| Coastal Surface Current Variability
|
The escalating interest in the coastal ocean has created
a requirement
for the acquisition of high-quality surface current data to improve the
understanding of
surface circulation, and to study their impact on a broad spectrum of
societal and environmental issues such as coastal pollution and oil spills
(Brink et al 1992, Smith and Brink 1994) and coastal
air-sea interactions (Rotunno et al 1996).
These environmental issues relating to the coastal ocean are
increasingly difficult to manage with respect to
water quality over large areas, which is directly related to submesoscale to mesoscale variability in the
spatially-evolving surface current fields. Inference of these spatial patterns is difficult
from single-point measurements
such as moorings or drifters, which propagate away from divergent flow regimes.
One approach that effectively measures spatially-evolving surface current
fields in near-real time
is the
Doppler radar technique, providing
spatial context and hence a dynamical framework for
mooring-, drifter- and ship-based measurements.
(From Shay, L.K., S.J.
Lentz, H.C. Graber, B.K. Haus: 1998, Current
Structure Detected by High Frequency Radar and Vector-Measuring Current
Meters, J Atmos. and Ocean. Tech, v15, 237-256.)
| Ocean Surface Current Radar (OSCR) |
A promising method that has evolved over the past four decades is the Doppler
radar technique originally described by Crombie (1955), who observed that
the echo Doppler spectrum consisted of distinct peaks symmetrically
positioned about the radar frequency. The concept is based on the premise
that pulses of electro-magnetic radiation are backscattered from the moving
ocean surface by resonant surface waves at one-half of the radar wavelength
or "Bragg waves". The two spikes resulting from Bragg resonant scattering
(constructive interference) originate from two targets travelling at
constant velocity on the ocean surface, one toward and the other awy from
the radar. Ocean surface-gravity waves of given wavelength propagate at a
constant speed in deep water. Stewart and Joy (1974) showed that hte
displacement of the Doppler peaks from their expected positions is related
to the underlying current flow modifying the phase speed of the surface waves.
The concept of using High Frequency (HF) radio pulses to probe the ocean
surface to deduce near-surface, has recieved considerable attention in
coastal oceanography experiments. The dual-frequency OSCR of the University
of Miami can use HF (25.4 MHz) and VHF (49.9 MHz) radio frequencies to map
surface current patterns over a large area in coastal waters. The
shore-based radar system consists of two units (Master and
Slave) which are deployed several kilometers apart. Each unit makes
independent measurements of current speed along radial beams emanating from
its phased-array antennae system. The data are then combined via UHF or
telephone communication to produce accurate vector currents (speed and
direction), stor them to disk, and display them in near real-time. The
emasurements can be made simultaneously at up to 700 grid points either at 1
km (HF mode) or 250 m (VHF mode) nominal resolution. The measurement
interval between each vector current map is 20 minutes. The measurement
cycle begins at the master site, where the transmitter sequentially sends
radio pulses over the illuminated field of view (IFOV) of the ocean for five
minutes. Simultaneously, the beams of the phased-array receiver elements are
electronically steered over the ocean's IFOV and acquire the radar echos.
Over the next five minutes, the same transmit and recieve sequence is
repeated at the slave site. Over the remaining 10 minutes of the 20-minute
cycle, radial currents are extracted from the Doppler spectra at a maximum
of 700 grid points. The specifications and capabilities of the OSCR system
are listed in Table 1.
(from Haus, B.K., H.C. Graber, L.K. Shay, J. Martinez, 1995:Ocean Surface
Current Observations with HF Doppler Radar during the DUCK94 Experiment,
RSMAS Tech Rep 95-010, 104 pp.)
| OSCR System Capabilities and
Specifications |
|---|
| Parameter | HF | VHF |
|---|
| Frequency | 25.4 MHz | 49.9 MHz |
| Range | 45 km | 10 km |
| Range Resolution | 1 km | 250 m |
| Azimuth Resolution | 8-11 º | 4-5.5 º |
| Measurement Cycle | 20 min | 20 min |
| Spatial Coverage | 700 km2 | 700 km2 |
| Max. number of measurement points | 700 | 700 |
| Measurement Depth | ~40 cm | ~20 cm |
| Data Storage | 120+ days | 120+ days |
| Transmitter Peak Power | 1 KW ERP | 1 KW ERP |
| Transmitter Average Power | 21 | 10 |
| Power Consumption (KW 240V) | <1 | <1 |
| Transmit Antennae Elements (Yagi; 6dB gain) | 4 | 4 |
| Recieve Antennae Elements (phased array) | 16 | 32 |
| UHF communication | 458 MHz | 458 MHz |
| Transmit Time | 293.6 s | 293.6 s |
Pulse length | 13.333 µs | 1.667 µs |
| Pulse repetition interval | 310 µs | 80 µs |
Accuracy |
| Radial Current | 2 cms-1 | 2 cms-1 |
| Vector Current (nominal) | 4 cms-1 | 4 cms-1 |
| Vector direction (max) | 5 º | 5 º |
(from Haus, B.K., H.C. Graber, L.K. Shay, J. Martinez, 1995:Ocean Surface
Current Observations with HF Doppler Radar during the DUCK94 Experiment,
RSMAS Tech Rep 95-010, 104 pp.)
Questions or problems, contact Tom Cook