A Wind Tunnel Investigation of the Growth of Graupel Initiated from Frozen Drops

Abstract

Abstract The formation of graupel by the riming of freely suspended frozen drops has been studied in the UCLA Cloud Tunnel. The following parameters were varied: 1) initial frozen drop size, 2) ambient temperature, 3) liquid water content, 4) growth time and 5) cloud droplet size. The various combinations yielded a total of 16 separate growth environments each consisting of approximately 10 individual growth events. In all of the growth environments, heat dissipation was efficient enough so that all the water acquired by the growing ice particle immediately turned into ice (“dry growth” regime). From the observed growth rates of the ice particles, collection kernels were calculated. These kernels are significantly different from those of drops colliding with drops or those derived from the theoretically determined efficiency with which plate-like and columnar ice crystals collide with drops. In addition, the densities of rime acquired by the frozen drops during their transition to graupel were measured and found to be somewhat higher than expected from the earlier work of Macklin (1962).