Turbulent transport at a simplified clear air/cloud interface

We consider a simplified physics of the could interface where condensation and evaporation are neglected and momentum, thermal energy and water vapor transport is represented in terms of the Boussinesq model coupled to a passive scalar transport equation for the vapor phase. The interface is modeled as a layer separating two isotropic turbulent regions with different kinetic energy and vapor concentration. In particular, we focus on the small scale part of the inertial range as well as on the dissipative range of scales which are important to the micro-physics of warm clouds (Reynolds number Re_lambda= 250). We have numerically investigated stably stratified interfaces by varying the local stratification at the cloud interface. The physical parameters are set to values met at an altitude of about 1000 meters (Prandtl number Pr=0.74, Schmidt number Sc=0.64) while the Froude number at the interface ranges from 0.8 to 8. The kinetic energy ratio is equal to 7. The initial evolution resembles the mixing in a non-stratified flow. However, as the buoyancy term becomes of the same order of the inertial one, the phenomenology of the system changes. We observe a spatial redistribution of the kinetic energy and a concomitant onset of a well of kinetic energy in the low energy side of the mixing layer. In this situation, the mixing contains two interfacial regions with opposite kinetic energy gradient, which in turn produces two intermittent layers in the velocity field. This generates a change in the structure of the fluxes, dissipation rate, temperature and water vapor with respect of the non stratified mixing: the communication between the two turbulent region is weak, and the growth of the mixing layer stops. These results are discussed and compared with laboratory and numerical results with and without stratification.