Turbulent mixing of clouds with the environment: Small scale two phase evaporating flow investigated in a laboratory by particle image velocimetry

In this report, experiments mimicking some aspects of cloud-clear-air mixing in a laboratory chamber are discussed. A saturated plume containing small droplets of water (a surrogate of an atmospheric cloud) is mixed with unsaturated environmental air in order to study effect of evaporative cooling at the interfaces between cloudy and clear air filaments on small-scale turbulence. Two components of the turbulent velocity at a small scale are determined using particle imaging velocimetry of the cloud droplets. Enhanced velocity fluctuations between the Kolmogorov ( η ≈ 0.8 mm ) and Taylor ( λ ≈ 8 mm ) microscales are observed. The characteristic feature of these microscales is anisotropy with a preferred vertical direction. A straightforward dependence of the observed effects on the relative humidity of the environment indicates the importance of buoyancy production by evaporative cooling. This conclusion is in agreement with previous numerical experiments described in the literature and suggests that, under certain conditions, effects similar to the above may influence the smallest scales of turbulence in natural clouds. ► Cloud-clear air mixing is investigated using particle imaging velocimetry of the cloud droplets. ► Small-scale velocity fluctuations are anisotropic with a preferred vertical direction. ► Dependence of the anisotropy on relative humidity indicates significance of evaporative cooling.