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 of3 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.     

Ship detection with high-resolution HF skywave radar

This paper presents an overview of ship detection by high-frequency (HF) skywave backscatter over-the-horizon radar (OTHR). Ships have been detected at ranges of 2000 km or more by OTHR that uses sufficient resolution in the radar spatial and Doppler frequency domains. The HF sea-echo Doppler spectrum limits the target signal-to-clutter ratio (SCR), as a function of the ocean wave-height distribution, wind direction, radio frequency, and ship target radial velocity. Maximum sea-clutter spectrum purity, and hence larger SCR, is achieved with the use of stable single-mode ionospheric propagation. Real-time measurement and interpretation of ionospheric propagation features therefore must guide the choice of OTHR operating frequency. Experimental data recorded at the ONR/SR1 Wide Aperture Research Facility (WARF) bistatic OTHR in central California demonstrate reliable ship detection in the Northeast Pacific Ocean. WARF transmits 1-MW average effective radiated power, using a linear frequency-modulated continuous-wave (FMCW) waveform, and receives with a 2.55-km broadside array of vertical monopole element pairs. Swept bandwidths as high as 200 kHz have been used. Sufficient spectral resolution is achieved with a coherent integration time (CIT) of 12.8 s. Longer CIT, and autoregressive (AR) spectral analysis techniques such as Marple's algorithm, have been used to improve Doppler resolution.     

Two tests of real-time ocean wave-height mapping with HF skywave radar

An estimate of the coverage efficiency of a high-frequency (HF) skywave (ionospheric) radar for mapping ocean wave height and surface wind direction at ranges between 1000 and 3000 km was made for two different but common operational ocean-monitoring applications. In the first test, three days in duration, wind direction and wave height were mapped over the entire coverage area. In the second test, four days in duration, wind direction was mapped over the entire coverage area and wave height was mapped at preselected small regions within the coverage area. On-line quality assessment and real-time ionospheric diagnostics helped select space-time-frequency windows with low ionospheric distortion. Wind direction over the 4 000 000-km²coverage area was mapped every day with the Wide Aperture Research Facility (WARF) skywave radar in about 1 h. For large-area wave-height mapping, 6 h was allowed, and for the coverage of a smaller grid, 3 h was allowed. Within these time windows, high-confidence wave-height measurements were obtained at 59 percent of the locations attempted, using on-line processing for both tests. When postexperiment processing was included, the coverage efficiency increased to 74 percent. Greater efficiencies would be possible operationally with more experience and more sophisticated radars. Eliminating the First day's results from the statistics shows the effects of increased operating experience. Then the overall on-line efficiency increases to 69 percent, and the off-line increases to 82 percent for the six days of the two tests.     

Ionospheric decontamination and sea clutter suppression for HF skywave Radars

In this paper, a cascaded correction and suppression method of reducing ionospheric phase path contamination and sea clutter to enable detection of targets travelling at speeds near the Bragg Doppler is addressed. The Hankel rank reduction (HRR) technique based on singular value decomposition (SVD) has been used to estimate the ionospheric phase distortion and suppress the sea clutter. Simulation results show that such a technique is helpful for the worse conditions when the target masking effect happens even after ionospheric phase decontamination. Finally, an attempt to combine another phase decontamination algorithm based on the piecewise polynomial phase modeling with the clutter cancellation stage for faster phase fluctuation is discussed briefly and some results are given.     

Studies of the sea using HF radio scatter

Radio signals of decameter wavelength resonantly scattered from waves on the sea surface are used to measure precisely the wavelength, frequency, and direction of travel of those waves. These measurements are not only important in themselves, but are also used to deduce currents, winds, and perhaps wind stress at the sea surface. Techniques for obtaining these measurements, as well as experiments to evaluate these techniques are discussed. Finally, scatter has been used to produce the first high-resolution measurements of the directional distribution of large ocean waves, measurements of ocean surface currents at ranges of 20 km, and of surface winds at ranges of 3000 km.     

