OCS Study MMS 2006-032 

Final Report 

CODAR in Alaska 

Principal Investigator: Dave Musgrave, PhD 

Associate Professor 

School of Fisheries and Ocean Sciences 

University of Alaska Fairbanks 

P.O. Box 757220 

Fairbanks, AK 99775-7220 

(907) 474-7837 

musgrave@ims.uaf.edu 

CoAuthor: Hank Statscewich, MS 

School of Fisheries and Ocean Sciences 

University of Alaska Fairbanks 

P.O. Box 757220 

Fairbanks, AK 99775-7220 

(907) 474-5720 

stats@ims.uaf.edu 

June 2006 
Contact information 

e-mail: carter@sfos.uaf.edu 

 phone: 907.474.1811 

 fax: 907.474.1188 

 postal: Coastal Marine Institute 

 School of Fisheries and Ocean Sciences 

 P.O. Box 757220 

 University of Alaska Fairbanks 

 Fairbanks, AK 99775-7220 

 ii
Table of Contents 

List of Tables ................................................................................................................................. iv 

List of figures................................................................................................................................. iv 

Abstract........................................................................................................................................... v 

Introduction..................................................................................................................................... 1 

Methods........................................................................................................................................... 2 

Results............................................................................................................................................. 4 

Spatial and Temporal Coverage ................................................................................................................4

Tidal Variability ........................................................................................................................................4

Subtidal Variability ...................................................................................................................................5

Model verification .....................................................................................................................................5

HF Radar – Drifter Comparisons ..............................................................................................................5

Discussion....................................................................................................................................... 6 

References....................................................................................................................................... 7 

 iii
List of Tables 

Table 1: Tidal Current Parameters for a single CODAR grid point.............................................................8

Table 2: Bulk tidal ellipse parameters from observations (HF radar), model results (Foreman) and the 

difference between the two. .............................................................................................................8

List of figures 

Figure 1: Study area for HF radar surface currents in Cook Inlet, Alaska. Boxes identifying Sites 1 and 

2 are the locations of the HF radar systems. They are 25 km apart and are located north and south 

of the Kenai River. This graphic shows data from 21:00 GMT, March 21, 2003. The pointing 

direction of the arrows indicate the direction of flow of the surface currents, and the arrow length 

indicates the magnitude of the current (magnitude scale is located in the lower right corner). The 

region of coverage for two dimensional velocities extends more than 10 km north of Site 1, more 

than 10km south of Site 2, and 45 km across the inlet, with a spatial resolution of about 2 km. 

Locations of the Drift River and Nikiski meteorological stations are also plotted................................9

Figure 2: Map of Cook Inlet showing the bathymetry (m) of the region...................................................10

Figure 3: Map of Cook Inlet HF Radar domain. Small dots represent total vector grid points and contour 

lines represent the Geometric Dilution of Precision (GDOP) in both the east-west and north-south 

directions. The plus symbols represent grid points where total vectors were observed greater than 

50% of the time and where the GDOP < 1.8. ......................................................................................11

Figure 4: Raw time series of (a) the number of current vectors at the Cook Inlet HF radar array (b) the 

tidal height amplitude from the Nikiski tide gauge and (c) wind speed from Nikiski.........................12

Figures 5: Tidal ellipses for a) K1, b) M2, c) N2 and d) S2 tidal constituents over the whole period. 

Every fifth grid point is plotted. Bathymetry (m) is indicated by the contour lines. ..........................13

Figures 6: Mean currents for a) the whole period, b) March and c) May. The surface current velocity 

scale is given in the lower right corner. The arrow on Kalgin Island gives the pseudo wind stress 

(magnitude = wind velocity squared, direction = wind direction). The scale for the wind stress 

magnitude is given in the upper left.....................................................................................................17

Figure 7: Current variance ellipses plotted over bathymetry contours. The principal axes of the subtidal 

current variability are polarized along the axis of the inlet with major axis magnitudes ranging 

between 20 and 40 cm s-1. A scale is found in the lower right corner ................................................20

Figure 8: Drifter trajectories for three drifters deployed in Cook Inlet during the HF radar experiment, 

2003. ....................................................................................................................................................21

Figure 9: Scatter plot of the drifter-HF radar velocity comparison. The upper (lower) figures give the 

regression of the HF radar velocities against the drifters for the longitudinal (meridional) velocities. 

