Temporal variability of tsunami arrival detection distance revealed by virtual tsunami observation experiments using numerical simulation and 1-month HF radar observation

The combination of a high-frequency ocean surface radar and a tsunami detection method should be assessed as the onshore-offshore distribution of tsunami detection probability, because the probability will vary in accordance with the signal-to-noise ratio (SNR) and the tsunami magnitude in addition to the radar system specifications. Here, we statistically examine the tsunami detection distance based on virtual tsunami observation experiments by using signals received by a high-frequency radar in February 2014 installed on the southern coast of Japan and numerically simulated velocities induced by a Nankai Trough earthquake. In the experiments, the Doppler frequencies associated with the simulated velocities were superimposed on the receiving signals of the radar, and the radial velocities were calculated from the synthesized signals by the fast Fourier transform. Tsunami arrival was then detected based on the temporal change in the cross-correlation of the velocities, before and after tsunami arrival, between two points 3 km apart along a radar beam. We found that the possibility of tsunami detection primarily depends on the kinetic energy ratio between tsunami current and background current velocities. The monthly average detection probability is over 90% when the energy ratio exceeds 5 (offshore distance: 9 km ≤ L ≤ 36 km) and reduces to 50% when the energy ratio is approximately 1 (L = 42 km) over the shelf slope. The ratio varied with the background current physics and SNR, which was mainly affected by ocean surface wave heights and ionospheric electron density.     

Propagating tsunami wave and subsequent resonant response signals detected by HF radar in the Kii Channel,Japan

Signals from the tsunami waves induced by the March 11, 2011 moment magnitude (M_w) 9.0 Tohoku-Oki earthquake and from subsequent resonances were detected as radial velocity variability by a high-frequency ocean surface radar (HF radar) installed on the eastern coast of the Kii Channel, at a range of about 1000 km from the epicenter along the eastern to southern coasts of Honshu Island. A time–distance diagram of band-passed (9–200 min) radial velocity along the beam reveals that the tsunami waves propagated from the continental shelf slope to the inner channel as progressive waves for the first three waves, and then natural oscillations were excited by the waves; and that the direction of the tsunami wave propagation and the axis of the natural oscillations differed from that of the radar beam. In addition, spectral analyses of the radial velocities and sea surface heights obtained in the channel and on the continental shelf slope suggest complex natural oscillation modes excited by the tsunami waves.     

Assessing SARAL/AltiKa Data in the Coastal Zone: Comparisons with HF Radar Observations

We present an initial assessment of SARAL/AltiKa data in the coastal band. The study focuses on the Ibiza Channel where the north-south water exchanges play a key role in controlling the circulation variability in the western Mediterranean. In this area, the track 16 of SARAL/AltiKa intercepts the domain covered by a coastal high-frequency (HF) radar system, which provides surface currents with a range up to 60 km. We evaluate the performance of the SARAL/AltiKa Ssalto/Duacs delayed-time along-track products compared to the HF radar surface velocity fields. SARAL/AltiKa data are retrieved at a distance of only 7 km from the coast, putting in evidence the emerging capabilities of the new altimeter. The derived velocities resolved the general features of the seasonal mesoscale variability with reasonable agreement with HF radar fields (significant correlations of 0.54). However, some discrepancies appear, which might be caused by instrumental hardware radar errors, ageostrophic velocities as well as inaccurate corrections and editing in the altimeter data. Root mean square (rms) differences between the estimated SARAL/AltiKa and the HF radar velocities are about 13 cm/s. These results are consistent with recent studies in other parts of the ocean applying similar approaches to Topex/Poseidon and Jason-1 missions and using coastal altimeter corrections.     

