Skywave Radar Research Articles

High-Frequency Skywave Radar Measurements of Hurricane Anita

We tracked and monitored hurricane Anita over a 5-day period by using the SRI-operated Wide Aperture Research Facility (WARF) high-frequency skywave radar. The WARF-derived positions for Anita agreed to within +/- 19 kilometers of the coincident temporal positions along the National Hurricane Center's smooth track. Hurricane Anita passed near the open ocean-moored buoy EB-71 of the National Oceanic and Atmospheric Administration, and measurements of wind direction, wind speed, and significant wave height made during this period at the WARF and in situ at the buoy showed agreement of 7 degrees , 0.4 meter per second, and 0.5 meter, respectively. The WARF estimates of longshore coastal surface currents showed good agreement with measurements made at a moored current meter.

Measuring rms wave height and the scalar ocean wave spectrum with HF skywave radar

Estimates of rms wave height and the scalar ocean wave frequency spectrum were made by inverting high‐frequency (HF) skywave radar‐measured sea‐echo Doppler spectra. Whereas low‐power surface‐wave radars can make these measurements out to approximately 100 km from the radar, coverage out to 3000 km can be obtained with skywave radars that illuminate the sea via a single ionospheric reflection. To demonstrate this capability, we used the Wide Aperture Research Faculty (WARF) HF skywave radar to measure Pacific Ocean sea backscatter near NOAA data buoy EB 20 during the passage of an at mospheric cold front. The height of the wind‐generated waves measured at the buoy doubled within a 6‐h period. Two new analysis techniques were used to derive rms wave height and the scalar ocean wave frequency spectrum from radar echoes. Estimates of rms wave height were made east and west of the front by using a power‐law relation that was derived from theoretical simulations of the sea‐echo Doppler spectrum for a wide range of wave conditions. Two of the rms wave height estimates were compared with the estimates made at EB 20 at two different times and are in agreement to within 7 and 17%, respectively. Scalar wave spectrum and rms wave height estimates were made by matching a theoretical Doppler spectrum derived in terms of a five‐parameter model of the wind‐wave spectrum to the measured Doppler spectrum. The radar and buoy estimates of the rms wave height agree within 7%. Agreement between the WARF‐derived and buoy‐derived rms wave height and wave spectra is within the combined experimental error of the buoy and the radar measurements.

HF RADAR MAPPING OF EXTENSIVE OCEAN WINDFIELDS

The possibility of deriving parameters of sea wave spectra remotely from characteristics of radio waves at high frequency (HF) scattered from the sea surface was first raised when Crombie (1955) correctly deduced that Doppler frequency shifts in the signal returned from short range in his HF radar resulted uniquely from components of the sea wave spectrum having wavelengths exactly one-half the radio wavelength, and travelling radially with respect to the radar. Since then the technique has been expanded in two directions: (a) The use of ionospherically propagated-JiF radio waves ('Skywave' HF radar) to^ examine extensive ocean areas out to some 4000 km from the observing site, to obtain oceanographic and meteorological data suitable for input to synoptic observation systems. This approach has been developed through the experimental work of Tveten (1967) and Ward (1969), and the empirical technique proposed by Long and Trizna (1973) to allow the simple extraction of sea surface wind vectors from Doppler spectra of the backscattered radio signals. (b) The determination of directional sea wave spectra and sea surface currents at short ranges with HF radars operating in the groundwave propagation mode, based on theoretical analyses of the scattering process such as those of Barrick (1972). The HF Skywave radar constructed and operated at Townsville by the Physics Department of James Cook University has been employed for some years now on research into the possibilities for mapping sea states and sea surface winds over ocean areas surrounding Australia (Ward, 1969; Ward and Dexter, 1976; Dexter and Casey, 1978).

Remote Measurement of the Position and Surface Circulation of Hurricane Eloise by HF Skywave Radar

Abstract The position and surface wind direction field of Hurricane Eloise was observed by measuring sea backscatter from the Gulf of Mexico through the use of an HF skywave radar situated in central California. A radar map of the surface wind direction field was compiled from the direction of the ocean gravity waves 8 m long, and compared to the surface wind directions measured at National Oceanic and Atmospheric Administration Data Buoy Office open ocean moored buoys EB10 and EB04. Agreement to within 10% was found. A radar position fix was determined from the radar-derived surface wind map and showed agreement to within 35 km of the position estimated from the official National Hurricane Center track.