Short pulse radar used to measure sea surface wind speed and SWH

A joint airborne measurement program is being pursued by NRL and NASA Wallops Flight Center to determine the extent to which wind speed and sea surface significant wave height (SWH) can be measured quantitatively and remotely with a short pulse (2 ns), wide-beam (60deg), nadir-looking 3-cm radar. The concept involves relative power measurements only and does not need a scanning antenna, doppler filters, or absolute power calibration. The slopes of the leading and trailing edges of the averaged received power for the pulse limited altimeter are used to infer SWH and surface wind speed. The interpretation is based on theoretical models of the effects of SWH on the leading edge shape and rms sea-surface slope on the trailing-edge shape. The models include the radar system parameters of antenna beam width and pulsewidth. Preliminary experimental results look promising and indicate that it may be possible to design a relatively compact airborne radar to infer, in real-time, the sea surface SWH and surface wind speed.     

Directional sea spectrum determination using HF Doppler radar techniques

Second-order features in HF radar Doppler spectral data are compared with a theoretical model of the radar spectrum. The model is the corner reflector double-scatter model which employs a more realistic directional sea spectrum model than those used in earlier works. It includes a frequency-dependent angular spreading function and assumes the existence of spectral energy over a full360degarising from an apparent second-order wave-wave interaction. Comparison is made with ground wave data collected at the NRL/NOAA/ITS San Clemente Island HF radar.     

卫星高度计海面风速的校准与验证

为了改善不同卫星高度计海面风速数据之间的一致性,以浮标数据为基准,对国内的HY-2A和国外的T/P、GFO、Jason-1、Envisat、Jason-2、CryoSat-2共7颗卫星高度计的海面风速数据进行了分析,给出了各个卫星高度计的海面风速校准公式,并对其校准效果进行了验证。验证结果表明:各个卫星高度计的海面风速在经过校准后,与浮标海面风速差异的均值和均方根都有所降低,其中HY-2A最为显著。经过校准后所有卫星高度计的海面风速与浮标海面风速差异的均值都在±0.2m/s以内。除了HY-2A、GFO和Jason-1,其余4颗卫星高度计校准后的海面风速与浮标海面风速差异的均方根都在1.6m/s以下。由此可以得出结论,利用本文的校准公式对各个卫星高度计(特别是HY-2A卫星高度计)的海面风速进行校准,可以有效减少其与浮标海面风速之间的差异。     

舟山海域高频地波雷达表层流探测试验

2015年4月7-30日,在浙江省舟山近海海域开展了“嵊山-朱家尖”小型阵列变频高频地波雷达系统的海上比测试验,通过雷达观测数据与定点ADCP海流资料的比对检验了地波雷达表层流探测性能。径向流比对结果显示,测点与雷达法向夹角越小,距离雷达距离越近,径向流比测结果越好,雷达探测的结果越可靠。嵊山站径向流与ADCP观测结果的各站总体平均误差为7.98 cm/s,平均均方根误差为15.34 cm/s,平均相关系数为0.89,朱家尖站径向流与ADCP观测结果的各站总体平均误差为6.24 cm/s,平均均方根误差为12.36 cm/s,平均相关系数为0.81。根据矢量流比对结果显示,矢量流速与ADCP观测结果的各站总体平均误差为4.82 cm/s,平均均方根误差为15.03 cm/s,平均相关系数为0.44。设置在嵊山、朱家尖两个雷达站双站探测的核心区域（两个雷达站连线的中垂线上,并且与两个雷达站构成一个近似直角三角形）的站点比测结果更加理想,当流速大于0.25 m/s时,对于核心区域平均后的流向均方根误差为24.9°。     

Remote sea current sensing using HF radar: an autoregressiveapproach

We present and demonstrate a new method for accurately determining the radial component of sea current, using information derived from HF radar returns. The method uses autoregressive modeling and constrains the estimated frequency separation of the first-order Bragg lines to be its required theoretical value of 2√(gf/πc), where f is the radar frequency     

