Parameterization of generalized cloud overlap for radiative calculations in general circulation models

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.