Moisture flux and condensation in turbulent flows

Moisture transport by baroclinic eddies in the atmosphere is a complicated phenomenon in which the eddy moisture fluxes are modified by large scale condensation and convection, and moisture in turn affects the eddies through latent heating. We consider a simplified problem in which turbulence advects moisture as a passive tracer subject to large scale condensation, but without fluctuations in the saturation specific humidity. A new closure is derived for the mean moisture flux and condensation based on a modified mixing length argument.

The effect of condensation on the moisture dynamics is examined in two flow fields. In the first case the moisture is advected by a one dimensional Brownian motion. The moisture field is initially saturated and then decays through condensation as individual particles enter regions of lower saturation specific humidity. In the second case the moisture is advected by isotropic, homogeneous, two-dimensional turbulence and a statistically steady moisture field is obtained.

The modified mixing length closure is tested for both flow fields. The closure is shown to capture basic effects such as condensation in the absence of mean supersaturation or mean flux convergence, as well as the effect of the saturation deficit on the magnitude of the eddy moisture flux.