The vertical turbulent structure of the Arctic summer boundary layer during ASCOS

The central Arctic environment, characterised by semi-permanent sea ice, presents unique atmospheric boundary layer conditions. The diurnal cycle, which influences the boundary layer structure in mid-latitude, is often absent or very weak but the ABL displays a large annual cycle. During summer stratiform low-level clouds are almost ubiquitous, with mean a cloud fraction as large as 80-90%. These clouds exert a major controlling influence on the surface energy budget and therefore on melting and freezing of the sea-ice. The low-level vertical structures during this period of the year typically presents either a well mixed, cloud topped boundary layer or a well-mixed cloud layer decoupled from a well-mixed surface-based layer by a shallow stable layer at an altitude of the order of 100-200m.

The Arctic Summer Cloud Ocean Study (ASCOS) project was designed to study the many interacting processes that govern the properties of Arctic clouds, including boundary layer mean and turbulent structure. Here we present details of the vertical structure of the boundary layer during the ASCOS field campaign. We use a combination of in-situ and remote sensing measurements to analyze the mean and turbulent structure with a focus on the processes controlling the coupling/decoupling of the surface-based and cloud layers. Two useful diagnostics are the Richardson number, derived from mean profiles, and the turbulent dissipation rate estimated from Doppler cloud radar measurements and a balloon-borne sonic anemometer.