The figures on the left are based on the predicted velocities based on the tidal predictions from the 

tidal analysis, and the figures on the right are for the actual HF radar data. .......................................22

Figure 10: Histograms of velocity component differences between HF HF hhhhhradar and drifter 

derived currents in Cook Inlet study for six drifters............................................................................23

 iv
Abstract 

From December 2002 through June 2003, 

surface currents were measured and analyzed 

near the mouth of the Kenai River in upper Cook 

Inlet (Figure 1). This was achieved by utilizing 

High Frequency (HF) radar systems 

manufactured by Coastal Ocean Dynamic 

Applications Radar (CODAR) Ocean Sensors, 

which operated continuously, aside from a 

seven-day dropout of data due to a damaged 

computer modem. Spatial coverage of the data 

return varied over the semidiurnal tidal period 

due to exposed beach seaward of the radar 

antennas during low tides, and over subtidal 

frequencies spatial coverage was reduced during 

periods of no or slight winds, presumably due to 

the absence of significant energy in the wave 

field at the Bragg wavelength. Tidal currents 

accounted for about 90% of the total current 

variance, and tidal ellipses for all tidal 

components were polarized along the axis of the 

inlet. The dominant component was the lunar 

semidiurnal (M2), with major axis magnitudes 

ranging from 30 – 200 cm s-1. Mean subtidal 

currents were as great as 25 cm s-1, and the 

current patterns were related to the wind 

direction. 

Particle excursions due to tides are between 

4 – 32 km in this section of Cook Inlet, and 

predicted tidal current ellipses agree quite well 

in magnitude and phase with the tidal model of 

Foreman et al. (2000). 

Comparison of surface velocities from the 

HF radar with drifters drogued at 7 m gave rootmean-square (rms) differences in the range of 40 

– 100 cm s-1. The large differences are probably 

due to large vertical shear in the upper 7 m of 

the water column. 

 v
Introduction 

Electromagnetic remote sensing of ocean 

currents from the shoreline using HF radar 

provides a complete mapping capability that has 

become a cost-effective solution for coastal 

current mapping. This technique was developed 

more than three decades ago and has received 

wide acceptance in the oceanographic 

community. Currently, there are approximately 

150 of these instruments used around the world 

for the purpose of measuring ocean currents. HF 

radar formally spans the frequency range of 3 – 

30 MHz (with wavelengths between 10 m at the 

upper end and 100 m at the lower end). A 

vertically polarized HF signal propagated along 

the electrically conductive ocean water surface 

can travel well beyond the line-of-sight, where 

more common microwave radars become blind. 

Rain or fog does not affect HF signals. 

Over the past 25 years, oceanographic radar 

techniques have been developed and improved 

so that detailed, gridded, two-dimensional maps 

of surface circulation can be provided and 

recorded in real time (Barrick 1978, Prandle 

1989). Currents play a critical role in the 

transport and fate of spilled oil, but there is a 

paucity of direct circulation measurements in 

some areas of Cook Inlet. Current meters 

provide data at only a few specific points and 

not at the water surface, where the oil would be 

found. HF radar techniques provide a measured 

equivalent of a gridded circulation model and 

can be used as input and validation for oil spill 

trajectory models. The accuracy of the surface 

current measurements has been reported to be 

between 4 – 20 cm s-1

These systems operate by transmitting a low 

power (40 watt average) radio signal frequency 

from land, which is reflected in all directions by 

the surface wave field. In accordance with 

Bragg scattering theory, reflected 

electromagnetic (em) waves that are exactly 

one-half the wavelength of the transmitted em 

waves add constructively to give a peak in 

radiation. This reflected Bragg peak is Doppler 

shifted in frequency by the combined effects of 

ocean waves and ocean currents. Utilizing well 

known wave dispersion theory, the speed of the 

surface gravity wave is subtracted from the total 

speed measured by the HF radar 

instrumentation. The resulting quantity 

represents the component of the surface ocean 

current which is traveling in a radial direction, 

either toward or away, from the transceiver. HF 

radar systems use radial components from at 

least two sites to determine the total current 

vector at a given point. 

The physical oceanography of Cook Inlet, 

Alaska, is characterized by complex circulation 

associated with variability at tidal, seasonal, 

annual and interannual time scales. As this 

region has the fourth largest tidal range in the 

world, the circulation is dominated by tidally 

driven flows with current speeds of up to 300 cm 

s

-1 (6 knots). Knowledge of the tidal and subtidal current patterns in Cook Inlet is essential 

for determining and predicting transport 

pathways affecting pollutants. In this study, 

surface currents measured by HF radar 

instrumentation are used to characterize the 

circulation of the area and assess the skill of 

numerical simulations of this system. 

