OCS Study 

•

MMS 2001-056 

Summary of Findings for using High 

Frequency Radar in Physical 

Oceanographic and Ecological 

Studies 

Final Technical Summary 

Final Study Report 

U.S. Department of the Interior 

Minerals Management Service

Pacific OCS Region 

Summary of Findings for using High 

Frequency Radar in Physical 

Oceanographic and Ecological 

Studies 

Final Technical Summary 

Final Study Report 

Authors 

Principal Investigators 

Libe Washburn 

Steven Gaines 

Prepared under MMS Cooperative 

Agreement No. 14-35-0001-30758 

by 

Coastal Marine Institute 

Marine Science Institute 

University of California 

Santa Barbara, CA 93106 

U.S. Department of the Interior 

Minerals Management Service Camarillo

Pacific OCS Region March 2001
Disclaimer 

This report has been reviewed by the Pacific Outer Continental Shelf Region, Minerals Management 

Service, U.S. Department of the Interior and approved for publication. The opinions, findings, 

conclusions, or recommendations in this report are those of the authors, and do not necessarily reflect 

the views and policies of the Minerals Management Service. Mention of trade names or commercial 

products does not constitute an endorsement or recommendation for use. This report has not been 

edited for conformity with Minerals Management Service editorial standards. 

Availability of Report

Extra copies of the report may be obtained from: 

U.S. Dept. of the Interior 

Minerals Management Service 

Pacific OCS Region 

770 Paseo Camarillo 

Camarillo, CA 93010 

phone: 805-389-7621 

A PDF file of this report is available at: 

http://www.coastalresearchcenter.ucsb.edu/CMI/ 

Suggested Citation 

The suggested citation for this report is: 

Libe Washburn and Steven Gaines. Summary of findings for using high frequency radar in physical 

oceanographic and ecological studies. MMS OCS Study 2001-056. Coastal Research Center, Marine 

Science Institute, University of California, Santa Barbara, California. MMS Cooperative Agreement 

Number 14-35-0001-30758. 31 pages. 
Table of Contents 

FINAL TECHNICAL SUMMARY ................................................................................... 1 

 Background..................................................................................................................... 1

 Objectives ....................................................................................................................... 3 

Description and Results .................................................................................................. 3 

 Significant Conclusions.................................................................................................. 9 

 Study Products................................................................................................................ 10 

 References....................................................................................................................... 12 

List of Figures - Final Technical Summary 

Figure 1. Locations of HF radars ...................................................................................... 4 

Figure 2. Spatial pattern of surface currents...................................................................... 5 

Figure 3. A. Distribution of dynamic height, 1998 

B. Vertical sections of temperature and geostrophic velocity

C. Abundance of 5 fish taxa in the region........................................................ 6 

Figure 4. A. Distribution of dynamic height, 1999 

 B. Distributions of fish taxa, 1999 ................................................................... 8 

Figure 5. Spatial mean currents in the Santa Barbara Channel and Santa 

 Santa Maria Basin, 2000 ............................................................................. 9 

FINAL STUDY REPORT .................................................................................................. 13 

Abstract.......................................................................................................................... 13 

Introduction................................................................................................................... 14 

Methods.......................................................................................................................... 14 

Results ........................................................................................................................... 17

 Correlation and radial velocity differences .............................................................. 17 

 Coverage variations.................................................................................................. 20 

 Spectral analysis....................................................................................................... 20 

 Bearing offsets.......................................................................................................... 23 

Discussion ...................................................................................................................... 25 

Conclusions.................................................................................................................... 29 

References...................................................................................................................... 30 

List of Tables and Figures - Final Study Report 

Table 1. HF Radar – mooring comparisons..................................................................... 18 

Figure 1. Locations of HF radars ...................................................................................... 15 

Figure 2. Time lines of the moored current time series used for comparison 

with the HF radar time series ...................................................................... 16 

Figure 3. Time series of radial currents............................................................................. 19 

Figure 4. Time series of coverage showing strong diurnal variations............................... 21 

Figure 5. Pairs of power spectra of radial vectors from HF radars and moorings............. 21 

Figure 6. Mean squared coherence spectrum.................................................................... 22 

Figure 7. Location of Point Conception radar and mooring.............................................. 24 

Figure 8. Location of Coal Oil Point radar and mooring .................................................. 24 

Figure 9. Bearing offset ∆θ versus bearing for all radars with more than 

 one mooring in coverage areas.................................................................... 25 

Figure 10. Uniform westward flow parallel to a straight coastline ..................................... 28 

Summary of findings for using high frequency radar in physical oceanographic and ecological studies 

FINAL TECHNICAL SUMMARY 

STUDY TITLE: “Application of High Frequency Radar for Surface Current Observations on

South-Central California Coast” 

REPORT TITLE: Summary of findings for using high frequency radar in physical 

oceanographic and ecological studies

CONTRACT NUMBER: 14-35-0001-30758 

SPONSORING OCS REGION: Pacific

APPLICABLE PLANNING AREA: Southern California

FISCAL YEARS OF PROJECT FUNDING: FY96, FY97, FY98, FY99 

COMPLETION DATE OF THE REPORT: 15 March 2001 

COSTS: FY 96 - $25,000, FY 97 - $91,246, FY 98 - $133,844, FY 99 - $51,727 

CUMULATIVE PROJECT COST: $301,817 

PROJECT MANAGER: Russell J. Schmitt

AFFILIATION: University of California, Santa Barbara

ADDRESS: Coastal Research Center, Marine Science Institute, University of California, Santa

Barbara, CA 93106

PRINCIPAL INVESTIGATORS: Libe Washburn, Professor, Institute for Computational 

Earth System Science, Dept. of Geography, University of California, Santa Barbara, CA 93106-

3060, email: washburn@icess.ucsb.edu; Steven Gaines, Professor, Dept. of Ecology, Evolution, 

and Marine Biology, Biology, California, Santa Barbara, CA 93106, email: 

gaines@lifesci.ucsb.edu 

1. BACKGROUND: 

1.1 High frequency radars for ocean current measurement 

Since the first observations of the Doppler spectrum of sea echo by [Crombie, 1955], high 

frequency (HF) radars have become a common technology for studying coastal circulation 

processes. Despite the increased use of HF radar, several issues remain about their performance

in measuring ocean surface currents. For example, the radars' ability to correctly place current

vectors on the sea surface, in less-than-ideal deployment situations, remains to be determined.

The factors affecting the accuracy and precision of HF radar measurements of surface currents 

are still not well understood, but in this research we made progress on these issues. 

Coastal HF radars for measuring ocean surface currents are divided into two types based on 

the method used to determine bearing to a sector on the ocean's surface: beam forming radars, 

such as the Ocean Surface Current Radar (OSCR), and direction finding radars, such as the 

Coastal Ocean Dynamics Application Radar (CODAR), the system used in our study. Beam

forming is accomplished by electronically steering a linear phased array of receive antennas to a 

sector of ocean surface. Direction finding of the CODAR system uses three receive antenna 

elements and a variant of the Multiple Signal Classification (MUSIC) algorithm [Schmidt, 1986] 

to determine bearing. Further discussion of the CODAR system can be found in [Paduan and 

1 
Final Technical Summary – Washburn and Gaines 

Rosenfeld, 1996], [Barrick and Lipa, 1997] and [Barrick and Lipa, 1999]. The beam forming 

method is discussed by [Graber et al., 1997], [Shay et al., 1998], and [Vesecky et al., 1998]. 

In 1997 with funding MMS and the W.M. Keck Foundation we began deploying an array of

five HF radars along the California coast in conjunction with a large coastal circulation study 

conducted by the Center for Coastal Studies at the Scripps Institution of Oceanography 

(CCS/SIO), which included an array of 9 current meter moorings (also MMS funded). We

obtained nearly 2 years of overlapping data from the Santa Barbara Channel and Santa Maria 

Basin that provided the opportunity to investigate the performance of HF radars over a range of 

conditions and deployment situations. We also used surface circulation data from the radars to 

address a number of research questions important to coastal marine ecology. 

1.2 Use of high frequency radar in ecological studies in the Santa Barbara Channel and 

Santa Maria Basin. 

We used a 3-year record of surface currents obtained by an array of HF radars to examine the 

surface circulation near Pt. Conception, California. We focused on aspects of the circulation 

important to larval transport and retention. In particular, we examined the role of a persistent

cyclonic eddy in the western Santa Barbara Channel. Evidence for larval retention within the

eddy came from mid-water trawling surveys for juvenile and late-stage larval fishes in June 

1998, 1999, and 2000. The trawling surveys were conducted by Mary M. Nishimoto of the 

Marine Science Institute at UCSB (MSI/UCSB) and were funded by the Biological Research 

Division (BRD) at no cost to this project. 

During the 1998 survey, a series of midwater trawls were conducted in and around the eddy, 

finding extremely high concentrations of fishes in its center. During the 1999 and 2000 surveys,

high concentrations of fishes were not found and the eddy circulation was not consistently 

observed. Time series of relative vorticity computed from the HF radar show that the cyclonic 

circulation was less persistent in 1999 compared with 1998 when it was relatively stable. 

