A simple model of subtropical stratocumulus feedbacks on climate sensitivity

Despite widespread recognition of the importance of subtropical stratocumulus clouds to the planetary energy budget, the strength and even the sign of stratocumulus response to increased levels of greenhouse gases is still unknown. This is due both to uncertainties in the relevant boundary layer (BL) processes that maintain the cloud and uncertainties in how the large-scale temperature, humidity and subsidence profiles and SST will change in these regions. Subtropical stratocumulus form over cold SSTs in the subsiding branch of the Hadley circulation. The above-BL temperature profile in these regions is strongly controlled by tropical deep convection in the Intertropical Convergence zone (ITCZ), so accurate prediction of changes in these clouds is predicated on accurate portrayal of changes to the ITCZ and to the Hadley circulation. In this study, we apply a weak temperature-gradient assumption and an above BL radiation-advection energy balance (with radiative fluxes computed by a 2-stream model) to derive above-BL thermodynamic and subsidence profiles for the Southeast Pacific stratocumulus region based on sea surface temperature (SST) in the ITCZ. By coupling these forcings to a cloud model it is possible to create a framework for predicting stratocumulus properties based on the SST in the ITCZ and in the stratocumulus region. One strength of this framework is its flexibility; this design would work equally well with a mixed layer model, a single column model, or a large eddy simulation.

Extensive comparisons between modeling assumptions and observations, reanalysis, and global climate model data are presented. Agreement between the various models and between models and observations is frequently marginal, highlighting the need for improved modeling of this region. The parameterizations of the current study are shown to be consistent with the available observations and, in general, with the model output.

Simulations with a mixed layer model (MLM) suggest that if heat flux into the ocean remains constant, the MLM surface energy balance can only be maintained if albedo and thus liquid water path remain constant. As a result, a constant ocean-atmosphere heat flux implies a low stratocumulus climate sensitivity. In this situation, the model predicts stratocumulus region SST to warm roughly half as much as in the ITCZ. Additionally, MLM results indicate that the choice of entrainment and drizzle parameterization have a large impact on the resulting mean state, but little influence on how the model responds to increasing SST. Because the validity of the mixed layer model depends on the mean state (the mixed-layer assumption fails for deep boundary layers), this suggests that while parameterization choice determines the range of model applicability, the feedbacks found in our results are parameterization-independent.