HURRICANES 



possible avenues of achieving benefi- 

 cial modification. Utilization of the 

 approach with the sounder basis of 

 scientific understanding has so far 

 been precluded by logistic considera- 

 tions. The second approach, which 

 involves complex but feasible logis- 

 tics, has been used in experiments 

 on three hurricanes with encouraging 

 but not yet definitive results even 

 though the detailed physical basis 

 for the approach is not completely 

 understood. 



Present Scientific Status 



Special observational efforts and 

 more intensive theoretical studies 

 during the past twenty years have 

 led to important advances in the 

 understanding of the physics of hur- 

 ricanes, but significant gaps remain 

 to be filled. Preliminary efforts at 

 constructing mathematical models of 

 the hurricane have been encouraging, 

 but serious defects remain. 



Data Base — For hurricanes in the 

 mature stage and in dissipating stages 

 over land, the descriptive data base 

 is good in the qualitative sense. The 

 principal data deficiencies consist of 

 quantitative measurements of such 

 items as: the distribution of water 

 in all phases as a function of tem- 

 perature in the storm; the fluxes of 

 heat and water vapor from the sea 

 to the air under the extreme condi- 

 tions present in the hurricane; and 

 the natural variability of various 

 meteorological parameters in the 

 inner regions of the hurricane as a 

 function of time-scales ranging from 

 an hour to a day or two. 



Basis for Modification — The most 

 significant addition to our scientific 

 knowledge of hurricanes in recent 

 years has been the convergence of 

 both theoreticians and empiricists on 

 the concept that the hurricane is the 

 complex result of the interaction of 

 physical processes on several dis- 

 tinctly different scales. It is now 

 agreed that these storms, whose 

 space-scale of a few hundred kilo- 

 meters and lifetime of a few days 



typify the synoptic-scale of atmo- 

 spheric systems, depend critically on 

 microscale (1 to 10 meters) turbulent 

 motions of the surface boundary 

 layer for the addition of heat and 

 water vapor from the sea surface, and 

 on mesoscale convective clouds, pri- 

 marily organized in the annular ring 

 surrounding the eye, for release of 

 the latent heat of water vapor as the 

 primary driving mechanism of the 

 storm. Furthermore, the combined 

 processes on these scales are influ- 

 enced by interactions with much 

 larger scale systems of the atmos- 

 phere. 



It is this dependence on microscale 

 turbulence and mesoscale convection 

 that has suggested the two avenues 

 to modification. Reduction of the 

 evaporation associated with the for- 

 mer would certainly result in reduc- 

 tion of hurricane intensity, but this 

 approach to modification has been 

 prevented by insurmountable logistic 

 problems. Redistribution of the 

 latent heat release associated with 

 the latter through the use of cloud- 

 seeding techniques shown to influ- 

 ence the structure and dynamics of 

 convective clouds is logistically feas- 

 ible and has been employed in ex- 

 periments on a small number of 

 hurricanes. There are residual un- 

 certainties and disagreements as to 

 the correct seeding techniques and 

 the interpretation of the experimental 

 results. 



Theoretical models of the hurricane 

 incorporating the various scales dis- 

 cussed above with varying degrees 

 of simplification have been developed. 

 Results of computer simulations 

 based on these models indicate qual- 

 itative success in modeling the physi- 

 cal processes responsible for the 

 formation and maintenance of the 

 hurricane. But significant quantita- 

 tive uncertainties remain. Further- 

 more, present models cannot con- 

 tribute significantly to problems of 

 hurricane motions. 



Interactions — Our present scien- 

 tific knowledge and understanding 



of the interaction of hi 

 other aspects of the atmospheric gen- 

 eral circulation, with other environ- 

 mental systems such as the ocean, 

 and with man and society are qualita- 

 tive and inadequate. For example, 

 it is known that rainfall associated 

 with hurricanes is often of consider- 

 able economic benefit, but it can also 

 lead to disastrous floods. We do not 

 know how the atmospheric circula- 

 tion would change if hurricanes did 

 not exist. Nor is it decided who in 

 society is to decide when and where 

 hurricane modification should be at- 

 tempted. 



Requirements for Scientific 

 Activity 



Significant scientific controversy 

 exists with respect to the following 

 aspects of hurricane modification: 



1. Can the effects of seeding ex- 

 periments be unequivocally de- 

 tected against the large natural 

 variability of hurricanes? 



2. How, exactly, does cloud seed- 

 ing redistribute latent heat re- 

 lease and how is this redistribu- 

 tion responsible for decreases 

 in hurricane intensity? 



3. Are the present mathematical 

 models and associated com- 

 puter simulations of hurricanes 

 sufficiently realistic to serve as 

 indicators of differences in ex- 

 pected behavior of natural and 

 seeded hurricanes? 



4. Are the amounts of super- 

 cooled liquid water necessary 

 if seeding techniques are to 

 result in significant redistribu- 

 tion of latent heat release ac- 

 tually present in the correct 

 portions of the storm, and is 

 this water actually frozen by 

 the seeding? 



The most urgently needed scien- 

 tific advances fall into two categories: 

 observations and theoretical model- 



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