Wave Measurement - C.O.Collins III

Wave Measurements

Wave measurements are key to understanding wave physics. In situ measurements are of utmost importance in providing "ground truth" to remote sensing methods and enhancing understanding of wave dynamics. Our group builds and deploys two unique, experimental buoys which host a variety of instruments for measuring near surface (air-sea) met-ocean parameters. Each buoy especially suited for particular applications.

Extreme Air-Sea Interaction (EASI) Buoy

Platform Overview - Wave Measurements
EASI is a customized NOMAD type hull buoy. These are engineered as very robust platforms, thought to be able to withstand the most extreme wind and wave conditions on the open ocean. The goal is to make measurements in environments where very little (or none at all) in situ measurements have been made. This would include environmental conditions with winds over 30 m/s and significant wave heights of over 10 m.

For surface wave measurements, EASI operates as single-point-triplet surface follower. On EASI, all 6 degrees of freedom are recorded with a motion package (local buoy reference frame) along with compass heading. The heave is tilt corrected (Anctil et al., 1994) and double integrated to produce sea-surface elevation. The compass heading is used to transform pitch and roll to an Earth-fixed coordinate frame and integrated to produce North-South and EAST-WEST sea-surface slopes. The surface elevation is cross-correlated with slope to calculate first 2 directional moments (Longuet-Higgins, Cartwright, & Smith, 1963).

Air-Sea Interaction Spar (ASIS) Buoy

Platform Overview - Wave Measurements
As spar platforms, ASIS buoys heave with waves of period >~8s and remains relatively stable for waves of period <~8s. ASIS senses the local water-surface elevation with an array of capacitance wave staffs. The outer array is a pentagon with sides of about 1 meter and the inner array is a triangle with sides of about 9 cm. The local sea surface elevation at each wave wire are transformed to Earth-referenced sea surface elevations by adjusting for the motions of buoy (Anctil, Donelan, & Drennan, 1994). The motion package includes 3-axis accelerometers, 3-axis gyroscopes, and a fluxgate compass. More information can be found in the following references: (Anctil et al., 1994; Drennan et al., 1998; Drennan et al., 2003; Graber et al., 2000; Pettersson et al., 2003). One may use an individual wave wire (or average several) to calculate 1-D spectral parameters. Array processing techniques, specifically the maximum likelihood method (MLM), may be used to construct a directional spectrum, from which 2-D spectral parameters are determined. This special design produces a very high resolution directional spectrum which is a unique in situ measurement on the open ocean.

Sensor Comparison

A fundamental part of understanding wave measurement platforms and hence wave dynamics is the comparison of wave sensors. I have done extensive wave sensor comparisons. 3 comparison studies are in my M.S. Thesis. I am currently writing up a comparison of 5 platforms (11 sensors) during the ITOP experiment. My philosophy is that there are two important aspects of sensor comparison. On one hand we want to know how well one sensor compares to another, and that the sensor reports basic parameters within given accepted ranges. This is immediately applicable in establishing confidence in measurements and reporting integral parameters from experiments. But by concentrating on relative differences of integral parameters we have only scratched the surface and perhaps put aside the more difficult issue: what is the "real" wave spectrum? More over, is traditional Fourier analysis (the longtime geophysical workhorse) the best perspective on of ocean waves considering the basic assumptions of stationary and linear conditions are rarely met. Confounding the problem is the lack of a reference sensor (i.e. some absolute "ground truth" over the entire wind wave spectrum) and the lack of good comparison methods. Considering this, I am trying to develop standard spectral comparison tools and apply novel analysis techniques so that we may learn more about to "real" ocean wave spectra.

References

Anctil, F., Donelan, M. A., & Drennan, W. M. (1994). Eddy-correlation measurements of air-sea fluxes from a discus buoy Journal of Atmospheric and Oceanic Science.

Collins, C. O., III. (2012). In situ wave measurements: Sensor comparison and data analysis. University of Miami). Open Access Theses, (Paper 372) (Open Access)

Drennan, W. M., Graber, H. C., Donelan, M. A., & Terray, E. A. (1998). Directional wave measurements from the ASIS (air-sea interaction spar) buoy OCEANS'98

Drennan, W. M., Kahma, K. K., Graber, H. C., Pettersson, H., Donelan, M. A., Icard, D., et al. (2003). ASIS-directional waverider comparison. In D. Hauser, K. K. Kahma, H. E. Krogstad, S. Lehner, J. Monbaliu & L. R. Wyatt (Eds.), MEASURING AND ANALYSING THE DIRECTIONAL
SPECTRUM OF OCEAN WAVES ()

Drennan, W. M., & Williams, N. J. (2008). An air sea interaction buoy for high winds No. NSF-OCE-0526442). National Science Foundation.

Graber, H. C., Terray, E. A., Donelan, M. A., Drennan, W. M., van Leer, J. C., & Peters, D. B. (2000). ASIS—A new Air–Sea interaction spar buoy: Design and performance at sea. Journal of Atmospheric and Oceanic Technology, 17(5), 708-720.

Pettersson, H., Graber, H. C., Hauser, D., Quentin, C., Kahma, K. K., Drennan, W. M., et al. (2003). Directional wave measurements from three wave sensors during the FETCH experiment. Journal of Geophysical Research, 108(C3), 8061.