Kenneth Hayes

Measurement of river surface currents with coherent microwave systems

River surface currents have been measured using coherent microwave systems from a bridge, a cableway, several riverbanks, a helicopter, and an airplane. In most cases, the microwave measurements have been compared with conventional measurements of near-surface currents and found to be accurate to within about 10 cm/s. In all cases, the basis for the microwave measurement of surface current is the Doppler shift induced in the signal backscattered from the rough water surface. In this paper, we outline the principles of the measurements and the various implementations that have been used to make microwave measurements of surface currents. Continuous-wave (CW) microwave systems have been used from a bridge to make long-term measurements of surface currents; these are compared with current-meter measurements and with time series of stage. A compact CW system has been developed and used on a cableway to measure surface currents at various distances across a river; these measurements have been compared with acoustic ones. Pulsed Doppler radars have been used to measure river surface currents from a riverbank, a helicopter, and an airplane. In the first two cases, comparisons with both current-meter and acoustic measurements have been made. We suggest that the CW system would be preferable to the pulsed Doppler radar to make such measurements from helicopters in the future. Finally, we consider the implications of our experiments for the measurement of surface currents from aircraft or satellites using interferometric synthetic aperture radars (INSARs). We find that a combination along-track, cross-track INSAR is necessary but that significant limitations are inherent in the technique.

Diverse Gastropod Hosts of Angiostrongylus cantonensis, the Rat Lungworm, Globally and with a Focus on the Hawaiian Islands

Eosinophilic meningitis caused by the parasitic nematode Angiostrongylus cantonensis is an emerging infectious disease with recent outbreaks primarily in tropical and subtropical locations around the world, including Hawaii. Humans contract the disease primarily through ingestion of infected gastropods, the intermediate hosts of Angiostrongylus cantonensis. Effective prevention of the disease and control of the spread of the parasite require a thorough understanding of the parasites hosts, including their distributions, as well as the human and environmental factors that contribute to transmission. The aim of this study was to screen a large cross section of gastropod species throughout the main Hawaiian Islands to determine which act as hosts of Angiostrongylus cantonensis and to assess the parasite loads in these species. Molecular screening of 7 native and 30 non-native gastropod species revealed the presence of the parasite in 16 species (2 native, 14 non-native). Four of the species tested are newly recorded hosts, two species introduced to Hawaii (Oxychilus alliarius, Cyclotropis sp.) and two native species (Philonesia sp., Tornatellides sp.). Those species testing positive were from a wide diversity of heterobranch taxa as well as two distantly related caenogastropod taxa. Review of the global literature showed that many gastropod species from 34 additional families can also act as hosts. There was a wide range of parasite loads among and within species, with an estimated maximum of 2.8 million larvae in one individual of Laevicaulis alte. This knowledge of the intermediate host range of Angiostrongylus cantonensis and the range of parasite loads will permit more focused efforts to detect, monitor and control the most important hosts, thereby improving disease prevention in Hawaii as well as globally.

Measurements of the Marine Boundary Layer from an Airship

Abstract In 1992 and 1993, the authors made measurements of the marine boundary layer off the coast of Oregon from an airship. In 1992, these measurements consisted of coherent microwave backscatter measurements at Ku band taken from the gondola of the airship and micrometeorological and wave height measurements made from an airborne platform suspended by a cable 65 m below the gondola so that it was between 5 and 20 m above the sea surface. In 1993, an infrared imaging system was added to the suite of instruments operated in the gondola and two narrowbeam infrared thermometers were mounted in the suspended platform. In both years, a sonic anemometer and a fast humidity sensor were carried on the suspended platform and used to measure surface layer fluxes in the atmosphere above the ocean. A laser altimeter gave both the altitude of the suspended platform and a point measurement of wave height. By operating all these instruments together from the slow-moving airship, the authors were able to measure atmos...