Analysis of surface circulation data from 2000 suggests temporal variability similar to 1999

and work on the 2000 biological data is continuing (BRD and UC Sea Grant funded). Patterns 

of surface currents filtered with a pass band of 7 – 28 days revealed small eddies propagating

through the western Santa Barbara Channel which appear to be linked to the stability of the 

larger cyclonic eddy. Horizontal scales of the eddies are of order 20 km and they propagate 

westward at about 7 km/day. In 1998 the small eddies began propagating through the region in 

late June, after the larval development period (approximately March through June). In 1999 the 

propagation began earlier, in mid-May. Analysis of 2000 data is not yet complete, but the onset 

of eddy propagation in 2000 appears similar to 1999. We hypothesize that the effectiveness of 

the eddy as a larval retention mechanism is linked to the stability of the eddy and the onset of the 

smaller propagating eddies. 

We also investigated relationships between settlement patterns of intertidal invertebrates (e.g. 

crabs, barnacles, limpets, etc.) and evolving patterns of coastal circulation. This analysis is 

continuing, but some patterns between settlement patterns and coastal circulation have emerged. 

For example, the typical pattern of offshore surface flow around Pt. Conception is consistent

with the fact that the Point is a northern range limit for several species. 

2 
Summary of findings for using high frequency radar in physical oceanographic and ecological studies 

2.0 OBJECTIVES: 

An overall objective our project was to evaluate the performance and feasibility of using high 

frequency radar systems to measure evolving patterns of surface currents in the waters off the 

south-central coast (SCC) of California. The study area encompassed the western Santa Barbara 

Channel and the Santa Maria Basin. 

Specific goals were to: 

1. To deploy HF radars at three sites along the between Pt. Conception and Pt. Buchon 

along the south-central coast of California. 

2. To compare HF radar-derived surface currents with other, independent measures of 

surface currents such as in situ current meters. 

3. To operate the HF radar systems continuously and obtain time series of sufficient length 

such that the important dynamic processes were resolved spatially and temporally. 

4. To compare HF radar-derived surface currents with the spatial and temporal patterns of 

nearshore plankton distributions, with a specific focus on the movement of larval stages 

of coastal invertebrates and fishes. 

All of these goals were accomplished during the project. 

3.0 DESCRIPTION AND RESULTS: 

3.1 HF radar deployments and evaluation. 

As discussed above an array of five HF radars were deployed in the Santa Barbara Channel 

over the course of the project. Three of the radars remain in operation; the other two radars were 

on loan (at no cost to the project) from NOAA's Environmental Science and Technology 

Laboratory in Boulder Colorado and had to be returned in February 2001. The coverage area of 

the HF radar array included eight moorings deployed in the western Santa Barbara Channel and 

Santa Maria Basin by CCS/SIO (open circles, Figure 1). The CCS/SIO moorings carried vector 

measuring current meters [Weller and Davis, 1980] at 5 and 45 meters depth providing hourly 

averages of current velocity. We used the 5 m data for comparison with the surface HF radar 

data. 

An additional mooring was deployed by UCSB investigators near the center of the Santa 

Barbara Channel from 20 May 1998 to 12 October 1999. The UCSB mooring carried an upward 

looking 1200 kHz acoustic Doppler current profiler (ADCP; manufactured by R.D. Instruments, 

San Diego CA) at 15 meters depth. The ADCP measured currents over 0.5 m depth increments 

(bins) to within 3-4 meters of the sea surface, every 20 minutes. These data were subsequently 

averaged into 1 hr blocks. Time series from occasional deployments of an electromagnetic 

current meter (model S4; manufactured by InterOcean, San Diego, CA) at 5 m depth on the 

UCSB mooring agreed closely with ADCP currents at 5 m. We compared HF radar time series 

with the time series from the ADCP bin nominally at 4 m depth. This was the shallowest depth 

consistently free from contamination due to surface reflections. 

3 
Final Technical Summary – Washburn and Gaines 

Figure 1. Locations of HF radars in the Santa Barbara Channel and Santa Maria Basin are indicated with

triangles: FBK is Fallback-22 near Pt. Sal, ARG is Pt. Arguello, PTC is Pt. Conception, RFG is Refugio Beach, 

and COP is Coal Oil Point. Circular sectors show nominal ranges of 42 km. Circles indicate locations of moored 

current meters used for comparison with HF radar currents. Naming convention of current meters is discussed in 

the text. Semi-circles show the extent of HF radar coverage. Bathymetric contours are plotted at 50, 100, 200,

300, 500, and 600 m. The inset shows the study site on the California coast.

We based our study of HF radar performance on comparisons of the radial component of 

currents observed by the radars with radial components observed by conventional current meters. 

We focused primarily on the radial current component rather than total velocity vectors because 

we wanted to evaluate the performance of individual radars. Calculation of total velocity vectors 

combines measurements of two or more radars and thus, makes evaluation of individual radars 

more difficult. 

To compare radial current vectors (radials) from a HF radar site VHF with those at a particular 

mooring, we computed the radial component of moored current velocity Vm, 

Vm = V • im (1) 

where • is the dot product, V is the mooring current velocity, and im is the unit vector pointing 

from the mooring toward the HF radar site. This procedure was repeated at each mooring for all 

radars having the mooring within range. A total of 18 paired mooring-HF radar time series were 

available with maximum record lengths exceeding one year. 

To examine performance of individual HF radars, we computed various statistics for each 

pair of HF radar and mooring time series. The square of the correlation coefficient (r2

) was 

computed between Vm and VHF for the sector containing the mooring, and for sectors 

surrounding the mooring. Typically, a single HF sector entirely contained the mooring watch 

circle (50 m to 300 m in radius depending on water depth for the CCS moorings; 750 m for the 

ADCP mooring). We interpret the value of r2

 as an overall measure of similarity between the 

4 
Summary of findings for using high frequency radar in physical oceanographic and ecological studies 

time series. Slopes and offsets of regression lines computed between Vm and VHF indicate 

systematic differences between the time series. Power spectra and squared coherence spectra of 

Vm and VHF compare the time series across a range of frequencies. 

The results of the comparisons between the HF radars and moored current meters are 

summarized in a paper (attached to this report), "Evaluating radial component current 

measurements from CODAR high frequency radars and moored in situ current meters", that has 

been submitted for publication [Emery et al., 2001]. 

3.2 HF radar in ecological studies of larval transport and advection 

In spring 1998, we observed a persistent cyclonic eddy in the western Santa Barbara Channel 

for several weeks preceding the annual midwater trawling survey conducted by Mary M. 

Nishimoto of the Marine Science Institute at UCSB (at no cost to this project) in June of that 

year. The feature was evident in surface current maps generated by HF radar (Figure 2A) and 

sea surface temperature (SST) imagery from satellite AVHRR (Figure 2B). The persistence of 

the eddy suggested the possibility that abundance patterns of pelagic juvenile fishes may be 

related in some way to this flow feature. Accordingly, several midwater trawls were conducted 

in the eddy and surrounding waters to test this conjecture. 

5 

120.8 120.6 120.4 120.2 120.0 119.8 119.6

A

Longitude

PC R

COP

Vorticity (x10-4 s-1)

Latitude

-1.0 -0.5 0 0.5 1.0

34.0

34.2

34.4

34.6 PA

B

Sea surface temperature (C)

Point Arguello

Point Purisima

Point Mugu

Santa Barbara

Californ

Point Conception ia

Figure 2. (A) Spatial pattern of surface currents at 1900 GMT on 2 June 1998 showing a strong cyclonic eddy in the western

anta Barbara Channel. Surface currents were mapped with HF radars at Coal Oil Point (COP), Refugio (R), and Point

onception (PC). Scale for current speed is at lower left. Circle (radius=3 km) at lower left is the area over which current 

vectors are averaged. (B) The cyclonic eddy is visualized by curving temperatures features wrapping around the eddy in the 

satellite sea surface temperature image at 0152 GMT on 3 June 1998. 

At the beginning of the trawling survey on 2 June 1998 the surface circulation (upper 1 m) in 

the Channel was strongly cyclonic, as determined by HF radar. The flow in the western Channel 

was dominated by an eddy centered at 120.1 W, 34.3 N with a diameter of about 30 km and 

maximum azimuthal currents of about 0.4 m s-1 (Figure 2A). The rotation of the eddy, quantified 

by its relative vorticity, ranged from 0.2 f (rotation period of ~ 9 days) on the periphery to about 

0.7 f (rotation period of ~3 days) in the center, where f is the Coriolis parameter. Strong rotary 

flow in the eddy was also indicated by alternating warm and cool streaks wrapped around the 

eddy as revealed by sea surface temperature (SST) imagery (Figure 2B). Time series of surface 

currents obtained from the HF radars showed that the eddy persisted throughout the juvenile fish 

survey. 

S

C

5 
Final Technical Summary – Washburn and Gaines 

The eddy circulation was not merely a surface flow, but extended to at least 200 m depth. 

The spatial pattern of dynamic height φ, determined from CTD casts during the trawling survey, 

had a minimum in the core of the eddy, consistent with the strong cyclonic circulation revealed 

by the HF radars (Figure 3A). Assuming a level of no motion at 200 dbar (~200 m depth), the 

sea surface (0 dbar) was lower in the center by about 0.05 m compared with surrounding waters. 

We defined the eddy center by φ<0.32 m.

