Abstract
Cloud feedbacks could influence significantly the overall response of the climate system to global warming. Here we study the response of warm convective clouds to a uniform temperature change under constant relative humidity (RH) conditions. We show that an increase in temperature drives competing effects at the cloud scale: a reduction in the thermal buoyancy term and an increase in the humidity buoyancy term. Both effects are driven by the increased contrast in the water vapor content between the cloud and its environment, under warming with constant RH. The increase in the moisture content contrast between the cloud and its environment enhances the evaporation at the cloud margins, increases the entrainment, and acts to cool the cloud. Hence, there is a reduction in the thermal buoyancy term, despite the fact that theoretically this term should increase.
Highlights
Cloud response to a warmer climate may determine the overall climate system state (Schneider et al 2017)
As opposed to the set of simulations with fixed relative humidity (RH) conditions (figure 2(a)), in the case of fixed Δqv, buoyancy term (BT) increases with warming (figure 4(a)) due to the increase in the latent heat release, as was shown in the adiabatic case. These results demonstrate that the decrease in BT with warming under constant RH conditions is due to the increase in moisture content contrast
We examine the response of the buoyancy terms, i.e. the drivers of the convection, to a warmer enviroment
Summary
Cloud response to a warmer climate may determine the overall climate system state (Schneider et al 2017). An increase in moisture contrast between the boundary-layer and the free atmosphere in a warmer climate (Van der Dussen et al 2015) and an increase in surface moisture fluxes (Held and Soden 2000, Xu et al 2010) can drive deepening and drying of the boundary-layer due to enhanced vertical mixing with the free troposphere This feedback will result in a reduction in cloudiness and acts as a positive feedback (Bretherton 2015, Rieck et al 2012, Sherwood et al 2014, Van der Dussen et al 2015, Qu et al 2015b). We examine the cloud buoyancy response to an idealized representation of a warmer climate, meaning a uniform increase in temperature under a constant RH, similar to what has been done in many previous works (Rieck et al 2012, Vogel et al 2016, Van der Dussen et al 2015) We note that this is a simplified representation of warmer atmosphere and it does not capture all environmental changes, such as changes in largescale vertical motion and inversion strength (Qu et al 2015a). Since buoyancy is the driving force for convection and is directly modulated by the environmental conditions, it could serve as a robust measure of the cloud-scale response to warming
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