Middle Cook Inlet is a dynamic region with 

significant sub-tidal circulation, including a 

south flowing buoyancy driven current along the 

western shore (Johnson and Okkonen, 2000). 

The bathymetry of the region is characterized by 

two 50 – 100 m deep troughs located in the 

center of the inlet and on the western side of 

Kalgin Island (Figure 2). 

This study's objectives were to investigate 

the utility of using HF radar in Cook Inlet for 

measuring surface currents. In addition, bistatic 

systems were investigated as a potential less 

expensive alternative to the monostatic system 

that we deployed. 

 1
Methods 

An array of two Seasondes (CODAR Ocean 

Sensors’ HF radar systems) measured hourly 

surface currents near the mouth of the Kenai 

River on the eastern shore of Cook Inlet. The 

systems were deployed in December 2002 and 

operated until June 2003. Each site consisted of 

a transmit antenna, a receive antenna, radar 

electronics and a computer for control and data 

logging. The radar electronics were housed in 

heated structures near the antennas and were 

powered by utilizing electricity from nearby 

private homes. The antennas were located as 

close to the ocean as possible, less than 100 

meters from the sea surface, to minimize signal 

attenuation by propagation over land. 

Unfortunately, at low tide long stretches of 

wetted sand were exposed during periods of 

large tidal fluctuations at the southern Seasonde 

site (Site 2, Kalifornski Beach). Signal 

attenuation during these periods was large and 

contributed to fluctuations in the spatial 

coverage of this radar unit. 

Extremely accurate GPS time 

synchronization permitted the two radar units to 

transmit concurrently at 13.51 MHz. At this 

frequency the transmitted signal has a 

wavelength of 22 m, yielding a wavelength (λ ) 

of the surface Bragg waves of 11 m. 

The Bragg scattering effect results in two 

discrete, well defined peaks in the Doppler 

spectrum. In the absence of any ocean current, 

the position of these echo peaks is symmetric 

and their frequency is given by 

λ

2C0 f B ±=

where 0 C g( /2

Δ = Vf r /2 λ

where is the radial component of the 

effective surface current along the look direction 

of the radar. 

Vr

Backscattered radar signals are Doppler 

shifted by an amount proportional to the intrinsic 

deep water phase velocity of 11 m ocean waves. 

Radial ocean current velocities are then found by 

calculating the difference between the measured 

and the intrinsic phase velocities. This 

difference corresponds to the average current 

from the surface to a depth of order λ 4/ π

(Stewart and Joy 1974), or approximately 90 cm 

at 13 MHz transmit frequency. Twodimensional surface currents can be computed 

by geometrically combining the components 

along radar beams emanating from two radar 

stations using the method described by Graber et 

al. (1997). 

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 Multiple 

Signal Classification (MUSIC) based processing 

algorithm divides these range cells into 

azimuthal sectors of ocean surface and estimates 

the currents in each sector (Schmidt 1986; 

Barrick and Lipa 1997). Each sector collected 

hourly current measurements at 1.5 km in range 

and 5° in bearing. The coverage of the systems 

typically extended 40 km and across the inlet to 

Kalgin Island. In contrast to in situ sensors, 

which essentially measure currents at a point, 

the sector areas associated with HF radar vary 

linearly from 0.2 – 5.5 km2

 as range varies from 

1.5 – 40 km. Current measurements represent 

an average of surface currents within the sector 

area. More precisely, the HF radar current 

estimates are based upon all of the spectral 

information obtained within the resolved area. 

Horizontal variations in the current field with 

scale lengths less than 1.5 km will result in 

broadened or multiple Doppler spectral peaks. 

A detailed discussion of the MUSIC processing 

algorithm and its limitations may be found in 

= λ π ) is the linear phase 

speed of the surface Bragg wave in deep water 

and is the acceleration due to gravity. A 

current underlying the surface waves causes a 

translation of the entire coordinate frame with 

respect to a stationary observer or shore based 

radar stations. Thus the Bragg peaks in the 

Doppler spectra are displaced from the Bragg 

frequency by an amount 

g

Bf

 2
Emery et al. 2004. In the case of the Cook Inlet 

study, Doppler spectra are computed every 10 

minutes and then processed to produce radial 

current estimates. The 10-min radial current 

data are then averaged to produce hourly radial 

vectors for each sector. The 256-point transform 

used to estimate cross spectra limits the radial 

speed resolution to 4 cm s-1. 

