Stratocumulus Cloud-Feedback to an Idealized Global Warming Scenario
Tuesday, 16 December 2014
The response of stratocumulus clouds to global warming as predicted by earth system models is uncertain. Recently, simulations of low clouds in the Hadley cell with single-column model versions of climate models were performed as part of the CGILS project. The results show a divergent behavior in the sign of the low cloud radiative feedback under an idealized global warming scenario. To understand these different model outcomes steady-state solutions of the stratocumulus-topped boundary layer are studied with a mixed-layer model (MLM) in a phase space consisting of the lower tropospheric stability and the free tropospheric specific humidity. In one set of experiments the sea surface temperature (SST) is increased. In case the entrainment rate is kept constant to the control case value it can be shown analytically from the MLM equations that the cloud liquid water path, the sensible and latent heat fluxes will increase. By contrast, if the entrainment rate is allowed to respond to the perturbed SST, both the LWP and the sensible heat flux may decrease. The latent heat flux will increase even further by a drying of the boundary layer due to an enhanced entrainment. Similar experiments were performed with five single column model versions of Earth System Models and the Dutch Atmospheric Large-Eddy Simulation model DALES. The findings strongly suggest that in a future climate changes in the free tropospheric specific humidity are key in controlling the response of stratocumulus cloud layers.