WVICA

Cloud feedbacks remain one of the largest sources of uncertainty in our understanding of climate sensitivity. Several recent studies point to the importance of low-cloud feedbacks in particular as an important source of uncertainty in evaluating equilibrium climate sensitivity. Recent studies have shown that much of the spread in model climate sensitivity can be attributed to the simulated strength of mixing between the lower and middle troposphere. Specifically, it has been found that mixing dehydrates the low-cloud layer at a rate that increases with warming. Scientists hypothesize that the stable isotopic composition of lower tropospheric water vapor should reflect the balance between mixing dehydration, shallow convection, and latent heat fluxes, and may provide a useful complement to existing convective drying indices that have been used to explain the diversity of low-cloud feedbacks in GCMs and, by extension, much of the variability in climate sensitivity between models. Because water vapor isotopic composition can be measured in the atmosphere and simulated in climate models, this approach may provide a unique link between climate model analysis and field measurements of the processes governing low-cloud feedbacks. In order to improve understanding of the processes governing boundary layer humidity in marine low-cloud settings, participating scientists will deploy the Los Gatos Research triple oxygen water vapor isotopic analyzer at the U.S. Department of Energy’s ARM observatory in the Azores. In marine stratocumulus settings like the Azores, the surface humidity is dependent on processes governing the overall moisture and temperature budgets of the stratocumulus-topped boundary layer. The isotopic data collected at this site was complemented by ARM’s ongoing measurements of vertical cloud structure, precipitation properties and solar radiation. Participants anticipated that the surface isotopic composition at the Azores site will depend on RH, SST, and possibly wind speed, as recognized in other settings, but also on the presence of drizzle, aerosol abundance, and on the degree of coupling between the stratocumulus layer and the surface moisture supply. The output of this study will provide new constraints for the analysis of low-cloud feedbacks in GCMs.

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