Measurements of meteorological variables 

were obtained from two sensors located within 

the HF radar coverage area. The Drift River 

station is operated by the National Data Buoy 

Center (NDBC). Hourly averaged 

measurements of wind speed and direction, air 

temperature, barometric pressure and dew point 

are available from the NDBC website. The 

National Oceanographic and Atmospheric 

Administration (NOAA) National Ocean Service 

(NOS) division operates another meteorological 

and tide gauging station in Nikiski, AK. 

Measurements found on the NOAA NOS 

website provide wind speed and direction, air 

temperature, barometric pressure and water level 

for Nikiski. 

We compared estimates of surface velocities 

from the surface current mappers with drifting 

buoys deployed in this area by Dr. Mark 

Johnson (UAF). Details of the design, 

deployment, processing of the data from the 

drifters can be found at 

http://www.ims.uaf.edu/research/johnson/cmi/co

okinletbuoys.html. 

 3
Results 

Spatial and Temporal Coverage 

An overall indicator of radar performance is 

spatial coverage over time. Coverage is defined 

as the number of sectors returning radials each 

hour. Since a two site radar network is required 

to produce two-dimensional surface current 

maps, we modify the definition of coverage to 

represent data recovery in a two-dimensional 

sense. In order to transform radial currents to 

vector maps, a rectangular grid is defined which 

extends throughout the maximum possible 

coverage area. Vector estimates were then 

attempted for each grid location using a least 

squares vector fit (Lipa and Barrick, 1983), 

which was applied to all data within a radius of 

3 km. An average is calculated and mapped 

onto the grid. These mapped vectors represent 

the “raw” data used in this study. Thus coverage 

is defined as the number of grid points where 

vector currents are measured. The hypothetical 

coverage area under which total vectors may be 

created is shown in Figure 3. In this figure small 

dots represent total vector grid points and 

contour lines represent the Geometric Dilution 

of Precision (GDOP) assumed when calculating 

a total vector from two radials (Chapman et al. 

1997). GDOP is an amplification factor of the 

error estimate placed on a particular vector 

current measurement. These errors arise as the 

angle of incidence between two radial currents 

deviates from 90°, as the ability to resolve 

perpendicular velocity components is not 

possible along the baseline of the two radial 

sites. 

 Gaps in the time series of vector currents 

arise when there were insufficient radial data 

nearby to estimate the vector current. Gaps in 

the radial data occur when: (1) the MUSIC 

direction finding algorithm cannot resolve all 

angular directions for a given range cell, (2) low 

signal to noise ratios cause an inability to 

discern Bragg sea echo, usually resulting in 

range fluctuations, and (3) the system is down. 

There were many data gaps caused by the first 

two reasons, but the third was not much of an 

issue as both sites operated 95% of the time 

during the experiment. There was one major 

power outage on the Kenai Peninsula on March 

28, 2003, which caused two failures within the 

northern radar system. The first was a power 

surge in the telephone line which damaged the 

computer modem, and the second was a total 

discharge of the Uninterruptible Power Supply 

(UPS) connected to the radar electronics. A 

new modem and UPS were installed at this site, 

but the failures caused a loss of data for 7 

consecutive days. The coverage ranged from no 

data during times of very low tides with no 

winds to excellent (Figure 4). 

The number of current measurements 

collected over any single hour in the Cook Inlet 

HF radar study area varies from 0 – 1200 

(Figure 4). Since propagation of the HF radar 

radio ground wave relies on seawater as a 

conductive medium, coverage is modulated by 

the extent of land the radio signal must travel 

over before reaching seawater. Time series of 

coverage, sea-level and wind speed from Nikiski 

indicate that there is a strong correlation 

between environmental conditions and system 

performance. First, at semidiurnal frequencies 

there is a high squared coherence (γ

2

 = 0.9) 

between coverage and water level due to the 

significant amount of exposed beach at low tide. 

This is similar to the findings of Emery et al.

(2004) although their coverage variations were 

expressed at diurnal frequencies. Variations 

resulted in patchiness and fluctuations in range 

as observed by Prandle et al. (1993) and Paduan 

and Rosenfeld (1996). Furthermore, high 

squared coherence (γ

2

 = 0.8) exists at a 2.5 day 

period between the number of totals and wind 

speed, suggesting that higher wind speeds create 

a more favorable environment for the HF system 

at 12 MHz. 

