Radar backscatter from mechanically generated transient breakingwaves. II. Azimuthal and grazing angle dependence

For Pt. I see ibid. vol. 26, pp. 181-200 (2001). This paper describes the results of experimental investigations into the microwave backscatter from mechanically generated transient breaking waves. The investigations were carried out in a 110 m×7.6 m×4 m deep model basin, utilizing chirped wave packets spanning 0.75-1.75 Hz. Backscatter measurements were taken by a K-band continuous wave radar (24.125 GHz) at 40° angle of incidence, and at azimuth angles of 0°, 45°, 90°, 135° and 180° relative to the direction of wave propagation. Grazing measurements were conducted using an X-band (10.525 GHz) FMCW radar at 85° angle of incidence, and azimuth angles of 0° and 180°. Results show that the maximum radar backscatter was obtained in the upwave direction prior to wave breaking and was caused by the specular or near specular presentation of the wave to the radar. After breaking, the backscatter transitioned from a specular or near-specular dominated scattering, primarily seen in the upwave direction, to a small scale roughness dominated scattering, observed at all azimuths. Physical optics solutions were found to correctly predict the backscatter for the specular or near-specular dominated scattering and the small perturbation method was found to accurately model the VV polarization post-break radar backscatter     

Polarimetric analysis and modeling of multifrequency SAR signaturesfrom Gulf Stream fronts

Using airborne synthetic aperture radar data from the 1990 Gulf Stream Experiment, this paper investigates the polarization and wavelength dependence of radar signatures for narrow fronts with converging flows occurring within the Gulf Stream. The signal-to-background ratios of the cross-polarization backscatter return from a convergent front were found much higher than those of copolarization returns, when the flight path is crossing the front. However, a second convergent front, imaged at 45°, showed that the signal-to-background ratios are nearly equal for co- and cross-polarizations. A polarimetric procedure, which has been successfully used to measure terrain slopes and to generate elevation maps, is applied to the convergent front to explain the polarization and imaging geometry dependence of these radar responses. A theoretical modeling of radar modulation using an ocean wave model and a composite-Bragg scattering model, which incorporates the effect of breaking waves, was developed. Calculations with the model agree reasonably well with the radar measurements at various polarizations for three radar frequencies: P-band (68 cm in wavelength), L-band (24 cm), and C-band (5.7 cm)     

Short-wave measurements by a fixed tower-based and a drifting buoysystem

The small-scale roughness of the sea surface acts as an important link in air-sea interaction processes. Radar and sonar waves are scattered by short surface waves providing the basis for remote sensing methods of the sea surface. At high wind speeds, breaking waves occur. Bubbles penetrate into the water and drastically increase acoustical reverberation, transmission loss and ambient noise. Thus, the development of short waves and wave breaking have to be known to apply radar remote sensing to the surface and to deduce from radar backscatter which sonar conditions prevail. To measure the wind dependence of short waves an experimental device was constructed for use from stationary platforms. It is nearly all-weather capable and can easily be handled by a crane. On the other hand, frequencies of short waves measured in a fixed position are extremely frequency shifted by currents. This limits the usefulness of tower-based measurements, e.g., the short wave modulation by wind and waves or currents can only be estimated in a rough approximation. Consequently, a buoy was developed to reduce the frequency shifts. The principle of the buoy is to drift in the local surface current and to follow the amplitudes of long waves. Therefore, short waves are measured in facets of long waves and the Doppler shifts are minimized. The wind is measured at a constant height above the long wave profile and relative to the moving facets. The paper describes the conventional measuring device and points out the necessity of the drifting buoy system. Examples of wind and wave spectra are presented and short wave modulations by long waves are depicted, too. From these measurements, new insights in short wave behaviour have to be expected     

