Nonturbulent Liquid‐Bearing Polar Clouds: Observed Frequency of Occurrence and Simulated Sensitivity to Gravity Waves

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 longterm 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.

Contrasting ice formation in Arctic clouds: surface-coupled vs. surface-decoupled clouds

“…Aeronet photometer measurements at Arctic sites show values for the Ångstrom exponent (440-870 nm) around 1.5 (not shown), indicating a fine-mode-dominated aerosol distribution. The Ångstrom exponent for typically coarse-mode-dominated marine aerosol centers around 0.5 (Smirnov et al, 2009;Yin et al, 2019). Both the small values of extinction coefficient and the high Ångstrom exponents are clear indications that the observed aerosol is not of typical marine origin.…”

Habit‐Dependent Vapor Growth Modulates Arctic Supercooled Water Occurrence

“…Liquid‐bearing cloud layers are identified where the measured relative humidity (RH), linearly interpolated onto a 15 m vertical grid spacing, exceeds 95%, which considers the instrument measurement uncertainty. This method for liquid‐bearing cloud detection was validated using the same data set against high spectral resolution lidar phase retrievals (Silber, Fridlind et al., 2020, Figure ).…”

Processes contributing to Arctic cloud dissipation and formation events that bookend clear sky periods

“…The formation of low clouds or fog bookending an Arctic clear sky period may bear similarity to the relatively nonturbulent, optically thin clouds reported in Sedlar (2014) and Silber et al (2020). During the cold, darker seasons, the low cloud or fog formations may in fact be ice fogs, a common occurrence in the Arctic (e.g., Gultepe et al, 2014;.…”

“…Simulations also suggest that very low cloud condensation nuclei (CCN) or cloud droplet number concentrations (~10 cm -3 or less) may result in cloud dissipation (Birch et al, 2012;Loewe et al, 2017;Stevens et al, 2018). Based on observations from the North Slope of Alaska (NSA) and complementary simulations, Silber et al (2020) hypothesize that low aerosol concentrations may also slow or inhibit the formation of turbulent liquid-bearing clouds from their optically thin, nonturbulent predecessors.…”