Clifford L. Rufenach

The effect of orbital motions on synthetic aperture radar imagery of ocean waves

The formation of wave-like patterns in synthetic aperture radar (SAR) images of the ocean surface caused by orbital motions is investigated. Furthermore, the degradation in azimuthal resolution due to these motions is calculated by applying a least square fit to the phase history. Formulas are given which describe the variation of intensity in azimuthal direction in the image plane as well as the degradation in azimuthal resolution as a function of ocean wave amplitude, wave frequency, direction of wave propagation, and radar wavelength, incidence angle, and integration time.

Wind vector retrieval using ERS-1 synthetic aperture radar imagery

An automated algorithm intended for operational use is developed and tested for estimating wind speed and direction using ERS-1 SAR imagery. The wind direction comes from the orientation of low frequency, linear signatures in the SAR imagery that the authors believe are manifestations of roll vortices within the planetary boundary layer. The wind direction thus has inherently a 180/spl deg/ ambiguity since only a single SAR image is used. Wind speed is estimated by using a new algorithm that utilizes both the estimated wind direction and /spl sigma//sub 0/ values to invert radar cross section models. The authors show that: 1) on average the direction of the roll vortices signatures is approximately 11/spl deg/ to the right of the surface wind direction and can be used to estimate the surface wind direction to within /spl plusmn/19/spl deg/ and 2) utilizing these estimated wind directions from the SAR imagery subsequently improves wind speed estimation, generating errors of approximately /spl plusmn/1.2 m/s, for ERS-1 SAR data collected during the Norwegian Continental Shelf Experiment in 1991.

The quiet geomagnetic field at geosynchronous orbit and its dependence on solar wind dynamic pressure

Vector magnetic fields at geosynchronous orbit were measured during 1980-1984 using the operational GOES 2, GOES 5, and GOES 6 spacecraft magnetometers. We corrected these spacecraft measurements for offsets due to spacecraft state and then used these field estimates to create a data base with 1-min resolution. Hourly quiet field values were calculated for these years from this data base using the ground-based geomagnetic index criteria AE < 120 nT and |Dst| < 20 nT. These quiet field components, rotated into dipole HVD coordinates, were approximated by the first two coefficients of a two-dimensional Fourier series in time of day and season. These Fourier harmonics provide a compact method of approximating the quiet synchronous field at any time of day and any season. The quiet geosynchronous field components, to first order, are given by mean values of about 90 nT, −60 nT, and 5 nT; and sinusoidal diurnal amplitudes of about 21 nT, 5 nT, and 5 nT, respectively, for H, V, and D where the spacecraft magnetometer was located near the geomagnetic meridian. The second harmonic diurnal amplitudes and the first and second harmonic seasonal amplitudes are typically of the order of a few nanoteslas or less except for the D component, which exhibits a larger seasonal variation. Furthermore, a one-dimensional Fourier series in time of day was used to study the quiet field dependence on solar wind dynamic pressure, Pd, by indexing the measurements into five pressure ranges during 1980. The H component of the quiet field increased 4.6 nT from 80.2 to 84.8 nT in its mean amplitude and 20.8 nT from 11.9 to 32.7 nT in its first harmonic amplitude for Pd increasing from 0.71 × 10−8 to 3.31 × 10−8 dyn/cm². These quiet H measurements, including the pressure dependence, are compared with a first-order field model (Mead, 1964) superimposed with a tail current, resulting in magnetospheric currents (magnetopause and tail) in agreement with previous model values. The measured field pressure dependence and the Mead model suggest a tail current dependence on pressure.

A radio scintillation method of estimating the small-scale structure in the ionosphere

Abstract The power spectra of the intensity fluctuations for the radio source Cygnus A were analyzed for 44 nights during the summer of 1970. When the scattering was weak, oscillations were observed in the power spectra. These oscillations are attributed to a Fresnel filtering effect. When present, these oscillations are used to calculate the velocity and to estimate the irregularity scale size and the rms electron density fluctuations for irregularities smaller than the Fresnel radius. On many occasions the power law intensity spectra for structure smaller than the Fresnel radius may be attributed to a power law electron density spectrum.

Shipboard active and passive microwave measurement of ocean surface slicks off the southern California coast

Ship-based radar scatterometer measurements at L band (20 cm) and X band (3.2 cm), radiometer measurements at C band (5.5 cm) and Ka band (0.86 cm), and microlayer chlorophyll fluorescence measurements on the southern California continental shelf were acquired across several surface slicks during October 1989. These measurements allowed the simultaneous extraction of both the short wave damping at Bragg wavelengths 24 cm and 4 cm and the chlorophyll a biological concentration with sufficient spatial resolution to observe features within individual slick bands. The measurements show that (1) the largest peaks in fluorescence, a measure of the peak chlorophyll a concentration, are colocated with the short wave damping at 24-cm and 4-cm wavelengths, (2) the maximum damping ratio, between the 24-cm and 4-cm waves is at least 16 for wind speeds of 3–4 m s−1, and (3) the radar roughness signatures at these short Bragg gravity wavelengths suggest that the slicks are caused by organic surface film damping rather than straining of these waves. The radar roughness changes in synthetic aperture radar imagery have usually been interpreted in terms of wave straining. The limited radiometer signatures are uncorrelated with the other microlayer measurements. Additional shipboard measurements are needed to further quantify these preliminary results.