CODAR wave measurements from a North Sea semisubmersible

CODAR, a high-frequency (HF) compact radar system, was operated continuously over several weeks aboard the semisubmersible oil platform Treasure Saga for the purpose of wave-height directional measurement and comparison. During North Sea winter storm conditions, the system operated at two different frequencies, depending on the sea state. Wave data are extracted from the second-order backscatter Doppler spectrum produced by nonlinearities in the hydrodynamic wave/wave and electromagnetic wave/scatter interactions. Because the floating oil rig itself moves in response to long waves, a technique has been developed and successfully demonstrated to eliminate to second order the resulting phase-modulation contamination of the echo, using separate accelerometer measurement of the platform's lateral motions. CODAR wave height, mean direction, and period are compared with data from a Norwegian directional wave buoy; in storm seas with wave heights that exceeded 9 m, the two height measurements agreed to within 20 cm RMS, and the mean direction to better than 15° RMS     

Microwave scattering by surface waves on water

The electromagnetic backscattering from small-amplitude water waves has been studied using a microwave model (2.5 GHz). The first- and second-order backscatter Doppler spectra have been measured and compared with the theoretical models, and good agreement has been found. In this paper, the specific case of two water waves propagating at right angles to each other will be presented, and the level of contribution of both the hydrodynamic and the electromagnetic effects on the backscatter radar Doppler spectrum will be considered.     

The effect of microwave backscatter uncertainty of satellite radar altimeter accuracy

The effect of variations in ocean surface roughness characteristics with upwind/downwind direction, reported by other investigators, is used to compute radar cross section (sigmadeg) and to assess the errors which may arise in present and planned altimeter sensors. Based on an analysis of the rough surface impulse response, the uncertainty between attitude angle andsigmadegasymmetry is found to cause height errors as large as 12 cm, depending on off-nadir angles and sea state. Additionally, the previously reported data in conjunction with computed facet backscatter are found to producesigmadegcharacteristics at large off-nadir angles which are in better agreement with experimental results than those predicted by physical optics Gaussian theory.     

一种适用于小入射角条件下的白冠海面后向散射模型

本文提出了一种白冠海面的小入射角星载雷达后向散射模型,模型包括海面非波浪破碎部分和波浪破碎部分的后向散射.在风的作用下,海浪破碎形成白冠,对星载雷达的后向散射信号造成影响.文中利用热带降雨测绘任务卫星搭载的降雨雷达(TRMM PR)和欧洲中期天气预报中心(ECMWF)的时空匹配数据集,拟合得出小入射角下星载雷达海面波浪...     

Four years of low-altitude sea ice broad-band backscatter measurements

The ability to use radar to discriminate Arctic Sea ice types has been investigated using surface-based and helicopter-borne scatterometer systems. The surface-based FM/CW radar operated at 1.5 GHz and at multiple frequencies in the 8-18-GHz region. Measurements were made at angles of10degto70degfrom nadir. The helicopter-based radar operated at the 8-18-GHz frequencies with incidence angles of0degto60deg. Extensive surface-truth measurements were made at or near the time of backscattar measurement to describe the physical and electrical properties of the polar scene. Measurements in the 8-18-GHz region verify the ability to discriminate multiyear, thick first-year, thin first-year, and pressure-ridged sea ice and lake ice. The lowest frequency, 9 GHz, was found to provide the greatest contrast between these ice categories, with significant levels of separation existing between angles from15degto70deg. The radar cross sections for like antenna polarizations, VV and HH, were very similar in absolute level and angular response. Cross-polarization, VH and HV, provided the greatest contrast between ice types, The 1.5-GHz measurements showed that thick first-year, thin first-year, and multiyear sea ice cannot be distinguished at10degto60degincidence angles with like polarization, VV, by backscatter alone; but that undeformed sea ice can be discriminated from pressure-ridged ice and lake ice. The effect of snow cover on the backscatter from thick first-year ice was also investigated. It contributes on the order of 0 to 4 dB, depending on frequency and incidence angle; the contribution of the snow layer increased with increasing frequency. Snow cover on smooth lake ice was found to be a major backscatter mechanism. Summer measurements demonstrate the inability to extend the knowledge of the backscatter from sea ice under spring conditions to all seasons.     

Induced-current effects on microwave backscatter

Microwave measurements have been made with a coherent radar in a wind-wave tank to determine the effect of induced current on backscatter. Perturbations were introduced into the wave structure by inducing a current in the water that flowed either with or against the wind. The effect of current on radar cross section was slight; the effect on the Doppler was much more pronounced. It was found that the wave components responsible for radar backscatter are predominantly free waves (that is, waves which travel at the dispersion velocity) rather than waves which are parasitic (or locked) to the dominant waves.     