The effects of space and time resolution on the quality of sea echo Doppler spectra measured with HF sky wave radar

Measurements using the SRI Wide‐Aperture Research Facility (WARF) HF sky wave radar show that the radar's azimuthal beam width and integration time play important roles in determining the quality of sky wave (ionospherically propagated) sea echo Doppler spectra. Using as a quality index the equivalent width of a portion of the Doppler spectrum in the vicinity of the stronger Bragg line, we find a reduction of ionospheric multipath spectral contamination as beamwidth and integration time decrease. For nominal (3 dB) beam widths of 1/2°, 2°, and 4° and 256‐s averaging, the mean equivalent widths of 104 spectra were 0.090, 0.099, and 0. 105 Hz, respectively. Experiments using 51‐s integration time gave average widths of 0.061, 0.075, and 0.079 Hz for the same three beam widths. The additional contamination observed with the larger beamwidths and the longer averaging time is often sufficient to preclude the extraction of ocean wave height from the second‐order spectral structure. A geometrical optics, rough‐surface model of the ionospheric reflection process explains this beamwidth dependence by relating the number and width of multipath spectral lines to the size, shape, and number of ‘reflective glints’ in the ionospheric area illuminated by the radar. The lateral dimension of this area increases with antenna beamwidth. The model also predicts a greater dependence of contamination on beamwidth as the integration time is reduced. We use ionospheric measurements made with CW Doppler sounders to estimate the statistical parameters of the rough surface model and the amount of contamination the model predicts for finite beamwidth and integration time. Our main conclusion is that the best way to avoid spectral contamination caused by short‐term ionospheric motions is to involve the smallest possible spatial and temporal samples of the ionosphere in the radar measurement. Specifically, a successful sea state radar should have a 3‐dB beamwidth of less than 2° and preferably as small as 1/2° and should use coherent integration times shorter than about 100s.

HF SKYWAVE RADAR MEASUREMENT OF HURRICANE WINDS AND WAVES

We measured significant wave height, and surface wind speed and direction for the first two Gulf of Mexico hurricanes of the 1977 season using a high frequency (HF) skywave radar. The radar measurements were made from California by using the SRI-operated Wide Aperture Research Facility (WARF). We recorded sea backscatter for hurricanes Anita and Babe, at distances more than 3000 km from the WARF, by means of single F-layer ionospheric reflection. We compiled real-time maps of the surface wind direction field within a radial distance of 200 km of the storm center, then estimated the hurricane position from these radar wind maps, and developed a track for Anita over a 4 day period between 30 August and 2 September 1977 as the storm moved westward across the Gulf of Mexico. The radar track was computed from 17 independent position estimates made before Anita crossed the Mexican coast, and was subsequently compared to the official track produced by National Hurricane Center (NHC). Agreement between the WARF position estimates and coincident temporal positions on the NHC smooth track was ±19 km. At approximately 0000Z on 1 September 1977, Anita passed within 50 km of the National Data Buoy Office (NDBO) open ocean moored buoy EB-71, and provided us with the opportunity to compare WARF estimates of the significant wave height, and surface wind speed and direction in all four quadrants of the storm with those made at the buoy. Agreement between the WARF and EB-71 measurements was within 10%. Two days after Anita crossed land, tropical storm Babe-a weaker, short-lived storm-developed. WARF estimates of the significant wave height, and surface wind speed and direction were made for selected regions of the storm.* No in situ wave measurements were available for comparison to the WARF measurements. WARF estimates of the wind speed were compared to wind speed measurements made at nearby oil platforms, and surface wind speeds computed from flight level winds (305 m) measured by a NOAA reconnaissance aircraft. Agreement was again within 10%. The purpose of this paper is to describe the capability of remotely monitoring hurricanes and other open ocean storms by using an HF skywave radar. We will describe the important aspects of the WARF skywave radar, the sea echo Doppler spectra, the method of analysis used to estimate the wave and wind parameters, and the accuracy of these radar-derived quantities.

