SECT. 2] LARGE-SCALE INTERACTIONS 235 



the surface pressure dro]) is produced by warming above 500 mb, and 50% by 

 warming above 300 mb. Thus the main function of the hot towers is to grow tall 

 and to pump large amounts of augmented heat-content air to great heights, 

 filling the upper regions of the core. The intervening clear spaces and evapora- 

 tional cooling which reduce middle-level temperatures several degrees below 

 the adiabatic value change the surface pressure by only a few millibars. The 

 structure, distribution and dynamics of the cumulonimbus towers have been 

 discussed at length in the hurricane literature following 1958 (see especially 

 Malkus, 1960; Malkus, Ronne and Chaffee, 1961). We shall be concerned here 

 mostly with the air-sea interaction phase and its dynamic implications. 



(ii ) Evidence for the extra oceanic heat source 



Actually, the existence of the extra heat source in hurricanes was pointed 

 out by Byers in 1944 and its dynamic importance was foreseen by Riehl in his 

 remarkable textbook on Tropical Meteorology (1954). Many published records, 

 from the early days of Depperman (1946), have proved that the surface air 

 temperature outside the eye is constant or decreases only very slightly toward 

 the center. If adiabatic expansion occurred during pressure reduction, on the 

 other hand, the temperature of surface air spiralling toward a hurricane center 

 should decrease markedly. For instance, air entering the circulation with the 

 mean tropical sub-cloud properties should reach the 960-mb isobar with a 

 temperature of 20.5°C and a specific humidity of 16.5 g/kg (dotted extension of 

 curve E, Fig. 68). Because of condensation, a dense fog should prevail at the 

 ground inward of the 985-mb isobar. But this is never observed. It follows that 

 the potential temperature of the surface air increases along the inward trajec- 

 tories. We also know that the specific humidity increases and that the cloud 

 bases remain between a few hundred and a thousand feet. 



The surface air thus acquires both latent and sensible heat during its travel 

 toward loiv pressure. Fig. 69 shows the variation of potential temperature, 6, 

 and specific humidity, q, during isothermal expansion at 26.0°C deduced from 

 observations in hurricane Daisy, 1958, and typical of the moderate-strength 

 hurricane. We may find the heat-energy increments in the core, or as the air 

 moves from 1000 to 960 mb, from the graph as follows : 



hQ = CpBd + LSq (51) 



= 0.24 X 2.7 + 600 x 3.2 x 10-3 

 = 0.65+1.9 ^ 2.6cal/g. 



We see that this is just the difference between the surface air in curves E and D 

 in Fig. 68. If the latter (29 = 960 mb, T = 26.0°C, ^^ = 21.8 g/kg) is lifted dry 

 adiabatically to the condensation level and from thence moist adiabatically, 

 we get a curve identical to D in the high levels and slightly warmer than D up 

 to 400 mb. We note from Fig. 69 this further remarkable fact : reading horizon- 

 tally from a given surface pressure {ps) ordinate, the potential temperature and 



