Single-column and mixed-layer model analysis of subtropical stratocumulus response mechanisms relevant to climate change
MetadataShow full item record
Subtropical stratocumulus clouds are important part of the Earth's energy budget. The response of low clouds to Earth's changing climate is one of the dominant uncertainties in global warming projections, due primarily to unresolved parameterized cloud processes in global climate models (GCMs). Improving our understanding of the role stratocumulus clouds play in response to climate change requires both a better understanding of the underlying physical mechanisms that govern stratocumulus clouds, and also better representing these processes in GCMs. This work addresses both aspects, using a range of models, from an idealized mixed-layer model (MLM) to a high-resolution large eddy simulation (LES). The aerosol indirect effect (AIE) for nondrizzling stratocumulus clouds is strongly dependent on an entrainment-sedimentation feedback that increases the entrainment efficiency for higher droplet concentrations, thereby decreasing the cloud amount. However, a single column model (SCM) derived from the CAM5 GCM exhibits the opposite sign of this response mechanism. Using an SCM we find that a combination of issues contribute to this, but the primary cause is due to a representation of cloud condensate within the PBL parameterization that is inconsistentwith other microphysical parameterizations that affect the cloud liquid water, rendering the entrainment efficiency insensitive to droplet concentration. The response of a representative stratocumulus to a variety of idealized climate-change perturbations is used in conjunction with an identically-forced LES to interpret the underlying mechanisms behind the observed sensitivity. The MLM and LES agree remarkably well for all cases where the boundary layer doesn't decouple in the LES. For doubling CO<sub>2</sub> forcing perturbations estimated from the CMIP3 multimodel mean, the MLM predicts a positive shortwave cloud feedback, like most CMIP3 global climate models. The cloud remains overcast but thins in the warmer, moister, CO<sub>2</sub>-enhanced climate, due to the combined effects of an increased lower-tropospheric vertical humidity gradient and an enhanced free tropospheric greenhouse effect that reduces the radiative driving of turbulence. Reduced subsidence due to weakening of tropical overturning circulations and a strengthening of the capping inversion partly counteract these two factors by raising the inversion and allowing the cloud layer to deepen. These compensating mechanisms may explain the large scatter in low cloud feedbacks predicted by climate models. The rapidity with which a stratocumulus cloud can respond to perturbations is important for understanding its response to perturbations that occur across a range of characteristic time scales. Using a MLM and LES, I show that there are three separate timescales: a slow adjustment timescale associated with boundary layer deepening (several days), an intermediate thermodynamic timescale (approximately 1 day), and a hitherto unidentified fast timescale (6-12 hours) for cloud water path adjustment associated with internal entrainment rate feedbacks. The fast timescale response is elicited by perturbations to overlying humidity and surface and atmosphere temperature, whereas purely radiative perturbations do not elicit an entrainment-liquid water path feedback. A range of MLM entrainment closures are shown to support a fast timescale, provided the entrainment rate is sensitive to the integrated buoyancy flux. The underlying entrainment-liquid flux adjustment mechanism suggests a cloud-thinning response to a uniformly warmed climate perturbation.
- Applied mathematics