174 



[OHAP. 4 



c. Vertical energy transport 



Fig. 36 shows the mean height distribution of Q = CpT + Agz + Lq in the 

 equatorial trough zone (dashed) and at 20° latitude away from it (solid). Taken 

 together with Fig. 35 it presents an apparently serious paradox precluding 

 maintenance of energy balance in the trough zone ! The decrease of Q in the low 

 levels in the tropics has been well known for many years. Of primary interest 

 is the increase above 400 mb ( ~ 25,000 ft) which, as brought out by Fig. 36, 

 does not exist merely in the trades but in the heart of the equatorial zone itself. 

 No problem was presented thereby for the upper trade-wind heat budget, since 

 advective and compressional warming were adequate to balance all losses, and no 

 significant upward flux by convection was required above the level of minimum 



78 80 82 84 86 



= CpT+ Agz + Lq (cal/g) 



88 



Fig. 36. Vertical profiles ofQ = CpT + Agz + Lq, total heat energy. (After Riehl and Malkus, 

 1958, Fig. 15.) 



Solid profile is for 20° latitude from trough. Dashed profile is for trough zone itself. 

 Abscissa in cal/g; left ordinate in lOO's of mb, right ordinate in lOOO's of ft. Solid 

 horizontal line denotes mean position of tropopause. 



Q. In the equatorial zone, on the other hand, huge quantities of heat energy are 

 exported aloft. Mass continuity demands ascent, but a uniform, gradually 

 rising circulation would produce cooling above the level of minimum Q. Radia- 

 tion losses, entered also in Fig. 35, further aggravate the difficulty. A heat 

 balance calculation, including both the mean lateral heat divergence and 

 radiation, shows that without some mechanism other than a gradual mass 

 circulation the upper equatorial troposphere would cool at a rate of about 

 2°Cperday! 



Nor is it possible to call upon diffusion to save the situation above the level 

 at which the vertical Q gradient reverses, even if convection in entraining 

 cumulus clouds is included. All trade-wind studies show that this mechanism 