Measurement of ocean surface currents using a long-range,high-frequency ground wave radar

High-frequency (HF) ground wave radar (GWR) is emerging as a significant tool for monitoring ocean surface conditions at ranges well beyond the line-of-sight horizon that limits conventional systems. An experimental GWR system at Cape Race, Newfoundland, Canada that has been operational since 1991, has the ability to performing routine surveillance of oceanic surface parameters and surface target detection. Operating in the frequency range between 5 and 8 MHz, the frequency modulated interrupted continuous wave (FMICW) radar has a nominal range capability of 200 km. An experiment was performed during the period of October 20-November 21, 1992 to test the surface current measuring capability of the Cape Race system. Here, near real-time radial surface current information is extracted from the Doppler spectra of the radar time series data and a comparison is performed to the Lagrangian velocities derived from the position-time tracks of Accurate Surface Tracker (AST) drifters. A wide range of oceanic conditions were experienced during the experimental period, and favorable results were obtained from the comparison regardless of the sea state conditions. The analysis shows the standard deviation in the radar radial velocity component to be approximately 5 cm/s     

Characterizing Observed Environmental Variability With HF Doppler Radar Surface Current Mappers and Acoustic Doppler Current Profilers: Environmental Variability in the Coastal Ocean

A network of high-frequency (HF) radars is deployed along the New Jersey coast providing synoptic current maps across the entire shelf. These data serve a variety of user groups from scientific research to Coast Guard search and rescue. In addition, model forecasts have been shown to improve with surface current assimilation. In all applications, there is a need for better definitions and assessment of the measurement uncertainty. During a summer coastal predictive skill experiment in 2001, an array of in situ current profilers was deployed near two HF radar sites, one long-range and one standard-range system. Comparison statistics were calculated between different vertical bins on the same current profiler, between different current profilers, and between the current profilers and the different HF radars. The velocity difference in the vertical and horizontal directions were then characterized using the observed root-mean-square (rms) differences. We further focused on two cases, one with relatively high vertical variability, and the second with relatively low vertical variability. Observed differences between the top bin of the current profiler and the HF radar were influenced by both system accuracy and the environment. Using the in situ current profilers, the environmental variability over scales based on the HF radar sampling was quantified. HF radar comparisons with the current profilers were on the same order as the observed environmental difference over the same scales, indicating that the environment has a significant influence on the observed differences. Velocity variability in the vertical and horizontal directions both contribute to these differences. When the potential effects of the vertical variability could be minimized, the remaining difference between the current profiler and the HF radar was similar to the measured horizontal velocity difference (~2.5 cm/s) and below the resolution of the raw radial data at the time of the deployment     

On the accuracy of current measurements by means of HF radar

The accuracy of surface current velocities measured by high-frequency (HF) radar is investigated. Data from the two radar systems of the University of Hamburg, CODAR (Coastal Radar) and WERA (Wellen Radar), are compared with in situ data. In one experiment, CODAR and a near-surface current meter were operated simultaneously over a 19-day period. In addition, WERA was operated for 6 days during that period. In the other experiment, WERA and a bottom-mounted current meter were operated simultaneously over a 35-day period. Both radars use frequencies of about 30 MHz where backscattering is due to ocean waves of 5 m wavelength. The influence of the orbital motion of underlying longer waves on radial velocity errors is investigated. In accordance with theory, the measured standard deviations of HF-measured current velocities depend on the sea state. Depending on the sea state, estimated errors range from 3 to 10 cm·s^-1 and explain only part of the rms difference of 10-20 cm·s^-1 found between HF and in situ current measurements. The rest is assumed to be due the differences of the quantities measured, e.g., the spatial averaging     

Directional wave information from the SeaSonde

This paper describes methods used for the derivation of wave information from SeaSonde data, and gives examples of their application to measured data. The SeaSonde is a compact high-frequency (HF) radar system operated from the coast or offshore platform to produce current velocity maps and local estimates of the directional wave spectrum. Two methods are described to obtain wave information from the second-order radar spectrum: integral inversion and fitting with a model of the ocean wave spectrum. We describe results from both standard- and long-range systems and include comparisons with simultaneous measurements from an S4 current meter. Due to general properties of the radar spectrum common to all HF radar systems, existing interpretation methods fail when the waveheight exceeds a limiting value defined by the radar frequency. As a result, standard- and long-range SeaSondes provide wave information for different wave height conditions because of their differing radar frequencies. Standard-range SeaSondes are useful for low and moderate waveheights, whereas long-range systems with lower transmit frequencies provide information when the waves are high. We propose a low-cost low-power system, to be used exclusively for local wave measurements, which would be capable of switching transmit frequency when the waveheight exceeds the critical limit, thereby allowing observation of waves throughout the waveheight range.     

