Supersaturation Intermittency in Turbulent Clouds

Abstract

It is hypothesized that bursts of high supersaturation are produced in turbulent, convective clouds through interactions between cloud droplets and the small-scale structure of atmospheric turbulence. This hypothesis is based on the observation that intermittency in the energy dissipation of turbulence at small scales is, in part, related to the presence of rare but intense vortex tubes. Scaling relationships for the size, lifetime, and intensity of vortex tubes observed in numerical simulations and laboratory studies of turbulence are presumed to hold at the high Reynolds numbers encountered in the atmosphere. Under this assumption a scale analysis shows that the tubes are sufficiently intense and persistent as to cause large flux divergences in the local concentrations of cloud droplets. When embedded in a mean updraft, a vortex tube will become a localized region of high supersaturation due to the low number concentration of cloud droplets (condensation sites). For typical cumulus conditions, water supersaturations may reach values of over 10% in the core of a vortex tube. Upon vortex breakdown, the localized regions of high supersaturation will lead to the formation of small concentrations of?superadiabatic? droplets in clouds. Finally, a threshold condition for this mechanism is derived and shown to be related to the droplet size distribution and the turbulent kinetic energy dissipation rate, both of which are commonly measured or calculated quantities in cloud field studies and cloud models. When the threshold condition is met, the concentration of superadiabatic droplets is expected to increase approximately linearly with height above cloud base.