William C. Keller

measuring stream discharge by non‐contact methods: A Proof‐of‐Concept Experiment

This report describes an experiment to make a completely non-contact open-channel discharge measurement. A van-mounted, pulsed doppler (10GHz) radar collected surface-velocity data across the 183-m wide Skagit River, Washington at a USGS streamgaging station using Bragg scattering from short waves produced by turbulent boils on the surface of the river. Surface velocities were converted to mean velocities for 25 sub-sections by assuming a normal open-channel velocity profile (surface velocity times 0.85). Channel cross-sectional area was measured using a 100 MHz ground-penetrating radar antenna suspended from a cableway car over the river. Seven acoustic doppler current profiler discharge measurements and a conventional current-meter discharge measurement were also made. Three non-contact discharge measurements completed in about a 1-hour period were within 1% of the gaging station rating curve discharge values. With further refinements, it is thought that open-channel flow can be measured reliably by non-contact methods.

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.

The dependence of microwave backscatter from the sea on illuminated area: Correlation times and lengths

During the Synthetic Aperture Radar (SAR) and X band Ocean Nonlinearities-Forschungsplattform Nordsee experiment, we mounted two continuous wave microwave systems on an elevator on the German Research Platform Nordsee for the purpose of investigating the dependence of microwave backscatter from the sea surface on illuminated area. The two systems operated at X and Ka bands (10 and 35 GHz) and collected HH and VV polarized backscattered signals simultaneously. The elevator system allowed us to vary the altitude of the two microwave systems above the sea surface from 7.5 to 27 m, always in the far field of the antennas. Most data were collected at a 45° incidence angle, which implied that the Ka band system illuminated areas from 0.4 to 6.0 m2 while the X band system viewed spots between 2.9 and 41.3 m2. We examined the dependence of the normalized radar cross section (σ0), its variance, and the bandwidth of the Doppler spectrum on illuminated areas. We were unable to detect any dependence of σ0 on area but found a definite decrease in its variance as area increased. At X band the variance divided by the square of σ0, the normalized variance, decreased from values near 12 for small areas to values near 2 for large areas. At Ka band, corresponding values were 40 and 2. The normalized variance was always slightly larger for HH polarization. By fitting the area dependence of the normalized variance to available theory, we deduce that correlation lengths are on the order of 10 times the microwave wavelength at both X and Ka band. Values for the normalized variance of an elementary scattering facet were also inferred and are presented in this paper. From the Doppler bandwidths we obtained radial velocity spreads over the illuminated areas and found that they agreed well at X and Ka band. These velocity spreads, which are inversely proportional to the correlation time of the backscatter, increased rapidly with illuminated area for small areas but tended to level off to values of about 0.5 m s−1 at large areas. This implies a decorrelation time for large illuminated areas of about 10 ms at X band and 3 ms at Ka band but somewhat larger values for small areas. The dependence of the velocity spread was found to be well explained by theory if an intrinsic velocity spread of 0.07 m s−1 was used to represent scatterer lifetime effects.

Bound waves and Bragg scattering in a wind-wave tank

We present optical and microwave measurements that show the presence of bound waves traveling at the speed of the dominant wave in a wind-wave tank. We suggest that when these bound waves are much shorter than the dominant waves, they are preferentially located on the leeward face of the dominant wave and hence have a mean tilt. We hypothesize that the turbulence associated with these bound waves suppresses freely propagating, wind-generated waves where bound waves are present so that we may divide the rough water surface into patches containing free and patches containing bound waves. This model is shown to account for the observed histograms of slope measured in the tank and, at least qualitatively, for the observed decrease in the probability of finding bound waves with increasing wind speed. Furthermore, if we add these bound, tilted waves to the free waves of the standard Bragg/composite-surface scattering model for microwave scattering from rough water surfaces, then the model can account for many otherwise unexplained features of the scattering. Principal among these features are the rapid decrease in polarization ratio and rapid increase in the first moment of the microwave Doppler spectrum with increasing wind speed when the antenna is directed upwind, features that occur to a much lesser extent when the antenna looks downwind.

A wave tank study of the dependence of X band cross sections on wind speed and water temperature

Measurements of normalized radar cross sections of wind-generated waves were made at X band for both vertical and horizontal polarization for incidence angles of 10°, 28°, 48°, and 68°. The study, conducted in the Naval Research Laboratory wind-wave facility, sought to measure effects on the backscatter of varying water temperature, wind speed, and wind stress. The cross-section measurements were averaged for 2.13 min simultaneously with wind speed and wind stress. Air and water temperature were measured periodically with mercury thermometers. The results were compared with the empirical model functions developed for the Seasat-A satellite scatterometer, SASS I and SASS II, and with the physically based models of Burden and Vesecky, Plant, and Donelan and Pierson. In order to use the SASS I and SASS II models for these comparisons, differences between Ku and X bands were assumed to be small; where possible, the other models were evaluated at X band. It was found that none of the models was consistently accurate at all wind speeds, incidence angles, and polarizations. Part of the inconsistency can be assigned to the fact that the models were developed for open ocean conditions with much higher sea states. However, Plants model can easily be adjusted to account for this effect by using a relationship between mean squared slope and wind stress appropriate for tank conditions. When this was done, the model did fit the data better but the improvement was not dramatic. This indicates that inaccuracies in the models are probably due to other factors as well. When plotted versus 19.5-m winds on a log-log scale, the measured cross sections do not fall on straight lines. Thus a power law dependence of cross section on wind speed is not a good representation of that relationship over our whole wind speed range. The data exhibit large variations at low wind speeds, however, which are not related to system noise. This may indicate that the statistics of backscatter depend markedly on wind speed. When these low wind speed data were omitted, a power law was found to fit the remaining data rather well. Although the water temperature was varied from 9°C to 36°C when the measurements were made at a 48° incidence angle, no temperature dependence was detectable above the low-wind-speed variability. The wave tank data compare well enough with 10 GHz, 3.0 cm (X band) aircraft measurements, and with the 14.6 GHz, 2.1 cm (Ku band) satellite data used in the SASS II model to cast doubt on the hypothesis that cross section depends on antenna altitude.

