The 1st ORCA, S2-3
May 17-18, 2012
Surface Current Measurement Using HF-Radar off Northeastern
Taiwan: Preliminary Result and Validation
Ying-Chih Fang1,2, Joe Wang3, Yiing-Jang Yang4
1
Institute of Oceanography, National Taiwan University, Taiwan ([email protected])
2
School of Fisheries and Ocean Sciences, University of Alaska Fairbanks, USA
3
Institute of Oceanography, National Taiwan University, Taiwan ([email protected])
4
Department of Marine Science, Naval Academy, Taiwan ([email protected])
Key words: HF-Radar, CODAR, Data Validating, Kuroshio
Abstract: Two long-range CODAR systems have been set-up respectively at Suao and
Han-Ben to monitor the surface ocean currents offshore the region northeast of Taiwan. After a
series of fine-tune, antenna pattern measurements and adjustment tasks, both stations are able to
provide the near real-time surface current map, in a radar-illuminated area with an 100 km
radius, since April 2011. In the context, we report preliminary statistical results of both the
spatial distribution and the long-term averaged characteristics of data quality of CODAR radial
velocities of these stations. Additionally, historic Ship-borne ADCP current data at 20-m depth
are used as a ground truth for validating CODAR measurements. Regression analysis, between
long-term averages of both ship-borne ADCP and CODAR data, showing a coherent
relationship infers that the surface current field derived from long-term averaged CODAR data
is reliable. To date, the operation of the whole system has been fully automated, and both the
near real-time current maps and the significant statistics on data quality are updated and
displayed on a web site hourly. The setting of antenna is critical to the reliability of CODAR
measurements, since the algorithm and calculation used to estimate the bearing of surface
current is sensitive to any variation of antenna pattern, which may be caused by natural hazards
or uncontrolled events in practice. Therefore, antenna pattern measurement is highly
recommended to conduct regularly (yearly or half-yearly) to ensure the data quality in Taiwan
region.
1. Introduction
Two sets of long-range type CODAR radar were installed at Suao and Han-Ben,
respectively, since 2010. Both two sites are located at the northeastern coast of Taiwan (Fig.1).
In this paper, we brief some of our learning experiences and preliminary results on operating
CODAR during past two years. Based on these measurements, we have found that the local
environment is seemingly an important factor influencing the quality of CODAR data
temporally and spatially. Statistics of our measurements show that it’s impossible to have a
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The 1st ORCA, S2-3
May 17-18, 2012
good validity on CODAR measurements if the antenna pattern is in suspicious, since the
magnitude of surface currents is accounted by virtue of the Doppler frequency shift, whose
estimation is more reliable than that of the bearing of currents, in CODAR principals. Details
of this will be addressed in the following context.
Suao
Han-Ben
Fig. 1: Locations of HF radar stations and schematic of ideal CODAR coverage off
northeastern Taiwan.
2. Data
The Suao site was operated in early February of 2010, but the operation of Han-Ben site
was deferred for a year due to administrative and electricity problems. Furthermore, the Suao
site, after 7 month smooth operation (leg A), was shut down at late August 2010, due to
disaster damages on the transmit antenna by lightning associated with the passage of a tropical
storm. The system was recovered and reworked in December 2010. After a series of fine-tune,
antenna pattern measurement and adjustment tasks, both sites are able to provide the near
real-time surface current map, in radar-illuminated area with a radius of 100 km, since April
2011. The data described here were collected after both sites had been fine-tuned and started
normal services, from 14 April 2011 to 20 May 2011 (leg B). For the purpose of a better
reliability on statistically analyzing local characteristics, the lengthened data in Suao site
before late August 2010 (Leg A) are also included into our statistic analysis.
3. Statistical Characteristics of Radial Current Data
Fig. 2 shows the spatial distribution of averaged radial currents of Suao and Han-Ben,
respectively. Suppose there is an imaginary vector normal to the local coastline of remote site
(the bearing of the normal vectors are 123° for Suao and 96° for Han-Ben sites, respectively,
both angles in azimuth), then one may find that radial currents flow away from the remote site
at north of the normal vector (with negative speed) while radial currents flow toward the
remote site at south of the normal vector (with positive speed). Near the normal vector, there
are seemingly no currents (the speed is nearly null) because of the direction of currents is
orthogonal to the propagation direction of CODAR radio waves, therefore no Doppler
frequency shift at all. These results imply the averaged currents flowing north-northeasterly,
which is consistent with our knowledge on Kuroshio, the main stream of which flows in
parallel to the eastern coast of Taiwan. Based on statistics of the radial current pattern, we may
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The 1st ORCA, S2-3
May 17-18, 2012
infer that the antenna pattern we used for both sites is reliable; and no severe systematic bias
on determining the bearing of currents has been found.