2-15 JUNE 1998

Potential

Temp. (C)

Geostrophic

Velocity (ms-1)

Purisima (2)

Arguello (3) West Entrance (5)

West edge (3)

Eddy (9)

East edge (5)

East Channel (4)

Mugu (5)

Mean no. haul -1 (+1 SE)

0

2000

4000

0

4000

8000

0

400

800

0

2000

4000

Northern lampfish

California smoothtongue

Rockfishes

Pacific hake

.32

.38

.38 .34

A

B

C

Figure 3. A) Distribution of dynamic height in the western Santa Barbara Channel during trawling surveys in 1998. B) Vertical

sections of temperature and geostrophic velocity along bold line in panel A. C) Abundance of 5 fish taxa in the region. High

concentrations of fishes were found in the center of the eddy. In panel A, P indicates Purisima, A is Arguello, W is west channel,

+ signs are east and west edges, E is east Channel N is north Channel, and M is Mugu. 

120.5 120.0 Longitude

180

160

140

120

100

80

60

40

20

702701700301302 303802

0

Depth (m)

Station code

120.5 120.0

Isolines of potential temperature T along an east-west section across the eddy showed the 

effects of the earth’s rotation by rising sharply in the eddy center (Figure 3B). The 10°C 

isotherm rose about 80 m in the center; similar deflections were also apparent for salinity and 

density. Horizontal density gradients in the eddy supported vertical shear in horizontal currents; 

the speed of these geostrophic currents changes by about 0.2 m s-1 over the 200 m measurement 

interval. Currents across the section show southward flow at depth west of the center and 

northward flow to the east, consistent with deep cyclonic flow (Figure 3B). Deflections of the 

deepest isolines indicated the cyclonic circulation of the eddy extended below 200 m. 

Figure 3. (A) Distribution of dynamic height in the western Santa Barbara Channel during trawling surveys in

1998. (B) Vertical sections of temperature and geostrophic velocity along bold line in panel A. (C)

Abundance of 5 fish taxa in the region. High concentrations of fishes were found in the center of the eddy. In

panel A, P indicates Purisima, A is Arguello, W. is west Channel, + signs are east and west edges, E is east 

Channel, N is north Channel, and M is Mugu.. 

During the two weeks of sampling, 2-15 June, net hauls were conducted in the eddy and 

surrounding waters to look for possible influences on juvenile fish abundance. In post-cruise 

analysis, hauls were grouped into seven geographic areas, including the eddy center that was 

defined by φ < 0.32 m. The abundance of each of the five dominant taxa was greatest in the 

eddy center (Figure3C). The fishes were patchily distributed, and occasional large catches 

6 
Summary of findings for using high frequency radar in physical oceanographic and ecological studies 

occurred at the southwest and northeast edges of the eddy and at the west entrance of the 

Channel. Catches of rockfishes (Sebastes spp.) and Pacific hake (Merluccius productus), two 

abundant taxa represented by late-stage larvae and pelagic juveniles, were 39 to 1.1x104

 haul-1 in

the center compared with north of the Channel and in the east Channel where catches were 

typically less than 100 haul-1. Mesopelagic fishes California smoothtongue (Leuroglossus 

stilbius), northern lampfish (Stenobrachius leucopsarus), and Mexican lampfish (Triphoturus 

mexicanus), represented by juveniles and adults, exhibited similarly high abundance and possible 

entrainment in the eddy center. 

The striking observation motivated us to repeat the survey in 1999 and 2000. Results from

the 1999 and 2000 surveys were characterized by very different larger-than-channel scale and 

mesoscale oceanographic conditions and catch distributions. Physical conditions in the Channel 

in June 1999 were much different than June 1998; perhaps the biggest difference was the 

stronger winds in 1999 associated with La Niña conditions.

Circulation in the Channel was highly variable in 1999 compared to the stable cyclonic eddy 

flow pattern observed in 1998. In 1999, temperature-salinity distributions changed rapidly as 

indicated by repeated sampling in the western Channel (Figure 4A). During the first leg of 

sampling, 6-21 June 1999, dynamic height was low in the Channel north of the San MiguelSanta Rosa Islands passage and corresponded with a cyclonic eddy feature observed by HF radar 

at the time of sampling. The gradient in dynamic height across the feature was similar to that in 

1998. Dynamic height at 5 m depth (5 dbar) assuming a level of no motion at 200dbar was lower 

in the center by about 0.05 m compared with surrounding waters. We defined the center as φ < 

0.34 m. Re-sampling during 24 June-1 July (leg 2) revealed that dynamic height increased in that 

area, and a low dynamic height feature with φ < 0.34 m as its center was observed west of the 

Channel entrance (Figure 4A). The area to the west of the Channel was beyond HF radar 

coverage and was not sampled during leg 1. 

The flow field in the few months preceding and during the 1999 survey was characterized by 

propagating trains of eddies with alternating cyclones (rotating clockwise) and anti-cyclones 

(rotating counterclockwise) along with increased flow from the east entrance through the 

Channel. These eddies were discussed in presentations at the 2000 AGU meeting in San 

Francisco by Beckenbach and Washburn and by Washburn et al.). Coherence of the spatial 

distributions of the dynamic height from legs 1 and 2 and HF radar data suggested that we 

sampled twice an eddy that propagated westward from within the Channel to the western 

entrance. The importance of these eddies is one of the focuses of our ongoing research. 

Abundance of the dominant fish taxa and their spatial distributions differed substantially 

between 1998 and 1999. Catch abundance in 1999 was lower by an order of magnitude 

compared to 1998). The five taxa comprised 98% and 96% of the mean total catch in 1998 and 

1999, respectively. Rockfishes were the fifth most abundant taxa in 1998. In 1999, sanddabs 

(Citharichthys spp.) and northern anchovy (Engraulis mordax) were slightly more abundant than 

rockfishes. 

We grouped the 1999 trawling stations by φ and geographic area as we did with the 1998 

samples. Furthermore, we divided the 1999 samples into two sampling periods (Figure 4B). In 

contrast to 1998, late-larval stage and pelagic juvenile rockfishes and Pacific hake were more 

7 
Final Technical Summary – Washburn and Gaines 

-121.0 -120.8 -120.6 -120.4 -120.2 -120.0 -119.8 -119.6 -119.4 -119.2 -119.0

33.8

34.0

34.2

34.4

34.6

34.8

-121.0 -120.8 -120.6 -120.4 -120.2 -120.0 -119.8 -119.6 -119.4 -119.2 -119.0

33.8

34.0

34.2

34.4

34.6

34.8

6-21 JUNE 1999

Cruises 2-8

24 JUNE - 1 JULY 1999

Cruises 9-11

n=3 4 16 2 12 4

Rockfishes

No. per haul (+/- SE)

0

2

4

6

8

10

Pacific hake

0

200

400

600

800

1000

Northern lampfish

0

20

40

60

80

100

120

140

Calif. smoothtongue

Area

Purisima

Center

Chan_leg1

West_low

Chan_leg2

Mugu

0

200

400

600

800

1000

n=3 4 16 2 12 4

Leg 1:

.32 .38

.36

.36

.32

.38 .40

A B

Figure 4.

Figure 4. A) Distribution of dynamic height in the western Santa Barbara Channel during trawling surveys of 1999. B) Distributions of fish taxa Fiduring 1999 surveys. gure 4. (A) Distribution of dynamic height in the western Santa Barbara Channel during trawling

surveys of 1999. (B) Distribut

abundant north of the Channel off Pt. Purisima than in the Channel, and their distributions did 

not show a relationship with dynamic height features in 1999. Northern lampfish and California 

smoothtongue, both represented by juvenile and adults, were more abundant in the western 

Channel during the first sampling period when it was occupied by a cyclonic eddy with a center 

of low dynamic height (“Low_leg 1”) than during the second sampling leg when the area is 

characterized by high dynamic height (“Chan_leg 2”). No consistent relationship between 

dynamic height areas and the spatial distribution of the dominant taxa in the Channel was 

observed in 1999. 

Leg 

1 2

Leg 2:

ions of fish taxa during 1999 surveys. 

Flows in the western Santa Barbara Channel and Santa Maria Basis were also compared with 

recruitment patterns of several invertebrate species. Recruitment was measured as part of the 

Partnership for Interdisciplinary Studies of the Coastal Ocean (PISCO) (at no cost to this project) 

along several sites on the south-central California coast including three in the HF radar coverage 

area (green squares, Figure 5). A typical regional flow pattern from summer 2000 shows strong 

equatorward and offshore flows outside the Channel consistent with prevailing upwelling 

favorable winds. The upwelling center around Points Conception and Arguello had mean 

offshore flows of about 10 cm/s when averaged over a six week period. The along shore 

distribution of settlement of 13 species showed a minimum at the recruitment site within this 

upwelling center (red arrow and bars in inset diagram of Figure 5). This suggests a strong link 

between settlement patterns of inter-tidal invertebrates and persistent coastal circulation features 

such as this upwelling center. We are continuing our investigation of links between settlement 

patterns and coastal circulation as part of PISCO. 

