Surface Current Mapping in the Lower Chesapeake
Teresa Garner, Larry Atkinson and Jose Blanco
Center for Coastal Physical Oceanography
Department of Ocean, Earth and Atmospheric Sciences
Old Dominion University,
Norfolk, VA 23529 USA
Two standard range 25 MHz CODAR SeaSonde antennas operated by Old Dominion University (ODU)
provide near real-time hourly observations of surface currents in the Lower Chesapeake Bay to the
public, maritime and scientific communities. Data from these two antennas are also combined with
data from a third site in the region operated by NOAA. Surface current information measured at this
high frequency over large spatial areas can be used in a wide variety of applications relevant to
research, government, commercial and recreational interests. The data are valuable for search and
rescue operations, maritime navigation, pollution tracking, fishing, research of coastal ocean
processes and improvement of numerical forecasting models. At this time, the Lower Bay velocity
maps are publically available and as part of the Mid-Atlantic Region Coastal Ocean Observing System
(MARCOOS), the data are sent to the National HFRADAR network. This report contains descriptions
of the Chesapeake Bay antenna sites, data products, processing and analysis as well as descriptions of
the measures taken to assess and improve data quality.
I. HFRADAR Antenna Sites
The locations of the three antenna sites in the Lower Chesapeake Bay are shown in Figure 1. The
first antenna site (CBBT) was installed on February 8 2007 on the 4th Island of the Chesapeake Bay
Bridge Tunnel. On April 9 2008, a second antenna site (VIEW) located at Ocean View Community
Beach became operational. A third site (CPHN) operated by the National Oceanic and Atmospheric
Administration was added in late February 2008 at Fort Story on Cape Henry. This third site
provided an excellent opportunity to incorporate more radials into the calculation of total velocity
vectors and improve data coverage.
+ CPH
N
Figure 1. RADAR site locations in the Lower Chesapeake Bay.
A. Ocean View Beach
(VIEW)
B. Chesapeake Bay Bridge Tunnel
(CBBT)
C. Cape Henry
(CPHN)
Figure 2. Antenna installations.
II. Data Products
Several data products are updated hourly on the project website
(http://www.ccpo.odu.edu/currentmapping) including the most recent surface velocity maps, subtidal surface currents, an animation of the past 12 hours of surface circulation, and Ocean View
alongshore currents as well as sample time series and particle trajectories at select locations. The
public can download ASCII text versions of the total velocities. These text files include unique
location ID tags so that the data may be easily incorporated into GIS.
A. Radials
The radial data output from the Lower Bay sites (Fig. 3) are sent to the National HFRADAR Network
where they are processed into total (actual) current velocities (Fig. 4) and then passed on to the Coast
Guard for inclusion into a database that uses the information to help define search areas for search
and rescue operations. Radial velocities are given at 1.5 km range intervals in 5-degree directional
bins. These vectors indicate water speeds moving towards or away from an antenna.
Figure 3. Radial velocities from the CBBT (red) and VIEW (blue) antennas.
Figure 4. Image from the National HFRADAR Network website maintained by
the UCSD Scripps Institute (http://cordc.ucsd.edu/projects/mapping/maps).
Total velocity vectors computed from radials are shown offshore and in the
lower Chesapeake Bay.
B. Surface Velocity Maps
A suite of processing routines is run every hour at ODU to compute and display images of the velocity
field in the lower Bay. These maps represent the basic RADAR product (Fig. 5).
Figure 5. Total vectors depicting surface current velocities in the Bay.
C. Sub-tidal Currents
Sub-tidal currents are calculated each hour for the past 50 hours of data (Fig. 6).
Figure 6. Surface current velocities averaged over a 50-hour period.
D. Alongshore currents
Ocean View alongshore currents are represented by VIEW site radial velocities in the directional bin
parallel to the beach (Fig. 7).
Fig. 7. Alongshore currents over a 48 hour period. The colored lines represent
time series at the locations marked in the top panel in the corresponding color
while the black line is a spatial average of currents at all locations along the line
shown above. The antenna location is designated by a square marker.
III. Data Processing
A community MATLAB toolbox for analyzing HFRADAR data is utilized to process and combine the
radial vectors from each site into a total velocity vector map. The total vector grid is the HFRADAR
National Network grid at nominal 2km point spacing. A suite of processing scripts runs each hour to
generate total velocity vector and graphics for the website. Over the project time period, scripts
were developed to remove outliers and anomalous data from the radial and total vector files. All of
the MATLAB hourly routine processing scripts were adapted to handle three sites so that CPHN was
fully incorporated into the ODU processing. This involved developing special “masks” for the grid file
depending on the combination of RADAR sites running for any particular hour. In the displays, all
vectors over land are masked as well as vectors failing to meet geometrical criteria related to
geometrical dilution of precision.
