Small-scale turbulent mixing of clouds with clear air:

Laboratory experiments and aircraft observations

Dr. Szymon Malinowski, Warsaw University, Poland

Small-scale turbulent mixing of clouds with clear air is investigated in a laboratory using a cloud chamber and by an ultra-fast thermometer mounted on an aircraft. The cloud chamber measurements apply the laser-sheet photography. Patterns of filaments created by mixing are anisotropic with the preferred direction in vertical. Thickness of an interface between cloudy and clear air filaments depends on the interface orientation.

High-resolution temperature measurements collected in real clouds show that in regions undergoing mixing there is also a lot of filamentation with temperature differences up to a few Kelvins on distances of the order of centimeters. Temperature power spectra show more energy in small scales than expected in the inertial subrange. These results indicate production of buoyancy at small scales due to droplet evaporation near the cloud-clear air interface. Sedimentation of cloud droplets seems to play an important role in this process..

Major results are:

(a) a linear dependence of 2-m temperature with elevation, that is closely related to static stability during the night,

(b) a dependence of 2-m temperature on surface cover, once the elevation effect is removed,

(c) interesting inconsistencies involving the two aircraft and surface data, some of which may be related to surface cover,

(d) the influence of surface properties on PBL growth, and

(e) a strong suggestion of direct heating of the PBL by radiative flux divergence during the late morning, especially for the day with nonuniform soil moisture. Mesoscale (order 10 km) eddies may have also played a role in heating on this day.