8 
Summary of findings for using high frequency radar in physical oceanographic and ecological studies 

Recruitment 1997-1999

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Paradise Cove

Port Hueneme

Ventura

Stearns

Ellwood

Gaviota

Boathouse

Avila

San Simeo

Av

n

erage number of settlers per

collector per sample interval

Elwood

Gaviota

Boathouse

Figure 5. Spatial mean currents in the Santa Barbara Channel and Santa Maria Basin during 6 June – 26 July, 2000. Inset shows inter-tidal

recruitment at several sites. Squares show recruitment sites within radar coverage area. Arrow in inset indicates low settlement in the upwelling

center between Points Conception and Arguello. Black circles and dots indicate HF radar sites.

poleward

4.0 SIGNIFICANT CONCLUSIONS: 

4.1 Radar performance 

Our investigation of the performance of HF radars for measuring surface currents supports 

the following conclusions: 

1. Radial currents obtained from the radars were significantly correlated with radial currents 

obtained from the moored current meters with r2

 in the range 0.39-0.77. Root-meansquare (rms) radial speed differences ranged from 7-19 cm s-1. A weak trend of 

increasing rms differences was found with increasing range. 

2. Significant coherence was found between current meter and radar-derived time series for 

frequencies below 2.2 cpd (11 hour period and longer). Power spectra showed similar 

magnitudes and slopes for frequencies below 2.2 cpd. 

3. A pointing error ∆θ ranging from –16o

 to19o was found for some of the radar sites where 

positive values indicate a clockwise error. ∆θ for a given radar was not constant, but 

varied with bearing. We speculate that ∆θ resulted from distortions of the receive antenna 

patterns in the near field. 

9 
Final Technical Summary – Washburn and Gaines 

4. We used a simple model of ∆θ versus bearing based on our observations to simulate 

errors in total velocity vectors computed from two radars. Using a uniform flow parallel 

to shore, ∆θ produced speed errors of up 15% and direction errors up to 9o in total 

velocity vectors We are working with colleagues at CODAR Ocean Sensors, Oregon 

State University, and the Naval Postgraduate School to correct these errors in processing 

algorithms for HF radar data. 

4.2 Application of HF radar to coastal circulation and marine ecology 

1. Persistent cyclonic circulation is a dominant flow pattern in the western Santa Barbara 

Channel consistent with results of [Dever et al., 1998; Harms and Winant, 1998; Winant 

et al., 1999]. 

2. Propagating trains of eddies occur in the western Santa Barbara Channel. The eddies 

have horizontal scales of order 15 km, current speeds of 0.1-0.2 m/s, and alternating 

relative vorticity of magnitudes comparable to the vertical component of the earth’s 

rotation rate. They propagate westward at ~7 km/day. Their high relative vorticity 

indicates that the eddies are not in simple geostrophic balance, but rotate rapidly enough 

to produce significant centripetal acceleration. We use the term "eddy" to describe the 

features, but they may be some type of wave such as a topographic Rossby wave. 

3. Animations of HF radar data show that surface currents respond rapidly to changes in 

local wind velocity, especially in the Santa Maria Basin. 

4. Very high concentrations of juvenile and late stage larval occur in the western Santa 

Barbara Channel under conditions of stable cyclonic flow 

5. Propagating eddies can prevent establishment of stable circulation patterns that might 

otherwise retain larvae. 

6. Coastal circulation features such as the upwelling center between Points Conception and 

Arguello limit settlement in the region due to strong persistent offshore advection driven 

by upwelling-favorable winds. 

5.0 STUDY PRODUCTS: 

Publications: 

Emery, B.M., L. Washburn, and J. Harlan, Evaluating CODAR high frequency radars for 

measuring surface currents: observations in the Santa Barbara Channel, Journal of 

Atmospheric and Oceanic Technology (manuscript submitted), 2001. 

Beckenbach, E.H. and L. Washburn, Propagating eddies in the Santa Barbara Channel, in 

preparation. 

Washburn, L. and S. Gaines. Summary of findings for using high frequency radar in physical 

oceanographic and ecological studies. MMS OCS Study 2001-056. Coastal Research 

Center, Marine Science Institute, University of California, Santa Barbara, California. 

MMS Cooperative Agreement Number 14-35-0001-30758. 32 pages. 

10 
Summary of findings for using high frequency radar in physical oceanographic and ecological studies 

Presentations: 

2000 Surface circulation near Pt. Conception, California: A mechanism for larval retention?, 

Washburn, L., M.M. Nishimoto, E.H. Beckenbach, and B.M. Emery, Fall Meeting, 

American Geophysical Union, San Francisco, 15-19 September. 

2000 Observations of propagating eddies in the Santa Barbara Channel, Beckenbach, E.H., L. 

Washburn, D. Salazar, and B.M. Emery, Fall Meeting, American Geophysical Union, 

San Francisco, 15-19 September. 

2000 Time series mapping of currents in the coastal ocean, Kosro, P.M., J.D. Paduan, and L. 

Washburn, Fall Meeting, American Geophysical Union, San Francisco, 15-19 September. 

2001 Surface circulation on the South-central coast: Mechanisms for larval transport and 

retention, seminar in the Dept. of Atmospheric Science, UCLA, 24 January. 

2001 Variability in mesoscale oceanographic processes corresponds to variability in the 

distribution and abundance of rockfish (Sebastes spp.) juveniles, Schroeder, D M , M.M. 

Nishimoto, and L. Washburn, American Society of Limnology and Oceanography, 

Albuquerque2001, Albuquerque, NM, 12-16 February. 

2001 The physical environment of the Santa Barbara Channel, seminar presented to volunteers 

of the Channel Islands National Marine Sanctuary", Chase Palm Park, 1 March. 

11 
Final Technical Summary – Washburn and Gaines 

6.0 REFERENCES: 

Barrick, D.E., and B.J. Lipa, Evolution of bearing determination in HF current mapping radars, 

Oceanography, 10 (2), 72-75, 1997. 

Barrick, D.E., and B.J. Lipa, Using antenna patterns to improve the quality of SeaSonde HF 

radar surface current maps, IEEE 6th Working Conference on Current Measurement, 

1999. 

Crombie, D.D., Doppler Spectrum of Sea Echo at 13.56 Mc/s, Nature, 175, 681-682, 1955. 

Dever, E.P., M.C. Henderschott, and C.D. Winant, Statistical Aspects of surface drifter 

observations of circulation in the Santa Barbara Channel, Journal of Geophysical 

Research, 103 (C11), 24,781-24,797, 1998. 

Emery, B.M., L. Washburn, and J. Harlan, Evaluating CODAR high frequency radars for 

measuring surface currents: observations in the Santa Barbara Channel, in preparation 

for Journal of Atmospheric and Oceanic Technology (manuscript available), 2001. 

Graber, H.C., B.K. Haus, R.D. Chapman, and L.K. Shay, HF radar comparison with moored 

estimates of current speed and direction: Expected differences and implications, Journal 

of Geophysical Research, 102 (C8), 18,749-18,766, 1997. 

Harms, S., and C.D. Winant, Characteristic patterns of the circulation in the Santa Barbara 

Channel, Journal of Geophysical Research, 103 (C2), 3041-3065, 1998. 

Paduan, J.D., and L.K. Rosenfeld, Remotely sensed surface currents in Monterey Bay from

shore-based HF radar (Coastal Ocean Dynamics Application Radar), Journal of 

Geophysical Research, 101 (C9), 20,669-20,686, 1996. 

Schmidt, R.O., Multiple Emitter Location and Signal Parameter Estimation, IEEE Transactions 

on antennas and propagation, AP-34 (3), 276-280, 1986. 

Shay, L.K., S.J. Lentz, H.C. Graber, and B.K. Haus, Current structure variations detected by 

high-frequency radar and vector-measuring current meters, Journal of Atmospheric and 

Oceanic Technology, 15, 237-256, 1998. 

Vesecky, J., C. Teague, D. Fernandez, J. Paduan, J. Daida, R. Onstott, K. Laws, and P. Hansen, 

Coastal Currents with HF radar, Backscatter, 13, 12-20, 1998. 

Weller, R.A., and R.E. Davis, A vector measuring current meter, Deep-Sea Research, 27, 565-

582, 1980. 

Winant, C.D., D.J. Alden, E.P. Dever, K.A. Edwards, and M.C. Henderschott, Near-surface 

trajectories off central and southern California, Journal of Geophysical Research, 104 

(C7): 15,713-15,726, 1999. 

12 
Summary of findings for using high frequency radar in physical oceanographic and ecological studies 

FINAL STUDY REPORT 

Evaluating radial component current measurements from CODAR high frequency radars 

and moored in situ current meters 

Brian M. Emery1

, Libe Washburn2, and Jack A. Harlan3

1

Institute for Computational Earth System Science, University of California, Santa Barbara, 

93106-3060; 2

Department of Geography, University of California, Santa Barbara, 93106-4060; 

3

NOAA Environmental Technology Laboratory, Boulder, Colorado 

Abstract 

The performance of a network of five CODAR (Coastal Ocean Dynamics Application Radar) 

high frequency (HF) radars is described based on comparisons with an array of nine moorings in 

the Santa Barbara Channel and Santa Maria Basin deployed between June 1997 and November 

1999. Eight of the moorings carried vector measuring current meters (VMCM’s), the ninth an 

upward-looking acoustic Doppler current profiler (ADCP). Coverage areas of the HF radars and 

moorings included diverse flow and sea state regimes. Measurement depths were ~1 m for the 

HF radars, 5 m for the VMCM’s, and 3.2 m for the nearest ADCP bin to the surface. Comparison 

of radial components of near-surface currents from 18 HF radar-mooring pairs yielded root mean 

square (rms) speed differences of 7-19 cm s-1. The radial components were significantly 

correlated with r2

 in the range 0.39-0.77. Other studies based on radar-mooring comparisons 

have found similar rms speed differences and r2

 based on total velocity vectors. Spectral analysis 

showed significant coherence for frequencies below 2.3 cycles day-1 (periods longer than 11 hr). 

