Metocean Extreme and Operating Conditions 3.3 Measurements 57
Part A | 3.3
ment of about 30 cm for significant wave height and
1:5 m s
1 for wind speed. Some of this difference is due
to buoy measurement error.
Synthetic aperture radar (SAR) has been sporadically deployed starting with Seasat. By far the most
successful application was the QuikSCAT (quick scatterometer) satellite which launched in 1999 and continued to operate until late 2009. Unlike altimeters which
only measure along their track, SARs measure over
a wide swath. In the case of QuikSCAT the swath was
1800 km, resulting in the coverage of 90% of the Earth’s
surface in a single day. Results were widely used to
improve forecast models, so archived model results are
one of the best ways to use QuikSCAT since native measurements have many hours between samples. Archived
QuikSCAT measurements are downloadable from the
web [3.60].
SAR from various satellites has also been used to
measure ice coverage, oil slicks, waves and currents as
described in [3.47]. Unlike the altimeter, SAR measures
wave direction in addition to wave height and period.
Furthermore, it makes those measurements over a wide
swath. Several commercial satellite wave databases include SAR measurements. SAR also has significant theoretical advantages over other sensors when it comes to
identifying near-surface currents. Unlike the altimeter,
SAR can identify non-geostrophic current fronts (i. e.,
currents that cause no detectable change in sea surface
height). However, analysis of SAR images is complex
in part because artifacts can be caused by natural surfactants, and also because a minimum wind threshold
is needed. These disadvantages have limited the use of
SAR for the measurement of waves and currents.
The wind velocity, wave height/period, sea surface
temperature, and sea surface height measurements from
satellites are routinely assimilated into ocean models to
provide nowcast and forecast products [3.61–65]. It is
probably in this form that the satellite results are the
most valuable, since the models are able to interpolate
between the large gaps in time and space that invariably
appear in all sources of satellite measurements. Some
of the more popular modeling products that are publicly
available are described in Sect. 3.4.
3.3.3 In Situ Measurements
Instruments are commonly deployed at a fixed offshore
location using quasi-permanent facilities like oil production jackets, moorings with subsurface or surface
buoyancy, or quasi-permanent coastal facilities.
No matter how instruments are deployed, the ocean
currents at a particular site are commonly measured
by an acoustic doppler current profiler (ADCP). The
accuracy and range of the instrument mostly depends
on the transmission frequency, which ranges from 38
to 1200 kHz for commercially available instruments.
ADCPs offer many advantages over earlier technologies. They are solid state instruments which are not
easily fouled by marine growth. Perhaps most attractive of all is their ability to accurately measure at up
to 1000 m from the instrument. That said, ADCPs can
yield problematic results which may not be obvious to
the untrained eye, especially in cases where the primary
sources of scatter are mobile like plankton or fish scatterers or where the scatterer is fixed (i. e., risers on an
offshore platform). Hogg and Frye [3.66] and Magnell
and Ivanov [3.67] give some good examples of artifacts
that can contaminate ADCP measurements.
High-frequency radar is a somewhat newer and
more expensive technology than ADCPs but its use has
grown rapidly in the past 5 years, largely because HF
radar (High Frequency radar) can map surface currents
over areas of the order of 1000 km
2 using only two
coastal installations. Dozens of HF radars have been installed along most of the eastern and western coastlines
of the US [3.68] and are available in real time from
NODC [3.69]. Paduan and Graber [3.70] discuss the
basic technology along with some of its limitations and
provide numerous references.
Surface gravity waves can also be accurately measured with ADCPs [3.71] and HF radar [3.70]. However, most historical measurements have been taken
with surface-following buoys equipped with accelerometers and perhaps augmented by roll sensors to measure directionality. Many developed countries with
coastlines have deployed such instruments for several
decades and the US results are available from the National Data Buoy Center (NDBC). Pandian et al. [3.72]
summarize the limitations of accelerometer-based systems. The most noteworthy is the tendency of the
smaller buoys to be pulled under water or be tossed
about in larger waves.
Wind velocity has typically been measured with
mechanical anemometers using some type of impeller.
Most offshore buoys are still equipped with this type
of sensor. While these measurements are useful most of
the time, questions have been raised about their accuracy during large wave events, especially for the smaller
buoys. Better sensors are needed to get detailed profiles. The least expensive of these better sensors are
based on LIDAR (light detection and ranging; optical) or SODAR (sonic detection and ranging; sound).
Both systems use a Doppler principle to determine
velocity and are capable of measuring multiple bins
over ranges of roughly 200 m above the sensor. Freeman et al. [3.73] compare a LIDAR profiler to more
traditional anemometers and show excellent accuracy.
