9k 



In the trade-wind stream, the cloud development ( Figure 21) is 

 restricted by the trade-wind inversion, so that only about 25 percent of 

 the latent heat gained is released by local precipitation. The remainder 

 is accumulated in a deepening moist layer and is shipped toward the equator 

 by the trades ( Figure 22). The rain release occurs at fairly low- levels, 

 and together with Qg, provides a net heat source and 'pressure head" which 

 sustains the lower trades . A preliminary mathematical model of this system 

 has been constructed (Malkus, 1956). The upper trades live on more sporadic 

 imports from the equatorial trough zone. 



Figure 23 summarizes the energetics of the region and the role of 

 exchange therein. Sensible heat plus potential energy ( h = GpT + Agz ) 

 is budgeted on the left smd latent heat L^ (q is specific humidity ) on 

 the right. Three layers are treated separately; in ascending order. These 

 are roughly consistent with the mixed layer, cloud layer and above-inversion 

 layer (although 500 mb is generally above cloud tops). The important point 

 is that the sum of Qe + Qs (l-^T units) is easily enough to balance the total 

 radiation loss (1.32 units) but cannot do so, since 1.4l units of latent are 

 exported. Half of Qg balances the radiation loss below cloud, and the other 

 half, together with all the precipitation, does not quite balance the radia- 

 tion loss of the middle layer. The heat import at high levels makes up the 

 difference . The upward - directed dashed arrows show the important "heat 

 pump" function of the cumuli . 



Convective clouds play an even more crucial role in the operation and 

 energetics of the equatorial zone; they provide the release mechanism for 

 most of the latent heat energy acquired from the sea over the entire tropics . 

 Here giant cumulonimbus towers abovmd (Figure 2k) but intermittently, bunched 

 into the wavelike and vortical perturbations, with horizontal diversions 

 ranging from about 200 to 2000 km. 



The heat budget for the equatorial zone is dissected in Figure 25. 

 Here we are examining total heat content ( Q = CpT + Agz + Lq ) The input 

 from the ocean is computed as TO percent that of the trade-wind belt. Qe 

 was obtained from the transfer formula and Qs was found as residual in an 

 energy budget which may have exaggerated its magnitude, but not by a two 

 factor. The main feature is the high heat export aloft. In their detailed 

 study, Riehl and Malkus (1958) showed that the penetrative cloud towers were 

 necessary to get this heat energy upward, balancing radiation losses and 

 providing the crucial export. 



In trade zone and trough together, the ocean provides a total Qs + Qe 

 of 3 -18 units. Of this, 73 percent is eventually lost locally by radiation 

 from the tropical atmosphere, leaving only 27 percent of the heat energy to 

 ship poleward across the subtropical ridge . Only a few percent of this 

 heat is ever converted into motions. As well as an inefficient heat engine, 

 we see that the atmosphere also operates a very leaky fuel pump I 



