The Impact of Temperature on the Adiabaticity and Coverage of a Single Shallow Cumulus Cloud

Abstract

The uncertainty of climate projection is significantly related to warm cloud feedback, which involves a complex interplay of various mechanisms. However, it is hard to unentangle temperature's impact on a single cloud with experiments, since the cloud dynamics always covary with environmental thermodynamical conditions. In this study, we investigate a simulated single shallow cumulus cloud's response to temperature using two perturbation methods, namely “uniform” and “buoyancy-fixed”, the latter of which keeps the buoyancy profile unchanged in temperature perturbation. High-resolution large eddy simulations show that uniform warming significantly increases cloud buoyancy, reducing cloud adiabaticity. If buoyancy is fixed, warming only reduces cloud area, leaving adiabatic fraction almost unchanged. Such a response can be explained by the Clausius-Clapeyron effect with an idealized 1D diffusion model, showing that warming increases the cloud-environment absolute humidity difference more than the increase in cloud liquid water content, resulting in a faster loss in both cloud coverage and total liquid water solely by lateral mixing. The responses of cloud coverage and total liquid water counteract, making adiabatic fraction insensitive to temperature change. Our work shows that the cloud adiabatic fraction's response to temperature is sensitive to the perturbed structure of the boundary layer, and the cloud coverage reduction by diffusion acts as a positive cloud feedback mechanism in addition to the adjustment processes of the boundary layer.