基于微波散射计观测的气候态海面风场和风应力场

收集了星载微波散射计NSCAT,QuikSCAT和SeaWinds on ADOES-II的全球海面风速和方向L2B数据,数据涉及的时间序列长度为11.5 a。通过对所收集数据的质量控制、Loess低通空间滤波和统计处理,构建了气候态的逐月全球海面风场、风应力场和风应力旋度场(简称为SCAT),其空间网格间距为0.25°×0.25°。SCAT资料与其他有关资料相比,包含了更丰富的海面风场中小尺度空间变化的信息,可广泛应用于海洋、气候、海气相互作用等方面的研究,特别适合应用于海洋中小尺度过程的研究。作为我国海洋二号(HY-2)卫星预研项目的成果之一,SCAT资料将由国家海洋局国家卫星海洋应用中心提供给有关用户。     

Measuring surface wind direction by monostatic HF ground-wave radar at the eastern China Sea

The extraction of full wind vectors from data obtained by single-site (monostatic) high-frequency ground-wave radar (HFGWR) is an ongoing challenge because of the inherent directional ambiguities. Here, a new algorithm for resolving the ambiguity of wind direction from monostatic data is presented. The true wind direction is determined by minimizing the sum of the difference among three wind directions derived from three different radar look angles. The wind directions estimated by applying the algorithm to data obtained from the OSMAR2000 HFGWR situated at the Eastern China Sea are compared with values obtained from ship-borne instrumentation. The mean difference between the ground-truthed values and those obtained from the radar data is approximately 20/spl deg/. The distance limit for wind direction sensing using the OSMAR2000 is about 200 km, which is the range for which signal-to-noise ratios typically exceed about 23 dB in the relevant first-order portions of the backscatter spectra.     

星载雷达高度计反演海面风速进展

因为星载雷达高度计风速资料有沿轨分辨率高、精度高的特点,对其进行深入研究有重要意义.在中国海洋二号动力环境卫星刚刚升空之际,对星载雷达高度计反演海面风速围内外研究进展作一个综述.首先介绍星载雷达高度计风速反演的理论依据及存在困难;然后以风速反演进展历程为主线,分别针对后向散射系数、波浪状态、降雨、白沫等物理量引入地球物...     

Measurement of ocean wave spectra using a ship-mounted HF radar

The work describes an inversion algorithm for HF radar measurement of nondirectional wave spectra using an omnidirectional receive/transmit antenna. Such a radar would be suitable for deployment on a stationary ship or drill rig. In this approach, wave information is extracted from the radar observations by numerically inverting the integral equation representing the backscatter return from the ocean. Test results of this technique applied to data collected using a 25.4-MHz radar installed on a ship have been very positive. For the two measurements collected, there is a high degree of correlation between the radar wave estimates and those of a WAVE-TRACK buoy     

基于ASCAT 散射计数据的2013 年南极周边海面风速特征分析

利用2013年1月1日~2013年12月31日的ASCAT散射计风速数据,从海面风速季平均分布特征、平均海面风速特征、海面风速大于10 m/s发生比例统计等方面,对55°S以南的南极周边海域开展了风速空间分布特性统计,得到以下结果:7月平均风速最大,为12 m/s,1月平均风速最小,为8 m/s。对于南极周边海域,区域平均风速主要在9~12 m/s,全年出现的时间大于280 d,约占全年的77%。风速大于10 m/s所占比例4~9月大于1~3月和10~12月。从全年来看,南极周边海域在4~9月风速普遍较大,且0°~60°W海域内风速明显比其他海域要小。在此基础上将2013年每日的区域平均风速与月平均风速与2012年做了对比,变化趋势与2012年基本一致。通过本项长期工作,可以了解南极风场环境基本情况,建立与更新基础资料和图件,得到南极地区风场要素时空分布、变化规律,服务于南极科学考察、全球变化等多学科研究应用。     

