Malcolm L. Heron

On the application of HF ocean radar to the observation of temporal and spatial changes in wind direction

The application of a 30-MHz narrow-beam ground-wave ocean radar to the observation of wind directions is described. It is found that the \cos^{s} (\theta/2) model for wind-wave directions does not apply in a specific case of shallow water where swell waves are behaving nonlinearly. To experimentally extract unambiguous wind directions from this model requires sampling three different beam angles simultaneously. In practice some time and space stationarity is assumed. Detailed analysis in time and space reveals structure in the transition of the cold front from sea to land which, although unexpected, agrees with coastline observations where they are available. The nature of the structure is only briefly discussed. The response of the \lambda = 5 -m wind waves to the frontal change was two orders of magnitude faster than time constants for similar events previously modeled using pitch-and-roll buoy data. This discrepancy needs to be reconciled before lower frequency radars can be used without ground truth for wind-direction changes.

An overview of developments and applications of oceanographic radar networks in Asia and Oceania countries

More than 110 radar stations are in operation at the present time in Asia and Oceania countries, which is nearly half of all the existing radar stations in the world, for purposes related to marine safety, oil spill response, tsunami warning, coastal zone management and understanding of ocean current dynamics, depending mainly on each country’s coastal sea characteristics. This paper introduces the oceanographic radar networks of Australia, China, Japan, Korea and Taiwan, presented at the 1st Ocean Radar Conference for Asia (ORCA) held in May 2012, Seoul, Korea, to share information about the radar network developments and operations, knowledge and experiences of data management, and research activity and application of the radar-derived data of neighbouring countries. We hope this overview paper may contribute as the first step to promotion of regional collaborations in the radar observations and data usages and applications in order to efficiently monitor the coastal and marginal sea waters along the western Pacific Ocean periphery.

Finite dispersal of a separative nepheloid plume by an internal hydraulic jump in a tropical mountainous river estuary

This paper investigates the dynamics of an internal hydraulic jump in a river plume and associated suspended sediment dispersal. Field investigations were undertaken into the river plume generated by the Herbert River, Australia, following a moderate flood event induced by Cyclone Fritz in 2004. The forced plume experiences an abrupt transition from supercritical to subcritical via an internal hydraulic jump, as defined by a mode-1 internal Froude number computed using the phase speeds from the Taylor-Goldstein equation. The hydraulic theory of a two-layer stratified flow was used to identify the plume shape and the mechanical energy loss within the jump. The hydraulic jump energy loss is primarily transferred to the buoyancy-driven potential energy, uplifting the river plume. Intense stratification decreases the bottom stress, damping the resuspension. Therefore, a separative nepheloid dispersal system occurs at the jump section. Both the upper and lower nepheloid flows are confined to the inner shelf, but have different dispersal behaviors and mechanisms. The upper nepheloid flow, which is primarily controlled by advection and settling, satisfies an exponential decay law of the total suspended sediment concentrations versus the offshore distance. The lower nepheloid flow dominated by deposition is detached seaward near the lift-off point of the river plume. A turbidity front associated with the jump may accumulate a large quantity of suspended sediments, enhancing sediment release from the river plume. These findings will promote in-depth understanding of both the cross-shelf sediment dispersal and muddy deposit on the shelf.

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

Line broadening on HF ocean surface radar backscatter spectra

Experimental results from an array of moored current meters and an HF ocean surface radar support the idea that line broadening on the radar spectra is caused by the velocity distribution within the radar target cell. The experiment was done in the wake of a small island where the velocity variations were severe. An estimate is made of the line broadening which can be expected. In a turbulent flow with dissipation rate of the order \epsilon \sim 10^{-10}m^{2}s^{-3} and target cell size 1 3000 m, the line broadening is \Deltaf \sim 10^{-3} Hz. This would be resolved with a radar time series of \sim 20 min and indicates that the HF ocean surface radar technique has potential in the observation of surface velocity distributions.

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.

Applying a unified directional wave spectrum to the remote sensing of wind wave directional spreading

Models for directional wind wave spectra can be used to assist in interpretation and analysis of Doppler sea echo spectra to determine wind direction over the oceans by remote sensing techniques. This paper compares and contrasts the data analysis required at different electromagnetic wavelengths. At large wavenumbers the observed spreading depends not only on the directional spreading at the Bragg wavenumber, but also on the weighted integrals over all shorter wavenumbers. This is discussed in terms of microwave and VHF Doppler spectra. At small wavenumbers (HF) the observed directional spreading should be directly predicted by the model. It is in this simple case that there is an inconsistency between the models and the observations, and this has a potential impact on the use of the models at all wavenumbers.

Short-wave ocean wave slope models for use in remote sensing data analysis

Recent developments in the directionality of sea waves are reviewed and adapted into a working model for radiometric remote sensing. There is a weight of evidence, both theoretical and observational, that a bimodality exists in the directional wave spectrum for sea surface gravity waves at wavenumbers greater than the value at the peak of the spectrum. We develop a model for the directional splitting which is consistent with observations but which extrapolates in a credible manner to the omnidirectionality observed at wavelengths around 0.3 m. At shorter wavelengths the directional spectrum becomes unimodal and has an approximately constant shape across the gravity-capillary wave regime. A working model is suggested for the directionality of the wave spectrum to improve the analysis of radiometric sea surface data. The model will also be useful for other analyses including active radar techniques which use Bragg scatter

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.

The Australian Coastal Ocean radar Network facility

The Australian Coastal Ocean radar Network (ACORN) is a monitoring network of HF radars which are being installed around Australia under a National Collaborative Research Infrastructure Strategy (NCRIS). It is a five-year project, at the end of which there will be five pairs of radar stations and one triplet installed and operating, enabled by the central pool of funding for the Integrated Marine Observing System (IMOS) which is a part of NCRIS, and augmented by funding from other sources. At each chosen site there is a pair (or triplet) of radar stations, mounted on the shore, which receive radar echoes from the rough sea. Four of the pairs of radar stations are phased array installations and one pair and the triplet are of the amplitude direction finding genre. The NCRIS strategy is to provide easily accessed data, freely available to researchers in quality controlled archives. The IMOS aim is to produce data which will support research into coastal dynamics and exchange between the open ocean and the continental shelf. In addition to research in coastal oceanography, the data are available for algorithm development and evaluation of new applications for HF ocean radar. There is potential for application of the data to management of coastal marine resources, and in marine safety areas. Real-time maps of surface currents and the prospect of short-term forecasting have the potential to reduce search areas in coastal waters and to make pollution/spill mitigation more effective. With the establishment of HF radar monitoring stations like those in ACORN, there is growing opportunity for researchers around the world to access data from well curated archives to carry out basic research on physical oceanography, or applications research without having direct access to the measuring facility. This feature brings the ACORN HF radars into GEOSS for coastal processes and dynamics.