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ACT Protocols for Wave Measurement Systems
July 2012
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mooring and communication systems. In these circumstances it will be important to evaluate the
full frequency-height response because there could be disparate effects on short and long waves.
These “extreme tests” will inform the NWS Weather Forecasting Office (WFO) as to the validity
of hindcast data used in understanding forecast model extreme events.
6.0 Verification Design
Initial generic protocols were further refined through direct discussions during an ACT Wave
Test and Evaluation Protocol Workshop held on 22-24 February, 2011 (described above). The
protocols will follow a format that includes field tests to evaluate performance under a variety of
environmental conditions. Laboratory tests will not be a component of this evaluation. Due to
the large scope of the evaluation, it has been broken down to 5 Tasks and presented in order of
feasibility and importance. Each of these components is described in detail in the sections that
follow.
7.0 Test Verification Factors
Based on the extensive discussions held in the workshop, the test protocols will be primarily
focused on the wave sensors performance in successfully recovering the “First-5” parameters.
Performance will be determined by comparing each sensor’s output with the reference method
over a predetermined range of wave frequencies. The First-5 refers to the total wave energy
(integrated over all frequencies), and the four directional parameters that define the low-order
directional moments of underlying directional distribution of wave energy.
Here, ACT will adopt an approach developed by the CDIP/SIO that evaluates the differences
between the first four directional parameters at specified frequency and energy bins (Figure 1).
This practical approach to evaluating two sets of wave observations deviates from the traditional
spectral representation of wave frequency-directional spectra as energy densities and instead
interprets the spectrum as quasi-2D spectral wave components. It is motivated by the realization
that directional wave instruments measure the frequency distribution of wave energy with greater
resolution than the directional distribution. By defining the specific bins of frequency and energy
to be compared, the ability of an instrument to measure the 4 directional parameters of each
component relative to a second benchmark instrument can be directly evaluated.
ACT’s purpose for adopting this methodology is to provide comprehensive information on the
sensors and their relative differences with the standard. We define the observed wave
components only by frequency and energy, and then look at the differences in the four
directional distribution parameters. The goal is to deploy sensors in appropriate locations and for
sufficient duration to fill out the frequency-energy matrix. This will provide an understanding of
how each sensor behaves for a variety of frequency/energy combinations.
The frequency dependence of the directional errors is particularly important because large
differences in particular frequency ranges may be obscured by integrating over all frequencies.
An example of this is shown in Figure 1 where differences between the Datawell GPS and MKII
wave measurement systems become much larger as frequencies increase above 0.35 Hz. While
this may not be important for an experiment in a swell dominated regime, it will be significant in
locations where the observed waves are fetch or duration limited. This methodology is consistent