A simple diagnostic calculation of marine stratocumulus cloud cover for use in general circulation models

An improved diagnostic calculation for determining the marine stratocumulus (SCu) cloud cover has been implemented for atmospheric and coupled general circulation models (GCMs). The approach aims to improve the simulated climatological features of the lower troposphere and to reduce warm sea surface temperature (SST) biases that develop along western continental coasts in the subtropical regions of coupled GCMs. In the new diagnostic calculation introduced in the present study, the SCu cloud cover was linearly regressed to atmospheric stability, and the temporal and spatial distributions of the regression coefficients were estimated beforehand using observational data sets. The upward transport of heat, moisture and momentum in subcloud layers accompanying the SCu was also simulated by controlling the vertical diffusion coefficients. Using the new calculation, the SCu cloud cover in an atmospheric GCM become more representative in both spatial and seasonal variations. In addition, the simulation of related atmospheric structures in the lower troposphere is considerably improved. Furthermore, when the new calculation is applied to a coupled GCM, SST values to the west of continents in subtropical areas display weakened warm biases and more realistic seasonal cycles resulting from the modification of the downward shortwave radiation flux at the sea surface. The new calculation introduces both local and large-scale improvements in model climatology over low-latitude and midlatitude regions. In the eastern Pacific, for example, a shallow meridional circulation across the equator is excited, and the strength and location of the intertropical convergence zone and the seasonal cycle of equatorial SST become more realistic.