X-band low-grazing-angle ocean backscatter obtained during LOGAN1993

This paper reviews X-band ocean microwave backscatter data from the LOGAN (LOw Grazing ANgle) experiment conducted on the Chesapeake Light Tower by the Naval Air Warfare Center. The data were collected under varied wind, sea, and swell conditions that provide some new insights into low-grazing-angle backscatter phenomena. Transient backscatter peaks called “sea spikes” have long been associated with deep-water breaking waves; however, they have yet to be fully reconciled with backscatter and hydrodynamic theories. New analysis techniques have been applied to the LOGAN data that take advantage of the unique characteristics of sea spikes and their dynamics. High-resolution Doppler spectra are organized relative to the space-time centroids of the sea-spike clusters and conditionally averaged by RCS strength. The mean Doppler variation of the strongest sea spikes then map the breaking-wave structure just as Doppler histories measured at moderate grazing angles map the dynamics of the dominant linear surface-wave components. While breaking waves are manifest to some degree in backscatter data at all grazing angles, a non-Bragg-scatter mechanism accentuates the crest scattering at low grazing angles. The phenomena potentially can be exploited for remote ocean sensing and imaging     

Synthetic aperture radar imaging of ocean waves during the marineland experiment

X- andL-band simultaneously obtained synthetic aperture radar (SAR) data of ocean gravity waves collected during the Marineland Experiment were analyzed using wave contrast measurements. The Marineland data collected in 1975 represents a unique historical data set for testing still-evolving theoretical models of the SAR ocean wave imaging process. The wave contrast measurements referred to are direct measurements of the backscatter variation between wave crests and troughs. These modulation depth measurements, which are indicators of wave detectability, were made as a function of: a) the settings used in processing the SAR signal histories to partially account for wave motion; b) wave propagation direction with respect to radar look direction for bothX- andL-band SAR data; c) SAR resolution; and d) number of coherent looks. The contrast measurements indicated that ocean waves imaged by a SAR are most discernible whenX-band frequency is used (as compared toL-band), and when the ocean waves are traveling in the range direction. Ocean waves can be detected by bothX- andL-band SAR, provided that the radar surface resolution is small compared to the ocean wavelength (at least 1/4 of the ocean wavelength is indicated by this work). Finally, wave detection withL-band SAR can be improved by adjusting the focal distance and rotation of the cylindrical telescope in the SAR optical processor to account for wave motion. The latter adjustments are found to be proportional to a value that is near the wave phase velocity.     

Wave kinematics for simulated shallow water storm waves - Analysis and experiments

Computation of wave kinematics at or near offshore structures is a vitally important consideration in the design of offshore structures. Design waves often include breaking and near-breaking storm waves in the presence of currents. It is important to predict the kinematics of these steep waves. Experiments were carried out in a wave tank with simulated steep waves with and without in-line current in which the wave profiles and the corresponding kinematics were simultaneously measured. The simulated waves represent shallow-water Gulf of Mexico storm waves. Many of these waves broke at or near the measuring instruments. Irregular stream-function theory was used to compute the wave kinematics and was found to generally predict the measured wave-current kinematics well. The differences found between the two are noted. Some of the noteworthy features of the breaking waves are also discussed.     

Detection of breaking waves using X-band pulse radar in the nearshore region

An X-band pulsed Doppler microwave radar has been used to determine the characteristics of breaking waves. Field experiments were conducted at the Shuang-Si estuary in the north of Taiwan in the winter of 2005. Analyses on maxima radar cross section and Doppler frequency shift are done to characterize wave breaking zones. Based on observations of breaking waves, the wave breaking zones are shown to be located at water depths of 1.8 to 2.2 m in the experimental site. In general, the results indicate that a radar system has the potential to delineate the spatial variation of breaking waves clearly and that this is sufficient to achieve a measurement operation for near-shore air-sea interaction events.     

