Global Wave Forecasts Using Spacecraft Data 



plus a wave propagation procedure that has new and desirable features for large 

 oceans. It will include procedures for specifying the wind field over the oceans, 

 and the turbulent structure of the winds. The wave spectra will be computed at 

 each grid point by three steps for each 2-hour time step. The three steps are 

 called: growth, dissipation, and propagation. The procedures under develop- 

 ment for specifying the wind field, and for the three steps of the wave spectra 

 computation will now be described. 



Wind Fields 



Presently, the wind field is described by a direction and speed at each grid 

 point. The speed is that which would supposedly be observed at an elevation of 

 19.5 meters over the sea surface under conditions of neutral stability, under the 

 assumption that the drag coefficient given by Sheppard (22) as a function of the 

 wind at 10 meters is correct. 



The surface pressure field is used to obtain a first estimate of the wind 

 field by means of a regression equation given by Thomasell and Welsh (3). The 

 weather ships are used next to correct the winds at the points on the grid closest 

 to the ships. The anemometer heights of the weather ships are used to compute 

 the wind at 19.5 meters under the above assumptions. 



All other ships with anemometers are then used. If a grid point is cor- 

 rected, subsequent ship reports are not allowed to change the wind at that grid 

 point. 



Finally, the transient merchant ships are used as if the reported wind had 

 been observed at 19.5 meters above the sea surface. These winds were also 

 first treated as if they had been observed at 10 meters above the sea surface 

 and increased according to the logarithmic wind profile to 19.5 meters above 

 the sea surface. This led to waves that were much too high. 



The wind fields are obtained on a JNWP grid (23) and interpolated to the 

 wave forecasting grid. 



In the system under development, the number of points on the JNWP grid 

 will be quadrupled by halving the distance between grid points. This will pro- 

 vide better resolution in areas of strong winds and strong wind shear. Also 

 more ships will be used in correcting the wind field from the regression analy- 

 sis. Recent Russian papers (and unpublished work by P. S. DeLeonibus) suggest 

 that air-sea temperature differences and wave height can be used to modify the 

 wind profile over the waves in a way that will influence the curvature and slope 

 of the profile so as to permit a modification of the growth rate for different 

 spectral components. 



Growth 



The growth portion of the computer program is based on the concept that if 

 the wind blows uniformly in direction and speed over a large enough area for a 



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