SECT. 2] 



LARfJE-SCALE INTERACTIONS 



223 



id) 



Fig. 56. Schematic illustrations of major features of "easterly wave" type of tropical 

 disturbance. (After Malkus, 1958a, Figs. 2, 3, 4 and 8.) 



(a) Siu'face streamlines of weak-to-moderate amplitude wave, which typically 

 moves toward west at speed of 10-12 knots or slightly slower than prevailing trade. 

 Its direction of motion is denoted by heavy arrow. Streamlines are drawn everywhere 

 parallel to the wind direction, so that ahead (west) of the wave trough, denoted by 

 the heavy solid line, winds blow from slightly north of east and to its rear (east) they 

 blow from slightly south of east. The wave trough line thus marks the wind shift. The 

 barbs on the wind arrows denote speed, each small one representing 5 knots. The area 

 of cloudiness and rain (indicated by triangular shower symbols) is commonly found to 

 the rear of the trough. The subtropical high pressure cell is the bean -shaped region 

 with "H" at its center. 



(b) 15,000-ft streamline pattern in typical moderate easterly wave. Note that the 

 wave amplitude is greater than at the surface. 



(c) Vertical cross -section going from west (right) to east (left) through wave shown 

 in (a) and (b). Cloud forms are shown schematically (not to scale). Winds (horizontal) 

 are denoted by barbed lines, each short barb representing 5 knots. Winds blow/?-o??i 

 the direction pointed by the tail, with north straight up, east to the right, south 

 straight down and west to the left, so that winds ahead of the wave are east or north 

 of east and winds to its rear are east or south of east. The wave trough is the heavy 

 solid line, while the top of the moist layer (trade-wind inversion) is denoted by the 

 light solid line. The triangles indicate showers and rain. 



(d) Schematic picture of a very deep easterly wave, showing streamlines and the 

 major cloud bands. Note the closed central vortex with wind turned all the way to 

 the southwest at its southern rim. Other svibsidiary cloud bands (not shown) are 

 numerous throughout the rain area, which is indicated by hatching. The central 

 surface pressure in such a disturbance may become as low as 1000 mb, with winds 

 up to 35-40 knots. About 10-20% of easterly waves, or 1-2 per month, reach this 

 intensity in the wet (summer) season. 



(Palmer, 1951), each with a trailing convergence zone filled with lines of hot 

 towers. In the Caribbean, these frequently trigger easterly waves to their north 

 (Fig. 57). Easterly waves disturb the wind field most at 15-20,000-ft elevation 

 and weaken toward the surface and high troposphere, while equatorial vortices 

 are frequently confined to the lowest 10-15,000 ft. Favorable upper-level 



