Wave and current measurements obtained using the

WERA and Pisces HF radar systems and Seaview

Sensing software are presented here to show that HF

radar can provide current and wave measurements with

useful accuracy. Pisces has been operational for over a

year in the Celtic Sea, providing data every hour.

HF Radar for Real-time Current,Wave and Wind Monitoring

By Lucy R Wyatt, Department ofApplied Mathematics, University ofSheffield, UK and Seaview SensingLtd, UK

Two recent articles in Hydro INTER-NATIONAL (June and December 2004)refer to oceanographic measurementcapabilities of HF radar systems. Itoand Kinoshita discussed the JapanCoast Guard system for ocean cur-rent observations. Grønlie providedan overview of ground-based wavemeasuring radar. I would like to com-ment on both of these.

Current Measurement

In the Japanese article, comparisonswere presented of radar currentswith those from an acoustic dopplercurrent profiler at 10m depth.Thecomparisons were limited and notvery convincing. Part of the reasongiven was that the measurements dif-fer both in terms of temporal andspatial averaging and depth.These are,of course, all reasons for difference.But I was surprised at the extent ofthis and wonder whether this is thereason for the comment that ‘thetechnology of surface-current ob-

servations with HF radar is not yetestablished.’ Although there may re-main a few uncertainties in the in-terpretation of HF radar currentmeasurement, there is plenty of evi-dence to suggest that the technolo-gy is very firmly established and isused by both scientists and the op-erational oceanography industry.It may be that there were just someteething problems with this particu-lar deployment. Figure 1 shows com-parisons of one year of radial currentsmeasured using Seaview Sensing soft-ware with the Pisces HF radar (avail-able from Neptune Radar Ltd) fromtwo sites and compared with the UKMet Office depth-averaged storm-surge model (no in-situ current mea-surements were available). Mean (andrms differences) are < 1cm/s in bothcases (10 and 38 cm/s).The region istidally dominated but with occasion-al wind influences that tend to bemuch stronger in radar-measured sur-face currents, leading to some differ-ences in radial currents between

model and radar.This data has alsobeen analysed for tidal constituentsby the Proudman Oceanographic Lab-oratory, with good agreement.

Wave Measurement

Grønlie claimed that ‘…the processesof backscatter (at HF) are too compli-cated (and not well enough understood)to allow the attainment of useful mea-surement accuracy.’ I disagree.There isa wealth of evidence that the theorydeveloped by Barrick in the ‘70s pro-vides a good description of thebackscattered signal over the fullrange of HF radar frequencies in lowsea states, and for low HF frequenciesin high sea states. My group at theUniversity of Sheffield have spentmany years developing methods thatuse this theory and have shown goodagreement with wave buoys using anumber of different HF radar systemsin a number of different locations.Twoof these systems, contrary to the re-mark made in the Grønlie article, areavailable commercially.WERA (devel-oped at the University of Hamburg)is available from Helzel MesstechnikGmbH in Germany and Pisces (men-tioned above) in the UK, both ofwhich suppliers are working withSeaview Sensing Ltd, licensor of thesoftware we have developed. Figure 2shows WERA wave-height and meandirection measured at the NorwegianCoast, compared with a buoy.Thereis more scatter in the radar mea-surement but the correlation is good.For the whole experiment, mean dif-ference (standard deviation) in wave-height was 19(44)cm, and in direction2º (35º).

Figure 1: Radial current components at the same location from two different directions

measured with Pisces (x-axis) compared with model currents.

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Long-range Data

Most recently we have been workingwith Pisces, which was specifically de-signed for long-range metocean mea-surements.This has been in operationon the south-west coast of the UnitedKingdom since December 2003, pro-viding wave, current and wind data every hour.This is a trial for theUK Department for Environment Foodand Rural Affairs (DEFRA), projectmanaged by the UK Met Office, whoare considering including HF radar sys-tems within the UK wave-monitoringnetwork,WAVENET. Figure 3 showswaveheight and peak direction mea-sured during a recent storm.The fur-thest range on the map is about 110kmfrom each radar site. Unfortunately,

there was nobuoy data at thistime, but earlier in the trial a mean dif-ference (standard deviation) in wave-height of 11(57)cm and in direction -9º(44º) were found.Wave buoys do notmeasure the full 2D directional spec-trum and we do not have sufficient de-tail in the buoy measurements to inferthis using, for instance, maximum en-tropy methods. Figure 4 show a direc-tional spectrum measured by the radarat 60km, with the buoy peak directionand directional spreading superimposedshowing good agreement.

Conclusion

I hope that I have demonstrated thatuseful oceanographic measurements

can be obtained using HF radar sys-tems. All the above measurementswere made using phased array radarsystems.Wave spectral measurementsin particular are more difficult to makewith compact antenna radar such asthe CODAR used in the Japanese sys-tem, although, as Grønlie says, these dohave the advantage of compactness andthus ease of deployment. However, al-though siting is more difficult, we haveobtained metocean measurementsfrom the phased-array systems Pisces,WERA and OSCR at several locationsaround European and US coasts. Someexpertise is needed to ensure the qual-ity of the data. Radio interference is aproblem, particularly at the low HF fre-quencies required for long-range ap-plications. Radar data quality monitor-ing and assessment systems to assistoperational users are needed; these areavailable and further developments areplanned.

Biography

Lucy Wyatt is a professor in the De-partment of Applied Mathematics atthe University of Sheffield. She hasBSc and MSc degrees in Mathematicsand a PhD in Physical Oceanography.Her work on HF radar began as a Re-search Fellow in the Department ofElectronic and Electrical Engineeringat the University of Birmingham in the1980s. She is technical director atSeaview Sensing Ltd.ı

E-mail: l.wyatt@sheffield.ac.uk

Figure 2: Waveheight and mean direction measurements at the Norwegian Coast in 2000.

WERA measurements (red) compared with a directional waverider (black).

Figure 3: Pisces storm waveheight and peak wave direction measurements in the

Celtic Sea.

Figure 4: Directional frequency spectrum measured by Pisces. Axes

show frequency in Hz and this spectrum shows wave propagation

to the east. Red square marks the peak and levels are shown at

0.1-0.9 of the peak. Arrow shows the buoy peak direction, and

buoy peak directional spread is indicated by black arc. These

intersect at the buoy peak frequency.

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