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Nonturbulent Liquid‐Bearing Polar Clouds: Observed Frequency of Occurrence...

Silber, I., A. M. Fridlind, J. Verlinde, L. M. Russell, and A. S. Ackerman (2020), Nonturbulent Liquid‐Bearing Polar Clouds: Observed Frequency of Occurrence and Simulated Sensitivity to Gravity Waves, Geophys. Res. Lett..
Abstract: 

A common feature of polar liquid‐bearing clouds (LBCs) is radiatively driven turbulence, which may variously alter cloud lifecycle via vertical mixing, droplet activation, and subsequent feedbacks. However, polar LBCs are commonly initiated under stable, nonturbulent conditions. Using long‐ term data from the North Slope of Alaska and McMurdo, Antarctica, we show that nonturbulent conditions prevail in ~25% of detected LBCs, surmised to be preferentially early in their lifecycle. We conclude that nonturbulent 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. We posit that the frequent occurrence of nonturbulent LBCs over the polar regions has implications for polar aerosol‐cloud interactions and their parameterization in large‐scale models. Plain Language Summary The presence of turbulent mixing in liquid‐containing polar clouds is commonly presupposed, but here, we show that a quarter of all liquid‐containing clouds over the North Slope of Alaska and McMurdo Station, Antarctica, are nonturbulent. Many of these nonturbulent clouds are likely in the first stages of their lifecycle after forming in stable, nonturbulent atmospheric layers. Unlike their often more mature counterparts, these nonturbulent clouds are frequently not very efficient at cooling themselves by radiating thermal energy, which among other factors, prolongs the time required to develop turbulence. Using model simulations, we show that oscillating vertical motions of air that are common under stable atmospheric conditions may enhance the formation and growth of cloud droplets such that the cloud radiative cooling efficiency becomes higher, which ultimately hastens turbulence formation. We conclude that it is likely necessary to properly represent a number of atmospheric parameters that control the concentration of cloud droplets, as well as the regional properties of the oscillating vertical air motions, to faithfully represent the polar atmosphere in regional and global models.

Research Program: 
Modeling Analysis and Prediction Program (MAP)