Thermally Indirect Motions in the Convective Atmospheric Boundary Layer

Abstract

The energetics of the dry convective boundary layer is studied by partitioning the turbulent heat flux into thermally indirect (w′θ′<0) and thermally direct (w′θ′>0) components as a function of z/Zi. It is found that except for the inversion transition region, the thermally direct and indirect fluxes each have linear profiles. The integrated profiles indicate that a fraction A of the buoyant production of thermal kinetic energy is available to do the work of entrainment, where A is the boundary layer entrainment coefficient. A simple mixing analysis shows that this would require the integrated production of energy due to surface heating to be independent of the entrainment process. Sub-partitioning the thermally indirect flux into two components (w′<, θ′>0 and w′>0, θ′<0) reveals that an upward flux of cold air is the dominant thermally indirect term throughout the bulk of the mixed layer. Further, in the inversion transition layer the measured negative mean entrainment beat flux (w′θ′)i is principally due to a net upward flux of locally colder air, and not to a net downward flux of locally warmer air. These results are interpreted in terms of a highly idealized dome-wisp model of the entrainment mechanism.