IV. Data Quality
A. Calibration with Antenna Pattern Measurements (APMs)
In late May 2007, antenna beam patterns were measured using transponders on a small boat. The
patterns are a measure of antenna response as a function of direction and are used by the CODAR
software direction finding algorithm, which parcels out observed radial speeds within each range cell
into 5 degree directional bins. Figure 8 shows the measured antenna patterns for both sites as well as
an ideal pattern for comparison. Note that the CBBT site is on a manmade island with a 360-degree
view of the lower Bay. The VIEW site measured pattern is similar to an ideal pattern while the CBBT
site pattern shows considerable distortion due to interfering structures within a wavelength (12
meters) of its antenna.
In November 2007, the CBBT antenna pattern was measured a second time during a test in which we
moved the antenna and raised it by 4 meters. This test proved that the antenna pattern was not
improved (closer to ideal) in the new position. However, it also showed that the pattern had changed
in the original antenna location due in part to the presence of two newly installed state police Yagi
antennas on the 4th island building rooftop. This new measured pattern was subsequently used for
data processing.
A
B
C
D
Figure 8. Antenna Patterns. A) May 2007 CBBT B) May 2007 VIEW C ) An ideal pattern D) Nov 2008 CBBT
(unsmoothed pattern). Note that May 2007 measured patterns are displayed with 10 degrees of
smoothing.
B. Baseline Comparisons
A basic check of data consistency with two facing CODAR HF RADAR instruments is a comparison of
data along the baseline, the line between the sites. Along this line the radial velocities ought to agree
well in magnitude and have opposite signs. In particular, at the middle of this baseline, where the
radial coverage areas are very similar, the data should match within the velocity errors of the
measurements.
A mid-baseline comparison for two months of data (June 7 2007 00:00 - August 7 2007 00:00 UTC)
for radials calculated using ideal antenna patterns demonstrates a lack of internal consistency
between sites (Figure 9). However, the same comparison for the same time period using measured
antenna patterns shows a dramatic improvement.
Figure 9. Baseline comparison at the mid-point between the two sites. Left panel shows ideal pattern radial
velocities and the right panel shows measured pattern radial velocities.
C. Real-time Diagnostic Monitoring
At the sites, a diagnostic tool written by CODAR is employed to monitor several variables including
equipment temperatures, disk usage, and power supply voltages at the field sites. The script
automatically notifies the operator by email if diagnostic values fall out of range.
Hourly updated displays on the project website provide diagnostic and quality control information.
Plots of radials, 50-hour total coverage, 50-hour radial coverage, equipment and radial diagnostics
are used to determine if operations are running smoothly and to assist in diagnosing problems when
they arise.
Quality can be roughly assessed through real-time plots of baseline comparisons along all three
baselines (Fig. 10) and real-time comparisons of total vectors with NOAA PORTS data at three ADCP
stations in the lower Bay (Fig. 11). However it is important to note that the real-time ADCP
comparison gives only a general idea of how the two instruments’ measurements compare because
the ADCP velocity is a point measurement at a depth of 6-7 meters while the CODAR velocity is a
spatial average over a radial cell in the upper meter of the water column. Further post-processing of
the data is required for a more refined comparison of these two instruments’ observations.
Figure 10. Baseline comparisons along each of the baselines in the Lower Bay.
Figure 11. A real-time comparison plot showing along-channel
ADCP data from the NOAA PORTS Cape Henry LB 2CH station (blue)
compared to CODAR total velocities (red).
D. Comparison of CODAR data to NOAA Acoustic Doppler Current Profiler (ADCP) Data
NOAA operates three Doppler current profilers on ATON buoys in the lower Chesapeake Bay through
the PORTS program. These instruments continuously collect data and disseminate it every six
minutes. Deployments last for several months and the time period between deployments is typically
only a few days so it is a lengthy and reliable time series record for comparison. In order to do an indepth comparison of CODAR and ADCP data, total vectors are calculated on grid points at each of the
ATON buoy locations and the total velocity vectors include antenna radials within a certain distance
of the buoy. A minimum of 3 radials is required within the specified radius in order to compute a
total current vector. The results from a 4 day comparison (July 27 00:00 -July 31 00:00 UTC) are
displayed in Figure 12.
Fig. 12 Comparison of NOAA ADCP and CODAR current speeds.
At each station, the search radius for radials was defined differently to achieve the minimum number
of required radials while still maintaining a small radius from the buoy. Particularly in the case of a
narrow channel such as the one at Thimble Shoals, the goal was to minimize the amount of radial data
coming from outside of the channel since the buoy measurements were taken within the channel.
