Dennis B. Trizna

Mapping of North Atlantic winds by HF radar sea backscatter interpretation

This work presents preliminary results on attempts to map winds of a storm at long range (500 to 1000 nmi) over a large area ( 10^{5} mi2) in the North Atlantic from the U. S. Naval Research Laboratory, HF radar research facilities at Chesapeake Beach, Md. [1] It appears that the short time response of the sea surface to local winds can be mapped by the analysis of a matrix of range-azimuth records containing frequency power spectra of HF radar signals backscattered from the sea surface via the ionosphere. This paper presents such a map based upon the ratio of the strengths of the first-order contributions to the backscatter spectrum (the approach-recede first-order Bragg lines) and is compared qualitatively with a U. S. Weather Bureau map of the area.

A model for Brewster angle damping and multipath effects on the microwave radar sea echo at low grazing angles

Brewster angle damping and local multipath effects are considered as sources of polarization differences in low grazing angle sea scatter characteristics. The authors show that at least five observed polarization differences can be explained by local multipath interference effects that occur due to the illumination of discrete nonlinear ocean surface features, such as bores and small scale breaking waves. The illumination gain factor (IGF) is defined at a point in space, as the total power at that point relative to the power in the incident plane wave. The IGF resulting from local multipath from the sea surface forward of a discrete scatterer produces strong interference patterns that can vary both with grazing angle and scatterer height. As a result, IGF values up to a factor of 16 (12 dB) can occur for horizontal polarization (HH) when the interference is constructive; a corresponding strong cancellation occurs for destructive interference. These extreme variations can cause strong HH NRCS amplitude modulations due either to a change of local wave slope or a change of scatterer shape with time. However, Brewster angle damping of the forward scatter path for grazing angles below 20/spl deg/ occurs for vertical (VV) polarization, and reduces the VV IGF in magnitude and dynamic range, eliminating such strong modulations. This effect scales with radar wavelength, and higher wave features are required to produce equivalent effects for radar frequencies far below 10 GHz. As an illustration, six radar bands are compared: L (1.4 GHz), S (3.5 GHz), C (5 GHz), X (10 GHz), K/sub u/ (15 GHz), and K/sub a/ (35 GHz), for a sea water dielectric. X-band results indicate that 12-dB IGFs can occur for water surface features just a centimeter above the mean surface. As an application of these results, the influence of these HH and VV IGF patterns is modeled for discrete scatterers distributed uniformly along an ocean gravity wave. The dynamic range of the HH IGF for a distribution of bore scatterers up to 5 cm high is found to he significantly larger than for VV at all locations on the long wave. Moreover, the IGF HHVV polarization ratio forward of the crest, where the largest number of small scale breaking wave scatterers occurs, is larger than at all other regions of the long wave, of the order of 20 dB. These results suggest that HH polarization may be sensitive to small scale breaking features on the ocean surface at low grazing angles, and thus may be a sensitive measure of air-sea fluxes.

Studies of dual polarized low grazing angle radar sea scatter in nearshore regions

Reports on experiments featuring horizontally (HH) and vertically (VV) polarized X-band marine radar sea scatter imagery in the coastal environments of Bermuda and La Jolla, under light to moderate winds and the absence of long gravity waves. These conditions allowed the study of fundamental scattering mechanisms from small scale roughness and short waves of a few meters wavelength or less, shorter than the radar pulse. While a large fraction of radar data collected revealed the presence of slick bands, the authors analyze the radar echo of ambient background outside of the slicks. Sea scatter data were digitally recorded for 360/spl deg/ azimuthal coverage for grazing angles between 1-3.5/spl deg/, and were converted to normalized radar cross section (NRCS or /spl sigma//spl deg/) images. The HH and VV polarizations show quite different spatial texture, with HH exhibiting a discrete character and VV being more nearly spatially homogeneous. Grazing angle dependencies are different for HH and VV: upwind-downwind differences of mean NRCS show ratios of just 4-6 dB for VV, but are equal or greater than the 16-dB noise-imposed limit for HH for the low wind conditions. HH NRCS grazing angle characteristics are shown to correlate with biological activity indicators of the coastal waters, with a fourth power dependence in relatively unproductive waters, to a nearly quadratic in productive waters. Arguments are presented that suggest different scattering mechanisms for the two polarizations: evenly distributed Bragg scatter patches for VV and scatter from small asymmetric bore features for HH. A multipath illumination model for small bore features is outlined, and scale sizes for the small scale breakers inferred from the experimental results presented are between 2 and 4 cm in height, with crest widths between 24 and 48 cm.