High-resolution mapping of oceanic wind fields with skywave radar

The direction of the mean surface wind field in the North Pacific Ocean was mapped on September 25 and 26, 1973, over an area of 3 \times 10^{6} (km)2by OTH-B HF radar. A spatial resolution of 60 km in range and 15 km in cross range was used at points spaced by 150 km in range and 80 km in cross range. Wind directions were inferred from the upwind/downwind first-order Bragg ratio and the measure of the maximum ratio occuring for radial winds at points near each observation. Over 90 percent of the recorded data were usable for this purpose.High spatial resolution is essential to make detailed measurements of the wind speed and direction across and along an atmospheric cold front. The location of the atmospheric cold front derived from the wind field agreed well with the ESSA VIII satellite frontal location.

High-resolution mapping of oceanic wind fields with skywave radar

The direction of the mean surface wind field in the North Pacific Ocean was mapped on September 25 and 26, 1973, over an area of 3 \times 10^{6} (km)2by OTH-B HF radar. A spatial resolution of 60 km in range and 15 km in cross range was used at points spaced by 150 km in range and 80 km in cross range. Wind directions were inferred from the upwind/downwind first-order Bragg ratio and the measure of the maximum ratio occurring for radial winds at points near each observation.

Measurement of oceanic wind speed from HF sea scatter by skywave radar

Remote measurements of the spatial mean ocean wind speeds were obtained using Doppler spectra resolved to 0.08 Hz from high-resolution HF skywave-radar backscatter measurements of the ocean surface. A standard deviation of 2.4 m/s resulted from the correlation of observed winds over the ocean and the broadening of the Doppler spectra in the vicinity of the higher first-order Bragg line. This broadening, for Doppler spectra unperturbed by the ionospheric propagation, is proportional to the increase in power caused by higher order hydrodynamic and electromagnetic effects in the vicinity of the Bragg line and inversely proportional to the square root of the radio frequency. A lower bound on the measure of wind speed was established at 5 m/s by the low resolution spectral processing and low second-order power. An upper limit is suggested by the steep slope in the region of the sea backscatter spectrum outside the square root of two times the first-order Bragg line Doppler.

Measurement of oceanic wind speed from HF sea scatter by skywave radar

Remote measurements of the spatial mean ocean wind speeds were obtained using Doppler spectra resolved to 0.08 Hz from high-resolution HF skywave-radar backscatter measurements of the ocean surface. A standard deviation of 2.4 m/s resulted from the correlation of observed winds over the ocean and the broadening of the Doppler spectra in the vicinity of the higher first-order Bragg line. This broadening, for Doppler spectra unperturbed by the ionospheric propagation, is proportional to the increase in power caused by higher order hydrodynamic and electromagnetic effects in the vicinity of the Bragg line and inversely proportional to the square root of the radio frequency. A lower bound on the measure of wind speed was established at 5 m/s by the low resolution spectral processing and low second-order power. An upper limit is suggested by the steep slope in the region of the sea backscatter spectrum outside the square root of two times the first-order Bragg line Doppler.

A broadband antenna array for sea scatter measurements

A nonuniformly spaced array of 25 side-terminated vertical half rhombics is synthesized for the entire high-frequency band. The array is to be used for both transmitting and receiving, as a sky-wave Doppler radar for sea scatter measurements. The array has the capability for providing a maximum transmitting-receiving product gain in the order of 60 dB at low take-off angles, a product azimuth pattern with a nominal beamwidth of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2\deg</tex> , a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">40\deg</tex> sectoral coverage, a final product sidelobe level of -38 dB, and a grating-lobe level of -30 dB.

Radio measurements of oceanic winds at long ranges: An evaluation

Oceanic winds were measured locally by anemometers and remotely by an HF skywave radar at ranges exceeding 3000 km. Wind velocity was deduced from sea backscatter Doppler spectra. These spectra were often perturbed by ionospheric motion, yet wind direction was measured to within ±16° and wind speed to within ±4 m sec−1. Better measures of wind speed should exist at shorter ranges or with better ionospheric conditions.