At higher frequencies no significant coherence was found. Comparisons revealed bearing errors 

in locating radial currents on the sea surface by some of the HF radars. These were typically 5-

10° with a maximum error of 19o. The effects of bearing errors on total velocity vector estimates 

were evaluated using a simple flow field and measured bearing errors, showing up to 15% 

differences in computed flow speeds, and up to ~9o differences in flow directions. 

1. Introduction 

Since the first observations of the Doppler spectrum of sea echo by (Crombie 1955), high 

frequency (HF) radars have become a common technology for studying coastal circulation 

processes. Despite its increased use, several issues remain about the performance of HF radar for 

measuring surface currents. For example, the radars ability to determine bearing to radial current

vectors on the sea surface in less-than-ideal deployment situations remains to be quantified. In 

1997, we began deploying an array of five HF radars along the California coast in conjunction 

with a large coastal circulation study conducted by the Center for Coastal Studies at the Scripps 

Institution of Oceanography (CCS/SIO), which included an array of 9 current meter moorings. 

Nearly 2 years of overlapping data from the moorings and the radars provides the opportunity to 

evaluate some of the performance characteristics of the HF radars. 

Coastal HF radars for measuring surface currents are divided into two types based on the 

method used to determine bearing to a sector on the ocean’s surface: beam forming radars, such 

13 
Final Study Report – Washburn and Gaines 

as the Ocean Surface Current Radar (OSCR), and direction finding radars, such as the Coastal 

Ocean Dynamics Application Radar (CODAR), the system used in this study. Beam forming in 

the OSCR system is accomplished by electronically steering a linear phased array of receive 

antennas to a sector of ocean surface (Kraus 1988). Direction finding of the CODAR system uses 

three receive antenna elements and a variant of the Multiple Signal Classification (MUSIC) 

algorithm (Schmidt 1986) to determine bearing. Further discussion of the CODAR system can be 

found in (Paduan and Rosenfeld 1996), (Barrick and Lipa 1997), and (Barrick and Lipa 1999). 

The beam forming method is discussed by (Graber et al. 1997), (Shay et al. 1998), and (Vesecky 

et al. 1998). We limit our discussion to HF radars that use ground wave propagation, via 

conductance along the ocean surface, rather than skywave or ionospheric propagation (e.g., 

Georges 1980). 

Despite different bearing determination methods used by the OSCR and CODAR radars, 

underlying physics produce operational similarities between the systems. Thus, some results 

obtained by comparisons between in situ current meters and either type of radar may apply to 

both systems. In particular (Graber et al. 1997) and (Shay et al. 1998) used an OSCR system to 

identify numerous sources of error common to both. These include differences in spatial and 

temporal sampling, horizontal variability of currents, near-surface vertical shear, Stokes drift, 

Ekman flow, and measurement errors due to current meters. Other comparisons have examined 

many of these factors including (Chapman et al. 1997), (Chapman and Graber 1997), (Shay et al.

1998), (Paduan and Rosenfeld 1996), and (Kosro et al. 1997). 

With few exceptions (e.g. Melton 1995 and Harlan et al. 2000), previous studies of HF radar 

performance compared total velocity vectors from in situ current meters and HF radars. Because 

total velocity vectors are computed from radial components from two (or more) HF radars, errors 

from individual radars are combined. Errors in total vectors also result from geometric dilution 

of precision (Chapman and Graber 1997). We compared radial components with moored current 

meters to evaluate performance of individual radars. 

The rest of this report is organized as follows: In section 2 we discuss the radar and mooring 

deployments and data analysis procedures; results of comparisons of radial currents from the 

radars and current meters are presented in section 3; section 4 is a discussion; and conclusions 

are in section 5. 

2. Methods 

In 1997 we began deploying HF radars along the western Santa Barbara Channel and southcentral California coasts (Figure 1). The first was installed at Coal Oil Point (COP) in June 1997, 

followed by Pt. Conception (FTC) in August 1997, then Refugio (RFG) in October 1997. The 

fourth and fifth were installed in November 1998 at Pt. Arguello (ARG) and Faliback 22 (FBK) 

near Pt. Sal. Each site consists of a transmit antenna, a receive antenna, radar electronics, and a 

computer for control and data logging. The radar electronics and computers are either housed in 

buildings (FTC and COP), or in weather proof enclosures. Antennas were placed as close to the 

ocean as possible to minimize signal attenuation by propagation over land. 

14 
Summary of findings for using high frequency radar in physical oceanographic and ecological studies 

FBK

ARG

PTC RFG

COP

ABOF

SAOF

SAIN

SAMI

AROF ARMI

SMIN

SMOF

ADCP

Pacific Ocean

34

35Latitude

Longitude 121 120

San Miguel Is.

Santa Rosa Is.

Santa Cruz Is.

Santa Barbara

Pt. Sal

Figure 1. Locations of HF radars in the Santa Barbara Channel and Santa Maria Basin are indicated with triangles: FBK is Fallback-22 near Pt. Sal,

ARG is Point. Arguello, PTC is Point. Conception, RFG is Refugio Beach, and COP is Coal Oil Point. Circular sectors show nominal ranges of 42 km. 

Circles indicate locations of moored current meters used for comparison with HF radar currents. Naming convention of current meters is discussed in text.

Semi-circles show the maximum extent of HF radar coverage. Bathymetric contours are plotted at 50, 100, 200, 300, 500 and 600 m. The inset shows the

study site on the California coast.

Figure 1. The Santa Barbara Channel and Santa Maria Basin. Locations of HF radars are indicated with

triangles: Fallback-22 near Point Sal (FBK), Point Arguello (ARG), Point Conception (PTC), Refugio Beach 

(REG), and Coal Oil Point (COP). Circular sectors show nominal radar ranges of 42 km. Circles indicate 

locations of moored current meters used for comparison with HF radar currents. Naming convention of

current meters is discussed in text. The inset shows the study site relative to the California coast. 

Each radar transmits at a different frequency in the range 12.2 - 13.6 megahertz. Following 

the Bragg principle, transmitted signals are backscattered from ocean surface waves with 

wavelengths of ~12 m, half the ~24 m transmitted wavelength. In the absence of any current, the 

backscattered radar signals are Doppler shifted by an amount proportional to the intrinsic phase 

velocity of the 12 m ocean waves, determined from the deep-water wave dispersion relation. 

Ocean current velocity is determined from the difference between the measured and intrinsic 

phase velocities. Based on the transmit frequency, this measurement corresponds to the velocity 

of the upper ~1 m of the ocean surface (Stewart and Joy 1974). Range to a sector of ocean 

surface is determined using frequency modulation, which divides the coverage area into 

concentric circular arcs called range cells. As implemented by CODAR Ocean Sensors, Ltd., the 

MUSIC algorithm divides these range cells into azimuthal sectors of ocean surface and estimates 

the bearing to each sector (Barrick and Lipa 1997, Laws et al. 2000). Radial currents are 

smoothed in bearing (azimuth measured clockwise from north) at 2° intervals with a 10° running 

average. For our study each sector spanned 1.5 km in range and 5° in bearing. The sector areas 

(depicted graphically in Figure 6a) varied linearly from 0.2 km2 to 5.5 km2 as range varied from

1.5 to 42 km. Radial current estimates were computed every 10 minutes and these were averaged 

to produce an hourly radial vector for each sector, although gaps typically occurred in the time

series. 

The coverage area of the HF radar array included eight moorings deployed in the western 

Santa Barbara Channel and Santa Maria Basin (open circles, Figure 1) from 1993 to 1999 by 

CCS/SIO. Time lines of mooring and HF radar data used for comparison are shown in Figure 2. 

The CCS/SIO moorings carried vector measuring current meters (Weller and Davis 1980) at 5 

15 
Final Study Report – Washburn and Gaines 

and 45 meters depth providing hourly averages of current velocity. We use the 5 m data for 

comparison with the surface HF radar data. As explained by (Harms and Winant 1998), the 

CCS/SIO moorings were named according to location and position on the continental shelf 

(Figure 1). The first two letters indicate mooring lines: AB for Avila Beach, SA for Pt. Sal, AR 

for Pt. Arguello, and SM for San Miguel Island. The second two letters indicate position within a 

mooring line: OF is offshore, MI is mid-shelf, and IN is inshore. For example, SAIN identifies 

the inshore mooring at Pt. Sal. 

Figure 2. (a) Time lines of the moored current time series used for comparison with the HF radar time series. (b-f)

Time series of coverage defined as the number of sectors returning data each hour for the five HF radars. Vertical

bars toward the end of the RFG time series denote the two week segment of Figure 4. 

