Mean and turbulence structure of the summertime Arctic cloudy boundary layer

Abstract

Abstract The structure of the summertime Arctic planetary boundary layer in the presence of a stratus cloud cover is described using the data from a series of aircraft observations made during 1975 and 1980 over the Beaufort Sea. The cloudy boundary layers that were studied can be classified into three groups: (1) a stable boundary layer with thin, patchy clouds that may occur in numerous layers; (2) a stable boundary layer, frequently containing a fog, that is surmounted by a cloud‐topped mixed layer; and (3) a cloud‐topped mixed layer that extends from the surface. Several existing models of the cloudy boundary layer that have been applied to the Arctic are discussed in light of the aircraft observations. Two cases with relatively high wind velocity signals have been chosen for detailed analysis of the mean and turbulence characteristics. The observed mean budgets of equivalent potential temperature were generally dominated by the radiative and advective terms, while the moisture budgets were dominated by advection and the turbulent fluxes. The processes that act to produce and destroy turbulence kinetic energy in the summertime Arctic cloudy boundary layer are also examined. The dominant mode of turbulence kinetic energy production in the Arctic cloudy boundary layer is the buoyancy produced within the cloud due to the cloud diabatic processes. This paper focuses particularly on the role of diabatic processes (radiative and condensational heating) associated with the cloud in modifying the turbulence structure of the boundary layer. A comparison is made of these two cases with the marine stratocumulus layers previously described in the literature.