SECT. 2] LAKGE-SCALE INTERACTIONS 215 



from cloud base should have several degrees temperature excess and strong up- 

 ward acceleration throughout. Yet only one trade cumulus in ten normally 

 reaches the inversion base ; the rest peter out ignominiously far below. A 

 braking mechanism is clearly suggested. The identification and pursuit of this 

 inhibitory mechanism and its implications has been the major contribution of 

 the Woods Hole group, as is well documented in the literature. 



Cumulus clouds interact with their surroundings primarily by exchange of 

 air. Drawing in drier air from outside or "entraining", they dilute away the 

 buoyancy-producing moisture which is their life blood ; even the vigorous 

 cloud in Fig. 54 has only a fraction of the excess warmth and water content 

 that a non-interacting air parcel would show. The original epoch-making 

 paper by Stommel (1947a) postulated the existence of entrainment by devising 

 a method to calculate its magnitude from the cloud profiles of the Wyman- 

 Woodcock expedition. Using it, we find that clouds like the one in Fig. 54 

 commonly incorporate a mass flux comparable to the total within their up- 

 draft in a vertical ascent of only 1-2 km ! Temperature lapse rate thus loses its 

 unique significance in prescribing conditions for cloud growth and the ambient 

 moisture structure becomes critical. 



But the exchange is a two-way process ; the brake upon cloud growth also 

 serves as the mechanism by which the cumuli alter their environment. Since 

 the updrafts retain about the same size with height, an efflux of cloudy air 

 approximately equal to the entrainment may be inferred. In this way moisture 

 and momentum brought up from lower levels are imparted to the surroundings 

 as the clouds shed their protuberances and erode away. Thus the moist layer 

 accumulates its water vapor and easterly momentum is spread upward from 

 the trade-wind maximum (Malkus, 1949, 1958). 



While demonstrably an adequate mechanism for moisture and momentum 

 transport, there is real doubt whether the ordinary trade-cumulus population 

 can fulfill the final important function of the cloud layer, namely release of 

 precipitation heating. The evidence suggests (Garstang, 1958; Riehl, Gray, 

 Malkus and Ronne, 1959) that all significant amounts of tropical oceanic rain 

 fall in synoptic-scale disturbances, where convergent flow and towering 

 cumulonimbus are found. To be sure, giant "hot towers" are rare, but under 

 special conditions they do occur, even in the trade-wind zones. Comparison of 

 such tower heights measured from photographs with temperature soundings 

 (Malkus and Ronne, 1954) showed that these giants cannot be significantly 

 diluted. Here the entrainment brake is somehow released ; the photographic 

 study pointed to cloud diameter as the critical factor in penetration. Aided by 

 a series of ingenious laboratory experiments (Scorer and Ronne, 1956; Wood- 

 ward, 1959) workers in cumulus dynamics have begun to develop this relation 

 mechanistically. 



The active, buoyant regions within a cloud body are called "thermals". 

 These may take the form of one or a succession of discrete, rounded elements 

 with an internal vortical circulation which, under particularly favorable 

 conditions, appear to join or elongate into more or less continuous plumes with 



