On the distribution of mean radiative cooling at the top of a stratocumulus‐capped mixed layer

Abstract

AbstractThe fraction, r, of the net in‐cloud radiative flux difference existing within the mean capping inversion of a stratocumulus‐topped mixed layer is calculated, assuming an exponential diminution of radiative flux below local cloud‐top height. Its dependence upon hoλ and Δh/ho is presented, where ho is the depth of the well‐mixed portion of the boundary layer, λ is the radiative extinction length, and Δh is the thickness of the mean capping inversion, or entrainment zone, within the context of ensemble averaging or horizontal averaging. the calculation makes use of laboratory observations on the distribution of mixed‐layer matter within the entrainment zone of a convectively mixed layer. the fraction r is shown to vary widely between its limits of 0 and 1 and under different configurations of stratocumulus mixed layers.For a given net radiative flux at the top of the cloud, it is shown that changes in λ and r have a modest effect upon the buoyancy flux profile and a strong effect upon the entrainment rate. Buoyant production of turbulence within the well‐mixed layer is found to be maximized when r lies in the vicinity of 0.3.