Midlatitude frontal clouds: GCM-scale modeling implications

The importance of subgrid-scale processes for the simulation of midlatitude frontal clouds by global models is investigated. The case chosen is a fron tal cloud associated with a cool change crossing the southern Australian co astline between 17 and 19 November 1983. The Commonwealth Scientific and In dustrial Research Organisation limited-area model, Division of Atmospheric Research Limited-Area Model. was run at horizontal resolutions of 30 and 30 0 km, and the results of the 30-km simulation were then averaged to 300-km resolution. Comparisons and evaluations of the simulations showed that the 300-km simulation failed to develop the frontal clouds. Comparison with the 30-km simulation averaged to 300 km showed the importance of the subgrid-s cale vertical motions fur this cloud development. In particular, it is foun d that the covariance of the subgrid-scale terms, although of smaller magni tude when compared with the larger-scale terms, needs to be parameterized t o capture correctly the frontal cloud development. It is suggested that par ameterization of the subgrid-scale dynamical forcing is important Fur the c orrect cloud development in general circulation models.