Joint Seminar: Cloud feedbacks and their impacts on atmospheric circulation

Cloud feedbacks remain the dominant source of uncertainty in climate change projections. Clouds affect not only the global-mean temperature response and climate sensitivity, but they also have profound impacts on the atmospheric circulation response to external forcing. This talk explores the spatial structure of cloud feedbacks, and its implications for the atmospheric circulation response. In the first part, I will discuss the mechanisms that determine the meridional structure of the shortwave cloud feedback, focusing on the negative feedback in high latitudes, which is a robust feature of global warming model simulations. I will show that a key driver of this negative cloud feedback is the suppression of ice-phase cloud microphysics upon warming in mixed-phase clouds. Suppression of ice-phase processes reduces the efficiency of the microphysical sinks of cloud liquid water through precipitation and conversion to ice, resulting in an increased cloud liquid water path and brighter clouds in high latitudes. In the second part, I will discuss how the spatial structure of the cloud feedbacks affects the atmospheric circulation response in an idealised aquaplanet model with a slab-ocean lower boundary. The effect of clouds is isolated by running 4xCO2 experiments with and without cloud feedbacks. In our model, cloud-radiative changes cause more than half of the total poleward expansion of the Hadley cells, jet streams, and storm tracks upon CO2 quadrupling. The effect of clouds on circulation is dominated by the shortwave cloud-radiative changes, which act to enhance the Equator-to-pole temperature gradient, causing a strengthening and poleward shift of the eddy-driven jet. Two important implications of these results are that (a) the atmospheric circulation responds sensitively to cloud-driven changes in meridional and vertical temperature distribution, and (b) the spatial structure of cloud feedbacks likely plays a dominant role in the circulation response to greenhouse gas forcing.

Date

27.01.2016

Time

13:30 h

Place

Bundesstr. 53, room 022/023
Seminar Room 022/023, Ground Floor, Bundesstrasse 53, 20146 Hamburg, Hamburg

Speaker

Paulo Ceppi, University of Reading, Department of Meteorology

Organizers

Bjorn Stevens

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