Atmospheric radiative transfer through global arrays of 2D clouds

Abstract

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 grid‐spacing 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.