Site
Cape Henry
Thimble Shoals
York Spit
Mean (cm/s)
16.2
13.2
13.9
Standard Deviation (cm/s)
14.0
11.2
10.0
Search Radius (km)
2
1
1.25
Table 1: Means and standard deviations of the speed differences where the difference is defined
as the absolute value of CODAR speed subtracted from the ADCP speed.
Note that NOAA ADCP depths for the first bins are 3.9, 4.0 and 3.4 meters for Cape Henry, Thimble
Shoals and York Spit stations respectively. Data at these depths are compared with CODAR velocities,
which represent the current in the upper meter of water.
E. Collaborative QA/QC Work
Collaborative efforts included active participation in MACOORA (Mid-Atlantic Coastal Ocean
Observing Regional Association) QA/QC teleconference calls. Modifications were made to site
settings in accordance with recommendations agreed upon within this QA/QC group.
V. Outreach to Users and Product Development
A. CODAR Data as a Resource for Numerical Modelers
One of the most promising uses of the RADAR current observations is as an input to numerical models
to improve forecasting capabilities. A modeler may also use the RADAR data to validate dynamics
within a numerical model by comparing model output with actual surface current observations.
Discrepancies between model output and observations may suggest modifications to the model or
shed light on its limitations.
One such comparative study was performed in the Lower Chesapeake Bay using model output from
an implementation of the ChesROMs model for the measurement period of August 22 to November 1
2007. Total velocity vectors were computed at model grid points and then compared to ChesROMS
model output. Both sets of velocities were then compared to buoy ADCP data with the result that
CODAR observations were much more correlated with the ADCP data. Based on these results, the
modeler has since adjusted model inputs and parameters in hopes of obtaining a better match of
model output to the CODAR data and NOAA ADCP data. A new comparative analysis of CODAR totals
and ChesROMS model output for the July 1 – August 31 2008 time period is now near completion. In
addition to the model modifications, the 2008 comparison benefits from the inclusion of CPHN site
radials into CODAR total vectors.
B. Further Product Development
New data products for the user community were discussed with pilot associations, sailing clubs, and
coastal engineers. Two examples of products targeted to users are a focused shipping channel
current map and a particle trajectory map (Fig. 13).
Fig. 13. Current velocities in the shipping channels (left) and particle trajectories within a current field
(right).
An animation of the Lower Bay surface circulation including navigational reference points was
developed based on a suggestion received through the Pilot’s Association (Fig. 14).
Figure 14. A single frame of an animation showing current velocities on a map including navigational
reference points.
C. Meetings/Outreach Summary
Sep 10-12 2007
Sep 09-Oct 04 2007
Oct 30 2007
Dec 04 2007
Dec 12 2007
Apr 09 2008
Apr 16 2008
May 01 2008
May 12-14 2008
Sept 03 2008
Sep 18 2008
Radiowave Operators Working Group (ROWG) meeting: presented two posters
MTS /IEEE Oceans: presentation and paper
MABPOM (Mid-Atlantic Bight Physical Oceanographers Meeting): presentation
Chesapeake Bay Observing System (CBOS) meeting: displayed two posters
SensorsWorld conference “Earth and Environmental Sensing” session:
presentation
ODU Research Day Expo: presented two posters
Presentation to Norfolk coastal engineers
Coast Guard/1st responders meeting: brief presentation
Chesapeake Bay Modeling Symposium: presentation
Norfolk Yacht and Country Club sailing committee: presentation
MTS /IEEE Oceans: presentation and paper
Conclusions
Two antennas were successfully installed in the Lower Chesapeake Bay to measure surface
circulation and provide near real-time hourly maps of surface currents to the public, maritime and
scientific communities. Several data products are displayed and updated hourly on the project
website. The total velocity vectors are freely available for download. On a national scale, the Coast
Guard receives this data to aid in search and rescue. Although some interest in the data was
expressed by local user groups including the sailing committee, pilots and numerical modelers, more
work remains to be done in cooperation with these groups in order to develop specific regional
products and recruit more local users.
Acknowledgments
Staff of the City of Norfolk, Virginia, the Chesapeake Bay Bridge Tunnel Authority, and the Department
of the Army are thanked for their cooperation. Mr. Chris Paternostro and Mr. Pat Burke at NOAA/COOPS are thanked for their collaborative work. The US National Oceanic and Atmospheric
Administration, the Center for Innovative Technology, and the Slover Endowment at Old Dominion
University are thanked for their support. Mr. Chris Powell with Old Dominion University, Mr. Mike
Muglia at the University of North Carolina and the Ocean Observing Group at Rutgers University are
also thanked for their support.