Observation of the Soya Warm Current using HF ocean radar

Three High Frequency (HF) ocean radar stations were installed around the Soya/La Perouse Strait in the Sea of Okhotsk in order to monitor the Soya Warm Current (SWC). The frequency of the HF radar is 13.9 MHz, and the range and azimuth resolutions are 3 km and 5 deg., respectively. The radar covers a range of approximately 70 km from the coast. The surface current velocity observed by the HF radars was compared with data from drifting buoys and shipboard Acoustic Doppler Current Profilers (ADCPs). The current velocity derived from the HF radars shows good agreement with that observed using the drifting buoys. The root-mean-square (rms) differences were found to be less than 20 cm s⁻¹ for the zonal and meridional components in the buoy comparison. The observed current velocity was also found to exhibit reasonable agreement with the shipboard ADCP data. It was shown that the HF radars clearly capture seasonal and short-term variations of the SWC. The velocity of the Soya Warm Current reaches its maximum, approximately 1 m s⁻¹, in summer and weakens in winter. The velocity core is located 20 to 30 km from the coast, and its width is approximately 40 km. The surface transport by the SWC shows a significant correlation with the sea level difference along the strait, as derived from coastal tide gauge records at Wakkanai and Abashiri. Deceased.     

HF radar observations of Arctic pack-ice breakup

This paper describes the first reported high-resolution remote measurements of sea-ice velocities during the summer Arctic pack-ice breakup, made with a high-frequency (HF) radar system (CODAR, for Coastal Ocean Dynamics Applications Radar) located on Cross Island, Alaska. Each 36-min observation also gives the positions of the ice edge, the moving ice, and the open water, with an azimuthal and distance resolution of5degand 1.2 km, respectively, to a range of 15 km. The statistical uncertainties in speed are typically 2-4 cm/s. The ice breakup was observed over a two-day period starting with low ice velocity and no open water and ending with ice and current velocities of approximately 40 cm/s. The position of the ice edge is verified by a simultaneous synthetic aperture radar (SAR) image. To compare the ice, current, and wind velocities, a uniform velocity model was fitted to the measurements of radial velocity. The speed of both ice and current under free drift conditions was found to lie between 2 and 5 percent of the wind speed and the direction within20degof the wind direction.     

Tidal currents in Baie de Seine: Comparison of numerical modelling and high-frequency radar measurements

Numerical modelling and HF radar technique are two developing methods for studying coastal circulation. They both have a wide space-coverage, and similar time and space discriminations (respectively a few minutes and a few km²). It is therefore particularly interesting to compare their results.An experiment using two high-frequency radars was made in Western Baie de Seine at the end of summer 1982, and three days of these measurements, covering a large range of tidal coefficients, have been compared with the corresponding results of a two-dimensional modelling of the whole Bay.Characteristics of this model, as well as the main methodological and technical points concerning HF radar measurements (especially a discussion of the measurements errors, evaluated to be 3 to 7 cm s⁻¹ on each radial component) are presented. A comparison is made at 6 points for the current vector, and at 20 points for one of the radial components. It concerns the half-day mode (amplitude, phase, hodograph).In most of the area, no significant phase lag is observed. The agreement is to within ca. 10 cm s⁻¹ for the amplitude, and to within less than 10° for the bearing of tidal ellipses, with smaller intervals for smaller tides. This agreement shows that the intrinsic accuracy of the model is comparable to that of other sensors such as current meters. What is even more satisfactory is that the model does not use as boundary conditions any measurements specific to the period studied except for sea level forecasts, which are given by a greater scale physical model.A significant difference is observed only close to the coast where the strong gradient of the current velocity (referred to the discrimination scale), is unfavourable both to the model and to the measurements.     

