approximately 30%. The methodology has been designed
and validated against calculations based upon the independent pixel approx
imation (IPA). The solution techniques and validation procedure are describ
ed in detail. The hourly radiative fluxes and heating rates from the parame
terizations and IPA have been compared for a 1-yr integration of CAM. The m
ean and rms errors in the hourly longwave top of the atmosphere (TOA) fluxe
s are -0.006 +/-0.066 W m(-2), and the corresponding errors in the shortwav
e TOA fluxes are -0.20 +/-1.58 W m(-2). Heating rate errors are O(10(-3)) K
day(-1). In switching from random to maximum/random overlap, the largest c
hanges in TOA shortwave fluxes occur over tropical continental areas, and t
he largest changes in TOA longwave fluxes occur in tropical convective regi
ons. The effects on global climate are determined largely by the instantane
ous changes in the fluxes rather than feedbacks related to cloud overlap.