Laboratory studies of radar sea spikes at low grazing angles

We report on measurements of low grazing angle radar sea spikes in a laboratory wave tank. Using an X band radar with 10-cm range resolution and multiple-range bin capability, the sea spikes were observed to have a horizontal/vertical polarization ratio much larger than unity. This is similar to observations in earlier field experiments, but is in contrast to a ratio of unity recently observed for moderate grazing angles (Jessup et al., 1990). Our experiment was designed to validate the low grazing field results and to establish design criteria for spatial-temporal measurement instrumentation; single-point measurements currently used as surface truth cannot provide water surface profiles needed for modeling of radar scatter from deterministic surface features. Surface scattering features were generated using a paddle-generated wave, combined with wind waves for a series of wind speeds. The surface feature responsible for the sea spike echo was visually identified as a short wave crest, occurring on the windward side of the paddle wave, driven to extreme steepness by the passage of the faster paddle wave crest. Long decorrelation times observed in the field are also seen in these studies, of the order of 500 ms for horizontal polarization and less than 5 ms for vertical. We suggest that rather than Bragg scatter, the occurrence of similar ubiquitous features on the ocean are responsible for horizontally polarized low grazing angle sea scatter at the X band for low to moderate winds, and perhaps at other frequencies as well.

Statistics of low grazing angle radar sea scatter for moderate and fully developed ocean waves

Results are reported of two deep ocean radar sea scatter experiments using calibrated marine navigation radars. The first experiment featured a continuous wind direction over a four-day period, satisfying time and fetch requirements for a fully developed sea. The second experiment experienced changing winds, which was expected to produce random seas. Radar cross section samples from a 60 degrees sector centered about the maximum clutter direction were assembled into cumulative distributions for several range bins, corresponding to different grazing angles between 1 degrees and 8 degrees . These distributions are bimodal for moderate to strong winds and were fitted by two Weibull distributions. The Weibull fit to the highest amplitudes was associated with discrete scatters, while that fitting the lower amplitudes was identified with distributed roughness. Models for scattering mechanisms are inferred from the results. >

A Study of the Wavenumber Spectra of Short Water Waves in the Ocean

Abstract Spatial measurements of capillary-gravity waves in the ocean were obtained using a scanning slope sensor mounted on a free-drifting buoy intended to minimize the flow disturbance. The data provide direct calculation of the wavenumber spectra of surface curvature in the capillary-gravity wave range. The results indicate that 1) a pronounced peak at the wavenumber k = 9 rad cm−1 is evident in the curvature spectra for wind speeds below 6 m s−1; 2) the slopes of the curvature spectra are 1 and −1 on the two sides of the spectral peak, 3) the spectral density and mean-square roughness properties increase linearly with wind speed; and 4) these observations suggest a spectral function of the form χ(k) = Au*c−2cmkmk−4, which is proportional to u*k−3 in the short gravity wave region and u*k−5 in the capillary wave region, where u* is the wind friction velocity, cm the minimum phase velocity of surface waves, and km the corresponding wavenumber. Capillary-gravity wave wavenumber spectra obtained from the ...

