A FAST RESPONSE CUP ANEMOMETER FOR MEASUREMENT 

 OF TURBULENT WIND OVER THE OCEAN 



R. G. STEVENS and L. F. SHODIN 



Woods Hole Oceanographic Institution 



Woods Hole, Massachusetts 



ABSTRACT 



A fast response cup anemometer suitable for 

 measuring turbulent winds over the ocean is 

 described. The electrical characteristics of 

 this instrument are suitable for use at remote 

 observing stations where low power drain and 

 telemetry are desirable. Dynamic response charac- 

 teristics of the cup anemometer are discussed 

 together with an ingenious method for dynamic 

 calibration. 



INTRODUCTION 



It should be remarked at the outset that mea- 

 surement of wind is a proper concern of the 

 oceanographer since the wind serves to transmit 

 considerable mechanical energy into the ocean and 

 strongly influences other processes such as evapo- 

 ration and mixing. In fact, wind driven phenomena 

 such as large current systems and wind generated 

 waves are certainly among the most spectacular 

 features of the ocean. 



Before discussing the details of the cup ane- 

 mometer it is worthwhile to examine some aspects 

 of the measurement of dynamic variables in ocean- 

 ography, particularly since it is becoming 

 increasingly fashionable to deal with dynamic 

 measurements by use of digital computers . It 

 should be apparent that meaningful measurement 

 of a variable implies a rather long term observa- 

 tion of its time history. If the time history 

 is interpreted in terms of its corresponding fre- 

 quency spectrum it is found that oceanographic 

 variables rarely contain frequencies above 10 cps . 

 A more usual upper frequency limit would be 2 cps 

 for phenomena near the surface, while the prac- 

 tical low frequency limits for many phenomena may 

 be measured in cycles per day or month. 



Obviously, low frequency data of this sort may 

 be processed conveniently and economically on a 

 general purpose digital computer as contrasted 

 with, say, acoustic data which may be analyzed 

 with so-called analog devices. On the other hand, 

 new technological capabilities have so vastly 

 expanded the capabilities for acquiring large 

 quantities of oceanographic data that the digital 

 computer becomes an essential element in the 

 interpretation, cataloging and analysis of data. 



Now, once the commitment is made to use a digital 

 computer in data analysis the computer becomes, 

 essentially, a part of the "instrument" itself. 

 This fact can be important in the design and 

 evaluation of the transducer and data acquisition 

 system since it is frequently possible to incor- 

 porate instrument corrections into the computer 

 program. Thus it is possible to apply corrections 

 for nonlinearity, temperature compensation, etc., 

 to the data in the digital computer. This pro- 

 cedure may serve to vastly simplify the trans- 

 ducer design or (because extremely complex correc- 

 tions may be made using a digital computer) it 

 may be possible to use transducers that would 

 otherwise be unsuitable. 



Cup anemometers have been a standard device 

 for measurement of wind speed for many years . In 

 recent years, however, their use has been extended 

 to the measurement of turbulent wind in micro- 

 meteorological research. While the cup anemometer 

 is a rugged and reliable device, serious ques- 

 tions have been raised regarding its suitability 

 for accurate dynamic measurements in turbulent 

 wind regimes . In particular, the linearity of 

 response to increasing and decreasing wind speed 

 has been questioned. 



The anemometer described here was designed for 

 use in conjunction with a study of wind generated 

 ocean waves. Since it was intended to subject 

 the wind speed data to power spectrum analysis, 

 the dynamic characteristics, particularly in 

 regard to linearity of response, had to be known. 

 This is particularly important since any instru- 

 ment nonlinearity must be removed before calcu- 

 lating the covariance function and the power 

 spectrum. However, as will be shown, the actual 

 performance of the cup anemometer did not exhibit 

 the anticipated difficulties. Nevertheless, it 

 seems worthwhile to point out the necessity for 

 careful evaluation of transducers intended for 

 dynamic measurements and the possible means of 

 compensating for undesirable transducer charac- 

 teristics . 



DESIGN SPECIFICATIONS 



The following criteria and specifications were 

 used in the design and construction of the 

 anemometer: 



Woods Hole Oceanographic Institution Contribution No. 1330- 



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