Fine-mesh and coarse-grid model simulations of a synoptic cloud system: Implications for cloud parameterizations in GCMs

Szeto, K.K., and H.Guan

J. Climate
(Submitted)

ABSTRACT

A winter oceanic cyclonic cloud system was simulated by using the MC2 model with either a combination of fine spatial resolutions and a sophisticated cloud microphysics package or a combination of coarse resolutions and a simplified cloud microphysics package typically used in current GCMs. Results from these simulations are inter-compared to examine the effects of the coarse model grid and the simplified model physics on the simulated large-scale storm environment. The observed fine-scale storm structures in the warm-frontal region were replicated quite well in the high resolution simulation. When aggregated to the an area approximately equivalent to the size of a grid box in current GCMs, the results from the two models differ significantly in the large-scale cloud and moisture profiles. In particular, the coarse-grid model underestimated the cloudiness and atmospheric moisture content in the warm-frontal region. Such differences in the large-scale model storm environment were consequences of the stronger cross-front circulation in the high resolution simulation. The stronger cross-front circulation was in turn a result of stronger frontogenetic processes over the region and dynamic influences of the mesoscale cloud bands on the parent storm. Since both the frontogenesis and the mesoscale frontal cloud bands are unresolved features in current GCMs, these results suggest that the parameterization of their bulk effects on the large scales shold be included in the representation of “stratiform” clouds in GCMs.