The Effect of Turbulence on the Accretional Growth of Graupel

Abstract

Abstract Wind tunnel experiments were carried out to investigate the influence of turbulence on the collection kernel of graupel. The collection kernel defines the growth rate of a graupel accreting supercooled droplets as it falls through a cloud. The ambient conditions were similar to those occurring typically in the mixed-phase zone of convective clouds, that is, at temperatures between −7° and −16°C and with liquid water contents from 0.5 to 1.3 g m−3. Tethered spherical collectors with radii between 220 and 340 μm were exposed in a flow carrying supercooled droplets with a mean volume radius of 10 μm. The vertical root-mean-square fluctuation velocity, the dissipation rate, and the Taylor-microscale Reynolds number of the turbulent flow were determined as urms = 0.13 m s−1, ε = 0.13 m2 s−3, and Rλ = 48, respectively. The collection kernels of tethered graupel grown under laminar and turbulent conditions revealed no measurable difference, indicating that turbulence has no effect on the growth of graupel in the investigated size range. A comparison of laminar collection kernels to theoretically calculated values from a continuous growth model showed that graupel growth is strongly dominated by the fast increase of the radius due to the accretion of rime ice with low density. It is assumed that, compared to a water drop growing by collision and coalescence, this causes a fast increase in the swept volume overcompensating all other effects such as the self-induced stochastic movements due to surface roughness and latent heat release, as well as the possible influence of the flow’s small-scale turbulence.