In contrast, de Noord et al. [3.74] raise serious con-
Part A | 3.3
ment of about 30 cm for significant wave height and
1:5 m s
1 for wind speed. Some of this difference is due
to buoy measurement error.
Synthetic aperture radar (SAR) has been sporadically deployed starting with Seasat. By far the most
successful application was the QuikSCAT (quick scatterometer) satellite which launched in 1999 and continued to operate until late 2009. Unlike altimeters which
only measure along their track, SARs measure over
a wide swath. In the case of QuikSCAT the swath was
1800 km, resulting in the coverage of 90% of the Earth’s
surface in a single day. Results were widely used to
improve forecast models, so archived model results are
one of the best ways to use QuikSCAT since native measurements have many hours between samples. Archived
QuikSCAT measurements are downloadable from the
web [3.60].
SAR from various satellites has also been used to
measure ice coverage, oil slicks, waves and currents as
described in [3.47]. Unlike the altimeter, SAR measures
wave direction in addition to wave height and period.
Furthermore, it makes those measurements over a wide
swath. Several commercial satellite wave databases include SAR measurements. SAR also has significant theoretical advantages over other sensors when it comes to
identifying near-surface currents. Unlike the altimeter,
SAR can identify non-geostrophic current fronts (i. e.,
currents that cause no detectable change in sea surface
height). However, analysis of SAR images is complex
in part because artifacts can be caused by natural surfactants, and also because a minimum wind threshold
is needed. These disadvantages have limited the use of
SAR for the measurement of waves and currents.
The wind velocity, wave height/period, sea surface
temperature, and sea surface height measurements from
satellites are routinely assimilated into ocean models to
provide nowcast and forecast products [3.61–65]. It is
probably in this form that the satellite results are the
most valuable, since the models are able to interpolate
between the large gaps in time and space that invariably
appear in all sources of satellite measurements. Some
of the more popular modeling products that are publicly
available are described in Sect. 3.4.
3.3.3 In Situ Measurements
Instruments are commonly deployed at a fixed offshore
location using quasi-permanent facilities like oil production jackets, moorings with subsurface or surface
buoyancy, or quasi-permanent coastal facilities.
No matter how instruments are deployed, the ocean
currents at a particular site are commonly measured
by an acoustic doppler current profiler (ADCP). The
accuracy and range of the instrument mostly depends
on the transmission frequency, which ranges from 38
to 1200 kHz for commercially available instruments.
ADCPs offer many advantages over earlier technologies. They are solid state instruments which are not
easily fouled by marine growth. Perhaps most attractive of all is their ability to accurately measure at up
to 1000 m from the instrument. That said, ADCPs can
yield problematic results which may not be obvious to
the untrained eye, especially in cases where the primary
sources of scatter are mobile like plankton or fish scatterers or where the scatterer is fixed (i. e., risers on an
offshore platform). Hogg and Frye [3.66] and Magnell
and Ivanov [3.67] give some good examples of artifacts
that can contaminate ADCP measurements.
High-frequency radar is a somewhat newer and
more expensive technology than ADCPs but its use has
grown rapidly in the past 5 years, largely because HF
radar (High Frequency radar) can map surface currents
over areas of the order of 1000 km
2 using only two
coastal installations. Dozens of HF radars have been installed along most of the eastern and western coastlines
of the US [3.68] and are available in real time from
NODC [3.69]. Paduan and Graber [3.70] discuss the
basic technology along with some of its limitations and
provide numerous references.
Surface gravity waves can also be accurately measured with ADCPs [3.71] and HF radar [3.70]. However, most historical measurements have been taken
with surface-following buoys equipped with accelerometers and perhaps augmented by roll sensors to measure directionality. Many developed countries with
coastlines have deployed such instruments for several
decades and the US results are available from the National Data Buoy Center (NDBC). Pandian et al. [3.72]
summarize the limitations of accelerometer-based systems. The most noteworthy is the tendency of the
smaller buoys to be pulled under water or be tossed
about in larger waves.
Wind velocity has typically been measured with
mechanical anemometers using some type of impeller.
Most offshore buoys are still equipped with this type
of sensor. While these measurements are useful most of
the time, questions have been raised about their accuracy during large wave events, especially for the smaller
buoys. Better sensors are needed to get detailed profiles. The least expensive of these better sensors are
based on LIDAR (light detection and ranging; optical) or SODAR (sonic detection and ranging; sound).
Both systems use a Doppler principle to determine
velocity and are capable of measuring multiple bins
over ranges of roughly 200 m above the sensor. Freeman et al. [3.73] compare a LIDAR profiler to more
traditional anemometers and show excellent accuracy.
In contrast, de Noord et al. [3.74] raise serious con-