Simultaneous Measurement of Ocean Winds and Waves with an Airborne Coherent Real Aperture Radar

Abstract A coherent, X-band airborne radar has been developed to measure wind speed and direction simultaneously with directional wave spectra on the ocean. The coherent real aperture radar (CORAR) measures received power, mean Doppler shifts, and mean Doppler bandwidths from small-resolution cells on the ocean surface and converts them into measurements of winds and waves. The system operates with two sets of antennas, one rotating and one looking to the side of the airplane. The rotating antennas yield neutral wind vectors at a height of 10 m above the ocean surface using a scatterometer model function to relate measured cross sections to wind speed and direction. The side-looking antennas produce maps of normalized radar cross section and line-of-sight velocity from which directional ocean wave spectra may be obtained. Capabilities of CORAR for wind and wave measurement are illustrated using data taken during the Shoaling Waves Experiment (SHOWEX) sponsored by the Office of Naval Research. Wind vectors...

Air/sea momentum transfer and the microwave cross section of the sea

Measurements of atmospheric fluxes of heat, moisture, and momentum were made simultaneously and coincidentally with microwave backscatter measurements from an airship flown over the Pacific Ocean in 1993. The measurement technique was well suited to measure fluxes at very low wind speeds because the airship required an air speed near 10 m s−1 in order to maintain altitude. The measurements show that very low wind speeds are always associated with very low microwave cross sections and very high air/sea drag coefficients. The occurrence of regions of very low wind speed is not usually correlated with either the sea surface temperature or the air/sea temperature difference. Nevertheless, these regions can remain in place for time periods of several hours. The rate of increase of the microwave cross section at very low wind speeds agrees with that predicted by Donelan and Pierson [1987], but the absolute value of the threshold wind speed appears to be lower than their prediction. The high drag coefficient at low wind speeds is due to the fact that the friction velocity is nearly constant for wind speeds below 4–5 m s−1. Thus, at these wind speeds the increase of the microwave cross section follows the behavior of the wind speed rather than the wind stress. At higher wind speeds, however, the behavior is reversed with the cross section following the wind stress at a constant wind speed. We suggest that this behavior can be understood if momentum transfer across the air/sea interface is supported by both viscosity and the entire spectrum of waves on the surface, as many investigators have indicated.

Characteristics of Internal Waves in the South China Sea Observed by a Shipboard Coherent Radar

In 2005 and 2007, a coherent, X-band radar was deployed in the South China Sea on two different ships. In both cases, the two parabolic antennas of the radar were fixed at grazing angles of approximately 2° looking toward the bow of the ship. The radar transmitted and received through a single antenna but alternated between the two antennas approximately every half second. One antenna was horizontally polarized and the other was vertically polarized. The data were analyzed by computing normalized radar cross sections and scatterer velocities as a function of ground range and time. Surface signatures of the internal waves were obvious in both types of image and at both polarizations as regions of enhanced cross sections or scatterer velocities. The collected imagery showed that at least two different types of internal waves exist in the South China Sea: small, nearly sinusoidal trains of waves and large soliton-like waves. These different types travel at very different speeds and interact with each other. The small nearly sinusoidal waves travelled at phase speeds near 1 m/s that increased as the small wave trains were overtaken by the faster solitons. Combined with other shipboard measurements, the radar measurements yielded the widths, maximum velocities, and strain rates of the solitons as well as the dependence of phase speed on amplitude. When the speeds of both the ship and the solitons were removed, the measurements showed that soliton full-widths at half-maximum ranged from about 0.5 to 4.5 km. These widths showed a dependence on the amplitude of the soliton. The phase speeds of the solitons also depended on their amplitude, reaching 3 m/s in deep water but only about 1.2 m/s in shallow water. CTD profiles were used to estimate an interface depth for a two-layer fluid model of the propagation of the solitons. The phase speeds predicted by this model agreed well with the observed dependence of the soliton phase speed on amplitude in both shallow and deep water.

