Strong cloud-circulation coupling explains weak trade cumulus feedback
AbstractShallow cumulus clouds in the trade-wind regions cool the planet by
reflecting solar radiation. The response of trade cumulus clouds to
climate change is a major uncertainty in climate projections. Trade
cumulus feedbacks in climate models are governed by changes in cloud
fraction near cloud base, with high climate-sensitivity models
suggesting a strong decrease in cloud-base cloudiness due to increased
lower-tropospheric mixing. Here we show that novel observations from the
EUREC4A field campaign refute this mixing-desiccation hypothesis. We
find the dynamical increase of cloudiness through mixing to overwhelm
the thermodynamic control through humidity. Because mesoscale motions
and the entrainment rate contribute equally to variability in mixing,
but have opposing effects on humidity, mixing does not desiccate clouds.
The magnitude, variability, and coupling of mixing and cloudiness differ
drastically among climate models and with the EUREC4A observations.
Models with large trade cumulus feedbacks tend to exaggerate the
dependence of cloudiness on relative humidity as opposed to mixing, and
also exaggerate variability in cloudiness. Our observational analyses
render models with large positive feedbacks implausible, and both
support and explain at the process scale a weak trade cumulus feedback.
Our findings thus refute an important line of evidence for a high