AEROGRAPHER'S MATE 3 & 2 



the United States. As a cold front moves off 

 the coast and over the Gulf Stream, it becomes 

 intensified, causing wave development to occur 

 near the Cape Hatteras area. This gives the 

 east coast of the United States much cloudiness 

 and precipitation. A similar situation occurs 

 off the east coast of Japan. That area in the 

 Pacific generates more cyclones than any other 

 area in the world. 



OTHER EFFECTS. — The movement of a 

 frontal system from one area to another often 

 has a great modifying effect, causing the front 

 to be regenerated in some instances and to be 

 dissipated in others. Transition affects waves 

 and cyclones as well as fronts. 



When dissipating, extratropical cyclones 

 enter regions of frontogenesis and cyclogenesis, 

 they are frequently regenerated into active 

 disturbances. This is usually caused by an 

 influx of warm moist air to the east and cold 

 air to the west of the center. In a situation in 

 which a well-defined cyclone, associated with 

 a front (or fronts), moves eastward over the 

 Rocky Mountains, the frontal system is usually 

 weakened by the time it descends the eastern 

 slopes. If there is an influx of warmer moist 

 air from the Gulf of Mexico, the frontal system 

 is regenerated as it moves eastward. If the 

 circulation to the east of the mountain range 

 is such that no moist air is drawn into the 

 cyclone or frontal system, frontolysis (the 

 process of a front weakening or dissolving) 

 takes place. 



Frontal systems moving from water to land 

 areas tend to weaken if an influx of moist air 

 is not brought into the situation. On the other 

 hand, a frontal system moving from land areas 

 to water areas is generally regenerated by 

 the influx of moist air. For example, a 

 frontal system may become quasi-stationary 

 in the vicinity of the east coast of the United 

 States. This frontal system is usually oriented 

 in a northeast- southwest direction and occurs 

 mostly during the summer and autumn months, 

 when outbreaks of cP air move southeastward 

 over the States. These fronts usually lose their 

 intensity over the Southern United States and 

 movement ceases. Frequently, stable waves 

 develop and travel along this frontal system, 

 causing unfavorable weather conditions. When 

 these waves move out to sea and warmer moist 

 air is brought into them, they become unstable 

 waves and are intensified as they move across 

 the ocean. 



TROPICAL SYSTEMS 



Phenomena to be discussed in this section 

 of the chapter will be the tropical systems 

 that have no mid-latitude characteristics, such 

 as the tropical wave, intertropical convergence 

 zone, and tropical cyclones. For a more 

 detailed discussion of these systems and other 

 tropical phenomena, the AG should refer to 

 AWS Technical Report 240. 



TROPICAL WAVES 



A tropical wave, which in past years has 

 been referred to as an "Easterly Wave", is 

 defined as a trough or a cyclonic curvature 

 maximum located in the easterly trade winds. 



Synoptic models for the tropics are not as 

 clear-cut as for mid-latitudes because data 

 on which to base them have been very sparse 

 until the advent of meteorological satellites 

 and high-level jet aircraft. Three models for 

 tropical waves in the Atlantic are given in 

 succeeding paragraphs. 



The oldest of the models describes a wave- 

 like pattern in the easterly current of the lower 

 troposphere which moves westward with an 

 average speed of 10 to 15 knots. These waves 

 reach a maximum intensity between 700 to 

 500 mb and have an eastward slope with height. 

 In the model case, the wave moves slower than 

 the basic lower level currents and faster than 

 the basic upper level currents. This results 

 in subsidence west of the wave trough and 

 convergence as well as disturbed weather east 

 of the wave trough. Figure 14-22 shows the 

 basic tropical wave model in the vertical and 

 horizontal. 



In another model, tropical waves move to 

 the west faster than the basic current. This 

 type causes convergence west of its axis and 

 is stronger at the axis with a varied cloud 

 pattern of cumuloform and cumulonimbus west 

 of the wave axis, with a dominant nimbostratus 

 cloud layer east of the axis. 



Another tropical wave model is the "Inverted- 

 V" formation. Its name is associated with the 

 herringbone arrangement of the cloud bands 

 associated with it. (See fig. 14-23.) This type 

 is best defined in the eastern and central North 

 Atlantic where it shows good day-to-day 

 continuity and moves about the speed of the 

 low-level trades which is 16 knots. 



336 



