Cirrus Outflow Dynamics

Abstract

Cirrus outflow from deep convection are analyzed as dynamically and thermodynamically active systems. The initial outflow is considered as an analog to wake collapse, in which a neutrally buoyant flow intrusion is flattened and stretched by its stratified environment, and the initially isotropic turbulence within it is converted to other forms. The early spread of the outflow is predicted on the basis of analytic and numerical calculations by Dugan et al. Strong radiative heat flux curvature then leads to maintenance or regeneration of buoyant turbulence in the collapsed outflow plume. Mixed layer analysis allow predictions of entrainment rates. In the case of strong net radiative heating, the mixed layer model predicts an encroachment condition, in which the upper boundary attains nearly the same temperature as the environment and grows into it rapidly. This tendency is countered by bodily ascent of the warm mesoscale plumes, the rate of which is predicted with the aid of a theory of drag on a flat plate in a stratified fluid. It is found that narrow plumes rise rapidly enough that their mean temperature is not much different from the environment, and their tops may be cold enough to contribute to drying of the lower stratosphere. Wider plumes follow the mixed layer analysis and develop a substantial temperature excess and an encroaching upper boundary condition. The critical width is estimated to be of order 150 km. The effect of cirrus precipitation represents an unknown limitation to this analysis.