Significant waveheights at Petten (FLI, 7.9MB)
This animation shows the wave behaviour near the Petten site. The measurement time runs from 01/11/96 at 00:00 (MET) to 13/11/96 at 23:40. The significant waveheights are colour-coded, and the arrows denote short-wave (wind) directions.
Directional spectra for Petten (FLI, 8.7MB)
The upper two panels of this animation show the directional distributions for the radar and wavebuoy. The lower panel compares frequency spectra and waveheight for the radar and buoy. Measurement time is from 25/11/96 at 00:00 to 03/12/96 at 23:40.
Short wave directions at Maasmond (FLI, 3.6MB)
The two radars that comprise the WERA system were located too far apart to be sure of sufficient data for the evaluation of WERA for wave measurement. The experiment was optimised to investigate the performance for currents and also to look at differences between beam-forming and direction-finding for current measurement. As a result there is only a small amount of wave spectral data from this deployment and it is of rather poor quality.
We have therefore concentrated on looking at the short wave direction and spread measurements that are obtainable using the same part of the signal that is used for current measurement ie the first order Bragg peaks. The method used is described in Wyatt, Ledgard and Anderson, Journal of Atmospheric and Oceanic Engineering, vol. 14, 1997. At the frequency of operation of the WERA radar short-wave direction is expected to be roughly aligned with the wind.
The animation shows these directions as arrows superimposed on the directional spreading which is colour coded. It will be seen that the region of coverage is rather variable. This is due to signal-to-noise variability and could also be due to spatial and temporal variabilities in surface current within a radar observation cell or to beam sidelobes both of which distort the first order signal. It will also be seen that there is often considerable spatial variability in both parameters.
During this experiment wind direction measurements were available at a location a little to the west of most of these measurements and there were two directional waveriders within the region. These provide an opportunity to verify the observed spatial variability of these short wave directional parameters.
The two time series below show the wind direction (from the same instrument on both plots), and the radar measurements at radar observation cell 024 corresponding to buoy measurements at the Euro15 site and those at 022, the MVN site close to the Rhine Outflow.
The buoy measurements are of the two parameters at a frequency of 0.5Hz, the largest available and approximately the same as the frequency of the radar measurements. What can be seen here are large temporal variations on, what appears to be, a tidal cycle in the buoy direction measurements with some evidence of this in the radar measurements at the MVN site. The radar data at site 024 is in good agreement with the wind data and there is rather more of it than at site 022.
The buoy measurements at the two sites show significant differences particularly during the 21–24 March providing some evidence that the spatial variability seen in the radar animation is real. It is interesting to note that the spreading is on average a little larger than was seen either at Holderness or Petten except in high sea-states.
These comparisons will be the subject of further work.
Directional spectra for Holderness (FLI, 1.1MB)
The upper two panels of this animation show the directional distributions for the radar and wavebuoy for the Holderness experiment. The lower panel compares frequency spectra and waveheight for the radar and buoy. Measurement time is from 20/12/95 at 18:00 to 23/12/95 at 17:00.