Atmospheric radiative transfer through global arrays of 2D clouds

Shortwave and longwave 2D radiative transfer calculations were performed using Monte Carlo radiative transfer models and output from a global climate model (GCM) that employed, in each of its columns, a 2D cloud system-resolving model (CSRM) with a horizontal gridspacing Δx of 4 km. CSRM output were sampled every 9 hours for December 2000. Radiative fluxes were averaged to the GCM's grid. Monthly-mean top of atmosphere (TOA) shortwave flux differences between 2D radiative transfer and the Independent Column Approximation (ICA) are at most 5 W m^-2 in the tropics with a zonal-average of 1.5 W m^-2. These differences are 2 to 10 times smaller than those stemming from the maximum-random overlap model and neglect of horizontal variability of cloud. Corresponding longwave differences are approximately 3 times smaller than their shortwave counterparts. Use of CSRM data with Δx < 4 km may roughly double the reported differences between 2D and ICA TOA SW fluxes.