PART VI — PRECIPITATION AND REGIONAL WEATHER PHENOMENA 



Figure VI— 1 1 — ARRAY FOR BARBADOS OCEANOGRAPHIC 

 AND METEOROLOGICAL EXPERIMENT (BOMEX) 



• Land Based Station O Current Stations 



Thermistor Array Moorings 



The deployment of instrument platforms for BOMEX is shown in the diagram. This 

 figure represents the consequence of designing a group of experiments of sufficient 

 scope and precision to test hypotheses and obtain useful new data from an 

 intermediate-scale system. The event is unique in human history. This experiment 

 was participated in by the Departments of Commerce, Defense, Interior, State, and 

 Transportation, the National Aeronautics and Space Administration, Atomic Energy 

 Commission, the National Science Foundation, National Center for Atmospheric 

 Research, and more than 10 universities. 



Basic Concepts 



The general character of the mon- 

 soons and their inter-regional varia- 

 tions reflect the juxtaposition of con- 

 tinents and oceans and the presence 

 or absence of upvvelling. However, 

 without the great mechanical and 

 thermal distortions produced by the 

 Himalayas and the Tibetan Plateau, 

 the vast northern-hemisphere deserts 

 would be less desert-like, central 

 China would be much drier and no 

 colder in winter than India, while 



even over the Coral Sea winter cloud 

 and rain would be uncommon. 



Within the monsoon area, annual 

 variations are seldom spatially or 

 temporally in phase. Even if these 

 variations were understood and their 

 phases successfully forecast, accurate 

 day-to-day weather prediction would 

 not necessarily be achieved, for the 

 climatological cycles merely deter- 

 mine necessary conditions for certain 

 weather regimes; synoptic changes 

 then control where and when the 



rain will fall, and how heavily, and 

 whether winds will be destructive. 



Synoptic-Scale Changes — Al- 

 though not new, a most important 

 concept is that of wide-ranging, 

 nearly simultaneous accelerations or 

 decelerations within a major vertical 

 circulation. Causes are elusive, al- 

 though the changes generally appear 

 to be triggered by prior changes in 

 the heat-sink regions of the vertical 

 circulation. This is a field of truly 

 enormous potential for numerical 

 modeling, on a time-scale between 

 synoptic and seasonal, in which fluc- 

 tuations in radiation and in air- 

 surface energy exchange might pro- 

 duce profound effects. 



The concept both explains previous 

 difficulty in maintaining continuity 

 of synoptic analysis and demands 

 that notions of day-to-day weather 

 changes be examined and probably 

 modified. Even during winter, fronts 

 seldom remain material boundaries 

 for long and air-mass analysis con- 

 fuses more often than not. 



That synoptic-scale disturbances 

 often appear to develop and to 

 weaken in response to changes in the 

 major vertical circulations might ex- 

 plain why many of the disturb- 

 ances are quasi-stationary. In turn, 

 synoptic-scale vertical motion deter- 

 mines the character of convection and 

 the efficiency with which energy is 

 transported upward from the heat 

 source. 



Synoptic-scale lifting, by spreading 

 moisture deeply through the tropo- 

 sphere, reduces the lapse rate and 

 increases the heat content in mid- 

 troposphere. Thus, though it dimin- 

 ishes the intensity of small-scale 

 convection and the frequency of thun- 

 derstorms, it increases rainfall and 

 upward heat transport. Conversely, 

 synoptic-scale sinking, by drying the 

 mid-troposphere, creates a heat mini- 

 mum there, hinders upward transport 

 of heat, and diminishes rainfall. How- 

 ever, the increased lapse rate favors 

 scattered, intense small-scale convec- 

 tion and thunderstorms. 



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