A. Prytz

Structure and influence of tropical river plumes in the Great Barrier Reef: application and performance of an airborne sea surface salinity mapping system

Input of freshwater from rivers is a critical consideration in the study and management of coral and seagrass ecosystems in tropical regions. Low salinity water can transport natural and manmade river-borne contaminants into the sea, and can directly stress marine ecosystems that are adapted to higher salinity levels. An efficient method of mapping surface salinity distribution over large ocean areas is required to address such environmental issues. We describe here an investigation of the utility of airborne remote sensing of sea surface salinity using an L-band passive microwave radiometer. The study combined aircraft overflights of the scanning low frequency microwave radiometer (SLFMR) with shipboard and in situ instrument deployments to map surface and subsurface salinity distributions, respectively, in the Great Barrier Reef Lagoon. The goals of the investigation were (a) to assess the performance of the airborne salinity mapper; (b) to use the maps and in situ data to develop an integrated description of the structure and zone of influence of a river plume under prevailing monsoon weather conditions; and (c) to determine the extent to which the sea surface salinity distribution expressed the subsurface structure. The SLFMR was found to have sufficient precision ( 1 psu) and accuracy (∼3 psu) to provide a useful description of plumes emanating from estuaries of moderate discharge levels with a salinity range of 16 to 32 psu in the open sea. The aircraft surveys provided a means of rapidly assessing the spatial extent of the surface salinity distribution of the plume, while in situ data revealed subsurface structure detail and provided essential validation data for the SLFMR. The combined approach allowed us to efficiently determine the structure and zone of influence of the plume, and demonstrated the utility of sea surface salinity remote sensing for studying coastal circulation in tropical seas.

A Method of Swell-Wave Parameter Extraction From HF Ocean Surface Radar Spectra

A new method for the extraction of swell-wave parameters from high-frequency (HF) radar spectra is presented. The method of extraction of the parameters, period, direction, and height, relies on a frequency-modulation approach that describes the hydrodynamic interaction of the swell waves with the resonant, shorter, Bragg waves. The analysis process minimizes the electromagnetic second-order interaction and a simulation model was used to validate the approach. This simplified method provides a fast means of examining swell conditions over large areas of the ocean surface. Data are acquired using a pair of coastal ocean surface radar (COSRAD) systems deployed at Tweed Heads, Qld., Australia. The radar covers a sweep (approximately 60deg) every 30 min with spatial resolution of the order of 3 km. A sample set of data from this deployment is used in a case study to show the extraction of swell direction and amplitude using these methods. The results support the use of the COSRAD HF radar for mapping swell in the near-shore zone

Tsunami observations by coastal ocean radar

When tsunami waves propagate across the open ocean, they are steered by the Coriolis effect and refraction due to gentle gradients in the bathymetry on scales longer than the wavelength. When the wave encounters steep gradients at the edges of continental shelves and at the coast, the wave becomes nonlinear and conservation of momentum produces squirts of surface current at the head of submerged canyons and in coastal bays. High frequency (HF) coastal ocean radar is well conditioned to observe the surface current bursts at the edge of the continental shelf and give a warning of 40 minutes to 2 hours when the shelf is 50 to 200 km wide. The period of tsunami waves is invariant over changes in bathymetry and is in the range 2 to 30 minutes. Wavelengths for tsunamis (in 500 to 3000 m depth) are in the range 8.5 to over 200 km, and on a shelf where the depth is about 50 m (as in the Great Barrier Reef (GBR)) the wavelengths are in the range 2.5 to 30 km. In the use of HF radar technology, there is a trade‐off between the precision of surface current speed measurements and time resolution. It is shown that the phased array HF ocean surface radar being deployed in the GBR and operating in a routine way for mapping surface currents, can resolve surface current squirts from tsunamis in the wave period range 20 to 30 minutes and in the wavelength range greater than about 6 km. An advantage in signal‐to‐noise ratio can be obtained from the prior knowledge of the spatial pattern of the squirts at the edge of the continental shelf, and it is estimated that, with this analysis, the time resolution of the GBR radar may be reduced to about 2.5 minutes, which corresponds to a capability to detect tsunamis at the shelf edge in the period range 5 to 30 minutes. It is estimated that the lower limit of squirt velocity detection at the shelf edge would correspond to a tsunami with water elevation of about 2.5 cm in the open ocean. This means that the GBR HF radar is well conditioned for use as a monitor of small, as well as larger, tsunamis and has the potential to contribute to the understanding of tsunami genesis research.

Evaluation of a New Airborne Microwave Remote Sensing Radiometer by Measuring the Salinity Gradients Across the Shelf of the Great Barrier Reef Lagoon

Over the last ten years, some operational airborne remote sensing systems have become available for mapping surface salinity over large areas in near real time. A new dual-polarized Polarimetric L-band Multibeam Radiometer (PLMR) has been developed to improve accuracy and precision when compared with previous instrument generations. This paper reports on the first field evaluation of the performance of the PLMR by measuring salinity gradients in the central Great Barrier Reef. Before calibration, the raw salinity values of the PLMR and conductivity-temperature-depth (CTD) differed by 3-6 psu. The calibration, which uses in situ salinity data to remove long-term drifts in the PLMR as well as environmental effects such as surface roughness and radiation from the sky and atmosphere, was carried out by equating the means of the PLMR and CTD salinity data over a subsection of the transect, after which 85% of the salinity values between the PLMR and CTD are within 0.1 psu along the complete transect. From offshore to inshore across the shelf, the PLMR shows an average cross-shelf salinity increase of about 0.4 psu and a decrease of 2 psu over the inshore 20 km at -19deg S (around Townsville) and -18deg S (around Lucinda), respectively. The average cross-shelf salinity increase was 0.3 psu for the offshore 100 km over all transects. These results are consistent with the in situ CTD results. This survey shows that PLMR provided an effective method of rapidly measuring the surface salinity in near real time when a calibration could be made.

