Abstract
AbstractThe mostly ice covered Arctic Ocean is dominated by low‐level liquid‐ or mixed‐phase clouds. Turbulence within stratocumulus is primarily driven by cloud top cooling that induces convective instability. Using a suite of in situ and remote sensing instruments we characterize turbulent mixing in Arctic stratocumulus, and for the first time we estimate profiles of the gradient Richardson number at relatively high resolution in both time (10 min) and altitude (10 m). It is found that the mixing occurs both within the cloud, as expected, and by wind shear instability near the surface. About 75% of the time these two layers are separated by a stably stratified inversion at 100–200 m altitude. Exceptions are associated with low cloud bases that allow the cloud‐driven turbulence to reach the surface. The results imply that turbulent coupling between the surface and the cloud is sporadic or intermittent.
Highlights
The central Arctic Ocean, characterized by a surface of semipermanent sea ice, presents unique atmospheric boundary layer (ABL) conditions
During summer the boundary layer usually has near-neutral stability (Persson et al, 2002; Tjernström et al, 2012) and is commonly capped by stratiform clouds, with a mean cloud fraction as high as 90% (Curry & Ebert, 1992; Liu & Key, 2016; Shupe et al, 2011; Tjernström et al, 2005, 2012; Wang & Key, 2004; Zygmuntowska et al, 2012). Both the clouds and surface fluxes are closely coupled to the structure of the atmospheric boundary layer, being in part controlled by it, and in turn modifying it (Bintanja et al, 2011, 2012; Vihma et al, 2014)
The Ri number data set has been qualitatively evaluated against eddy covariance turbulence measurements from a sonic anemometer on a 30 m mast and against turbulent kinetic energy (TKE) dissipation rates estimated from a tethered balloonbased sonic anemometer and derived from a ground-based Doppler radar, with encouraging consistency
Summary
The central Arctic Ocean, characterized by a surface of semipermanent sea ice, presents unique atmospheric boundary layer (ABL) conditions. During summer the boundary layer usually has near-neutral stability (Persson et al, 2002; Tjernström et al, 2012) and is commonly capped by stratiform clouds, with a mean cloud fraction as high as 90% (Curry & Ebert, 1992; Liu & Key, 2016; Shupe et al, 2011; Tjernström et al, 2005, 2012; Wang & Key, 2004; Zygmuntowska et al, 2012) Both the clouds and surface fluxes are closely coupled to the structure of the atmospheric boundary layer, being in part controlled by it, and in turn modifying it (Bintanja et al, 2011, 2012; Vihma et al, 2014). An intricate balance between their radiative properties at solar and infrared wavelengths and the highly reflecting surface
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