Ocean wave spectral distortion in airborne synthetic aperture radar imagery during the Norwegian Continental Shelf Experiment of 1988

C band radar images of ocean gravity waves off the Norwegian coast were processed into one-dimensional azimuth spectra. These spectra were used to measure the azimuth spectral (width) cutoff on the basis of a least squares fit to a Gaussian spectral shape. The widths were calculated for a range of wave heights (2–5 m) and wind speeds (2–18 m/s) during 3 days in March, 1988. Velocity smearing (συ) estimates were extracted, independent of R/V and incidence angle, based on an imaging model and the measured azimuth cutoffs with συ values varying from 0.4 to 0.7 m/s. Quantitative velocity smearing estimates are important as input to models describing the distortion in wave imagery. We propose a first-order model which neglects velocity bunching for ocean swell with peak wavelengths longer than about 250 m. This model is offered as a first estimate of when ocean wave swell will be detected by the C band SAR on board the ERS 1 spacecraft. The model predicts that this swell will be imaged under light winds of the order of 2–4 m/s. Higher wind speeds cause larger smearing, which may result in significant distortion of the imaged swell provided that the swell is traveling near the direction of the spacecraft ground track.

Extraction of marginal ice zone thickness using gravity wave imagery

Synthetic aperture radar (SAR) imagery of gravity waves in the marginal ice zone (MIZ) was acquired during 1987 and 1989 using aircraft flying over the Greenland Sea. The dominant gravity wavelengths acquired during 1987 exhibit a systematic lengthening with penetration distance into the MIZ for three different regions examined. The extracted dominant wavelengths were analyzed using a linear least squares fit with penetration distance and a flexure-gravity wave model to estimate mean ice thickness. The extracted mean thickness varies from about 1±0.4 m near the ice edge to approximately 2±0.4m near the deepest detectable penetration of the waves (about 20 km). These thicknesses are in agreement with 31 in situ measurements taken in the Fram Strait MIZ within 2 weeks of the SAR measurements. We also acquired wave imagery across a 150-km section of newly forming ice called the Odden, a tongue-shaped region, during 1989. These SAR images at L, C, and X bands display small wave modulation signatures at the northern edge of the Odden which we interpret as arising from rapidly growing new ice.

SYNTHETIC APERTURE RADAR WAVE OBSERVATIONS DURING GOASEX

A comparison of airborne and satellite synthetic aperture radar wave imagery and coincident surface wave observations indicates that for the range of environmental conditions encountered, the radar estimates of dominant wavelength and direction are of equal or better accuracy than the corresponding in-situ estimates.

Technical note Observation of internal waves in LANDSAT and SEASAT satellite imagery

Abstract Available LANDSAT and SEASAT images of the North American east and west coasts were analysed for internal wave patterns during the summer months of 1978. These images are from the satellite sensors: (i) multispectral scanner on board the LANDSAT satellite and (ii) synthetic aperture radar on board the SEASAT satellite. The Gulf of California during these months provided an excellent opportunity to compare the detectability of ocean internal wave patterns in these two sensors since the internal waves were tidally generated every 12·45 hours and cloud contamination in the LANDSAT imagery was negligible. Internal wave patterns were frequently visible in SEASAT imagery but only occasionally visible in LANDSAT imagery, a fact which cannot be explained by cloud contamination. Furthermore multiple wave packets with 12·45 hours spacing were observed from SEASAT but not always from LANDSAT. One possible reason for the infrequent detection of internal wave patterns from LANDSAT is that a specialized Earth-...

Comparison of aircraft synthetic aperture radar and buoy spectra during the Norwegian Continental Shelf Experiment of 1988

Aircraft synthetic aperture radar (SAR) measurements from seven flight paths at two different altitudes were acquired over a trimodal ocean wave system off the coast of Norway on March 11, 1988. These three wave systems traveling in different directions were also measured by a directional buoy with the buoy spectra rotated into the radar coordinates for comparison with SAR spectra. Fifteen subspectra were averaged to obtain the measured SAR spectra, and sixteen spectral bands were averaged to obtain the measured buoy spectra. The seven flight paths in conjunction with the three wave systems resulted in a nearly uniform distribution of azimuth peak directions varying from 0° to 83° and R/V varying from 28 to 110. The comparison of the SAR- and buoy-derived peak wavelength and direction shows some scatter. We consider the scatter to be due primarily to the statistical uncertainty in the spectral estimates, the wave number resolution, and differences in the temporal and spatial averaging. Scanning distortion is shown to cause significant bias in the SAR and buoy comparison for slow-flying aircraft imaging ocean wave swell. Correction of the SAR spectra for this special radar-ocean condition gives significant improvement in the SAR-buoy comparison. We investigate the relative importance of the motion-related mechanisms (velocity bunching, velocity smearing, and scanning distortion), and the tilt contributions using four nondimensional imaging parameters.