76 



In Figure 6, low- level wind patterns begin to delineate the intriguing 

 paradox: equatorward - blowing trades carry the water vapor fuel unconverted 

 into the convergent trough zone (solid line) - it is largely not processed 

 locally in the input zone and from there exported poleward, but goes on 

 this roundabout circuit because of the air's movements and differing cloud- 

 building abilities . 



Trade-wind clouds are normally stunted cumuli, as depicted in Figure 'J, 

 while in the equatorial regions, high cumulonimbus towers (Figure 8) often 

 flourish. The latter are excellent latent heat converters and not the former 

 which usually evaporate without dropping precipitation back into the ocean, 

 which is the prerequisite that the heat stay in the air. 



We see that the cloud and precipitation processes control the role of 

 sea-air exchange products in the atmosphere - so again we return to air 

 structure and motions and this time ask how these interact with clouds . In 

 1957 we made an air'jome photogrammetic study in the Pacific to begin an 

 attack on this question; the complete results have just been published 

 (Malkus and Riehl, 196^4-) . From the carefully time-lapsed movies, cloud maps 

 were constructed and compared with sounding and synoptic data. Figures 9 and 

 10 show two typical trade-wind cases, the first with weak winds and the 

 second with normally vigorous flow. In both, the ocean is presumed but not 

 known to be warmer than the low- level air. To assess the role of sea-air 

 interaction, comparison with Avsec's (1939) extension of Benard's classical 

 convection cell studies was made. Figure 11 shows one of Avsec's laboratory 

 experiments, with uniform heating from below and weaJs. translation (weak 

 shear between convecting fluid and lower boundary) . Figure 9 suggests a 

 similar admixture of polygonal cells with cells becoming elongated into rolls. 

 In Figure 10, the stronger flow has presumably caused the rolls to predominate; 

 our Pacific study confirmed the conditions, related to shear, for their 

 orientation . 



From satellite pictures, Krueger and Fritz (1961) have identified 

 polygonal cells in some way apparently similar to those studied in the 

 laboratory. Figure 12 is from one of the few cases where they had enough 

 supplementary data to relate these patterns partially to sea-air interaction. 

 The sea-air temperature excess was large ( -v 3°c ) and a strong trade-wind 

 inversion confined the convection - its horizontal scale increased toward the 

 southeast as the moist layer deepended. Three puzzling features were found, 

 however, prohibiting the naive extrapolation of laboratory results. 



Firstly, the winds were strong (15-20 knots) in the region (Figure I3) 

 \*Ere,the polygonal pattern was photographed - why were not the polygons 

 elongated into rolls? Secondly, the horizontal cell size runs about 30 times 

 the depth of the convective layer, exceeding the laboratory ratio by an order 

 of magnitude. Thirdly, Figure 12 (upper) suggests each cell wall contains 

 several cloud elements, or more than one scale of motion. 



