Stable Isotopes in Hailstones. Part II: Embryo and Hailstone Growth in Different Storms

Abstract

Hailstone collections were made in seven storms well documented by radar measurements. This sections were prepared of over 2000 hailstones for crystallographic analysis of embryos and growth layers. The majority of the embryos were frozen drops, embedded in a clear layer (2A type). Selected hailstones were also analyzed for their deuterium and oxygen-18 isotope content. The results, plotted in a δD-δO18 diagram give information on the extent to which embryos and growth layers evaporated and/or condensed during growth. Frozen drop embryos and clear growth layers showed a significantly stronger evaporation during growth than did graupel and opaque layers. Evidence is presented that some frozen drop embryos are soaked and melted graupel or large aggregates rather than drops formed by coalescence or shed from hailstone surfaces. Using the isotopic cloud model (ICM) of Part I, an absolute temperature scale was attributed to the isotope values and hailstone trajectories wore calculated. The growth ranges of embryos and hailstones are compared with the storms? radar structure. Good agreement was found for most storms, and it is concluded that the ICM gives more realistic results than the adiabatic model used earlier. With the ICM, the large height deviations, obtained with the adiabatic model, are considerably reduced, eliminating the deduction of hailstone growth below ?40°C which is impossible. The overall growth range for the 86 hailstones analyzed for D and O18 lies between ?2.5 and ?30.5°C, but hailstones grow mainly between ?15 and ?25°C in a wet mode. The trajectories of the large hailstones are surprisingly flat, indicating that an approximate balance is maintained between updraft and the increasing terminal velocities of the growing hailstones. Measurements of the isotope content of subcloud vapor showed variations of 10?15% even on days without precipitation and the crystallographic method for determining δ0 and thereby fixing the temperature scale is recommended.