高频地波雷达观测的苏北辐射沙洲南部海域夏季表层海流特征

本文利用高频地波雷达获得的江苏如东海域大范围长期海流观测资料对苏北辐射沙洲南部烂沙洋海域夏季表层海流特征进行了分析。分析结果表明:研究海域表层海流靠近近岸一侧为往复流,流向总体上呈西北-东南向,靠近外海一侧为旋转流;海域潮流动力较为强劲,夏季表层海流实测最大流速达1.47 m/s,涨潮平均流速介于0.44~0.55 m/s,落潮平均流速介于0.38~0.52 m/s,海域西北部区域涨落潮平均流速明显大于其他区域;表层潮流为正规半日潮流,M₂分潮为最主要分潮,其潮流椭圆长轴范围为0.57~0.71 m/s,远大于其他分潮,其次为S₂分潮;该海域夏季表层余流呈现近岸大离岸小的分布趋势,余流流向基本指向近岸方向,从离岸到近岸余流流向呈现逆时针偏转。     

SeaSonde Radial Velocities: Derivation and Internal Consistency

This paper describes the methods presently used to produce unaveraged radial velocity maps from radar voltage cross spectra measured by a SeaSonde, including a discussion of the multiple signal classification (MUSIC) algorithm as it is applied to SeaSonde data and methods employed to alleviate difficulties associated with the use of measured antenna patterns. We also describe internal consistency checks including visual observation of the radial velocity map, consideration of the computed uncertainties and quantitative tests of radial velocities measured by two radars along the baseline between the systems. Baseline analysis is illustrated by application to two SeaSonde networks, with contrasting results that lead to a better understanding of SeaSonde output     

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     

On the performance of a shipborne current mapping HF radar

High-frequency (HF) radars have been developed to map surface currents offshore by means of land-based stations. Presently available radar systems use frequencies between 25 and 30 MHz and allow a spatial resolution of 1 km and ranges of up to 50 km. This paper reports on the experience with a shipborne radar and discusses problems which arise for the azimuthal resolution on a metal ship, the correction for the ship's speed, and limitations due to pitch-and-roll motions. Current measurements during cruises to the North Atlantic are presented. It has been found that, with the support of the satellite-supported Global Positioning System, the shipborne HF radar can measure surface current velocities with an accuracy of some 5 cm·s^-1     

HF radar data quality requirements for wave measurement

HF radar wave measurements are presented focussing on theoretical limitations, and thus radar operating parameters, and quality control requirements to ensure robust measurements across a range of sea states. Data from three radar deployments, off the west coast of Norway, Celtic Sea and Liverpool Bay using two different radar systems, WERA and Pisces, and different radio frequency ranges, are used to demonstrate the wave measurement capability of HF radar and to illustrate the points made. Aspects of the measurements that require further improvements are identified. These include modifications to the underlying theory particularly in high sea states, identification and removal of ships and interference from the radar signals before wave processing and/or intelligent partitioning to remove these from the wave spectrum. The need to match the radio frequency to the expected wave peak frequency and waveheight range, with lower radio frequencies performing better at higher waveheights and lower peak frequencies and vice versa, is demonstrated. For operations across a wide range of oceanographic conditions a radar able to operate at more than one frequency is recommended for robust wave measurement. Careful quality control is needed to ensure accurate wave measurements.     

Tsunami hazard and mitigation analysis for bathing beaches in China based on numerical simulations

Bathing beaches are usually the first to suffer disasters when tsunamis occur, owing to their proximity to the sea. Several large seismic fault zones are located off the coast of China. The impact of each tsunami scenario on Chinese bathing beaches is different. In this study, numerical models of the worst tsunami scenarios associated with seismic fault zones were considered to assess the tsunami hazard of bathing beaches in China. Numerical results show that tsunami waves from the Pacific Ocean could affect the East China Sea coast through gaps between the Ryukyu Islands. The Zhejiang and Shanghai coasts would be threatened by a tsunami from Ryukyu Trench, and the coasts of Hainan and Guangdong provinces would be threatened by a tsunami from the Manila Trench. The tsunami hazard associated with the Philippine Trench scenario needs particular attention. Owing to China’s offshore topography, the sequential order of tsunami arrival times to coastal provinces in several tsunami scenarios is almost the same. According to the tsunami hazard analysis results, Yalongwan Beach and eight other bathing beaches are at the highest hazard level. A high-resolution numerical calculation model was established to analyze the tsunami physical characteristics for the high-risk bathing beaches. To explore mitigating effects of a tsunami disaster, this study simulated tsunami propagation with the addition of seawalls. The experimental results show that the tsunami prevention seawalls constructed in an appropriate shallow water location have some effect on reducing tsunami hazard. Seawalls separated by a certain distance work even better. The analysis results can provide a scientific reference for subsequent preventive measures such as facility construction and evacuation.     