An additional mooring, designated ADCP in Figures 1 and 2, was deployed by UCSB 

investigators near the center of the Santa Barbara Channel from 20 May 1998 to 12 October 

1999. The UCSB mooring carried an upward looking 1200 kHz acoustic Doppler current profiler 

(ADOP; manufactured by R.D. Instruments, San Diego CA) at 15 meters depth. The ADCP 

measured currents over 0.5 m depth increments (bins) to within 3-4 meters of the sea surface, 

every 20 minutes. These data were subsequently averaged into 1 hr blocks. Time series from

16 
Summary of findings for using high frequency radar in physical oceanographic and ecological studies 

occasional deployments of an electromagnetic current meter (model S4; manufactured by 

InterOcean, San Diego, CA) at 5 m depth on the UCSB mooring agreed closely with ADCP 

currents at 5 m. We compared HF radar time series with the time series from the ADCP bin 

nominally at 3.2 m depth. This was the shallowest depth consistently free from contamination 

due to surface reflections. 

To compare radial current vectors VHF (radials) from a radar site with those at a particular 

mooring, we computed the radial component of moored current velocity Vm, 

 Vm = V • im (1) 

where • is the dot product, V is the mooring current velocity, and im is the unit vector pointing 

from the mooring toward the HF radar site. This procedure was repeated at each mooring for all 

radars having the mooring within range. A total of 18 paired mooring-HF radar time series were 

available with maximum record lengths exceeding one year (Figure 2 and Table 1). 

To examine performance of individual HF radars, we computed various statistics for each 

pair of HF radar and mooring time series. The square of the correlation coefficient (r2

) was 

computed between Vm and VHF for the sector containing the mooring, and for sectors surrounding 

the mooring. Typically, a single radar sector entirely contained the mooring watch circle (50 m 

to 300 m in radius depending on water depth for the GCS/SIO moorings; 750 m for the ADCP 

mooring). We interpret r2

 as an overall measure of similarity between the time series. Rms 

differences were computed with means removed, with the difference in the means reported as the 

bias. Biases, along with slopes and offsets of regression lines computed between Vm and VHF

indicate systematic differences between the time series. Power spectra and squared coherence 

spectra of Vm and VHF compare the time series across a range of frequencies. 

3. Results 

3.1 Correlation and radial velocity differences 

Time series of VHF and Vm typically showed strong tidal variations as in the 2-week example 

from FBK-SAMI (Figure 3a). The time series were clearly similar and exhibited significant 

correlation for this short interval (r2

 =0.81, N=314; Figure 3b). Table 1 summarizes the statistical 

comparisons between VHF and Vm for all HF radar-mooring pairs over much longer time scales. 

Variations in the comparison periods (columns 3 and 4) mainly resulted from changing coverage 

for some of the radars due to hardware problems. Values of r2

 fell in the range 0.39-0.77 (column 

8), and root-mean-square (rms) speed difference ranged from 7-19 cm s-1 (column 11). Biases 

(column 12) were typically less than 2 cm s-1, with a maximum of 6 cm s-1. Slopes m of 

regression lines, defined such that VHF = Vm • m + b, were in the range 0.31-0.88 with intercepts b 

in the range 4.5-8.4 cm s-1 (columns 13 and 14). 

17 
Fin

al Study Report – Washburn and Gaines 

18 

Table 1. HF radar – mooring comparisons. Summary of comparison statistics for 18 HF radar - moored current meter pairs. HF site name and mooring name abbreviations are described in the text. (1) HF radar - mooring pairs used in the calculation of the 

Mean squared coherence spectrum γ2, (Figure 6). 
Summary of findings for using high frequency radar in physical oceanographic and ecological studies 

Figure 3. (a) Time series of radial currents Vm from mooring SAMI (solid line) and radial currents VHF from the 

FBK radar (dots) for 1-14 September 1999. (b) Scatter plot of Vm versus VHF in panel (a). Vm and VHF are 

significantly correlated with r2 = 0.81, N= 314 and are related such that VHF = 0.846 Vm+0.535. 

Good operating conditions and high signal to noise ratios resulted in low rms radial speed 

differences (9-13 cm s

-1) and generally high r2

 (0.59 - 0.70) for the FBK radar (Table 1). 

Comparable rms differences (7-10 cm s-1) and r2

 (0.63-0.77) were found for the PTC radar for 

the period 1 June 1998 through 28 January 1999 when unlimited access to the site allowed 

routine maintenance and repair. Before and after this period, hardware problems reduced the 

performance of the PTC radar. This is indicated for the PTC-AROF pair by the lower r2

 (0.58) 

and greater rms differences (14 cm s-1) for the full time series, 1 August 1997 through 3 October 

1999. Similar results were found using the full time series when PTC was paired with the other 

moorings of Table 1 (results not shown). The greatest rms differences (11-19 cm s-1) and lowest 

r

2

 (0.39-0.62) were observed between the ARG radar and surrounding moorings due to reduced 

signal to noise ratios caused by antenna and cable problems. 

The longest time series, each with more than 6000 hourly data points, were obtained from the 

ADCP mooring and the RFG and COP radars. These radars had consistently high signal-to-noise 

ratios during the mooring deployment period, but with r2

 (0.50 - 0.60) and rms differences (12 - 

11 cm s-1) comparable to the other pairs. This may have resulted from the large watch circle of 

the ADCP mooring (radius ~750 m); we were unable to account for mooring motions. 

19 
Final Study Report – Washburn and Gaines 

3.2 Coverage variations 

An overall indicator of radar performance is spatial coverage over time. Coverage is defined 

as the number of sectors returning radials each hour (Figure 2b-f). Some moorings were near the 

range limits of the radars (42 km) or near the edge of angular coverage range (e.g., ABOF in 

Figure 1). Angular coverage typically extended to within a few degrees of the surrounding 

coastline. Consistently high coverage obtained at FBK corresponded to high signal to noise 

ratios, low rms differences, and high r

2

. At ARG, low, intermittent coverage during the 

comparison period corresponded to lower r

2 and higher rms differences. An increase in coverage 

at ARG in November 1999, after the comparison period, resulted from antenna and cable 

replacement. Causes of coverage variability include power outages, antenna collapse, or other 

hardware failures. For example, at RFG in April 1998 animals bit partially through the transmit 

cable, causing poor transmission, lower signal-to-noise ratios, and frequent signal loss. Transmit 

and receive cables were then enclosed in electrical conduit. Coverage variations may also result 

from changing noise sources or variations in the environment around the radar antennas. 

High frequency coverage variations were apparent in the time series (Figure 2b-f). An 

expanded view of a 2-week period of the RFG time series shows that these had a strong diurnal 

component (Fig 4a). Strong diurnal peaks occurred in the spectra of coverage time series from all 

sites (data not shown). Hourly coverage maps (Figure 4b and 4c) show that the diurnal variation 

resulted from patchiness in coverage, as well as extensions and contractions of range, as 

observed by (Paduan and Rosenfeld 1996). For RFG the diurnal coverage reached a maximum

around 1400 local time each day and a minimum about 12 hours later (Figure 4a). In addition to 

diurnal variations, large fluctuations extending over a range of time scales are evident in the 

coverage record, often obscuring the diurnal pattern. 

3.3 Spectral analysis 

Power spectra of VHF and Vm were computed for subsets of the time series of Figure 2 to 

compare variance levels over time scales of 2 hours to ~20 days. Subsets were chosen to avoid 

large data gaps occasionally lasting over a month (Figure 2). For each subset of data, up to 30% 

of data were missing, and these were filled with zeroes. Representative power spectra for three 

HF radar-moorings pairs are shown in Figure 5. Spectral levels generally agreed for frequencies 

less than 2.3 cycles day

-1 (cpd), but levels of VHF tended to be somewhat less than Vm such as for 

PTC-SMOF in Figure 5. This is consistent with the observation that regression line slopes were 

less than 1. The diurnal (K1) and semidiurnal (M2) tidal peaks in Vm and VHF were well resolved

for FBK-SAOF and PTC-SMOF, and SMIN, but not for the ARG radar. Shoulders in the FBKSAOF spectra suggest a poorly resolved peak near the inertial frequency (1.13 cpd). Above ~2.2 

cpd VHF spectra become nearly white and depart sharply from Vm spectra. At these frequencies 

Vm spectra have slopes in the range -3 to -2. 

20 
Summary of findings for using high frequency radar in physical oceanographic and ecological studies 

Figure 4. (a) Time series of coverage at RFG for 16-30 November 1999 showing strong diurnal variations. 

Coverage was defined as the number of sectors returning radial currents each hour. Vertical lines indicate local

noon. Arrows indicate times of high coverage, shown in (b), and low coverage (c). 

Figure 5. Pairs of power spectra of radial vectors from three HF radars (bold lines) and three moorings (light lines).

From top to bottom radar-mooring pairs are FBK-SAOF, PTC-SMOF, and ARG-SM1N. Vertical lines denote the K1

and M2 tidal frequencies, and f indicates the inertial frequency. The upper two pairs of spectra are offset vertically by 

factors of 102

 and 104, respectively. 

