Effect of Cloud Overlap and Inhomogeneity on Climate Simulations

Atmospheric radiation budgets are strongly affected by the horizontal and vertical distributions of cloud systems. With the horizontal resolution of several hundred kilometers in general circulation models (GCMs), parameterization of cloud vertical overlap and horizontal inhomogeneity in radiation schemes has long been a major challenge for climate simulations (Stephens 1984; Stephens et al. 2004). Numerous studies have focused on developing methods to approximate the effects of subgrid interactions into the GCM radiation schemes (Geleyn and Hollingsworth 1979; Stephens 1984; Liang and Wang 1997; Barker et al. 1999; Morcrette and Jakob 2000; Li 2000; Fu et al. 2000; Collins 2001; Li and Barker 2002; Stephens et al. 2004). However, the evaluation and implementation of these methods are limited due to the lack of consistent, fine-resolution observations of cloud-radiation interactions. The radiation parameterization is further complicated by the uncertainties associated with the parameterization of convection and clouds in GCMs. The development of cloud-resolving models (CRMs) and Atmospheric Radiation Measurement (ARM) observations provides an opportunity to make progress on this problem. Our research objective is to generate long-term comprehensive and physically consistent data that will facilitate quantifying the effects of subgrid cloud-radiation interactions and, ultimately, develop a physically based treatment of cloud vertical overlap and horizontal inhomogeneity for the radiation scheme in GCMs. This will contribute to the fundamental goal of the ARM Program, i.e., “to improve the treatment of radiation and clouds in the models used to predict future climate, particularly the general circulation models.”