For the purpose of validating the radial currents, we use Sb-ADCP (Ship-borne Acoustic
Doppler Current Profiler) data from May 1992 to June 2008 (in the region 120°E~124°E,
22°N~27°N, and at the depth of 20 m, provided by Ocean Data Bank of National Science
Council, Taiwan) as the ground truth. The region has been further sub-gridded with a grid
interval of 0.2°. Afterwards, all the Sb-ADCP and CODAR data are sorted to corresponding
grids. Figs. 3 and 4 are results of averaged radial currents calculated from Sb-ADCP and
CODAR data sets, respectively. The averaged radial currents from two different sources are
very similar to each other, except the mean radial currents calculated from Sb-ADCP data are
stronger in some regions; we suspect the difference is likely due to an uneven distribution of
Sb-ADCP data, those exaggerated values are usually associated with gridded points contained
too few data. To solve the problem, we pre-exclude those gridded points whose Sb-ADCP data
amount is less than 10, and after this kind of sub-sampling, the number of qualified grid points
becomes 183 and 105 for Suao and Han-Ben, respectively.
Fig. 2: The left and right panels represent the spatial distribution of averaged radial currents at
Suao and Han-Ben sites, respectively.
Fig. 3: Spatial distribution of 0.2˚ gridded averaged radial currents calculated from Sb-ADCP
data (left: radial velocity relative to Suao; right: radial velocity relative to Han-Ben).
Fig. 4: Spatial distribution of 0.2˚ gridded averaged radial currents calculated from CODAR
data (left: radial velocity relative to Suao; right: radial velocity relative to Han-Ben).
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The 1st ORCA, S2-3
May 17-18, 2012
Afterwards, a linear regression is used to analyze the relationship between radial velocity
obtained from CODAR and that from Sb-ADCP. The analysis is shown in Fig. 5; from which
we may infer that the averaged radial currents calculated from CODAR are highly correlated
to that of the historical Sb-ADCP data, since the coefficient of determination (R2) is larger
than 0.7 for both sites. Also, no significant bias has been found from the analysis. The end
product of CODAR, the vector current field, is derived by combining radial velocities from
multiple sites; so this kind of pre-validation may ensure the reliability of the final end product.
Fig. 5: Linear Regression of radial currents, where the x-axis is the averaged radial velocity
derived from Sb-ADCP, y-axis from CODAR (left: Suao; right: Han-Ben).
4. Summary
Two long-range CODAR surface current monitoring stations had been set-up at Suao and
Han-Ben respectively. Preliminary results of statistical analyses, such as the spatial
distribution and the long-term averaged characteristics of data quality of CODAR radial
velocities of both sites, are reported in the context. Additionally, a comparison of long-term
averages of CODAR measurements and that of ship-borne ADCP data at 20-m is made, the
result shows a highly correlated and unbiased relationship; which may imply the surface
current field derived from long-term averaged CODAR data is reliable. So far the operation of
the whole system has been fully automated, and both the near real-time current maps and the
significant statistics of observation data are updated and displayed on a web site hourly. The
magnitude of the radial current measured by CODAR would be reliable; however, the bearing of
the current is derived from the DOA (Direction of Arrival) function via the multiple signal
classification (MUSIC) algorithm (Lipa et al., 2006), which is sensitive to the antenna pattern
used. It is recommended to conduct antenna pattern measurement (APM) yearly or half-yearly,
to ensure the data quality of CODAR in Taiwan area.
Acknowledgement
This study was supported by National Science Council, Taiwan; under grant NSC
99-2623-E002-006-D.
References
Lipa, B. J., B. Nyden, D. S. Ullman and E. Terrill, 2006: SeaSonde radial velocities: derivation
and internal consistency. IEEE J. Ocean. Eng., 31, 850-861.
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