Measuring and Modeling the NRCS of the Sea for Backscatter

Recent ship-based measurements using a calibrated, coherent, dual-polarized, X-band radar illuminated the behavior of the normalized radar cross section of the sea, NRCS or s0, at incidence angles of 88deg to 89deg. For wind seas s0(VV) at these incidence angles behaves much like those at lower incidence angles while s0(HH) behaves very differently, being largest looking upwind and smallest looking downwind. Fits of a multiscale scattering model to these data and data at lower incidence angles show that over the range of incidence angles from 0deg to 89deg, s0(VV) is adequately explained by the model while s0(HH) is generally higher than the model predicts at incidence angles above about 45deg. Thus scattering phenomena exist on the ocean that affect HH backscatter very strongly at high incidence angles while impacting VV backscatter only slightly. We show that when the ocean surface is disturbed by currents s0(HH) can exceed s0(VV) by as much as 15 dB for long periods of time and over large spatial areas. We examine phenomena that might account for this behavior and how they might affect the wind sea case.

Characteristics of internal waves in the South China Sea Observed by a shipboard coherent radar

In 2005 and 2007, a coherent, X-band radar was deployed in the South China Sea on two different ships. In both cases, the two parabolic antennas of the radar were fixed at grazing angles of approximately 2° looking toward the bow of the ship. The radar transmitted and received through a single antenna but alternated between the two antennas approximately every half second. One antenna was horizontally polarized and the other was vertically polarized. The data were analyzed by computing normalized radar cross sections and scatterer velocities as a function of ground range and time. Surface signatures of the internal waves were obvious in both types of image and at both polarizations as regions of enhanced cross sections or scatterer velocities. The collected imagery showed that at least two different types of internal waves exist in the South China Sea: small, nearly sinusoidal trains of waves and large soliton-like waves. These different types travel at very different speeds and interact with each other. The small nearly sinusoidal waves travelled at phase speeds near 1 m/s that increased as the small wave trains were overtaken by the faster solitons. Combined with other shipboard measurements, the radar measurements yielded the widths, maximum velocities, and strain rates of the solitons as well as the dependence of phase speed on amplitude. When the speeds of both the ship and the solitons were removed, the measurements showed that soliton full-widths at half-maximum ranged from about 0.5 to 4.5 km. These widths showed a dependence on the amplitude of the soliton. The phase speeds of the solitons also depended on their amplitude, reaching 3 m/s in deep water but only about 1.2 m/s in shallow water. CTD profiles were used to estimate an interface depth for a two-layer fluid model of the propagation of the solitons. The phase speeds predicted by this model agreed well with the observed dependence of the soliton phase speed on amplitude in both shallow and deep water.

X-band backscatter from the ocean at low-grazing angles

In 2005 and 2006, we mounted an X- band Doppler radar on ships that operated in the South China Sea and off the coast of New Jersey, respectively. The measurements were made only at W polarization in 2005 but at both HH and W polarization in 2006. On average, VV normalized radar cross sections, sigma0(VV), behaved at grazing angles between 1 and 2 degrees in much the same manner that they do at higher grazing angles. In particular, they showed the second-harmonic dependence on the angle between the antenna-look direction and the wind direction that is characteristic of scatterometry. sigma0(HH), on the other hand, showed little evidence of the second harmonic component, maximizing with the antenna looking into the wind and minimizing in the opposite direction. For both polarizations, sigma0 was generally well above that expected from Bragg scattering and the polarization ratio sigma0(VV) / sigma0(HH) was much smaller. Surface signatures of internal waves (IWs) off the New Jersey coast were much weaker when the antennas looked in the direction of internal wave propagation than when they looked opposite this direction. Interestingly, for the nonlinear internal waves found in the South China Sea, the opposite phenomenon occurred: W signatures were stronger looking in the direction of IW propagation than opposite to it.

Shipboard Measurements of Coherent Microwave Backscatter from the Ocean

Two different coherent X-band radars have been operated from ships to collect microwave back-scatter from the ocean at low grazing angles. The data show not only the properties of the mean backscatter but also temporal and spatial effects of both surface and internal waves