SECT. 2] LAR(;E-SCALE INTERACTIONS 177 



ever-changing active regions and carried out therein by such evanescent 

 elements as tropical clouds must be pondered soberly by those sanguine souls 

 who aspire to control their vagaries. An order of magnitude calculation brings 

 out the concentration of the equatorial firebox function. 



The undilute upward mass flux through the 500-mb surface, required in 

 Fig. 35, is about 18 x lO^^ g/sec. If the protected towers ascend with the con- 

 servative speed of 5 m/sec (Malkus, 1960), they occupy an area of roughly 

 4 X 1014 cm2, or 0.1% of the equatorial belt 10° latitude in width! An intriguing 

 descending hierarchy of fractional area occupied by the various scales of 

 phenomena is thus suggested: 



Area of equatorial zone = A = 4x 10^'^ cm^ 



Area occupied by synoptic disturbances = 10"!^ = 4 x lO^^ cm^ 



Area occupied by active rain= 10~^A = 4 x lO^^ cm^ 



Area occupied by undilute towers = 10~^A — 4x 1014 cm^ 



Photographic cloud-mapping surveys (Riehl et al., 1959) undertaken to test 

 these figures have revealed a striking organization of the penetrative giant 

 clouds. Over the tropical oceans, conditions for producing precipitating 

 cumulonimbus are met only in the convergent zones of disturbances. Here 

 the inversion lid and entrainment brakes are released. Large elements grow 

 and shoot explosively to the tropopause, lined up in "spiral arms" winding 

 about the vortex center. From the area tabulation above, we see that about 30 

 synoptic disturbances (wave length ~ 1350 km) are needed at a given time in 

 the equatorial belt. With diameters of 3-5 km, energy balance requires 50-150 

 rising towers per disturbance. The most striking result of this work is, in fact, 

 that summing for the whole 10° belt only about 1500-5000 active giant clouds 

 are needed to convert the water-vapor fuel imported from the trades, to main- 

 tain the warm core and heat budget of the trough and to provide for its high- 

 level poleward export. Each cloud tower pumps about 0.5-1.0 x 10^2 cal/sec 

 through the mid-troposphere and in its half-hour lifetime releases a net amount 

 of condensation heat energy greater than that of a hydrogen bomb (~2.4x 

 1014 cal). 



In addition to the vast energy they combust, these cloud cylinders differ 

 further from their better behaved man-made counterparts ; they depend for 

 their very lives upon large-scale instabilities in the machinery, upon flow 

 perturbations which wind up the easterlies into amplifying wave and vortex 

 in preferred but variable localities as the trades flow into equatorial latitudes. 

 The instability of low-latitude air currents to synoptic-scale disturbances lies 

 today at the dimly-explored frontier of tropical meteorology. Unfortunately, 

 however, for the peaceful pursuit of oceanography, this scale of atmospheric 

 perturbation flourishes in all marine regions except the lower trades, bringing 

 its time-dependent complexity to the main driving force of sea motions, namely 

 the momentum input from the atmosphere. The next section is concerned with 

 this most intimate linkage between the two planetary fluids. 



7— s. I 



