Laboratory Measurements and Parameterizations of Supercooled Water Skin Temperatures and Bulk Properties of Gyrating Hailstones

Abstract

The accretional growth of gyrating hailstones was studied in a pressure- and temperature-controlled icing wind tunnel, starting with oblate ice spheroids, under cloud conditions and at free-fall speeds. Measured parameters were hailstone surface temperature (with infrared radiometric microscope), net collection efficiency, ice fraction, surface roughness, and final shape of the particle. Surface temperatures of water skins covering growing deposits were found to be supercooled up to ?2.5°C, depending on experimental conditions. Over the range of variables covered, the transition from solid to spongy ice deposits occurred at surface temperatures below 0°C. This revised Schumann-Ludlam limit is approximated by a linear liquid water content-air temperature relationship. The final shape of the hailstones varied with icing conditions, and different surface roughness characteristics appeared depending on the gyration rates. Other main findings are that spongy deposits grow while millimeter-sized drops are shed and the type of deposit is strongly dependent on the experimental conditions. It is also concluded that nonhomogeneous and nonisotropic growth conditions have to be considered to establish a heat budget. The measured surface temperatures, ice fractions, and net collection efficiencies were parameterized as functions of the liquid water content, air temperature, and gyration rates and are available now to assess the sensitivity of the heat transfer to ambient conditions and other variables, and for microphysical parameterization in cloud modeling.