A method to determine the amounts of cloud-top radiative and evaporative cooling in a stratocumulus-topped boundary layer

The turbulent processes of a stratocumulus-topped marine boundary layer are not yet fully understood. Among the issues is the relative importance of the effects of cloud-top radiative and evaporative cooling in driving the stratocumulus-topped marine boundary layer turbulence, and in producing cloud break-up. A better way to analyse the relevant observations is needed. Based on the concepts of mixing fraction and mixing-line analysis, a new method to quantitatively determine the radiative and evaporative cooling of the entrained air parcels near cloud top is proposed. Quantities 6(s,)~, which includes evaporative cooling and entrainment mixing warming, and ~ ( s , ) R . which includes both radiative cooling and the condensation warming due to radiative cooling, are defined to measure the relative importance of entrainment and radiation effects on parcel buoyancy. Provided that radiative cooling does not depend on mixing fraction for a solid cloud, slopes of the mixing lines from the linear regression of aircraft data, rather than the jumps from soundings, can be used to determine S(S,)E and S ( S , ) R . In this way, the information from an extra level above cloud is not needed to determine 6(.s,)~ and B ( s , ) R . and thus the ambiguities associated with the jumps are by-passed. The applications of the method to a large-eddy simulation case and an aircraft data set suggest that radiative cooling is the dominant contributor to the negative buoyancy of the entrained parcels for both cases. They also raise the question as to how much a single sounding can be trusted quantitatively to determine the jumps across the inversion.