SECT. 2] 



LARGE-SCALE INTERACTIONS 



147 



gradually wins the battle as the air flows equatorward. As illustrated schemati- 

 cally in Fig. 24, the moist layer is itself vertically subdivided. A well-stirred 

 turbulent subcloud layer about 600 m deep is topped by the cloud layer, where 

 the picturesque trade cumuli grow day and night, in bunches or in streets 

 elongated parallel to the wind. 



The low-level trades are the world's steadiest surface wind system ; they 

 couple a horizontally homogeneous air mass by vertical mixing to a uniform 

 sea over 31% of the globe or 62 million square miles. Near the ground, variations 

 in temperature and humidity are slight and synoptic disturbances, for the most 

 part, weakly developed. Here, if anywhere, the daily flow patterns resemble 

 the climatic mean, so that steady-state models may be attempted. Only the 

 concentration of rainfall into two or three days per month suggests that at 



EAST WIND 



DRY, STABLE, SINKING AIR ^"^^'V "'(""^ 



INVERSION f 7 



I * 



jJhts^ 



EVAPORATION 7 TURBULENT EDOIES ~l 5m ph 



SEA SURFACE 



Fig. 24. Schematic vertical cross-section along the path of the trade winds. (After Malkus, 

 1958, Fig. 1.) Typical wind speeds at various levels are indicated by arrows at the 

 right. The moist layer deepens by about 1000 ft in 500 miles horizontal distance ; clouds 

 are thus drawn much larger than to actual scale. NE stands for northeast and SW for 

 south-west, denoting the trajectory orientation of the Northern Hemisphere trades. 



upper levels the regularity in tropical flows gives way to restlessness and 

 fluctuations. Interestingly enough, the wind steadiness takes its sharp drop 

 just above cloud tops, or through the inversion layer. Aloft the meridional cell 

 shows marked longitudinal and time variations in its return, poleward-moving 

 branch. 



The Meteor Expedition of 1924-26 was an epoch-making event in atmos- 

 pheric as well as oceanographic science since it opened tropical meteorology 

 and first revealed the role of the trades as water-vapor accumulators (Ficker, 

 1936, 1936a). Since then, several ship and aircraft expeditions of the Woods 

 Hole Oceanographic Institution (Wyman et al., 1946; Bunker et al., 1949; 

 Malkus, 1958) have explored the turbulent eddy structure near the air-sea 

 boundary, tropical clouds and their role in energy transports, which will be 

 incorporated further on. However, for a large-scale energy-budget study 

 using theequations and methods of Sections 3 and 4 (pages 100-144), the data 

 requirements exceed the scope of a single expedition and, except in rare cases, 



