Data

Your access to the IMOS Ocean Radar data discovery and exploration is through the Australian Ocean Data Network (AODN) Portal.

The data streams collected from the direction finding (SeaSonde) and phased array (WERA) radars are used to create surface current maps, significant wave height measurements, directional wave spectrum and derived parameters, and wind direction. All of the stations provide data in real time that is available through the Ocean Portal. Quality controlled delayed mode data is also available for the Coffs Harbour, Capricorn Bunker Group, South Australia Gulfs, and Rottnest Shelf stations.

Surface currents

The basic current data provided by either type of radar is a radial current – i.e. the component of the ocean current along the direction from the radar site to the measurement position. This is obtained from the frequency shift of the first order peaks in the Doppler spectrum with respect from its position as predicted by linear wave theory. Files with this information are generated at each radar sites and sent to the Ocean Radar Facility together with some additional quality control and assessment data. These are then packaged into NETCDF files and sent on to the IMOS archive for access through the data portal.

In the case of SeaSonde data radial currents from the pair of sites are combined at the facility using CODAR software providing ~1-hour averaged vector current maps. These too are transferred as NETCDF files to IMOS. Basic quality-control procedures are carried out at this stage to ensure an acceptable level of data quality.

A similar procedure for the WERA data is carried out by eMII using their own matlab code. A number of quality control measures are recommended in the literature [1, 2, 3] and on manufacturer’s websites, when carrying out this procedure.

When the full data files have been collected from the WERA radar sites they are reprocessed using Ocean Radar Facility -developed software - WERA quality control.

  1. Lipa, B. J., and D. E. Barrick, Least-square methods for the extraction of surface currents from CODAR crossed-loop data:  Application at ARSLOE, IEEE J. Oceanic Eng., 0E-8(4), 226-253, 1983.
  2. Gurgel K-W. and G. Antonischki, Measurement of surface current fields by HF radar, IGARSS’97 conference proceedings, 1820-1822, 1997
  3. Chapman, R.D.,  L. K. Shay, H . C. Graber, B . Edson, A. Karachintsev, C. L. Trump, and D. B. Ross: On the accuracy of HF radar surface current measurements: Intercomparisons with ship-based sensors.Journal of Geophysical Research,  102, 18,737-18,748, 1997.

Below are two examples of surface current maps from each of the radar genres. 

Surface current map for the Turquoise Coast site, as derived from two SeaSonde (direction finding) radar stations (GHED, Green Head; LANC: Lancelin), overlain to sea-surface temperature maps. SeaSonde radars share the transmit frequency through a GPS time synchronization tool that allows simultaneous operation of two or more systems. This sample is a daily (24-hours) average of hourly current maps for 25 February 2013, created from hourly vector current maps.
Surface current map of the Rottnest Shelf site, derived from two WERA (phased array) radar stations at Fremantle and Guilderton, Western Australia. Fremantle and Guilderton WERA stations operate on the same frequency (9.330 MHz with 33 KHz bandwidth), with each station alternating 5 min on and 5 min off. This particular map represents the daily-average current field for 05 May 2013, and was derived from hourly-averaged, QC radar radial maps.

Waves

Ocean radar has two software packages for extracting wave information from the WERA radar signal.

  • The basic package provided by the manufacturer, Helzel GmbH, is based on a semi-empirical method described in [1]. This provides single radar estimates of the wave frequency spectrum from which significant waveheight and dominant period are extracted. These single radar estimates are calculated at the sites and sent to the Ocean Radar facility where they are packaged into the same NETCDF files as the currents. The option to combine these files to try to reduce the ambiguities that are present in single radar data has not yet been implemented.
  • The Seaview Sensing wave measurement package, which is offered as an option by Helzel GmbH, is based on methods described in [2, 3]. The full directional spectrum and derived parameters, e.g. significant waveheight, peak period and direction, can be obtained provided the signal to noise is sufficient.  Single radar estimates can extend the coverage for waveheight and period albeit with some ambiguities.

New data stream

The Ocean Radar Facility is now producing NetCDF files with wave data for each WERA site. These are hourly averages and are available when there is sufficient signal-to-noise and limited interference. They are not available in near real-time because of bandwidth limitations. We are working our way through the archived radar data. These files are available from http://thredds.aodn.org.au/thredds/catalog/IMOS/ACORN/catalog.html

The data are organized into the following directories.

gridded_1h-avg-wave-station-map_QC contains files with the single station wave parameters

These data are obtained using the method described in Wyatt, An evaluation of wave parameters measured using a single HF radar system, Canadian Journal of Remote Sensing, 28, 205-218, 2002. This file contains wave parameters that have been extracted using empirical methods and data from one station only. The figure below shows significant waveheight from the GUI station in WA.

gridded_1h-avg-wave-site-map_QC contains files with the dual station (site) wave parameters

