3. HAIL 



Hailstorm Research and Hail Suppression 



Hailstorms belong to those atmos- 

 pheric phenomena whose life history 

 originates and terminates in the 

 mesoscale range — i.e., their size 

 ranges from about 1 to 100 kilo- 

 meters. Phenomena of this scale pre- 

 sent great difficulties for observation 

 and description, and the means and 

 instrumentation for that purpose are 

 only now being developed. 



Radar, the oldest tool of mesoscale 

 observation, has been somewhat dis- 

 appointing when quantitative data are 

 required. A system that combines 

 airborne radar with data derived from 

 the aircraft's doppler navigation sys- 

 tem has proved to be a powerful tool 

 for storm studies. The radar helps to 

 delineate the precipitation echo of 

 the storm while the doppler system 

 provides the wind vector at flight 

 level. Thus, on circling the storm, 

 the line integrals for divergence and 

 vorticity can be solved, and these 

 yield the inflow into the storm 

 throughout its life history. 



The improved means of storm ob- 

 servation have de-emphasized the 

 classical approach to storm research. 

 This approach attempts to find, 

 through observation and deduction, 

 one valid storm model that satisfies 

 all hailstorms. The last such model 

 was derived by Browning from radar 

 observations of one storm in England. 

 It was characterized by a slanted 

 updraft and an echo-free vault — i.e., 

 an area where the main updraft 

 speed was concentrated and where, 

 due to the high updraft speed, no 

 large particles accumulated that 

 would cause radar reflections. 



Hailstorm Characteristics 



Nowadays we know that hail- 

 storms appear in many manifesta- 



tions. The energy source is always 

 the latent energy of condensation, 

 but in the exploitation of that energy 

 the vertical wind profile appears to 

 assume an important role. Over the 

 Great Plains of the U.S., hailstorms 

 usually travel from west to east. 

 They can grow and form new cells 

 from the leading (eastern) edge or 

 from their trailing (western) edge; 

 thus, they can actually grow from 

 the rear. It appears that their updraft 

 is usually upright and not slanted 

 even under conditions of strong wind 

 shear; more and more, they are re- 

 garded as aerodynamic hindrances in 

 the large-scale atmospheric flow re- 

 gime, with the wind going around 

 and over the storm. Thus, the up- 

 draft tower may be eroded on the 

 outside by the horizontal wind but 

 remain undisturbed in the interior. 



The air intake into a growing cell 

 is of the order of 10 cubic kilometers 

 per minute. High wind velocity in 

 the anvil level appears to be the 

 mechanism that prevents early decay 

 of the cell, since precipitation and 

 liquid water are carried away from 

 the cell and, consequently, do not fall 

 back into and "suppress" the updraft. 

 It has been shown that hailstorms 

 occur with special frequency in jet- 

 stream regions of the United States, 

 Europe, and India and that the com- 

 bination of convective storms and 

 jet stream can produce a very efficient 

 and abundant precipitating cloud sys- 

 tem. There are indications that the 

 effectively producing hailstorm is 

 characterized by high latent instabil- 

 ity, inflow from the right rear quad- 

 rant, and strong wind shear aloft. 



Very poorly understood is the way 

 hailstorms become organized. As yet, 

 we do not know under what condi- 

 tions many small storms or a few 



big ones form, what causes the storms 

 sometimes to align themselves in 

 rows and sometimes to form in clus- 

 ters. It has been speculated that 

 differences of surface temperature be- 

 tween sunlight and shadowed areas 

 may cause local seabreeze-type cir- 

 culations which contribute to the 

 organization of inflow areas. 



Some conditions lead to self- 

 enhancement of storm intensity. For 

 example, when the storm moves over 

 its own precipitation area and en- 

 trains moist air, the base level is 

 lowered, which in turn increases the 

 buoyancy. This will increase the in- 

 flow into the storm, which then leads 

 to an increased diameter of the up- 

 draft column. This causes an increase 

 of updraft speed for the same latent 

 instability because the ratio between 

 buoyancy forces and drag forces has 

 shifted in favor of the buoyancy 

 force. 



Theoretical Studies 



Theoretical studies of the dynamics 

 of storms extend in two general di- 

 rections: 



Analytical Studies — These studies 

 deal with the influences of buoyancy 

 and water-loading on updraft speed 

 and radial divergence when the 

 buoyancy term is compensated by the 

 weight of the cloud and precipitation 

 water. Essentially, this research aims 

 at appraising the existence of an 

 "accumulation level" of cloud water 

 in the upper regions of the storm. 



According to Soviet scientists, the 

 accumulation level is characterized by 

 a high liquid-water content, since the 

 local derivative of the updraft speed 



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