Small-scale turbulence in clouds

Small scale turbulence in clouds plays a major role for mixing processes and the interaction between cloud droplets and the turbulent flow. Mixing processes include internal mixing but also entrainment of dry and sub-saturated air into the cloud (so-called “entrainment”). On smallest scales close to the dissipation scale (typically 1 mm or so under atmospheric conditions) turbulence can lead to the accumulation of droplets in regions of low vorticity whereas in regions of strong vorticity the local droplet number density is reduced (so-called “inertial clustering”). However, the nature of cloud turbulence on sub-meter scale has not yet been investigated in more detail. Most experimental data is based on fast-flying aircraft yielding a spatial resolution of about one meter and, therefore, the validity of classical turbulence theory in clouds (Kolmogorov, 1941, 1962) has not yet been experimentally proven on the sub-meter scale. In this paper, we present first in-situ data of the three-dimensional wind vector and temperature with a spatial resolution on the decimeter range. The data was taken during balloon- and helicopter-borne experiments in shallow cumulus clouds of different life stages.

Since this work focus on the small-scale turbulence local values of the energy dissipation rate are derived which characterize the degree of turbulence in the different cloud regions with a spatial resolution of about 10 to 15 m. This local values are semi-logarithmic distributed in space indicating the highly intermittent character of the turbulence which is typically for high Reynolds number flows. The range of local energy dissipation rates covers up to four orders of magnitude inside the same cloud which makes the use of mean values questionable and has important consequences for cloud modeling, especially for the calculation of the droplet collision kernel. Furthermore, the isotropy of the flow is analyzed by means of the ratio of the power spectra of longitudinal and transversal velocity vector components.

A second focus was set on the edges of freshly evolving cumulus clouds where regions of down-drafts (due to subsidence) and the up-draft regions are localized close together resulting in strong shear turbulence. It was found that the energy dissipation rates in that regions are often much higher compared with the pure up-draft regions in the middle of a cloud.