SECT. 2] LARGE-SCALE INTERACTIONS 91 



(2) Budget studies, in which some of the hydrodynamic equations are used 

 as conservation laws, particularly those stating continuity of mass, heat, 

 kinetic energy, momentum and often vorticity. The various terms in the equa- 

 tions are generally evahiated from observations and physical hypotheses ; the 

 results of large-scale field expeditions in meteorology and oceanography com- 

 monly come into their first valuable fruition when utilized in this manner. 

 These investigations have the great value of shedding light upon the energy 

 sources and transports of a geophysical fluid system, and of demonstrating 

 Avhat processes and scales of motion are important in their operation. In 

 meteorology, such studies have surged ahead since World War II due to the 

 efforts of E. Palmen, H. Riehl and their collaborators, and have been largely 

 enabled by extended observational networks and the use of instrumented air- 

 craft. Examples involving oceanic influence include the trade-wind system 

 (Riehl et al., 1951), the equatorial trough (Riehl and Malkus, 1958), several 

 hurricanes (Palmen, 1958; Gangopadhyaya and Riehl, 1959), and, for the first 

 time, a joint air-sea budget study of the Caribbean region (Colon, 1960), most 

 of which will be discussed in some detail later. Oceanographic studies of this 

 sort have been undertaken for whole ocean basins, and often include budgets of 

 various solutes such as salt, carbon dioxide and other gases (Wiist, 1957). 



All these treatments have the basic limitation of not being predictive dynamic 

 or thermal-dynamic models, thus precluding real understanding of time changes, 

 stabilities and fluctuations, and thereby of causality: but they are usually 

 required before it is known what are the important variables to incorporate in 

 the more formal models, and how to parameterize meaningfully, which is in 

 fact the key to successful treatment of a planetary geophysical problem. They 

 suffer three further operational limitations, namely, first the specification of 

 boundaries, since most geophysical systems are open ; secondly, that radiation 

 energy sources and sinks are at best known on a long-term (monthly or seasonal) 

 basis (daily fluctuations therein are beyond conjecture) ; and, thirdly, the 

 effects of the turbulent (size, meters or less), convective (meters to several 

 kilometers) and even meso-scale (kilometers to about 100 kilometers) motions 

 are difficult to assess directly with existing observational networks and coverage. 



(3) Quantitative descriptive and statistical studies, directed toward finding 

 "What is there?" and "What happens when?" in terms of numerical values, 

 such as where is the Gulf Stream, how warm is its core, and how do these 

 measurables fluctuate? Or what are the rainfall patterns in Hawaii, are they 

 composed of a uniform monthly and daily precipitation regime, or is the 

 average figure made up of a cruel see-saw between occasional flood and usual 

 starvation? Do the statistical moments of the distribution suggest primarily 

 orographic rainfall, or are storms required to enable the mountains to build up 

 rain-clouds? As these questions are phrased, it is implied that such studies are 

 generally most valuable if they either suggest or test a physical hypothesis. 



(4) Purely descriptive treatments, in which one attempts to say what 

 happens in words, generally drawing on physical laws, results of fragmentary 

 and purely visual observations, and analogies with other known phenomena. 



