Wind Shear Enhancement of Entrainment and Refractive Index Structure Parameter at the Top of a Turbulent Mixed Layer

Abstract

A model for inversion layer turbulence properties of a cloud-free entraining mixed-layer with wind shear at the top is developed. Using the approach of Wyngaard and LeMone (1980), expressions for the average values of ? and σw2 in the entrainment region are developed in terms of a mixed layer scaling velocity Wm, the entrainment velocity We and several mean profile scaling parameters including the flux Richardson numberwhere ?Sk is the inversion wind shear, Γ the lapse rate of ?v above the inversion and ??v the buoyancy jump at the inversion. Deardorff's empirical relation for We/ σw is used to close the set of equations and to obtain a parameterization for We which applies it Rf is greater than a critical value approximately equal to one-half. The wind shear enhancement of entrainment leads to an increase in the refractive index structure parameter, CN2, in the interfacial region. This increase in CN2 may be significant under conditions of strong geostrophic forcing combined with a low-level inversion or large baroclinic effects associated with horizontal gradients of mixed-layer temperature or inversion height.