Time‐resolved hailstone analyses and radar structure of Swiss storms

Abstract

AbstractTime‐resolved, quenched, hailstone size distributions from five hailstorms simultaneously monitored by 10cm and 3cm radar (in the PPI and RHI mode respectively) have been measured. A total of 1220 hailstones have been sectioned and the embryo types (rimed embryos or frozen drop embryos) were classified according to the crystallographic and bubble structure of the centre unit and the initial growth layer. For three hailstones the trajectories deduced from crystallography have been checked by deuterium analyses.The most important findings were: (1) In storms with relatively warm cloud bases a big‐drop zone containing large drops which freeze and become efficient hail embryos exists. (2) The time variation of the parameters of the hailstone spectra reveals the fundamental storm dynamics (multicell or steady state). (3) Variability of embryo types is smaller in steady‐state storms than in multicellular storms, implying that hail formation zones are regions of rather uniform temperature and liquid water content in steady‐state storms whereas in multicell storms growth occurs in a more heterogeneous environment, possibly in different updraught branches. (4) The two embryo growth mechanisms – riming, and freezing of supercooled drops – coexist in the storms. (5) A characteristic change of embryo type with time is observed consistently in the storms. (6) The size frequency distributions of those frozen drops whose diameters could be measured, showed a maximum at 2mm; this can be explained by ‘freezing’ an exponential spectrum of raindrops according to the stochastic hypothesis.