PART VI — PRECIPITATION AND REGIONAL WEATHER PHENOMENA 



leases usually cite only the damage 

 they cause. 



Role of the Tropics in the General 

 Circulation — The role of the tropics 

 in hemispheric or global circulation 

 is known to be important. The long- 

 term (five days or more) prediction 

 models now being developed require 

 a tropical data input that is not vet 

 adequate. In particular, studies are 

 needed of (a) how constant the 

 tropics are as a source of energy and 

 momentum, and (b) the appropriate 

 way to include in the models the 

 energy infusions into the atmosphere 

 from more or less point sources ■ — 

 i.e., from small-area cumulonimbus 

 cloud systems. In addition, ways 

 must be found to represent surface 

 interface processes, not only evapo- 

 ration and sensible heat transport, 

 but also momentum exchange, espe- 

 cially on the equator itself. 



Hemispheric Interchange — Any 

 interconnection between the extra- 

 tropical regions of both hemispheres 

 must take place via the tropics. From 

 the spread of various tracers or aero- 

 sols, over a scale of weeks or months, 

 we know that an exchange of air does 

 take place. An understanding of 

 these exchanges is particularly rele- 

 vant to problems of air pollution. 

 Actual pollution problems in the 

 tropics are not likely to become 

 severe because of the unstable strati- 

 fication, although tropical countries 

 are, of course, exposed to sedimenta- 

 tion or washout of pollutants by pre- 

 cipitation. In the longer view, how- 

 ever, the ability of the air to transport 

 pollutants across the equator requires 

 serious study of the air exchange — 

 its mobility, the magnitude of the 

 exchange, preferred paths, and the 

 like, with a view to eventual control 

 of such transports. 



Needed scientific activity under 

 each of these major categories is dis- 

 cussed in the sections that follow. 



Tropical Water Supply 



Some parts of the tropics are sub- 

 ject to recurrent, severe droughts. 



Those of northeast Brazil, which have 

 led to large out-migrations of popu- 

 lation and great economic and politi- 

 cal instability in all of Brazil, are an 

 example. These droughts, superim- 

 posed on an average rainfall that is 

 itself marginal for a tropical economy, 

 have lasted from two to as long as 

 nine consecutive years. The longer 

 droughts have occurred in the more 

 recent part of the climatological rec- 

 ords, suggesting a secular drying out 

 — a most unfavorable circumstance. 



Many scientific questions remain 

 before such tropical droughts can be 

 understood, much less controlled. 

 The droughts (as well as the inter- 

 mittent heavy rainfall years) must be 

 related somehow to the anomalies of 

 the northern- and southern-hemi- 

 sphere general circulations and pos- 

 sibly to some oceanic temperature 

 distributions that do not follow di- 

 rectly from anomalous surface trade- 

 wind speeds and directions, as well 

 as to associated surface divergences 

 or convergences. But the controls 

 that govern these relationships are 

 completely unknown at present. Lack 

 of data over the tropics, the southern 

 oceans, and even the North Atlantic 

 at lower latitudes has prevented any 

 definitive study. Little adequate use 

 has been made of such information as 

 is available — surface temperature, 

 pressure, and precipitation anomalies 

 over wide areas, as well as recent 

 findings on wind anomalies over the 

 equatorial Atlantic. 



Data Base — New data are accumu- 

 lating very fast for all parts of the 

 tropics, eliminating the old excuse 

 that lack of observations prevents 

 progress. Data have been accumu- 

 lating from the rapidly growing num- 

 ber of commercial flights over tropi- 

 cal areas. Programmed new satellite 

 data are adding even more rapidly to 

 the pile. An energetic attack on the 

 discovery of the controls of equatorial 

 dry zones and variable rainy seasons 

 should be possible in the 1970's as a 

 result of these new data. Once the 

 controls are known, it will be possible 

 to see whether prediction of the con- 



trol functions can be achieved with 

 synoptic-statistical modeling tech- 

 niques, although direct deterministic 

 prediction does not appear in the 

 picture for the foreseeable future. 



Cloud Modification — The ques- 

 tion of cloud-modification potential 

 in the tropics remains unresolved. 

 Nonprecipitating cumulus congestus 

 may be a preferred cloud form over 

 many semi-arid tropical areas. But 

 past efforts to study the possibilities 

 of modifying such clouds have been 

 rather sporadic. Early interest in Aus- 

 tralia has lagged. A few serious cu- 

 mulonimbus studies have been made 

 in the Caribbean, but these relate 

 to the atmosphere over open sea; 

 since surface heat sources are much 

 stronger over land, these oceanic ex- 

 periments cannot be applied directly 

 to the tropics, although they may be 

 useful indirectly if they are successful 

 in making cumulonimbus grow. 



Quite apart from modification ex- 

 periments, it would be of value 

 merely to learn the cloud composition 

 at different locations in order to 

 assess what might be termed the 

 "stimulation potential." Even in this 

 respect, knowledge has remained de- 

 ficient. There exists on this subject a 

 great need not only for scientists but 

 also for adequate instrumentation 

 (notably radar) and good technicians. 

 Good radar technicians actually avail- 

 able for meteorology are rare, and in 

 tropical countries they tend to be 

 either nonexistent or insufficiently 

 skilled. The World Meteorological 

 Organization has a large technician- 

 training program, which merits sup- 

 port. 



Tropical Storms 



Tropical storms are notoriously 

 variable in frequency from year to 

 year and region to region. (See Figure 

 VI-12) Sometimes the connection with 

 the general circulation is obvious, but 

 not always. The role of hurricanes in 

 the general circulation is not yet fully 

 determined, and general-circulation 

 research, with a focus on general cir- 



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