Mechanisms of the negative shortwave cloud feedback in mid to high
نویسندگان
چکیده
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. We assess the importance of microphysical processes for the negative optical depth feedback by perturbing temperature in the microphysics schemes of two aquaplanet models, both of which have separate prognostic equations for liquid water and ice. We find 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. We verify our findings by investigating the relationship between LWP and temperature in CMIP5 models and observations, and find very strong positive correlations between monthly-mean LWP and temperature in mixed-phase cloud regions in all models as well as in observations. We conclude that the LWP increase with warming, and the associated optical depth increase, result primarily from the temperature-dependence of ice-phase microphysical processes that deplete cloud liquid water, rather than increasing adiabatic water content as suggested in some previous studies. 10
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