Tidal Variability 

 In the following discussions, we used data 

from the grid (Figure 3) that had greater than 

50% coverage over the entire period of 

operation. The matlab program “t_tide” 

(Pawlowicz et al., 2002) was used at each grid 

point in the domain to calculate the tidal 

 4
constituents. Table 1 shows the results for one 

point near the center of the domain. Tidal 

currents are dominated by the M2 tidal 

constituent with current magnitudes ranging 

from 30 – 200 cm s-1. Tidal currents account for 

approximately 66 – 95% of the total current 

variance with a mean of 89%. Due to the strong 

north-south polarization of the tidal currents, 

only 67% (mean over the domain) of the 

variance is predicted in the longitudinal (eastwest) direction; however 90% of the variance is 

predicted in the meridional direction. Tidal 

current ellipses (Figure 5) for the four major 

tidal constituents reveal a strong polarization of 

tidal currents in the north-south direction, 

consistent with steering by local bathymetry and 

orientation of Cook Inlet. The magnitudes are 

generally greater and more polarized in the mid 

inlet region than near the sides for all 

constituents. Particle excursion distances, δ = U 

* T / π , where U is the magnitude of the major 

axis and T the period, are between 4 and 32 km. 

Tidal analyses over monthly periods have 

similar results. 

Subtidal Variability 

For analysis of subtidal flows, a 33 hour 

filter was applied to time series of data at each 

grid point (Beardsley et. al, 1983). The mean 

flow over the whole period is shown in Figure 

6a. The mean wind stress (actually pseudo-wind 

stress which is the magnitude squared in the 

direction of the wind) measured at Drift River is 

northerly (blowing from the north) and is 

aligned with the axis of Cook Inlet. The current 

pattern shows persistent southward currents 

along the northeast side of Kalgin Island with 

speeds up to 25 cm s-1, northward currents in the 

center of the Inlet and southward flow near Site 

2 in the eastern Inlet. The monthly averaged 

currents all show the persistent southward flow 

along Kalgin Island, but when the winds are 

strong in a southward direction (Figure 6b), the 

northward flow in the center of the Inlet is 

decreased, and the southerly flow near Site 2 is 

greater. When the winds are reversed (Figure 

6c), the mid inlet northward flow is greater and 

the southward flow near Site 2 is less. While 

subtidal current speeds are significantly weaker 

than tidal currents, they may dominate transport 

processes at time scales longer than the 

dominant tidal periods due to their persistence. 

The principal axes of the subtidal current 

variability are polarized along the axis of the 

inlet with major axis magnitudes ranging 

between 20 and 40 cm s-1 (Figure 7). The 

principal axes are more polarized in midinlet. 

Model verification 

Tidal predictions from HF radar 

instrumentation are used to assess the accuracy 

of the Foreman (2000) barotropic numerical tide 

model for the region. Differences between the 

major axes of HF radar derived and modeled 

tidal current ellipses fall into the range of 0.6 – 

24 cm s-1 and phase differences are between 35o 

and 234o (Table 2). Large discrepancies 

between the tidal constituent parameters from 

the model may be a result of inadequate 

representation of the complex bathymetry of the 

region. 

HF Radar – Drifter Comparisons

Three current-following drifters were 

deployed in the coverage area of the HF radar 

instrumentation during the study and persisted in 

the area for periods ranging from 2 – 4 weeks 

(Figure 8). The drifters were of a holey sock 

design and were deployed in the vicinity of the 

central tide rip at the deepest section of the inlet, 

drouged 10 m below the surface. Collocated HF 

radar surface current measurements show a high 

degree of scatter with RMS velocity differences 

in the range of 40 – 100 cm s-1 (Figure 9). 

Vertical shear in the water column due to the 

presence of strong stratification appears to limit 

the usefulness of extrapolating full water column 

velocities from surface current measurements in 

Cook Inlet. Relatively large differences in the 

drifter-HF radar velocities may skew these 

average statistics (Figure 10). 

 5
Discussion 

As expected, tidal currents dominate the 

flows in Cook Inlet and account for about 70 – 

90% of the total variance. All variability (tidal 

and subtidal) is polarized along the axis of the 

inlet. Patterns of mean surface currents show 

across inlet variability with (generally) 

southward flow in the west, a reversal of flow in 

mid inlet and southward flow in the east, 

although winds can affect this pattern. 

Comparison of Foreman’s tidal model 

(2000) with the HF radar-derived tidal 

constituents shows good agreement, while the 

agreement between drifters drogued at 10 m and 

the HF radar surface currents is poor due to large 

vertical shears. 

 6
References 

Barrick, D.E. 1978. HF radio oceanography – 

A review. Bound.-Lay. Meteorol. 13(1–

4):23–43. 

doi: 10.1007/BF00913860 

Barrick, D.E., and B.J. Lipa. 1997. Evolution 

of bearing determination in HF current 

mapping radars. Oceanography 10(2):72–75. 

