TM No. 377 



Henry Stommel in his book The Guif Stream (1958) states? "In spite of the 

 intensive effort that has been made to measure the stress of the wind on the 

 sea j there is so much scatter in the various determinations that the stress is 

 still not well known „ It is hard to imagine an object of study more important 

 to the physical oceanography of ocean currents „" 



Despite such statements 9 the understanding of the physical mechanism re- 

 sponsible for the generation 8 the wind stress and energy transfer, and the 

 decay of wind waves seems to be at an impasse, A disproportionate emphasis 

 has been placed upon empirical characterization of various spectral parameters, 

 which can offer little understanidng of the physical mechanisms within the 

 wave regime. Certainly a substantial effort should be made to directly measure 

 the wave motions and to provide at least a statistical description thereof. 

 The pursuit of this latter purpose may require certain bold, if not bizarre, 

 innovations and improvements in both instrumentation and techniques. 



Why have conventional current meters not been used to measure turbulent 

 fluctuations associated with wind-driven waves? Examination of this question 

 reveals a paradoxical situation. Host near-surface current measurements are 

 made to observe mean flow or 9 at most, much lower frequency oscillations than 

 those peculiar to the wind wave regime (e.g., tidal currents). In fact, es- 

 timates of flow fluctuations are usually avoided because this tends to compli- 

 cate the otherwise smooth and well-behaved data. For this reason most modern 

 current meters 8 such as the Savonius rotor (used in the Richardson type self- 

 recording system) s have built-in integrating systems that average the values 

 of current speed and direction over periods of 10 minutes to several hours. 



Moreover 9 most ocean current meters, when held on tethered moors 9 are 

 adversely affected by oscillatory motions. This gives rise to potential 

 biasing of low frequency or steady flow components. The Savonius rotor only 

 senses speed of flow. Hence s any oscillatory flow about the rotor tends to 

 bias its integrated output to give a magnified average value of flow, i,e,, 

 the rotor tends to half-wave rectify the oscillatory motions and add this to 

 the lower frequency components, A similar effect occurs with a Roberts or 

 Von Arx type meter. The oscillation cf the meter in its pivoted bridle, in 

 response to vertical oscillatory motions 8 causes spurious biasing of the 

 quasi-continuous record. 



As more is learned of the natural transfer processes in the ocean and the 

 atmosphere s statistics on fluctuations of the motions are seen to be cf equal 

 or greater importance than those relating to the mean (or statistically smooth) 

 motions. This is demonstrated both in the study of large scale transfer pro- 

 cesses in the atmosphere (Starr, 1956) and in the study of the energy balance 

 of the Gulf Stream (Webster, 1961), 



It seems evident that the success of any study of the gross transfer of 

 dynamic properties at the air-sea interface is dependent upon actual high 

 resolution measurements of the fluctuating motions of the two fluids. This 

 study of particle motions in ocean waves is therefore based upon in situ 

 measurements. 