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...

Tower-based measurements of normalized radar cross section from Lake Ontario : evidence of wind stress dependence

We report here the dependence of the normalized radar cross-section (NRCS) on incidence angle, azimuth angle, wind speed, wind stress, and atmospheric stratification for Ku band microwave backscatter from a lake. The measurements were made in autumn 1987 on Lake Ontario, using a rotating microwave system mounted on a research tower operated by the Canada Centre for Inland Waters. The results show that at intermediate incidence angles the NRCS on the lake generally increases faster with wind speed than it does on the ocean. We attribute this to the larger atmospheric drag coefficients which exist on the lake compared with the ocean, and we show that the results are more consistent with a dependence of the NRCS on wind stress than on wind speed near the surface. We find a stratification dependence of the NRCS similar to that previously reported at C band and show that at 40° and 60° incidence angles this dependence can be removed by parameterizing the NRCS in terms of either the friction velocity or neutral wind speed. At a 20° incidence angle the stratification dependence is not removed by this procedure.

The normalized radar cross section of the sea at 10/spl deg/ incidence

Measurements of the normalized radar cross section of the sea at K/sub u/ band at an incidence angle of 10/spl deg/ were performed from a manned airship off the Oregon coast in September and October of 1993. The cross section at this incidence angle is often assumed to have little dependence on windspeed and direction. Their measurements, however, indicate that at windspeeds below 6-7 m/s, the cross section is in fact dependent on these quantities, and the azimuthal modulation can reach values on the order of 5-8 dB. Comparisons of the measured values with the predictions of the quasispecular scattering model are presented. The theory is shown to be accurate in predicting the azimuthal modulation and the strength of the backscatter if the effects of swell are included or if measured wind directions are ignored and the upwind direction is forced to be near the maximum cross section. Values of mean-square wind-wave slope and effective-reflection coefficient required to obtain these fits are very close to those obtained by previous investigators. In particular, mean-square wind-wave slopes are about 70-80% of those of Cox and Munk (1954) because the radar responds only to facets larger than about 10 cm, with smaller ripples acting to reduce the reflection coefficient. If swell is included, they find that mean-square slopes in the direction of the swell, that are as much as ten times the measured swell slopes, are required to fit the model to the cross-section data at low windspeeds. They suggest that this may be due to high-order effects of the hydrodynamic modulation of short waves by the swell. They believe that this explanation is more likely than assuming that wind directions were incorrectly measured.

Radar imaging of thermal fronts

Abstract The paper investigates the physics of radar backscatter across sea surface temperature fronts using a simple drag model followed by actual airborne radar observations. The model predicts large changes in radar cross section for low wind speed conditions. The X-band (9–43 GHz) Real Aperture Radar (RAR) observations in the vicinity of the sea surface temperature fronts show that radar cross-section variations are largely dependent on the local wind direction with respect to the front and the change in wind stress response at the front and feedbacks from the larger scale buoyancy driven circulation. Other radar imaging mechanisms are discussed.

Microwave and acoustic scattering from parasitic capillary waves

We report simultaneous microwave and acoustic Doppler backscattering measurements made in a wind-wave tank. The microwave system operated at 35 GHz (0.857 cm), while the acoustic system transmitted at 190 kHz (0.777 cm). The two systems were mounted to view the surface at the same incidence angle, which was varied. The measurements showed that when both systems looked upwind, horizontal transmit and receive polarization (HH) microwave backscatter from the rough water surface was 2 to 12 dB stronger than acoustic backscatter, depending on incidence angle and wind speed. When the acoustic system looked downwind, however, its backscattering level was consistently about 1 dB lower than that of the upward-looking microwave system. We interpret these results to indicate that both the acoustic and microwave systems were scattering from parasitic capillary waves in addition to freely propagating, wind-generated waves. The tilt of the parasitic capillary waves accounts for the observed differences in microwave and acoustic backscatter. We show that Bragg scattering theory predicts both the intensity and the Doppler shift of the microwave and acoustic signals very well using known properties of parasitic capillary waves. Spectral densities of the parasitic capillary waves derived from this Bragg scattering model are in good agreement with those predicted recently by Fedorov and Melville [1998] and observed by Fedorov et al. [1998].