Beardsley, R.C., R. Limeburner and L.K. 

Rosenfeld. 1985. Introduction to the 

CODE-2 moored array and large-scale data 

report. In R. Limeburner [ed.], CODE-2: 

Moored Array and Large-Scale Data Report. 

Woods Hole Oceanographic Institution 

Technical Report No. 38, WHOI-85-35. 

Chapman, R.D., L.K. Shay, H.C. Graber, J.B. 

Edson, A. Karachintsev, C.L. Trump and 

D.B. Ross. 1997. On the accuracy of HF 

radar surface current measurements: 

Intercomparisons with ship-based sensors. J. 

Geophys. Res. 102(C8):18,737–18,748. doi: 

10.1029/97JC00049 

Emery, B.M., L. Washburn and J.A. Harlan. 

2004. Evaluating radial current 

measurements from CODAR highfrequency radars with moored current 

meters. J. Atmos. Ocean. Tech. 21(8):1259–

1271. doi: 10.1175/1520-

0426(2004)021<1259:ERCMFC>2.0.CO;2 

Foreman, M.G.G., W.R. Crawford, J.Y. 

Cherniawsky, R.F. Henry and M.R. 

Tarbotton. 2000. A high-resolution 

assimilating tidal model for the northeast 

Pacific Ocean. J. Geophys. Res. 

105(C12):28,629–28,652. doi: 

10.1029/1999JC000122 

Graber, H.C., B.K. Haus, R.D. Chapman and 

L.K. Shay. 1997. HF radar comparisons 

with moored estimates of current speed and 

direction: Expected differences and 

implications. J. Geophys. Res. 

102(C8):18,749–18,766. doi: 

10.1029/97JC01190 

Johnson, M.A., and S.R. Okkonen [eds.]. 2000. 

Proceedings Cook Inlet Oceanography 

Workshop. November 1999, Kenai, Alaska. 

Final Report. OCS Study MMS 2000-043, 

University of Alaska Coastal Marine 

Institute, University of Alaska Fairbanks and 

USDOI, MMS, Alaska OCS Region, 118 p. 

Lipa, B.J., and D.E. Barrick. 1983. Leastsquares method for the extraction of surface 

currents from CODAR crossed-loop data: 

Application at ARSLOE. IEEE J. Oceanic 

Eng. 8(4):226–253. 

Paduan, J.D., and L.K. Rosenfeld. 1996. 

Remotely sensed surface currents in 

Monterey Bay from shore-based HF radar 

(Coastal Ocean Dynamics Application 

Radar). J. Geophys. Res. 101(C9):20,669–

20,686. doi: 10.1029/96JC01663 

Pawlowicz, R., B. Beardsley and S. Lentz. 

2002. Classical tidal harmonic analysis 

including error estimates in MATLAB using 

T_TIDE. Computers and Geosciences. 

28(8):929–937. 

doi: 10.1016/S0098-3004(02)00013-4 

Prandle, D., and D.K. Ryder. 1989. 

Comparison of observed (HF radar) and 

modelled nearshore velocities. Cont. Shelf 

Res. 9(11):941–963. doi: 10.1016/0278-

4343(89)90001-0 

Prandle, D., S.G. Loch and R. Player. 1993. 

Tidal flow through the Straits of Dover. J. 

Phys. Oceanogr. 23(1):23–37. doi: 

10.1175/1520-

0485(1993)023<0023:TFTTSO>2.0.CO;2 

Schmidt, R.O. 1986. Multiple emitter location 

and signal parameter estimation. IEEE 

Trans. Antenn. Propag. 34(3):276–280. 

Stewart, R.H., and J.W. Joy. 1973. HF radio 

measurements of ocean currents, p. 64–67. 

In Proceedings IEEE International 

Conference on Engineering in the Ocean 

Environment, 25–28 September, Seattle, 

Washington. 

 7
Table 1: Tidal Current Parameters for a single CODAR grid point.

Tidal 

Constituent 

Period (hours) Major Axis 

(cm s-1) 

Minor Axis 

(cm s-1) 

Inclination (deg) 

of the major axis 

Phase 

(deg, 

relative to 

UTC) 

Signal to 

Noise 

Ratio 

M2 12.42081729 126.962 -14.697 88.52 322.98 66 

S2 12.00048002 26.555 1.954 75.89 310.3 3.1 

K2 11.96744854 23.023 1.694 75.89 245.21 2.3 

K1 23.93489708 14.737 1.007 82.93 193.85 24 

N2 12.65822785 10.931 2.33 97.85 301.25 0.5 

P 24.06738869 9.554 0.653 82.93 245.82 10 

O1 25.81977795 6.349 -0.874 80.56 235.88 4.5 

Q1 26.86727566 1.671 0.242 109.7 311 0.33 

Table 2: Bulk tidal ellipse parameters from observations (HF radar), model results (Foreman) and the 

difference between the two. 

