Cloud-clear air interfacial mixing: anisotropy of turbulence generated by evaporation of liquid water. Laboratory observations and numerical modeling

Saturated air, containing small water droplets (cloud), undergoes mixing with the unsaturated environment (clear air). Turbulent mixing driven by turbulence cascading down from the larger scales influences fluids undergoing mixing creating the evolving cloud and clear air filaments. There are two various transport mechanism acting across the interface between these filaments: 1)molecular diffusion of water vapor (and heat); 2)gravitational sedimentation of cloud droplets from cloud to clear air filaments (liquid water transport). Both mechanisms lead to evaporation of cloud droplets. Evaporative cooling at the interface effects in density gradients. As a result, local production of the small-scale turbulence occurs. This process has already been investigated independently by numerical experiments (spatial resolution down to 2.5mm) and the laboratory cloud chamber measurements by means of Particle Image Velocimetry (PIV) method (with 1.2mm spatial resolution). Results of both investigations has shown, that such turbulence is highly anisotropic (preferred vertical direction) substantially influences turbulence cascading down from the large scales. In the present study we compare of statistical properties of turbulence observed in the cloud chamber and modeled with the numerical experiments. There are far going similarities in results; both: laboratory and numerical indicate significant anisotropy of small-scale turbulence. Typically, <(u')2> is about two times smaller than <(w')2>. Here u' stays for turbulent velocity fluctuations in horizontal and w' denotes such fluctuations in vertical. The probability distribution functions of w' are spreaded wider than those of u', which is in agreement with the statement that small-scele turbulence is generated by buoyancy forces. In conclusion we state, that both data sets: results from the numerical simulations and the data from the laboratory cloud chamber indicate that small-scale turbulence in clouds undergoing mixing with the unsaturated environment differs significantly from the usually assumed isotropic turbulence.