Unsteady ship waves in shallow water of varying depth based on Green–Naghdi equation

Based on Green–Naghdi equation this work studies unsteady ship waves in shallow water of varying depth. A moving ship is regarded as a moving pressure disturbance on free surface. The moving pressure is incorporated into the Green–Naghdi equation to formulate forcing of ship waves in shallow water. The frequency dispersion term of the Green–Naghdi equation accounts for the effects of finite water depth on ship waves. A wave equation model and the finite element method (WE/FEM) are adopted to solve the Green–Naghdi equation. The numerical examples of a Series 60 (C_B=0.6) ship moving in shallow water are presented. Three-dimensional ship wave profiles and wave resistance are given when the ship moves in shallow water with a bed bump (or a trench). The numerical results indicate that the wave resistance increases first, then decreases, and finally returns to normal value as the ship passes a bed bump. A comparison between the numerical results predicted by the Green–Naghdi equation and the shallow water equations is made. It is found that the wave resistance predicted by the Green–Naghdi equation is larger than that predicted by the shallow water equations in subcritical flow , and the Green–Naghdi equation and the shallow water equations predict almost the same wave resistance when , the frequency dispersion can be neglected in supercritical flows.     

结合椭圆型缓坡方程模拟近岸波流场

波浪向近岸传播的过程中,由波浪破碎效应所产生的近岸波流场是近岸海域关键的水动力学因素之一.结合近岸波浪场的椭圆型缓坡方程和近岸波流场数学模型对近岸波浪场及由斜向入射波浪破碎后所形成的近岸波流场进行了数值模拟.计算中考虑到波浪向近岸传播中由于波浪的折射、绕射、反射等效应使局部复杂区域波向不易确定,采用结合椭圆型缓坡方程所给出的波浪辐射应力公式来计算波浪产生的辐射应力,在此基础上耦合椭圆型缓坡方程和近岸波流场数学模型对近岸波流场进行数值模拟,从而使模型综合考虑了波浪的折射、绕射、反射等效应且避免了对波向角的直接求解,可以应用于相对较复杂区域的近岸波流场模拟.     

Internal flow structure of short wind waves

Characteristic features of the internal flow field of short wind waves are described mainly on the basis of streamline patterns measured for four different cases of individual wave. In some waves a distinct high vorticity region, with flow in excess of the phase speed in the surface thin layer, is formed near the crest as shown in Part I of this study, but the streamlines are found to remain quite regular even very near the water surface. The characteristics of flow in the high vorticity region are investigated, and it is argued that the high vorticity region is not supported steadily in individual waves but that growth and attenuation in individual waves repeats systematically, without no severe wave breaking. Below the surface vorticity layer a quite regular wave motion dominates. However, this wave motion is strongly affected by the presence of the high vorticity region. By comparing the measured streamline profiles with those predicted from wave profiles by the use of a water-wave theory, it is found that the flow of the wind waves studied cannot be predicted, even approximately, from the surface displacements, in contrast to the case of pure irrotational water waves.     

Ocean wave parameters retrieved directly from compact polarimetric SAR data

We aim to directly invert wave parameters by using the data of a compact polarimetric synthetic aperture radar(CP SAR) and validate the effectiveness of ocean wave parameter retrieval from the circular transmit/linear receive mode and π/4 compact polarimetric mode. Relevant data from the RADARSAT-2 fully polarimetric SAR on the C-band were used to obtain the compact polarimetric SAR images, and a polarimetric SAR wave retrieval algorithm was used to verify the sea surface wave measurements. Usin...     

A three-frequency scatterometer technique for the measurement ofocean wave spectra

A microwave technique for the measurement of ocean wave spectra has been compared with wave gauge output during extensive field testing. The method is based on the dual-frequency technique for detecting long ocean waves by matching the modulation of short waves with the beat wavelength between two transmitted microwave frequencies. The new method, however, utilizes three microwave frequencies in order to reduce mean backscatter not related to short-wave modulation. Two prototype scatterometers have been built using three frequencies at L-band and at K_u-band. Wave spectra have been measured by both radar systems which, when properly normalized, agree well with simultaneous in situ measurements taken by conventional wave gauges at the pier site. Thirteen sets of spectra have been computed, five of which correspond to a situation in which a local wind sea was generated and then decayed. The present experiment does not demonstrate the directionality of this new technique