Tidal 

Constituent 

Period 

(hours) 

Major Axis 

(cm s-1) 

Minor Axis 

(cm s-1) 

Phase 

(deg) 

Inclination 

(deg) 

M2 12.4206 HF radar 102.3536 8.7606 -37.9433 76.8797 

Foreman 126.7034 7.6395 -13.0339 76.1858 

Foreman - radar 24.3498 -1.1212 24.9094 -0.6939 

S2 12.0005 HF radar 23.8658 0.831 -47.1677 72.7341 

Foreman 45.4339 2.7879 24.7659 76.1949 

Foreman - radar 21.5681 1.9569 71.9337 3.4608 

K2 11.9672 HF radar 20.6916 0.7205 -112.2577 72.7341 

Foreman 10.6274 0.6643 15.9802 76.9211 

Foreman - radar -10.0642 -0.0561 128.2379 4.187 

K1 23.9345 HF radar 12.555 0.9161 -175.9291 74.467 

Foreman 17.1127 0.8856 59.0164 -99.6886 

Foreman - radar 4.5578 -0.0304 234.9455 -174.1556 

N2 12.6583 HF radar 10.3086 0.3696 -91.1233 79.0191 

Foreman 25.6134 1.5303 -41.1007 76.2084 

Foreman - radar 15.3048 1.1607 50.0226 -2.8107 

P1 24.0659 HF radar 8.1394 0.5939 -123.9591 74.467 

Foreman 2.5099 0.0609 -88.1216 70.6166 

Foreman - radar -5.6295 -0.5329 35.8375 -3.8504 

O1 25.8193 HF radar 3.7673 0.5777 -126.0075 87.1827 

Foreman 13.4784 0.6131 60.0469 -101.4094 

Foreman - radar 9.7112 0.0354 186.0545 -188.5921 

Q1 26.8684 HF radar 1.404 0.1768 -74.9323 77.638 

Foreman 0.8131 0.0278 79.6649 -73.4447 

Foreman - radar -0.5909 -0.149 154.5973 -151.0826 

 8
15' 152o

W 45' 30' 15' 12'

18'

60o

N

24.00'

30'

36'

42'

Site 1

Site 2

250 cm/s

Nikiski Winds

Drift River Winds

Velocity

Scale

Kalgin I

Figure 1: Study area for HF radar surface currents in Cook Inlet, Alaska. Boxes identifying Sites 1 and 2 

are the locations of the HF radar systems. They are 25 km apart and are located north and south of the 

Kenai River. This graphic shows data from 21:00 GMT, March 21, 2003. The pointing direction of the 

arrows indicate the direction of flow of the surface currents, and the arrow length indicates the magnitude 

of the current (magnitude scale is located in the lower right corner). The region of coverage for two 

dimensional velocities extends more than 10 km north of Site 1, more than 10km south of Site 2, and 45 

km across the inlet, with a spatial resolution of about 2 km. Locations of the Drift River and Nikiski 

meteorological stations are also plotted. 

 9
15' 152o

W 45' 30' 15' 12'

18'

60o

N

24.00'

30'

36'

42'

-10

-10

-10

-10

-10

-10

-10

-1

-1

0

0

-10

-10

-20

-20

-20

-20

-20

-20

-20

-20

-20

-20

-20

-30

-30 -30

-30

-30

-30

-30

-30

-30

-40

-40

-40

-40

-40

-40

-40 -40

-40

-50

-50 -50 -50

-50

-50

-50

-50

-60

-60

-60

-60

-70

-70

-70

-70

-80

-80

-80

West

Forelands

East

Forelands

Nikiski Tide Gauge

Site 1

Site 2

Drift River

Meteorological

Station

Kalgin I

Figure 2: Map of Cook Inlet showing the bathymetry (m) of the region. 

 10
Figure 3: Map of Cook Inlet HF Radar domain. Small dots represent total vector grid points and contour 

lines represent the Geometric Dilution of Precision (GDOP) in both the east-west and north-south 

directions. The plus symbols represent grid points where total vectors were observed greater than 50% of 

the time and where the GDOP < 1.8. 