东中国海业务化海流雷达观测及数据服务平台

由武汉大学研发的海洋状态监测及分析雷达（OSMAR）,被布置于东中国海沿岸的6个雷达站点,用于观测海表面速度（海流,波浪,风）。本研究以雷达观测的流场为例,阐述了一个业务化海表面流雷达观测及数据服务平台,给出了一个从数据获取、传输、处理、可视化以及服务的业务化流程。详细描述了业务化平台中包含三个系统（雷达观测系统、数据服务系统、可视化服务系统）,以及各系统间的数据流。各站点获取的流速将在雷达观测系统中数据接收和预处理中心进行集成,然后传输到数据服务系统进行质量控制。用户可以在数据服务系统的主界面上对数据进行浏览,也能够获取这些数据。可视化服务系统能够在球体平台上对数据产品进行直观展示。通过业务化平台可以对东中国海的海流进行实时监测,也能够对海流的日变化以及季节性变化进行研究。     

高频地波雷达生成海洋表面矢量流图

武汉大学研制的双站高频地波雷达系统OSMAR2000利用测得的两幅单站径向海流图生成矢量海流图。经典矢量流图生成方法不能直接应用到OSMAR2000系统中。本文提出一种先在极坐标系下用自然三次样条函数将径向流插值到公共网格上然后直接进行矢量合成的矢量海流图生成方法。OSMAR2000在东海的表面矢量流实测结果与作对比验证的传统海流计测量结果十分吻合。对比数据表明，该方法是可行的，且优于先进行径向流线性插值后矢量合成的矢量流图生成方法。这也是国内首次利用高频地波雷达实现海洋表面矢量流的实时监测。     

Reconstructing tsunami run-up from the characteristics of tsunami deposits on the Thai Andaman Coast

Tsunamis can leave deposits on the land surface they inundate. The characteristics of tsunami deposits can be used to calculate tsunami run-up height and velocity. This paper presents a reconstruction of tsunami run-up from tsunami deposit characteristics in a simple mathematical model. The model is modified and applied to reconstruct tsunami run-ups at Ao Kheuy beach and Khuk Khak beach, Phangnga province, Thailand. The input parameters are grain-size and maximum run-up distance of the sediment. The reconstructed run-up heights are 4.16–4.91 m at Ao Kheuy beach and 5.43–9.46 m at Khuk Khak beach. The estimated run-up velocities (maximum velocity) at the still water level are 12.78–19.21 m/s. In the area located 70–140 m inland to the end of run-up inundation, estimated mean run-up velocities decrease from approximately 1.93 m/s to 0 m/s. Reasonably good agreements are found between reconstructed and observed run-up heights. The tsunami run-up height and velocity can be used for risk assessment and coastal development programs in the tsunami affected area. The results show that the area from 0 to 140 m inland was flooded by high velocity run-ups and those run-up energies were dissipated mainly in this area. The area should be designated as either an area where settlement is not permitted or an area where effective protection is provided, for example with flood barriers or forest.     

多普勒雷达资料在近海强台风模拟中的同化试验

利用中尺度数值模式(WRF),并同化了多普勒雷达反射率和径向速度资料以及非常规的观测资料,对近几年登陆于浙闽沿海的4例强台风进行了数值模拟。通过高时空分辨率的模拟结果对比分析表明:雷达资料的同化,对近海登陆台风路径和降水模拟以及中尺度降水特征都有进一步改进的效果;模拟较好的揭示了台风近中心螺旋云带中的强中尺度对流系统。通过模拟分析表明,在台风近中心的螺旋云带中,低层有一条强辐合线存在,它与实况多普勒雷达给出的低层平显(PPI)强度回波带有较好的对应关系,也与沿海地区中尺度暴雨系统紧密联系,并由此看到近海海域降水带和强对流区的存在。