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The parametrization of entrainment driven by surface heating and cloud‐top cooling
Author(s) -
Lock A. P.,
Macvean M. K.
Publication year - 1999
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1002/qj.49712555315
Subject(s) - mechanics , entrainment (biomusicology) , parametrization (atmospheric modeling) , radiative cooling , turbulence , radiative transfer , convective boundary layer , convection , radiative flux , atmospheric sciences , buoyancy , environmental science , boundary layer , physics , planetary boundary layer , meteorology , optics , rhythm , acoustics
Abstract A numerical modelling study of entrainment in the convective atmospheric boundary layer has been undertaken. to isolate the effects of long‐wave radiative cooling at cloud‐top from those associated with phase changes of water, simulations of dust clouds have been performed. the radiative properties of these dust clouds were chosen to be broadly similar to those of stratocumulus. A wide range of interfacial and overlying stabilities were modelled, with turbulence driven by different combinations of surface heating and radiative cooling. the results of the simulations have been used to develop parametrizations both of the entrainment flux and the entrainment velocity. Where the boundary layer was driven by surface heating alone, the results are consistent with the widely accepted view that the magnitude of the entrainment buoyancy flux is a constant fraction (about 0.2) of the surface flux. When both driving mechanisms were active simultaneously, they appear to generate entrainment independently so that parametrizations based on a cubic average of ‘external’ velocity‐scales, characterizing the strength of the forcing, are found to be more successful than those based on measures of the actual strength of the turbulence, such as the generalized convective velocity‐scale. the assumption of a discontinuous inversion structure can lead to predictions of the entrainment rate which are much less accurate than those of the entrainment flux. In the worst cases, the predictions may be too low by a factor of two. the situation can be substantially improved, however, by allowing for a linear variation within an inversion of finite depth. Radiative cooling is then found to promote deepening of the boundary layer both directly (by cooling the air within the horizontally averaged inversion) and indirectly (through the buoyant production of turbulence).