 11
Figure 4: Raw time series of (a) the number of current vectors at the Cook Inlet HF radar array (b) the 

tidal height amplitude from the Nikiski tide gauge and (c) wind speed from Nikiski. 

 12
5A 

−50

−40

−40

−30

−30

−20

−20

20 cm/s

−75

 151° 152 151°

 36’ W 12’ W °

 00’ W 

 60°

 42’ N 

 60°

 36’ N 

 60°

 30’ N 

 60°

 24’ N 

 60°

 18’ N 

 151° 151 24’ W °

 48’ W 

Site 2

Site 1

Tidal Ellipses for K1

Figures 5: Tidal ellipses for a) K1, b) M2, c) N2 and d) S2 tidal constituents over the whole period. 

Every fifth grid point is plotted. Bathymetry (m) is indicated by the contour lines. 

 13
5B 

−50

−40

−40

−30

−30

−20

−20

120 cm/s

−75

 151° 152 151°

 36’ W 12’ W °

 00’ W 

 60°

 42’ N 

 60°

 36’ N 

 60°

 30’ N 

 60°

 24’ N 

 60°

 18’ N 

 151° 151 24’ W °

 48’ W 

Site 2

Site 1

Tidal Ellipses for M2

 14
5C 

−50

−40

−40

−30

−30

−20

−20

20 cm/s

−75

 151° 152 151°

 36’ W 12’ W °

 00’ W 

 60°

 42’ N 

 60°

 36’ N 

 60°

 30’ N 

 60°

 24’ N 

 60°

 18’ N 

 151° 151 24’ W °

 48’ W 

Site 2

Site 1

Tidal Ellipses for N2

 15
5D 

−50

−40

−40

−30

−30

−20

−20

40 cm/s

−75

 151° 152 151°

 36’ W 12’ W °

 00’ W 

 60°

 42’ N 

 60°

 36’ N 

 60°

 30’ N 

 60°

 24’ N 

 60°

 18’ N 

 151° 151 24’ W °

 48’ W 

Site 2

Site 1

Tidal Ellipses for S2

 16
6A 

Figures 6: Mean currents for a) the whole period, b) March and c) May. The surface current velocity 

scale is given in the lower right corner. The arrow on Kalgin Island gives the pseudo wind stress 

(magnitude = wind velocity squared, direction = wind direction). The scale for the wind stress magnitude 

is given in the upper left.

Mean Currents 12/2002−6/2003

 152o

W 50’ 40’ 30’ 20’ 

 20’ 

 25’ 

 60o

N 

 30.00’ 

 35’ 

 40’ 

Site 1

Site 2

25 cm/s

Wind Str 20 m2/s2

 17
6B 

Monthy Mean Currents 3/2003

 152o

W 50’ 40’ 30’ 20’ 

 20’ 

 25’ 

 60o

N 

 30.00’ 

 35’ 

 40’ 

Site 1

Site 2

25 cm/s

Wind Str 20 m2/s2

 18
6C 

Monthy Mean Currents 5/2003

 152o

W 50’ 40’ 30’ 20’ 

 20’ 

 25’ 

 60o

N 

 30.00’ 

 35’ 

 40’ 

Site 1

Site 2

25 cm/s

Wind Str 20 m2/s2

 19
Figure 7: Current variance ellipses plotted over bathymetry contours. The principal axes of the subtidal 

current variability are polarized along the axis of the inlet with major axis magnitudes ranging between 20 

and 40 cm s-1. A scale is found in the lower right corner 

−50

−40

−40

−30

−30

−20

−20

20 cm/s

−75

 151° 152 151°

 36’ W 12’ W °

 00’ W 

 60°

 42’ N 

 60°

 36’ N 

 60°

 30’ N 

 60°

 24’ N 

 60°

 18’ N 

 151° 151 24’ W °

 48’ W 

Site 2

Site 1

Current Variance Ellipses for filt

 20
Figure 8: Drifter trajectories for three drifters deployed in Cook Inlet during the HF radar experiment, 

2003. 

 21
Figure 9: Scatter plot of the drifter-HF radar velocity comparison. The upper (lower) figures give the 

regression of the HF radar velocities against the drifters for the longitudinal (meridional) velocities. The 

figures on the left are based on the predicted velocities based on the tidal predictions from the tidal 

analysis, and the figures on the right are for the actual HF radar data. 

 22
Figure 10: Histograms of velocity component differences between HF radar and six drifter-derived 

currents in Cook Inlet study.

 23