Atmospheric Science Seminar: “Using Satellite and Ground-Based Observations to Test Whether Low-Clouds will Become More Reflective With Global Warming Over the Middle and High Latitudes”

Presented by: Christopher R. Terai, University of California

Abstract:

Most climate models predict that the optical depth of low clouds in the middle and high latitude regions increases with global warming, which represents a negative cloud feedback and acts to dampen the warming due to increased greenhouse gas concentrations. This study builds on results that suggest the cloud optical depth response to temperature is timescale invariant for low-level clouds. The timescale invariance allows one to then use satellite observations to constrain the models' optical depth feedbacks.

Three satellite retrievals are compared against simulations from eight climate models that contributed to the most recent assessment report of the United Nations Intergovernmental Panel on Climate Change. Our study confirms that the low-cloud reflectivity response is timescale invariant, which means that the response of clouds due to long-timescale global warming is roughly equal to the cloud response to monthly and year-to-year timescale temperature fluctuations. Compared to satellite estimates, most climate models overestimate the increase in reflectivity with warming at the monthly and year-to-year timescales, while underestimating the response due to changes in the strength of the boundary layer inversion. A simple replacement of the models' optical depth sensitivities with the satellites' sensitivities reduces the negative shortwave cloud feedback by at least 50% in the 40°–70°S latitude band and by at least 65% in the 40°–70°N latitude band.

To better understand the mechanisms that are behind the the observed cloud optical depth response to temperature, we also analyze retrievals of cloud properties from three middle and high latitude observation sites managed by the U.S. Department of Energy’s Atmospheric Radiation Measurement Program. We test whether there is evidence in observations that support three mechanisms that have been proposed to drive a cloud optical depth changes with global warming and determine what their relative roles are in modulating the observed cloud optical depth. 

Category

Atmospheric Science Seminars

Time

Location

Dunn 101