SECT. 2] LARGE-SCALE INTERACTIONS 221 



most or all of the troposphere. Observations suggest that this may result from 

 an accidental superposition or from a disturbance in either layer working its 

 way up or down. However, these attempts fail far more often than they succeed. 

 Unfortunately neither the necessary nor sufficient conditions for deep develop- 

 ment have been dehneated. 



Tropical disturbances may be defined as regions of convergent and divergent 

 flow (order of IQ-^ sec-i) with associated patterns of vertical motion on the 

 200-2000-km scale (area-averaged W'^ 1-50 cm/sec). They are recognizable on 

 the synoptic weather charts for a lifetime ranging from two days to four weeks ; 

 the identifying deformation in the streamline field may have the shape of a 

 wave, a closed cyclonic vortex, or merely a shear line where two parallel 

 currents of different speed are closely juxtaposed. The frequency of each type 

 varies with region, season and elevation, and the different forms may interact, 

 combine or superpose vertically in a multiplicity of permutations. 



Despite this complexity, the outstanding effect of all types, weatherwise and 

 physically, is that their presence means alterations in the thickness of the moist 

 layer and thereby in the conditions for cumulus convection. In regions of low-level 

 convergence, the inversion lid may be destroyed altogether and a deep moist 

 layer and hot towers build all the way to the tropopause, while where low-level 

 divergence prevails, the inversion is lowered and strengthened, and convection 

 is suppressed. 



In contrast to the middle -latitude cyclone, tropical disturbances do not 

 contain fronts, but form and live their lives entirely within tropical air (except 

 for the escaped hurricane). Thus they are not able to feed upon stored potential 

 energy in pre-existent horizontal temperature contrasts, but must either 

 create their own density gradients by differential latent heat release or draw 

 their kinetic energy and momentum from other scales of motion. The relative 

 importance of these processes is not well known, except in deep disturbances of 

 the wet season where the major energy source is demonstrably condensation 

 (Riehl and Gentry, 1958; Riehl, 1959; Malkus and Riehl, 1960). This is not to 

 imply that even here dynamic factors may not be vital triggers. 



In winter, when westerlies overlie most of the trades, disturbances most often 

 move eastward and have an extra-tropical origin. They consist of shearline 

 remnants of cold fronts, or downward penetrations of polar troughs or "cut-off 

 cold lows" from jet-stream undulations. The latter two types are usually best 

 developed aloft, with the ground-dweller often just seeing their effects in 

 cloudiness and precipitation (cf. hot towers described on pp. 219-220). 



In summer, when the easterlies are deep and more decoupled from mid- 

 latitude influences, tropical perturbations commonly travel toward the west, 

 the significant ones at speeds of 10-12 knots, or somewhat slower than the 

 trade current. The famous "easterly wave" discovered by Dunn (1940) and 

 described extensively in the literature of the 1940's and early 1950's by Riehl 

 (1954a) and his collaborators, is illustrated schematically in Fig. 56. An im- 

 portant source of low-level disturbances is the equatorial trough, which in its 

 downstream active portions often consists of a series of cyclonic vortices 



