Mechanisms of the negative shortwave cloud feedback in middle to high latitudes

• Published in: Journal of Climate
• Main Authors: Ceppi, P, Hartmann, DL, Webb, MJ
• Format: Article in Journal/Newspaper
• Language: English
• Published: American Meteorological Society 2015
• Subjects: Science & Technology; Physical Sciences; Meteorology & Atmospheric Sciences; Physical Meteorology and Climatology; Climate change; Climate sensitivity; Cloud microphysics; Cloud water; phase; Clouds; Feedback; EDDY-DRIVEN JET; OPTICAL-THICKNESS; CLIMATE FEEDBACKS; STRATIFORM CLOUDS; CARBON-DIOXIDE; PART I; MODEL; RADIATION; SCHEME; LARGE-SCALE MODELS; SOUTHERN-OCEAN; CLIMATE MODELS; TEMPERATURE; 0401 Atmospheric Sciences; 0405 Oceanography; 0909 Geomatic Engineering; Southern Ocean
• Online Access: http://hdl.handle.net/10044/1/75995; https://doi.org/10.1175/JCLI-D-15-0327.1

Increases in cloud optical depth and liquid water path (LWP) are robust features of global warming model simulations in high latitudes, yielding a negative shortwave cloud feedback, but the mechanisms are still uncertain. Here the importance of microphysical processes for the negative optical depth feedback is assessed by perturbing temperature in the microphysics schemes of two aquaplanet models, both of which have separate prognostic equations for liquid water and ice. It is found that most of the LWP increase with warming is caused by a suppression of ice microphysical processes in mixed-phase clouds, resulting in reduced conversion efficiencies of liquid water to ice and precipitation. Perturbing the temperature-dependent phase partitioning of convective condensate also yields a small LWP increase. Together, the perturbations in large-scale microphysics and convective condensate partitioning explain more than two-thirds of the LWP response relative to a reference case with increased SSTs, and capture all of the vertical structure of the liquid water response. In support of these findings, a very robust positive relationship between monthly mean LWP and temperature in CMIP5 models and observations is shown to exist in mixed-phase cloud regions only. In models, the historical LWP sensitivity to temperature is a good predictor of the forced global warming response poleward of about 45°, although models appear to overestimate the LWP response to warming compared to observations. The results indicate that in climate models, the suppression of ice-phase microphysical processes that deplete cloud liquid water is a key driver of the LWP increase with warming and of the associated negative shortwave cloud feedback.