HURRICANES 



The release of energy in a devel- 

 oping tropical storm involves a num- 

 ber of links in a chain of actions, 

 each of which must unfold in a timely 

 and effective manner if the storm is 

 to develop. First, the environment 

 must be structured to support the 

 spin that tries to develop locally in 

 the wind circulation when pressure 

 first begins to fall. Second, the en- 

 vironmental winds must be able to 

 distribute systematically the heat re- 

 leased by the large cumulus clouds 

 that spring up near the area of max- 

 imum spin. It is the systematic dis- 

 tribution of this heat, not its release 

 per se, which generates fresh kinetic 

 energy for intensification of the storm 

 system. 



As the tropical storm intensifies 

 further and approaches hurricane 

 force, the system depends uniquely 

 on a continuous flow of heat energy 

 from the sea to the air. These proc- 

 esses involve a subtle interaction be- 

 tween the scales of motion charac- 

 teristic of temperate-latitude storms 

 and those characteristic of cumulus 

 clouds only a few miles in diameter. 

 This interaction is difficult to model, 

 as is the flow of heat energy from the 

 sea to the air. The primary purpose 

 of project BOMEX (Barbados Ocean- 

 ographic and Meteorological Experi- 

 ment) conducted from May through 

 July of 1969, was to gain better 

 understanding of the exchange proc- 

 esses across the ocean/atmosphere 

 interface. 



The modeling problem, especially 

 in connection with the tracking of 

 undeveloped disturbances, is further 

 complicated by the fact that in the 

 tropics there is essentially a two- 

 layer atmosphere, with disturbances 

 in the lower layer sometimes travel- 

 ing in a direction opposite to those 

 in the upper layer. 



Because of all these complications, 

 no model yet exists that can predict 

 in real-time the moment and develop- 

 ment of hurricane seedlings. A num- 



ber of diagnostic models have been 

 produced which seem to simulate, in 

 a research environment, many of the 

 physical processes that occur during 

 this development and that charac- 

 terize the behavior of the full-grown 

 hurricane. However, forecasting pro- 

 cedures for tropical disturbances and 

 storm systems still depend primarily 

 on the identification, description, 

 tracking, and extrapolation of the 

 observed movement of the system. 



Present-Day Techniques — Fortu- 

 nately, the digital computer provides 

 the forecaster with rapid data- 

 processing which enables him to as- 

 sess the immediate behavior of storm 

 systems and how this may reflect 

 on the future movement and devel- 

 opment potential. Because of the in- 

 creasing use of machines for data- 

 processing, it is now possible to make 

 more extensive use of analogues to 

 compare the present storm system 

 with similar systems from historical 

 records and thereby compute the 



probable movement and intensifica- 

 tion to be expected. 



Figure V-3 is an example of one 

 such method developed during 1969 

 at the National Hurricane Center. 

 In this case, the computer is required 

 to search historical records for all 

 storms that were similarly located 

 and whose characteristics were com- 

 parable to the storm system for which 

 a forecast must be made. From the 

 historical record, a most-probable 

 track for periods up to 72 hours is 

 determined and a family of prob- 

 ability ellipses is computed show- 

 ing expected deviations from the 

 most-probable track (50% and 25% 

 probability areas). This family of 

 ellipses is used to identify the area 

 of the coastline to be alerted initially 

 to the threat of a hurricane. 



Other more sophisticated tools, us- 

 ing statistical screening techniques, 

 are also used by the forecaster to 

 guide his judgment in predicting hur- 

 ricane movement. 



105W 



Figure V-3 — PROBABILITY FORECASTS FOR HURRICANES 

 100W 95W 90W 85W 80W 75W 



70W 



35 N 



30N 



25 N 



20N 



In this relatively crude warning technique, a computer searches historical data to 

 find a hurricane situation with similar characteristics to the one under observation. 

 It then prognosticates future positions for 12, 24, 36, 48, and 72 hours, as shown 

 in the figure, based on the history of the earlier hurricane. The size of the proba- 

 bility ellipses indicates the magnitude of error that is involved in the use of this 

 technique. 



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