Coarse-Grid and Cloud-Resolving Simulations of a Midlatitude Cyclonic Cloud System: Implications for the Parameterization of Layer Clouds in GCMs

Abstract

A winter oceanic cyclonic cloud system was simulated by using the mesoscale compressible community (MC2) model with different combinations of model resolutions and cloud microphysics packages. Results from these simulations are intercompared to examine the effects of the coarse model grid and simplified model physics on the simulated large-scale storm environment. When aggregated to an area approximately equivalent to the size of a grid box in current GCMs, the results from the models differ significantly in the large-scale cloud and moisture profiles. Although the effects of using different stratiform cloud schemes on the coarse-grid results are appreciable, the effects of different model resolution are shown to be greater on the large-scale frontal cloud field. In particular, the coarse-grid models 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 mean cross-front circulation and mesoscale cloud features 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. Because both the frontal zones and the mesoscale cloud bands are unresolved features in current GCMs, these results suggest that the parameterization of their bulk effects on the large scales should be included in the representation of frontal layered clouds in climate models.