Occurrence of shallow cold flows in the winter atmospheric boundary layer of interior of Alaska

Abstract

During winters, the absence of solar radiation combined with clear skies and weak synoptic forcing enables cold pooling in the complex topographic basins of interior Alaska. Under these conditions, shallow, small-scale cold flows originating within, or flowing from, north-facing semi-enclosed basins are able to penetrate the frigid atmospheric boundary layer (ABL) of the open south-facing basins. This paper introduces the Winter Boundary Layer Experiment carried out during three consecutive periods in Fairbanks (2009–2011) and examines observational results illustrating the changes in the mean and turbulent state of the ABL during the occurrence of shallow flows. Observations introduced here demonstrate that during flow penetration, surface layer stratification is destroyed allowing mixing and thermal stabilization of the basin cooling regime. Evidence of upper level ABL thermal turbulence related to shear driven flow is introduced and discussed. Basin-scale turbulent heat fluxes are shown to reach −20 Wm−2 during flow occurrence.