Extended Abstract (292K)P3.6
Some aspects of turbulence-microphysics interaction in numerically simulated stratocumulus clouds
Shouping Wang, NRL, Monterey, CA; and Q. Wang and G. Feingold
Condensation and turbulent liquid water transport in stratocumulus clouds involve complicated interactions between turbulence dynamics and cloud microphysical processes, and play es-sential roles in defining the cloud structure. This work aims at understanding this turbulence-microphysical interaction and providing information necessary for parameterizations of the en-semble mean condensation rate and turbulent fluxes of liquid water variables in a turbulence parameterized model. The approach is to simulate non-precipitating stratocumulus clouds with a coupled large-eddy simulation and explicit bin-microphysical model, and then perform a budget analysis for four liquid water variables: mean liquid water content, turbulent liquid water flux, mean cloud droplet number concentration and the number density flux. The results show that the turbulence contribution to the mean condensation rate comes from covariance of the integral cloud droplet radius and supersaturation, which enhances condensation in turbulent updrafts and reduces evaporation in the downdrafts. Turbulent liquid water flux results from a close balance between turbulence dynamics and microphysical processes. Consequently, the flux can be parameterized in terms of the common diffusive down-gradient formulation, fluxes of conservative thermodynamic variables and a condensation time scale, which depends on the droplet spectrum. The droplet number concentration flux can also be formulated in a similar fashion. In addition, the results suggest that the liquid water flux depends, to some degree, on the droplet number concentration, because the larger the number concentration is, the more condensation (evaporation) occurs in turbulent updrafts (downdrafts), leading to stronger condensation fluctuations. Therefore, the turbulence dynamics may be affected by the microphysics (mainly, the CCN number) even in the absence of drizzle. Furthermore, since a saturation adjustment cloud model instantly condenses (evaporates) all available water vapor (liquid water) surplus, there seems to be a systematic difference between the liquid water flux resolved with this type of model and that with a supersaturation-based cloud scheme for which a finite condensation time scale applies.
Poster Session 3, Cloud Physics Poster Session III (Parallel with Joint Poster Sessions JP2 and JP3)
Wednesday, 5 June 2002, 1:00 PM-4:00 PM
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