Riming of Graupel: Wind Tunnel Investigations of Collection Kernels and Growth Regimes

Abstract

Laboratory experiments were carried out in the vertical wind tunnel in Mainz, Germany, to study the collision coalescence growth of single spherical ice particles having initial radii between 290 and 380 ?m while they were freely floated in a laminar flow containing a cloud of supercooled droplets with radii between 10 and 20 ?m. The experiments were performed in a temperature range between ?8 and ?12°C, where riming proceeds in the atmosphere, and with cloud liquid water contents lying between 0.9 and 1.6 g m?3 (i.e., values typically found in mixed-phase clouds). The collection kernels were calculated from the mass increase of the rimed ice particles and the average liquid water content during the experiments. Surface temperature measurements of growing graupel indicated that a dry growth regime prevailed during the whole set of growth experiments. The collection kernels of rimed ice particles attained values between 0.9 and 2.3 cm3 s?1 depending on their collector momenta (mass ? fall velocity of the riming ice particles), which had values between 0.04 and 0.1 g cm s?1. It was found that the collection kernels of ice particles determined from the present set of experiments were higher than the collection kernels of liquid drops. To correct for this discrepancy, an empirical factor depending on the cloud droplet radii was extracted from the newly measured data as well as from the old data. For the investigated size ranges of ice particles and droplets, these corrected collection kernels of ice particles can be incorporated in cloud models for the corresponding size ranges.