Liquid-bearing polar clouds: A non-turbulent class

 

Submitter:

Silber, Israel — Pacific Northwest National Laboratory
Fridlind, Ann M. — NASA - Goddard Institute for Space Studies

Area of research:

Cloud Processes

Journal Reference:

Silber I, A Fridlind, J Verlinde, L Russell, and A Ackerman. 2020. "Non‐Turbulent Liquid‐Bearing Polar Clouds: Observed Frequency of Occurrence and Simulated Sensitivity to Gravity Waves." Geophysical Research Letters, 47(10), e2020GL087099, 10.1029/2020GL087099.

Science

The presence of turbulent mixing in liquid-containing polar clouds is commonly presupposed, but here we find that a quarter of all liquid-containing clouds over the North Slope of Alaska and McMurdo Station, Antarctica, are actually non-turbulent. These non-turbulent clouds are preferentially thin, and most of them are likely in the first stages of their life cycle after forming in stable polar atmospheric layers. During the development of such clouds, we also find that vertical wind motions associated with atmospheric gravity waves may increase cloud droplet number concentrations when atmospheric aerosol particles (on which cloud droplets form) are sufficiently small, as observed over McMurdo Station, Antarctica.

Impact

Since properly representing liquid-bearing polar clouds in climate models is expected to be important to predicting arctic sea ice evolution and polar temperatures, we posit that climate models may need to faithfully represent aerosol particles and gravity waves, and add the impact of gravity waves on cloud droplet formation. In addition, the general tendency of initially non-turbulent liquid-bearing clouds in a warmer environment to more rapidly develop turbulence and thereby substantially increase their opacity suggests a possible aerosol-mediated cloud-climate feedback mechanism (e.g., an added warming to unlit sea ice surfaces) that has not been previously described in the literature to our knowledge.

Summary

A common feature of polar liquid-bearing clouds (LBCs) is radiatively driven turbulence, which may variously alter cloud life cycle via vertical mixing, droplet activation, and subsequent feedbacks. However, polar LBCs are commonly initiated under stable, non-turbulent conditions. Using long-term data from the North Slope of Alaska and McMurdo, Antarctica, we show that non-turbulent conditions prevail in ~25% of detected LBCs, surmised to be preferentially early in their life cycle. We conclude that non-turbulent LBCs are likely common over the polar regions owing primarily to atmospheric temperature and stability. Such stable environments are known to support gravity wave activity. Using large-eddy simulations we find that short-to-intermediate-period gravity waves may catalyze turbulence formation when aerosol particles available for activation are sufficiently small.