Ultrawide-band radar observations of multipath propagation over the sea surface

An ultrawide-band radar system centered at 10 GHz has been developed for sea-scatter research and was recently deployed on a research pier in the North Atlantic. The radar is based on a time-domain reflectometer module for a sampling oscilloscope. Using transient excitation of a traveling wave-tube amplifier, the system generates 200-ps wide pulses with a 10-GHz center frequency and a peak power of approximately 1 KW. The resulting range resolution is approximately 3 cm. The system was used to investigate low-grazing angle, multipath effects in the ocean environment using a trihedral corner reflector, mounted 45 cm above the water surface. The ultrahigh-range resolution of the system allows spatial separation of the direct and indirect echoes from the trihedral. In addition, a comparison of the indirect vertically- (VV) and horizontally- (HH) polarized echoes illustrates the effects of Brewster angle damping. The implications of these effects for sea-clutter statistics are briefly discussed.

Errors in bathymetric retrievals using linear dispersion in 3-D FFT analysis of marine radar ocean wave imagery

The phenomenon of ocean wave-shoaling, and the associated reduction of ocean wave phase speed with decreased water depth, provides useful information for inferring water depth D (bathymetry) in coastal environments. One strategy for relating D to phase speed C and wave-vector K of long wave length ocean waves involves using the one-dimensional (1-D) linear (gravity wave) dispersion relationship C 2 = g * tanh(KD)/K. In principle, this approach has limitations because the approach is based on a WKB approximation, so it cannot be applied when D varies appreciably over the wavelength of a shoaling wave. Also, the approach is restricted to waves that have small wave height. In the present paper, we use a set of marine radar image sequences and apply this linear approximation, using a three-dimensional (3-D) FFT analysis of 88 sets of image sequences spaced half an hour apart. We invert the dispersion relation to solve for D. Depths between 3.6 and 5.8 m were tested, for root mean square (RMS) wave heights offshore between 8 and 3 m. We show that for low to moderate wave heights, the approach does generally retrieve the correct depth in water depths of 5 m and greater for moderate wave RMS heights. However, an increase in the RMS wave height from 1 m to 3.5 m produced a much poorer depth estimate, proving the need for an application of a nonlinear wave model to the problem. The errors also increase with shallower depths as expected, as the error dependence on depth and wave height is determined.

A model for Doppler peak spectral shift for low grazing angle sea scatter

A model is formulated for Doppler spectral characteristics of radar sea scatter for low grazing angles, and is compared with previous radar measurements reported in the literature. The Doppler model is based upon the two-scale model for radar scatter, with scatterer motions hypothesized as due to the orbital wave velocity of the large-scale waves, Stokes and wind drift currents, and the phase velocity of the small-scale Bragg scatterers. Expressions for Doppler shifts due to these motions are derived, and are given as a function of wave height, wave period, and wind speed. Although this model appears to account for the peak Doppler shift of the sea-scatter Doppler spectrum for vertical polarization, it is insufficient to describe horizontally and cross-polarized data, which have larger mean Doppler shifts. However, these two cases are found to scale very closely with the nearly simultaneous vertically polarized data for the variety of environmental conditions reported. Implications of the extension of these results to higher-angle remote-sensing applications are discussed.

Spatial measurements of short wind waves using a scanning slope sensor

Abstract The spatial structures of short wind-generated waves from 0.6 to 12 cm were measured with a scanning optical slope sensor and the wavenumber-frequency spectra of surface slope and curvature under various wind velocities were computed. The results show that the energy densities of both the frequency and wavenumber spectra increase with the wind-friction velocity. The rate of increase with wind-friction velocity is linear in the gravity and short capillary regions and between square to cubic in the gravity-capillary (centimeter waves) region. The drop-off of the wavenumber ( k ) spectrum varies from k −4 in the gravity region to k −8 in the capillary region, with a sharp break between the two regions occurring at the dividing wavelength from 0.8 cm at low winds to 1.6 cm at high winds. The magnitudes of the mean-square slope and mean-square curvature at various wind velocities are also presented. The fractional contribution to the mean-square slope from short waves of 0.6–12 cm in length is more than 50%, and the ratio shows an increase with with velocity