Seminar

Abstract

Shallow 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 key 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 owing to increased lower-tropospheric mixing. Here we show that new observations from the EUREC4A (Elucidating the role of cloud-circulation coupling in climate) 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 markedly 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 climate sensitivity.

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Raphaela Vogel is a research scientist and lecturer at the Meteorological Institute of the Universität Hamburg. She received her MSc in atmospheric and climate science from ETH Zürich and her PhD from the Max Planck Institute for Meteorology in Hamburg. Dr. Vogel's research focuses on better understanding clouds and convective processes. She combines observations, high-resolution numerical simulations and simple conceptual models to study how shallow cumulus clouds respond to changes in their large-scale environment, for example under climate change or across the diurnal cycle. She's particularly interested in constraining the influence of rain and mesoscale convective organization on cloud feedbacks and thus climate projections.