2121 
Final Study Report – Washburn and Gaines 

Squared coherence spectra γ

2

 were computed between several time series of VHF and Vm

(denoted by superscript 1, column 2 of Table 1) to examine correlation versus frequency; the 

envelope of these spectra is indicated by shading in Figure 6a. Spectra of γ

2

 were averaged 

together to produce γ

2

 as an overall measure of coherence versus frequency (bold line, Figure 

6a). The standard deviation σ at each frequency was computed as a measure of variability in γ

2

(thin solid lines indicate γ

2

 ± 1 σ in Figure 6a). γ

2

 fell in the range 0.3-0.7 up to frequencies of 

about 2.3 cpd where it dropped below the threshold labeled 95%. The threshold was defined such 

that γ

2

 between two unrelated time series would exceed this level only 5 times out of 100. This 

result indicates significant coherence between Vm and VHF for frequencies less than ~2.3 cpd, the 

same frequency range over which their power spectra agree (Figure 6). The phase spectrum was 

nearly zero over this range and became highly variable at frequencies where γ

2

 was not 

significant (Figure 6b). 

Figure 6. (a) Mean squared coherence spectrum γ

2

 between radial currents obtained from the radars VHF and radial 

currents obtained from the moorings Vm (bold line). Time series used in computing γ

2 are indicated in column 2 of 

Table 1 by superscript 1. γ

2

 ± 1 standard deviation are shown (thin lines) along with envelope of all γ

2

 spectra 

(shading). γ

2

 between radial currents measured by the acoustic Doppler profiler at 3.2 m and 8.2 m depth is also

shown (dashed line). Vertical lines denote the K1 and M2 tidal frequencies, and f indicates the inertial frequency.

Dotted line shows threshold for significance at 95% confidence. (b) Mean phase spectrum (bold line) ± 1 standard

deviation (thin lines) between VHF and Vm corresponding to γ

2

 spectrum of panel (a). 

22 
Summary of findings for using high frequency radar in physical oceanographic and ecological studies 

To investigate how vertical shear near the surface might the affect coherence between VHF

and Vm, we computed γ

2

 between ADCP time series at 3.2 m and 8.2 m depths (dashed line, 

Figure 6a). This depth difference is comparable to the difference in measurement depths of the 

radars and VMCM’s. The rms difference between the ADCP time series at these depths was 5 

cm s

-1 over the 12,048 hr deployment period. The γ2

 spectrum for the ADCP over this depth 

range was higher than γ

2

 for Vm and VHF, but had a somewhat similar shape. Around the diurnal 

peak, levels representing the γ

2

 +1 σ were comparable to γ

2

 for the ADCP as were some of the 

individual γ

2

 spectra based on the envelope. At higher frequencies γ

2

 for the ADCP dropped off 

more slowly. 

3.4 Bearing offsets 

The sector with the highest r2 between VHF and Vm often did not contain the mooring where 

Vm was measured, but was displaced in bearing. This displacement suggests errors in the radar’s 

determination of direction to sectors on the ocean’s surface. A similar result was reported by 

(Kosro et al. 1997) from an OSCR system. We argue that in the absence of directional errors, the 

highest r2

 will coincide with the sector containing the mooring. A broad peak in r2 may occur if 

the spatial correlation scales of the velocity field are large in the azimuthal and radial directions. 

We define the displacement in bearing, or bearing offset ∆θ, as, 

∆θ = θr – θm, (2) 

where θr is the bearing to center of the sector with maximum r

2

 and θm is the bearing to the 

mooring. Positive ∆θ indicates that the sector with maximum r2 is displaced clockwise from the 

mooring. Column 9 of Table 1 shows ∆θ for each of the 18 HF radar-mooring pairs. ∆θ ranged 

from -16° to 19° with an average absolute value of 7°, although ∆θ could only be determined to 

within the 5° sector width. 

An example of a large bearing offset, ∆θ = -16°, was found for PTC-SMIN (Figure 7a). The 

maximum in r2

 was broad, but its peak was clearly offset from SMIN as indicated by r2 profiles 

along constant range lines of 4.5, 6.0, and 7.5 km (Figures 7b, 7c, and 7d, respectively). The 

maximum r2

 occurred in the same 1.5 km range cell as SMIN, such that the offset was in bearing 

only. A small bearing offset was found for COP-ADCP with ∆θ = -1°. Here the sector with 

maximum r2

 contained the mooring location (Figure 8a) and a broad maximum extended over 

bearing and range (Figures 8a, 8b, and 8c). Figure 9 shows how ∆θ changed with bearing for the 

PTC, ARG, and FBK radars, which all had more than one mooring within their coverage areas. 

At PTC, with four moorings in its coverage area, ∆θ increased roughly monotonically with 

bearing from ∆θ = -15° at bearing 160° to ∆θ = 9° at 291° (open squares, Figure 9). Values of 

∆θ were only determined near the end points of this range, however. At ARG, ∆θ also increased 

with bearing up to a maximum of ∆θ = 19° at 311° followed by a decrease to ∆θ = 10° at 355°. 

At FBK, ∆θ reached a minimum of -11° at 216° then increased to 16° at 299°. 

23 
Final Study Report – Washburn and Gaines 

Figure 7. (a) Location of Point Conception radar PTC (triangle) and mooring SMIN (circle). Radial current vectors

VHF were found for sectors 1.5 km in radius by 5° in azimuth (gray lines). Radial currents Vm were measured in situ

at the mooring. The diamond indicates the sector with the highest r2 between VHF and Vm. The arrow shows bearing 

offset ∆θ between the mooring location and the sector with highest r

2

. Dashed arcs show the locations of r2 profiles 

plotted in panels (b), (c), and (d). Profile of r

2

 between VHF and Vm along ranges of (b) 4.5 km, (c) 6.0 km, and (d)

7.5 km. 

Figure 8. (a) As in Figure 7, but for the Coal Oil Point radar COP and mooring ADCP. Profiles of r

2

 between VHF

and Vm along ranges of (b) 16.5 km, (c) 18.0 km, and (d) 19.5 km. 

24 
Summary of findings for using high frequency radar in physical oceanographic and ecological studies 

Figure 9. Bearing offset ∆θ versus bearing for all radars with more than one mooring in coverage areas: Point 

Conception (PTC, squares, gray line is a linear least square fit), Point Arguello (ARG, diamonds, solid black line), 

and Fallback-22 (FBK, triangles, dashed line). 

4. Discussion

An operational characteristic of surface current measuring HF radars is variability in range 

and azimuthal coverage. (Paduan and Rosenfeld 1996) suggest this results from range variations 

and failure of the direction finding algorithm to differentiate radial currents as a function of 

bearing within range cells. We found changes in range and azimuthal coverage spanned a range 

of time scales, but a diurnal variation was prominent, consistent with results of (Prandle et al.

1993) and (Paduan and Rosenfeld 1996). Changes in sea state and variability in the distribution 

of the waves producing Bragg scattering from the sea surface may be responsible. Coverage 

variability may also result from changes in the environment around the antennas, such as daily 

variations in moisture content in surrounding soils and vegetation, or changes in the atmosphere 

as suggested by (Prandle et al. 1993). Additionally, it is well known that the ionosphere’s lowest 

layer, the D-region, has a diurnal cycle. The D-region nearly disappears at night, allowing HF 

radio waves to travel over long distances (~103

 to l04 kin) by reflecting off higher ionospheric 

layers (e.g., Davies 1990). HF transmissions from great distances can become external noise to 

HF radars, leading to lower signal-to-noise ratios and poor coverage. As an example, an 

investigation of unusually strong diurnal coverage variations, and poor signal-to-noise ratios, at 

the PTC radar revealed media broadcasts as the cause. 

The rms differences we observe between radial velocity components VHF and Vm (Table 1) 

are comparable in range (7 - 19 cm s-1) and mean (12 cm s-1) to rms differences in total velocity 

components reported elsewhere. (Shay et al. 1995) reported rms differences of 11 - 15 cm s-1

between OSCR observations and in situ current meters off the North Carolina coast. (Shay et al. 

1998) found somewhat larger rms differences, 18-19 cm s-1 on the inshore boundary of the 

Florida Current using 05CR, although the depth difference between the HF radar and current 

meter depths was 15 m. (Paduan and Rosenfeld 1996) reported rms differences of 6.2 and 10.8 

cm s-1 around Monterey Bay using CODAR systems and a moored ADCP. Graber et al. (1997) 

25 
Final Study Report – Washburn and Gaines 

compared OSCR with several current meters moored in the depth range 5-30 m and report rms 

differences in the range 10-20 cm s-1. 

The weak trend of increasing rms differences between VHF and Vm with increasing range 

(Table 1) is consistent with the effects of horizontal variability in the velocity field. When data 

from ARG were not included, rms differences were correlated with range (r2

 =0.32, N = 16). As 

discussed by (Graber et al. 1997), differences in spatial sampling between current meters and HF 

radars can produce measurement differences. The VMCM’s sample at points around mooring 

watch circles and HF radars sample sectors of varying size, depending on range. For our radar 

sites, sectors containing moorings had widths from 0.5 km to 3.9 km corresponding to ranges of 

5.7 to 44 km (Table 1, Figure 1). Assuming the moorings were up to 1⁄2 the sector width or length 

(1.5 km) away from the moorings, the moorings and points within sectors could have been as 

much as ~2 km apart. Effective separations could have been larger if, for example, an average 

hourly radial were dominated by individual radial estimates from a sector edge and the mooring 

were at an opposite edge of the sector. Figure 10 of (Graber et al. 1997) shows an expected rms 

difference derived from OSCR total velocity data of about 5-6 cm s-1 resulting solely from a 

horizontal separation of 2 km between measurements. Kosro (1987) computed the structure 

function of horizontal velocity differences using shipboard ADCP data in an upwelling regime to 

find expected difference of 7 cm s-1 for a 2 km separation. 

