Low-Cloud Feedbacks from Cloud-Controlling Factors: A Review

Abstract

The response to warming of tropical low-level clouds including both marine stratocumulus and trade cumulus is a major source of uncertainty in projections of future climate. Climate model simulations of the response vary widely, reflecting the difficulty the models have in simulating these clouds. These inadequacies have led to alternative approaches to predict low-cloud feedbacks. Here, we review an observational approach that relies on the assumption that observed relationships between low clouds and the ''cloud-controlling factors'' of the large-scale environment are invariant across time-scales. With this assumption, and given predictions of how the cloud-controlling factors change with climate warming, one can predict low-cloud feedbacks without using any model simulation of low clouds. We discuss both fundamental and implementation issues with this approach and suggest steps that could reduce uncertainty in the predicted low-cloud feedback. Recent studies using this approach predict that the tropical low-cloud feedback is positive mainly due to the observation that reflection of solar radiation by low clouds decreases as temperature increases, holding all other cloud-controlling factors fixed. The positive feedback from temperature is partially offset by a negative feedback from the tendency for the inversion strength to increase in a warming world, with other cloudcontrolling factors playing a smaller role. A consensus estimate from these studies for the contribution of tropical low clouds to the global mean cloud feedback is

Observations of Local Positive Low Cloud Feedback Patterns and Their Role in Internal Variability and Climate Sensitivity

“…Our observational analysis when considered together with these modeling results implies stronger latent warming and possibly higher climate sensitivity than current estimates for many models because the SST‐LCC feedback of current models is too weak. SST‐LCC feedback being too weak in many models suggests that a higher equilibrium climate sensitivity might be more realistic (see Klein et al, 2017, and references therein). Indeed, we find that models capturing better the SST‐LCC coupling, as measured by the spatial correlation, have higher values of equilibrium climate sensitivity (Figure S7), which may be a result of both stronger overall positive cloud feedback and stronger latent warming.…”

Influence of the Subtropical High and Storm Track on Low-Cloud Fraction and Its Seasonality over the South Indian Ocean

“…Previous studies have indicated that climatologicalmean distribution of LCF and its seasonality are well explained by lower-tropospheric stability, whose enhancement acts to increase LCF (e.g., Klein and Hartmann 1993;Wood and Bretherton 2006;Koshiro and Shiotani 2014). Climatological-mean distributions of EIS defined in (1) are thus shown in Figs.…”

“…For example, stratocumulus clouds prevail over the eastern portion of each of the subtropical ocean basins (e.g., Klein and Hartmann 1993;Wood 2012), which is located east of a surface subtropical high that accompanies persistent midtropospheric subsidence and equatorward surface winds (e.g., Nakamura 2005, 2010). The equatorward winds induce coastal upwelling and upper-ocean mixing in addition to surface evaporation, acting to maintain relatively low sea surface temperature (SST).…”

“…Combination of the low SST and enhanced midtropospheric subsidence, which acts to warm the free troposphere, maintains a strong temperature inversion at the top of the boundary layer, inhibiting cloud-top entrainment of dry air. In fact, the temperature inversion can explain seasonality of low-cloud fraction (LCF) in the eastern portion of an ocean basin (Klein and Hartmann 1993;Wood and Bretherton 2006). Those equatorward surface winds yield cold advection, thus destabilizing the surface layer, and thereby facilitating shallow convection in the boundary layer, to further increase LCF.…”

“…The occurrence of fog and stratus is, by contrast, more likely under warm advection that renders surface air temperature higher than SST underneath (Norris and Klein 2000;Norris and Iacobellis 2005;Tokinaga et al 2009;Tanimoto et al 2009;Koshiro et al 2017). It has been shown that seasonal-mean LCF can be explained well by lowertropospheric stability (Klein and Hartmann 1993;Wood and Bretherton 2006;Koshiro and Shiotani 2014).…”