under neutral stability conditions; however, a certain amount of mixing 

 must occur at the interface to prevent formation of a density gradient in 

 the lowest portion of the mixed layer. The stability index of 2 F used 

 in this prediction method is the nearest interval to neutral stability 

 conditions (At = ). The mixing force necessary to insure decay resist- 

 ance at the region of interface may occur at this low At interval. 



In order to clarify this idea, suppose a 30-knot wind and a fully 

 developed sea exist in the prediction area. Stability index at the time 

 of prediction is 7 F; furthermore, the sea state parameter 77 , deter- 

 mined from Table 5> is 266. The predicted mean thermocline depth from 

 the normal curve in Figure 17 is 165 feet. If there is a 25-knot wind and 

 a fully developed sea on the following day, significant wave height (HWd) 

 will be at least 13»7 feet, T rnax will be 12.2 seconds, and 77 will equal 

 1^0. The mixing force is smaller than it was on the previous day; conse- 

 quently, the thickness of the mixed layer will not be increased unless 

 convergence is produced by the 30-knot wind field. Figure 15 (At = 2° F) 

 shows that mixing could effectively penetrate the mixed layer to 200 feet 

 at this 77 value. Sufficient mixing takes place in the lowest part of the 

 mixed layer to prevent decay (formation of density gradient), and the 

 thermocline depth will remain 165 feet, as long as the latter surface con- 

 ditions persist. Suppose the wind is 20 knots 2 days later. The sea 

 state parameter 77 for fully developed sea is then 65, and the effective 

 mixing penetrates to only 135 feet (Figure 15). This means that the mixed- 

 layer thickness should decrease from l65 r feet to 135 feet, if the same 

 surface conditions persist for several days. 



Mixed-layer decay has been predicted quite successfully during tests 

 of this method; however, data are insufficient to recommend its use as 

 standard procedure. The method may be correct, but numerous other factors 

 may be involved. Decay depends not only on residual mixing but also on 

 flow in the thermocline, on convergence or divergence, and on other factors 

 which are not usually known. 



Persistence 



This prediction method does not require knowledge of previous tempera- 

 ture distribution and mixed-layer thickness. If a prediction is made every 

 day over a long period of time in the same area and proper verifications 

 can be made with BT observations, "persistence" (of the previously observed 

 thermocline depth) can be taken into account. Predictions can then be 

 adjusted, especially if there is a tendency towards convergence or diver- 

 gence which is not clearly apparent from weather charts. Persistence is 

 valid as long as the same surface conditions exist. Since changes of ther- 

 mocline depth usually occur slowly, the persistence method of forecasting 

 is successful a large percentage of the time. However, if mixing energy 

 overcomes stability at the interface or if favorable conditions occur for 

 decay of the mixed layer, persistence disappears. A prediction can then 

 be based only on the changing factors. 



Although a prediction method based on mixing and stability factors is 

 always preferable, persistence should be consulted for adjustment of the 

 prediction if there is sufficient reason to believe that such an adjust- 

 ment is necessary or useful. 



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