Vertical separation of HF radar and mooring measurements may have also contributed to 

differences between VHF and Vm. The 5 cm s

-1

 rms difference between the 3 m and 8 m bins at 

the ADCP mooring is likely a lower limit for typical near-surface vertical current shear in the 

radar coverage area. The ADCP was moored in an area of lower winds in the Santa Barbara 

Channel compared with winds at the VMCM moorings. Dorman and Winant (2000) show the 

highest annual average winds occur in the western portion of our coverage area, in the vicinity of 

Pt. Conception and Pt. Arguello. Assuming that vertical current shear is primarily wind driven, 

the 5 cm s

-1

 rms difference we measured probably represents a lower bound on the difference 

between the surface and 5 m measurements. 

Limited resolution of the HF radar’s Doppler spectra also contributes to rms differences 

between VHF and Vm. The length of the time series used to compute the Doppler spectrum (256 s 

for these data) sets the radar’s spectral resolution (Barrick 1980). This effectively limits the 

resolution of VHF to discrete levels separated by ∆VHF = 4.3 cm s-1. The conversion of

continuously valued currents into discrete levels produces rms errors of (1/12)1/2 • ∆VHF = 1.2 cm 

s

-1 [Bendat J.S., 2000 #64]. It also produces uncertainty in spectral levels (figure 5)

corresponding to 0.13 cm2 

s

-2 (cpd)-1.

The small effect of the limited Doppler spectral resolution, along with the significant effect 

of horizontal and vertical separation, may account for a significant portion of the observed rms 

differences. Assuming errors of 5-6 cm s-1 due to near surface vertical shear, and 5-7 cm s-1 due 

to horizontal velocity differences, about 50% of the typical rms difference (~12 cm s

-1) between 

VHF and Vm could be explained by the different sampling techniques of the radars and current 

meters.

The bearing offset ∆θ may result from several causes. Easily dismissed explanations include 

misalignment of the receive antenna, and compass errors in the mooring current meters. The HF 

radar receive antennas were carefully oriented in bearing through a calibration procedure. 

Repeated checks indicated antenna alignments are constant in time to within ~l°. Misalignment 

26 
Summary of findings for using high frequency radar in physical oceanographic and ecological studies 

of the receive antenna would result in constant ∆θ at each of the moorings around a given radar. 

Figure 9 shows that this is not the case: ∆θ varies in magnitude and sign for each mooring-radar 

pair. Similarly, current meter compass error cannot explain the variation of ∆θ with bearing. If 

compass error alone accounted for ∆θ, then pairs using the same VMCM would show similar ∆θ. 

Table 1 shows this is not true: for example, for the FBK-SAOF pair ∆θ = -8°, but for ARGSAOF ∆θ = 19°. 

A more likely explanation of ∆θ is given by (Barrick and Lipa 1986), who examined the 

influence of antenna patterns on radial currents. Antenna patterns describe the directional 

response of the receive antenna to incoming HF radiation. Ideally, only the antenna’s design 

determines the antenna pattern. However, the patterns are also affected by conductors located 

within about one wavelength of the antenna (~25 m), referred to as the near field. These 

conductors couple with the antenna, distort the antenna pattern, and produce errors in bearing. 

For example, (Barrick and Lipa 1986) found severely distorted antenna patterns during a 

deployment on an offshore oil platform, with rms bearing offsets ~35°. We measured antenna 

patterns at PTC, RFG, and COP by moving a transponder in a small boat along circular arcs 

within the coverage areas of the radars. Antenna patterns from these sites exhibited varying 

levels of distortion (data not shown). Antenna patterns measured at REG and PTC were typical 

of patterns found elsewhere, while patterns at COP were moderately distorted (D. Barrick, 

personal communication, 1999). 

In principle, antenna pattern distortions can be accounted for in the MUSIC algorithm, and 

techniques for doing so are in development (e.g., Barrick and Lipa 1999). During the data 

collection phase of this study, corrections for antenna pattern distortion were unavailable, and 

HF data used to compute ∆θ assumed ideal antenna patterns. We compared observations of ∆θ

with measured antenna patterns to look for a direct relationship between distortions in the 

measured patterns and non-zero values of ∆θ. For example, following a suggestion by J. Paduan 

(personal communication, 2000), a relationship between ∆θ and the rate of change of the 

measured antenna responses versus bearing was investigated. Extensive experimentation failed 

to reveal a consistent relationship between ∆θ and measured antenna patterns. When procedures 

for incorporating measured antenna patterns into estimates of VHF are available, we intend to test 

them by mapping r2

 around moorings as in figures 7 and 8. We hypothesize that accounting for 

antenna pattern distortions would result in lower ∆θ. 

Another factor affecting ∆θ is phase calibration of the three-element receive antenna. Each 

element responds to incoming signals with differing voltage phases. Phases were initially 

determined at PTC from sea echo as part of the CODAR data processing procedures (Barrick and 

Lipa 1986). Later, phases were directly measured with the transponder during the antenna pattern 

measurements and used in processing. We computed ∆θ between PTC and SMIN for each of 

these time periods to test the effect of phase settings. Using sea echo phases resulted in ∆θ

~=30°, while transponder phases resulted in ∆θ ,~=15°. The reduction in ∆θ using the 

transponder phase suggests a strong link between receive antenna characteristics and ∆θ. 

Bearing offsets ∆θ in HF radial data ultimately produce errors in total velocity vectors 

determined from two or more radars. We examined effects of variable ∆θ on total velocity 

vectors by simulating its effect on an idealized flow field. For simplicity, we used a uniform

westward flow along a straight coast (gray arrows, Figure 10). Patterns of VHF as functions of 

range and bearing were computed at two sites labeled A and B in Figure 10. At site A, bearings 

27 
Final Study Report – Washburn and Gaines 

to sectors on the sea surface were distorted by ∆θ using the slope of the linear least square fit for 

the PTC radar (gray straight line, Figure 9): over bearings 90° - 270°, ∆θ in the range -l8° to 14° 

were added to the bearing. Total velocity vectors (black arrows, Figure 10) were computed on a 

2 km grid, from VHF at sites A and B. Following the method of (Gurgel 1994), a total velocity 

vector at each grid point was computed from all VHF within a circle (3 km radius) centered on the 

grid point. A similar procedure was used by Paduan and Rosenfeld (1996). The mean difference 

in flow speeds between the original vectors (gray arrows, Figure 10) and distorted vectors (black 

arrows, Figure 10) was ~7% of the original uniform flow speed with a maximum difference of 

~15%. The mean difference in flow direction was ~2.5°, with a maximum of ~9°. Simulations 

with other simple flow fields produced comparable errors in total velocities (results not shown). 

These errors are significant and reinforce the need for incorporation of antenna pattern 

corrections in the direction finding algorithms used with these radar systems. 

34.5 A B

34.4 

34.3 

Latitude 

34.2 

34.1 

-120.7 -120.6 -120.5 -120.4 -120.3 -120.2 -120.1 -120.0 -119.9 

Longitude

Figure 10. Gray arrows (offset for clarity) show uniform westward flow parallel to a straight coastline (bold line and

shading) used for modeling effects of bearing offset ∆θ on total velocity vectors. Arrows were placed on grid with 2

km spacing. Black arrows show total vectors computed from radar sites located at A and B (triangles). Radial 

currents VHF measured at A were distorted by bearing offset ∆θ as explained in text. 

28 
Summary of findings for using high frequency radar in physical oceanographic and ecological studies 

5. Conclusions 

We compared radial components of near-surface ocean currents (radials) from 18 pairs of 

moored current meters and five HF radars (transmitting at frequencies of 12-13 Mhz) from the 

Santa Barbara Channel and Santa Maria Basins. Comparisons were based on observations 

between June 1997 and November 1999 with record lengths of 5 to 424 days. Eight vector 

measuring current meters moored at 5 m depth and one acoustic Doppler current profiler with its 

shallowest bin at 3.2 m were compared with HF radar currents at ~1 m. Our analysis supports the 

following conclusions: 

1. Radials obtained from the radars were significantly correlated with radials obtained from 

the moored current meters with r2 in the range 0.39-0.77. Root-mean-square (rms) radial 

speed differences ranged from 7-19 cm s-1. A weak trend of increasing rms differences 

was found with increasing range. 

2. Significant coherence was found between current meter and radar-derived time series for 

frequencies below 2.2 cpd (11 hour period and longer). Power spectra show similar 

magnitudes and slopes for frequencies below 2.2 cpd. 

3. A pointing error ∆θ ranging from -l6° to19° was found for some of the radar sites where 

positive values indicate a clockwise error. ∆θ for a given radar was not constant, but 

varied with bearing. We speculate that ∆θ resulted from distortions of the receive antenna 

patterns in the near field. 

4. We used a simple model of ∆θ versus bearing based on our observations to simulate 

errors in total velocity vectors computed from two radars. Using a uniform flow parallel 

to shore, ∆θ produced speed errors of up 15% and direction errors up to 9° in total 

velocity vectors. 

29 
Final Study Report – Washburn and Gaines 

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Final Study Report – Washburn and Gaines 

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