The use of HF radar surface currents for computing Lagrangian trajectories: Benefits and issues

Surface coastal currents mapped by a pair of high frequency ground-wave radars (HFR) have been used to predict Lagrangian trajectories in the proximity of Heron Island (Capricorn Bunker Group, Great Barrier Reef, Australia), and to compare with the current data measured by an Acoustic Doppler Current Profiler (ADCP) at three mooring stations. Overall the HRF and ADCP absolute current speeds showed a difference less than ±0.15 m s−1 for 68% of the observations. A good agreement between HFR (at a depth of 1.5 m) and ADCP (at a depth of 5.5 m) data were observed for the u-component (cross-shelf) which presented a stronger tidal signal, while a poor comparison was found for the v-component (north-south) more influenced by the south-easterly and northerly winds. The HFR allowed inclusion of not only the temporal, but also the spatial current variability in the tracking computation. This proved to be crucial because the Lagrangian trajectories were very sensitive to the starting position and time in the studied area, where the currents exhibit a large spatial variation imposed by tides, winds, large scale circulation and topography. One challenge in applying HFR data for Lagrangian tracking consists of estimating the missing values and including the effects of small scale fluctuations.

Coherence scales for microwave Bragg scatter

The transfer functions used in most inversion schemes for calculating ocean surface directional wave spectra from SAR images are simple functions of k, the wave number, and are based on three modulation processes which are tilt modulation, hydrodynamical modulation and the velocity bunching effect. Alpers et al. (1981) indicated that the effect of velocity bunching on the SAR resolution on the sea surface was not included in his analysis. Heron and Amadon, (1992) showed that spatial resolution of the SAR system is adversely affected by velocity bunching. The present authors show that the tilt modulation mechanism has an impact on limiting the coherence scale of the Bragg scattering waves on the sea surface. For typical sea conditions they find that the coherence lengths are of the order of 1-10 metres. This calculation of coherence lengths gives a firm theoretical basis for the assumption that the coherence length is greater then the Bragg wavelengths but shorter than the SAR resolution scale.

PortMap: a VHF ocean surface radar for high spatial resolution

A VHF ocean surface radar system operating at 152.2 MHz with a bandwidth of 1.5 MHz is producing surface currents on a spatial grid of 100 m. This is an appropriate spatial resolution for current map products in Ports and Harbours and around natural or built headland features. The design parameters for the new PortMap radar are discussed and sample data are shown using an early prototype for a narrow tidal inlet with a shipping channel and mudflats on each side. We show that eddy structures develop in the ebb flow which have special consequences for sediment transport in the alongshore direction across the mouth of the inlet. A second case study is shown where an alongshore current interacts with a headland to produce a recirculation of water on the leeward side. In both cases the 100 m spatial scale of the data points gives a well-sampled image for surface current.

REMOTE SENSING OF SEA SURFACE SALINITY: A CASE STUDY IN THE BURDEKIN RIVER, NORTH-EASTERN AUSTRALIA

The principles of remotely sensing sea surface salinity are briefly reviewed. The airborne instrument used for this study is a Scanning Low Frequency Microwave Radiometer (SLFMR). It has spatial resolution of typically 500m (depending on the altitude of the aircraft) and a salinity resolution of about 1 psu. This configuration is suitable for studying the dynamics of river plumes as they form on the continental shelf. The Burdekin River is one of Australias major rivers with about 2.4% of the annual runoff from the continent. This river is in the dry tropics and has severe transient peaks which last for days to weeks during the summer monsoon season. The case study shows the river plume during growth and decay phases and is supported with in situ vertical profiles of water salinity and temperature. The river plume forms a classic low density layer at the surface, but its development and movement along the coast is a feature of the regional oceanography.

Calibration of scanning low frequency microwave radiometer

The scanning low frequency radiometer (SLFMR) is a narrow-band (24 MHz) radiometer operating at 1.413 GHz. It uses a Dicke-switched reference load and a null sensor to match a noise temperature to the brightness temperature of a target. A Butler matrix is used to steer an 8 /spl times/ 8 phased array antenna into eight beam directions. Calibration is required to obtain sea surface salinity estimates from the instrument as it is flown over an area to be mapped. Sea surface temperature, sea state, beam incidence angle and downwelling brightness temperature of the air affect the instrument readings. Consideration must also be given to the effect of solar and galactic radiation reflected off the sea surface into the instrument. This paper focuses on instrument calibrations which are needed to account for the effects of various temperatures measured by sensors at key locations within the instrument. Calibration of the SLFMR was performed by Prosensing before delivery, but it became clear that re-calibration was necessary for each flying campaign. Long-term stability of the instrument and appropriate calibration parameters in light of those suggested by the manufacturer are discussed. These are second-order corrections which arise from small variations in temperature

Operational requirements for oceanographic ground-wave HF radars: Experiences from the Australian Coastal Ocean Radar Network

the whole cabinet has a feedback control loop to hold the cabinet temperature at a nominal 40/spl deg/C. A multivariate linear approach is used to calibrate the instrument for a set of coefficients associated with the temperature measurements. Coefficients were evaluated for all beam positions and checks were made with the SLFMR pointing upwards to scan across known sources of the sun, moon and the centre of the galaxy. Further calculations were made in a room where the target temperature was kept constant. These calibration procedures removed imbalances between the responses at different beam positions. Success of the procedure is demonstrated with some early salinity maps made in the Great Barrier Reef Lagoon in north-east Australia.