The Impact of Temperature on the Adiabaticity and Coverage of a Single
Shallow Cumulus Cloud
Abstract
The uncertainty of climate projection is significantly contributed by
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 covaries
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
simulation shows 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 response can be explained by 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 works shows that 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 positive cloud feedback mechanism in
addition to the adjustment processes of the boundary layer.