apparently steer the tropical cyclone. The 700 mb. wmd flow was most 

 valuable when the tropical cyclone began to become extratropical. Typ- 

 hoons Gwen and Jean were typical. 



It was shown that no one level can be selected for steering. Some 

 tropical cyclones have a closed circulation extending above 40,000 ft., 

 while others were definitely only 10,000 ft. and were easily steered by 

 the 700 mb. winds. Each particular tropical cyclone must be analysed 

 each day to find the proper level for steering. 



A useful aid in forecasting the movement of typhoons is the asym- 

 metrical distribution of wind velocities around the typhoon. The rule is 

 this : if the wind is symmetrical, the typhoon is nearly stationary ; if the 

 wind \'elocity varies in the different quadrants, the typhoon will move 

 parallel to the strongest wind, and the speed of movement will be pro- 

 portional to the difference between the wind speed in the strong quadrant 

 and that of the weak quadrant. A simple explanation of this rule is as 

 follows : in the strong quadrant the general steering current, or move- 

 ment \^ector, is added to the cyclonic current of the typhoon ; in the 

 w^eak quadrant the general current is subtracted from the cyclonic current. 



This rule can be applied to some of the typhoons of 1947, although it 

 was not generally used by the forecaster. Many reports note the vari- 

 ation in the distribution of wind speed around the typhoon. Clouds and 

 weather are also often distributed asymmetricallv. Transport aircraft as 

 well as regular reconnaissance aircraft remarked on this feature frequently. 



This principle seems to be another manifestation of the steering 

 principle ; the steering current intensifies the side parallel to it, weakens 

 the side opposed to it. However, there is an important difference in the 

 application of the principle, and therefore some of the difficulties encount- 

 ered in employing the steering effect of the free air current are avoided. 

 No search need be made for the proper steering level and no attempt 

 need be made to evaluate its force. The manifestation of the free air 

 current is observed directh' in its effect on the symmetry of the typhoon. 

 Steering by the wind differential method is considered one of the best 

 methods available. It has a theoretically sound basis, the determining 

 factors can be observed, often without the aid of instruments, and its 

 verification from actual use is relatively high. 



The use of the surface synoptic map in steering was found quite useful, 

 since the most data was available for these maps. In so far as the upper 

 air situation can be estimated from the surface map, the use of the surface 

 map is successful. (This may lead to a vicious circle, as in the case of 

 Rosalind.) A point which must be kept in mind here is that the surface 

 synoptic pattern is a function of the typhoon. The energy of the typ- 

 hoon must be integrated with the forces controlling the synoptic picture 

 to arrive at a forecast. 



The present rules that typhoons dissipate rather rapidly after moving 

 north of 30° N. latitude held up well in 1947. All of those which moved 

 north of 30° N. in this year, only Donna, Gwen, Kathleen, and Rosalind 

 had any significant life north of 30° N., and these weakened rapidly upon 

 reaching and passing that latitude. An examination of climatological 

 records for Japan also verify that typhoons almost never retain their 

 severity, experienced at lower latitude stations such as Iwo Jima and 

 Okinawa, upon reaching Honshu. This finding is somewhat different 

 from on the eastern coast of the U.S., where hurricanes sometimes remain 

 intense well above 30° N. latitude. A possible explanation of this differ- 

 ence is contained in the curvature of coast-line of south-west Honshu. It 



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