Sea surface temperature and subsidence effects on steady-state stratocumulus to shallow cumulus cloud transitions

Clouds forming in the atmospheric boundary layer play a crucial role in the Earth's energy balance. Climate projections are sensitive to the amount of low-cloud cover and small variations in stratocumulus area coverage can produce energy-balance changes comparable to those due to greenhouse gases. The objective of this study is to understand the relation between boundary layer cloudiness and large-scale circulation. We follow the canonical problem of Chung et al. (2012) and investigate statistically steady state marine boundary layers with varying subsidence and sea surface temperature. The boundary layer is modeled using large-eddy simulation. Each simulation differs with respect to the SST and subsidence, which span a typical subtropical-value parameter space. To reduce the problem parameters, the free tropospheric profiles, i.e., the lapse rate/stability above the boundary layer, depend on the large-scale subsidence. The balance of the various cloud-controlling parameters is studied using the column-integrated liquid water potential temperature equation. Established relations between atmospheric stability and boundary layer cloudiness are discussed in the context of the present statistically stationary boundary layers.