高风速下海面阻力系数的研究

利用由于海面摩擦造成的台风角动量的损失，推算出高风速下的海面阻力系数．高风速条件下的阻力系数迅速增大，与风速的平方成正比．此结论对于研究高风速下的海浪、风暴潮有着实际意义．     

HF radar detection of tsunamis

This paper demonstrates that HF radar systems can be used to detect tsunamis well before their arrival at a coastline. We solve the equations of motion and continuity on the ocean surface using models to simulate the signals produced by a tsunami approaching the east U.S. coast. Height and velocity profiles are derived along with expressions for the radar-observed current velocities in terms of bathymetry and tsunami height and period. Simulated tsunami-generated radial current velocities are superimposed on typical maps of radial velocity generated by a Rutgers University HF radar system. A detection parameter is defined and plotted to quantify the progress of the tsunami, which is shown to be detectable well before its arrival at the coast. We describe observations/warnings that would have been provided by HF radar systems at locations in the path of the 2004 Indian Ocean tsunami.     

The role of the gravity-wave dispersion relation in HF radar measurements of the sea surface

Recent experimental and theoretical findings raise interesting questions about the applicability of the normal gravity-wave dispersion relation at wave frequencies that exceed the spectral peak frequency. The use of the dispersion relation in analysis of HF radar Doppler sea echo is examined in this paper. Drawing on the results of perturbation theory for wave-wave nonlinear interactions, we show that this relation, so essential to echo interpretation in terms of current and wave information, can be employed with no degradation in accuracy for current measurement when the dominant wave frequency is considerably less (by as much as 10) than the radar Bragg resonance frequency. This finding is supported by comparisons of currents measured by HF radar with "surface truth;" the first-order echo must only be identifiable in order to be used accurately. Wave-height directional spectral information can be extracted from the second-order echo at a given radar frequency up to the point (in wave height) where the perturbation solution employed in the inversion process fails; then a lower radar frequency must be used. On the other hand, most conventional wave measuring instruments should not use the dispersion relation for interpretation of data well beyond the spectral peak, because they do not observe wave height as a function of both space and time independently, as does HF radar.     

Detection of meteorological fronts over the North Sea with Valensole skywave radar

The skywave radar located at Valeusole is described. The front localization method is reviewed and discussed. Wind direction was mapped over the North Sea during a three-day experiment. Thirty maps were collected and two cold fronts were successfully detected and tracked. Data corresponding to one front are presented, discussed, and compared to French Meteorological Office (FMO) surface charts. The measurements show that this front did not move with a constant velocity. Front detection is finally discussed in terms of spatial localization accuracy, resolution, and detectability.     

Surface current measurements by HF radar in freshwater lakes

HF radar has become an increasingly important tool for mapping surface currents in the coastal ocean. However, the limited range, due to much higher propagation loss and smaller wave heights (relative to the saltwater ocean), has discouraged HF radar use over fresh water, Nevertheless, the potential usefulness of HF radar in measuring circulation patterns in freshwater lakes has stimulated pilot experiments to explore HF radar capabilities over fresh water. The Episodic Events Great Lakes Experiment (EEGLE), which studied the impact of intermittent strong wind events on the resuspension of pollutants from lake-bottom sediments, provided an excellent venue for a pilot experiment. A Multifrequency Coastal HF Radar (MCR) was deployed for 10 days at two sites on the shore of Lake Michigan near St. Joseph, MI. Similarly, a single-frequency CODAR SeaSonde instrument was deployed on the California shore of Lake Tahoe. These two experiments showed that when sufficiently strong surface winds (2 about 7 m/s) exist for an hour or more, a single HE radar can be effective in measuring the radial component of surface currents out to ranges of 10-15 km. We also show the effectiveness of using HF radar in concert with acoustic Doppler current profilers (ADCPs) for measuring a radial component of the current profile to depths as shallow as 50 cm and thus potentially extending the vertical coverage of an ADCP array