PART V — SEVERE STORMS 



versus height is negative 



(£<•) 



above that level and positive below 

 it. As long as the maximum updraft 

 speed is greater than 10 meters per 

 second, water drops will neither de- 

 scend below the accumulation level 

 nor ascend much above it. Therefore, 

 liquid water may become trapped at 

 a certain layer and provide conditions 

 for the rapid growth of hailstones. 

 While the existence of such a level 

 is possible, the rapidly increasing 

 water-loading will, for continuity rea- 

 sons, cause a strongly divergent flow 

 that discharges the accumulating wa- 

 ter content radially in a short time. 



Numerical Studies — Several at- 

 tempts are under way to expand one- 

 or two-dimensional numerical cumu- 

 lus-cloud models into convective 

 storm models. Even two-dimensional 



models, however, are much too prim- 

 itive for the simulation of a phe- 

 nomenon as complex as a hailstorm. 

 The best model to date appears to 

 be a time-dependent, two-dimensional 

 model developed by Orville; however, 

 even this model puts severe strains 

 on computer capacity and memory. 

 There can be no question that these 

 attempts are only first steps and that 

 much research and data collection is 

 required to make them realistic. 



Microphysical Studies 



Microphysical studies aim, partic- 

 ularly, at an explanation of hailstone 

 structure and the application of hail- 

 stone features to explain the condi- 

 tions under which it has grown. It 

 is hoped that hailstones can be used 

 as aerological sondes which even- 

 tually may reveal their life history 



and, consequently, the environmental 

 conditions inside the hail cloud. (See 

 Figure V-ll) 



Here the investigator is confronted 

 with complexities related to greatly 

 varying growth conditions of ice due 

 to accretion of supercooled water. 

 The most thoroughly conceived the- 

 ory has been developed by List from 

 actual growth conditions in a hail 

 wind tunnel. However, List gives 

 consideration only to the accretion 

 of supercooled cloud water; ice struc- 

 tures resulting from the accretion of 

 a mixed cloud (ice crystals and water 

 droplets) or of aggregation of smaller 

 hail or graupel have not been studied. 



The following general statements 

 may be made with caution: 



Hailstone Structure — Most hail- 

 stones show a hail embryo in their 



Figure V-1 1 — STRUCTURE OF HAILSTONE EMBRYOS 



At the heart of almost every hailstone there is a distinct growth unit 5-10 millimeters 

 in diameter known as the embryo. The illustration shows the three most common 

 types: (1) Conical embryos consist of opaque crystals larger than 2 millimeters in 

 diameter, indicating formation between -20C and C. These embryos fall in a 

 stabilized position, blunt end downward, so they collect droplets on only one 

 surface. This category represents about 60% of the hailstones studied. (2) Spherical 

 embryos of clear ice (25% of the hailstones studied) consist of large crystals or a 

 single crystal, indicating growth in clouds with temperatures above -20 C. Many 

 of these embryos have cracks caused by the freezing of internal liquid water. (3) 

 Spherical embryos of opaque ice (10% of the hailstones studied) have crystals of 

 intermediate size and air bubbles showing no particular arrangement. They may 

 have had a more complicated origin than other embryos, involving partial melting 

 and refreezing or even collection of snow crystals. Because they tumble as they 

 fall, they collect droplets equally on all surfaces. 



150 



