PART V — SEVERE STORMS 



sometimes approach 200 miles per 

 hour may cause storm surges of 20 

 to 30 feet or so, the development of 

 strong coastal currents which erode 

 the beaches, and the onset of moun- 

 tainous waves. Once the latter three 

 elements are in being, they are far 

 more destructive than the winds and 

 are usually responsible for the greater 

 damage. Their destructive power 

 varies directly with the speed of the 

 winds. 



Damage due to sea forces and to 

 winds is concentrated along and near 

 the seacoast; even the damage at- 

 tributed to winds alone usually drops 

 off drastically within a relatively few 

 miles of the coast when a hurricane 

 moves inland. Damage from rain- 

 caused floods, on the other hand, may 

 extend far into the interior and is 

 particularly acute in mountainous 

 regions traversed by the remnants of 

 a hurricane. This is especially true 

 in situations where rain-induced 

 floods originate in mountains near 

 the coast and arrive at the coastal 

 plain before the ocean waters have 

 receded. In view of the difficulty of 

 building structures to resist all these 

 destructive elements, efforts have 

 lately concentrated on reducing the 

 destructibility potential of hurricanes. 



If the present program for modi- 

 fying hurricanes to reduce their in- 

 tensity should prove effective, the 

 potential benefit/cost ratio could be 

 of the order of 100:1 or 1,000:1. It 

 should be emphasized that the modifi- 

 cation program has no intention of 

 either "steering" or completely de- 

 stroying hurricanes. The rainfall 

 from hurricanes and tropical storms 

 is an essential part of the water bud- 

 get of many tropical and subtropical 

 land areas, including the southeastern 

 United States. The hope is to reduce 

 a hurricane to a tropical storm by a 

 reduction in the speed of the concen- 

 trated ring of violent winds near the 

 center, leaving the rainfall and total 

 energy release of the over-all storm 

 essentially unchanged. 



Details of the Project 



The groups active in Project 

 STORMFURY, a joint effort of the 

 U.S. Navy and the National Oceanic 

 and Atmospheric Agency (NOAA), 

 conducted experiments on hurricanes 

 in 1961, 1963, and 1969. In each 

 case, the objective was to reduce 

 the maximum winds of the hurricane. 

 The technique called for seeding a 

 hurricane with silver iodide crystals 

 in order to cause supercooled water 

 drops to freeze and release their latent 

 heat of fusion. In the earlier years, 

 the experiments consisted of seeding 

 a hurricane one time on each of two 

 days. The results appeared favorable 

 but were inconclusive, since the 

 changes were of a magnitude that 

 often occurs naturally in hurricanes. 



In August 1«60, the STORMFURY 

 group seeded Hurricane Debbie five 

 times in a period of eight hours on 

 the 18th and 20th of the month, with 

 no experiment on the 19th. Following 

 the seedings, maximum winds at 

 12,000 feet decreased within six 

 hours by 31 percent on the 18th and 

 15 percent on the 20th. The storm 

 regained its original intensity on the 

 19th. While changes of this mag- 

 nitude have happened in hurricanes 

 on which there was no experiment, 

 they have been quite rare. When one 

 considers the entire sequence of 

 events in 18-20 August, one can say 

 that such a series of events has not 

 happened in previous hurricanes 

 more than one time in 40. Thus, 

 while we cannot state that the Debbie 

 experiments proved that we know 

 how to modify hurricanes, the results 

 were certainly very encouraging. 



Along with the experimental pro- 

 gram, there has been an intensive 

 effort to develop models which simu- 

 late hurricanes. The best of these 

 models now reproduce many features 

 of a hurricane quite well. One devel- 

 oped by Rosenthal has been used to 

 simulate seeding experiments, includ- 

 ing the one performed on Debbie. 

 The STORMFURY experiment was 

 simulated by adding heat at appro- 



priate radii at the 500 and 300 milli- 

 bar levels (approximately 19,000 and 

 32,000 feet, respectively) over a pe- 

 riod of ten hours. The amount of 

 heat added was believed to be com- 

 parable to the amount of latent heat 

 that can be released by seeding a 

 hurricane. Within six hours after 

 cessation of the simulated seeding, 

 the maximum winds at sea level 

 decreased about 15 percent. The 

 time-scale for the decrease in max- 

 imum winds was roughly the same 

 as that in the Debbie experiments. 



Evaluation of Results 



The net results of the various field 

 experiments and the implications 

 from modeling experiments give 

 strong reason for believing that at 

 least some degree of benefical modifi- 

 cation was achieved in the Debbie 

 experiments. Unfortunately, how- 

 ever, we cannot say the matter is 

 proved nor can we claim the results 

 are statistically impressive at some 

 high level of significance. 



The modeling results are most in- 

 teresting and highly suggestive, but 

 there are certain deficiencies in the 

 model which require that one be 

 cautious in interpreting them. First, 

 a highly pragmatic parameterization 

 of cumulus convection is used. Sub- 

 stantial improvements in this area 

 must await increased understanding 

 of both cumulus convection and its 

 interaction with larger scales of mo- 

 tion. Second, the major simplifying 

 assumption of circular symmetry 

 used in the model precludes direct 

 comparison between model calcula- 

 tions and specific real tropical cy- 

 clones. Real cyclones are strongly 

 influenced by interaction with neigh- 

 boring synoptic systems, and these 

 vary markedly in character and in- 

 tensity from day to day. 



When one looks at parameters 

 other than the winds for further 

 verification of seeding effect, either 

 the data were not collected in Hur- 

 ricane Debbie or insufficient data are 



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