These are obtained using the method described in Wyatt, A relaxation method for integral inversion applied to HF radar measurement of the ocean wave directional spectrum, International Journal of Remote Sensing, 9, 1087-1095, 1990; Green and Wyatt, Row-action inversion of the Barrick-Weber equations, Journal of Atmospheric and Oceanic Technology, 23, 501-510, 2006 . This file contains wave parameters that have been extracted from the measured directional wave spectra which are available in the gridded_1h_avg-wave-spectra_QC directory. Directions are all towards which the wave is propagating.  The figure below shows significant waveheight and peak direction from the SAG radars in SA.

gridded_1h-avg-wind-map_QC contains files with wind direction and short wave direction and spread data for each site every hour. These are obtained using the method described in Wyatt et al, 1996, Maximum Likelihood estimation of the directional distribution of 0.53Hz ocean waves, Journal of Atmospheric and Oceanic Technology, 14, 591-603, 1996. Directions are all towards which the wave is propagating or the wind is blowing. The figure below shows wind direction and short wave spread from the SAG radars in SA.

This directory contains the measured directional wave spectra, and the frequency spectra and direction spectra obtained by integrating that spectrum with respect to direction and frequency respectively. These are obtained using the method described in Wyatt, A relaxation method for integral inversion applied to HF radar measurement of the ocean wave directional spectrum, International Journal of Remote Sensing, 9, 1087-1095, 1990; Green and Wyatt, Row-action inversion of the Barrick-Weber equations, Journal of Atmospheric and Oceanic Technology, 23, 501-510, 2006. Directions are all towards which the wave is propagating. Wave parameters derived from these spectra are available in the gridded_1h-avg-wave-site-map_QC directory.

The figure below shows (top figure) the frequency spectrum (blue) and mean direction at each frequency (green dashed) at 9 different locations for the SAG radars in SA, and (bottom figure) the directional spectra at these locations.

The Ocean Radar Facility has not yet validated wave measurement capability from SeaSonde radars. With this system measurements can be made over an annular ring of fixed range (or small number of ranges) close to the radar sites where sufficient signal to noise is available. The CODAR software normally provides a few wave parameters at this position although methods to obtain the full directional spectrum have been developed [4, 5].

The important quality control criterion for wave measurement is the 2nd order signal to noise level.  Other quality measures which will need to be taken into account when these data are made available through the portal are discussed in [6].

 

References

  1. Gurgel, K-W, H-H Essen and T. Schlick, 2006: An empirical method to derive ocean waves from second-order Bragg scattering: prospects and limitations. IEEE Journal of Oceanic Engineering, 31, 804-811.
  2. Green, J.J. and L.R. Wyatt, 2006: Row-action inversion of the Barrick-Weber equations. Journal of Atmospheric and Oceanic Technology, 23, 501-510.
  3. Wyatt, L. R., 2002: An evaluation of wave parameters measured using a single HF radar system.Canadian Journal of Remote Sensing, 28, 205-218.
  4. Lipa, B. and B. Nyden, 2005: Directional wave information from the SeaSonde. IEEE Journal of Oceanic Engineering, 30, 221-231.
  5. Lipa, B, Nyden, B., D. Barrick and J. Kohut, 2008: HF radar sea-echo from shallow water. Sensors, 8 4611-4635.
  6. Wyatt L.R., J.J. Green and A. Middleditch, 2011: HF radar data quality requirements for wave measurement. Coastal Engineering, 58, 327-336.

More Papers on wave measurements Waves

Winds

Wind direction is a parameter that has been measured with HF radars over many years. It is obtained by taking the ratio of the amplitude two first order Bragg peaks (the same signal that is used for current measurement except there we use the frequency of the peaks) and applying a directional wave model. It is the short wave direction that is actually being estimated but, in most circumstances, these waves are expected to be aligned with the wind and hence the direction can be interpreted as wind direction. For the WERA data this measurement can be made with the Seaview package [1,2, 3] which is currently being implemented. In the meantime the amplitude of the two peaks is included in the single radar NETCDF files and the ratio of these can be used to provide qualitative information about the changing wind directions.

Accurate wind speed estimation is still a research area for HF radar. The Seaview package when implemented will provide an estimate but it is not expected to be very accurate particularly in low wind/swell cases.

References

[1] Wyatt L.R., L.J. Ledgard, C.W.  Anderson, 1997: Maximum likelihood estimation of the directional distribution of 0.53Hz ocean waves. Journal of Atmospheric and Oceanic Technology, 14, 591-603.

[2] Wyatt L.R., 2012: Short wave direction and spreading measured with HF radar. To appear in Journal of Atmospheric and Oceanic Technology.

[3] Heron M.L., 1987: Directional spreading of short wavelength fetch-limited wind waves. Journal of Physical Oceanography, 17,281-285.

More papers on wind measurements > Winds