SURFACE-TEMPERATURE VARIATIONS OF GYRATING HAILSTONES AND EFFECTS OF PRESSURE-TEMPERATURE COUPLING ON GROWTH

Simultaneous measurements of surface temperatures in the equatorial an d polar regions of gyrating hailstones, growing in a closed circuit wi nd tunnel, were performed using two infrared microscopes. Results indi cate that a temperature difference as high as 5.9 degrees C existed be tween these regions, with maximum differences occurring in the liquid water content range of 1.5 to 6.4 g m(-3). Under such conditions growt h was generally characterized by a dry, cold regime at the poles and a wet, warmer regime at the equator. Thus, contrary to standard assumpt ions, the surface temperature of growing hailstones cannot be consider ed homogeneous. A comparison of the hailstone growth results from the present study, performed at laboratory pressure, with those of Garcia- Garcia and List (1992), who varied air pressure with temperature accor ding to the Denver soundings (Beckwith, 1960), indicates that the mass growth rate is enhanced at lower pressures due to the combination of increased terminal velocity and collection efficiency. Smaller ice fra ctions in deposits were measured at lower air pressures for comparable air temperatures and liquid water contents. This investigation demons trates the difficulty of extrapolating from laboratory pressure to hai l growth regions at lower pressure. An examination of the governing eq uations shows that such a simulation is physically impossible.