Currents in the ocean are much like wind in the air...carrying objects from one place
to another.
In coastal waters, knowing the speed and direction of coastal ocean currents is essential to
critical operations such as search and rescue and responding to hazardous spills—as well
as ongoing applications such as coastal research and management.
Operations such as these require data 24/7, 365 days a year.
Traditionally, sensors on offshore moorings have been the standard for measuring surface currents.
In the last decade, high-frequency radar—known as “HF radar”—has been increasingly
used to complement these single-point measurements with detailed maps of ocean surface currents.
While satellites can provide large-scale ocean circulation patterns, they cannot directly
measure currents near the coastline...nor can they provide the near-real-time 24/7 data
needed for many applications.
Two high-frequency radar sites can map the current flowing over hundreds of square miles
of coastal waters every hour.
Additional sites extend the area even further.
Coverage like this would be cost-prohibitive using traditional mooring-based sensors.
Imagine that a pipeline carrying oil from an offshore rig ruptures, allowing hundreds
or even thousands of gallons of oil to flow into coastal waters.
As first responders are being notified, NOAA’s Emergency Response Division tries to determine
where the free-floating oil will go—so the proper amount of equipment and personnel can
be brought in to fight the spill.
Ocean currents can be very complicated, with swirling eddies of many sizes and differing
spin directions, both clockwise and counterclockwise.
Because HF radar provides such a detailed picture, we can get a much better understanding
of these currents.
HF radar uses a well-understood effect in physics, the Doppler shift, to measure the
speed of surface currents.
Unlike more familiar radars, such as police speed guns, HF radar transmits signals that
follow the curvature of the earth, travelling far beyond the horizon—routinely reaching
distances of 200 km (or 120 miles) from shore.
They operate best when located right at the ocean shoreline and near water level.
Velocity data from the HF radar network are combined on standard grids supporting National
Weather Service and U.S. Coast Guard operations as well as oil spill monitoring.
The data are also publicly available for private, public and academic use.
Returning to our oil spill emergency…
NOAA can use HF radar data as one of the inputs to their spill trajectory prediction system
which tells the response teams where and how fast the spill may spread.
For example, a cracked oil pipeline near Santa Barbara, California, leaked oil into the ocean
near a state beach park in 2015.
The coastal waters there have a complex circulation pattern that changes readily due to winds
and currents.
The existing HF radar network covered this area, and their data were used in modeling
the spill.
The ocean current speed data are one of the types of data brought into a NOAA response
model that forecasts the spill’s likely path.
The U.S. Integrated Ocean Observing System—known as IOOS—operates the world’s largest HF
radar network made up of approximately 140 HF radar sites in nearly every coastal state
and Puerto Rico.
IOOS data are distributed by servers at both the SCRIPPS Coastal Observing Research and
Development Center and the National Data Buoy Center.
These servers also import data from radar sites in Mexico and Canada.
Setting up new HF radar sites requires coastline access and availability of electricity and
communications services.
Signals are unaffected by rain, snow, wind or other atmospheric events but since they
travel a long distance and operate in a crowded area of the radio-spectrum, they are vulnerable
to interference from other users and must be operated within strict parameters.
Also, because of radar transmitting principles, signals cannot be measured over the first
2 or 3 km from the radar instruments.
More than 90% of radar sites are relatively small and require a very small physical footprint,
an advantage in many coastal environments.
As impacts from storms and other coastal events become more important, real-time, accurate
data on surface currents from HF radar provide a critical contribution to coastal intelligence.
With this information, coastal communities can prepare for, and react to, long-term and
immediate risks with confidence.