Effective Sea Clutter Spectrum Extraction Method for HFSWR in Adverse Conditions

An effective approach in solving the sea clutter spectrum extraction problem is studied in the paper.Different from the conventional signal to noise ratio（SNR）method based on Doppler frequency or range domain information,a method is developed to characterize the differences between the sea echo and those interferences are by signal to interference plus noise ratio（SINR）which jointly utilizing the range,Doppler frequency and azimuth domain information.Furthermore,these differences can be adaptable to adverse conditions by forming the necessary boundaries and constraints in searching of the maximum SINR,which greatly promotes the extraction of sea clutter spectrum.The real high frequency surface wave radar（HFSWR）data demonstrate that the proposed method is less influenced by those interferences and can effectively extract the sea clutter spectrum even under the adverse conditions.Furthermore,it has been shown as an effective method for ship detection and sea state remote sensing of HFSWR.

Doppler Spectrum Analysis of Time-Evolving Sea Surface Covered by Oil Spills

Based on the model of a contaminated sea surface that was proposed by Lombardini et al., the influence of the damping effect of oil films on the sea surface roughness spectrum and the geometrical structure of the sea surface is examined in detail by comparing with a clean sea surface. Fhrthermore, based on a quasi-stationary algorithm, a time series of backscattered echoes from a time-evolving sea surface covered by oil slicks is obtained by utilizing the frequency-domain numerical method of the parallel fast multiple method. Then, the Doppler spectrum is evaluated by performing a standard spectral estimation technique. Finally, the influence of the oil film damping effect on the Doppler spectrum of the backscattered echoes from time-evolving sea surface is investigated in detail by making a comparison of the Doppler spectrum of an oil-covered sea surface with the Doppler spectrum of a dean sea surface. The numerical simulations show that the damping effect of oil films has an influence on the Doppler spectrum signature for both horizontal-to-horizontal and vertical-to-vertical polarizations.

Bandwidth Scale for Frequency Spectrum of Sea Waves

It is well known that energy spectrum bandwidth should be able to reflect the degree of energy concentration. However, the commonly used bandwidth factors defined by Longuet-Higgins could not fit the concept satisfactorily. A new kind of spectrum bandwidth scale factor with a clear physical meaning is given in the present paper and a constant is obtained which reveals the intrinsic characteristics of sea waves. Thereby a universal relationship between significant wave height of sea waves and spectrum bandwidth is established.

SINGULAR CROSS SPECTRUM APPLIED ON DIAGNOSTIC ANALYSIS OF AIR-SEA INTERACTION

The Singular Cross Spectrum Analysis (SCSA) method was employed to investigate the coupledperiods of air-sea/mid-low circulation interaction using 1951-1993 500-hpa geopotential heights and the seasurface temperature (SST) in the Northern Hemisphere. Results show that air-sea correlation is noticeable on the3-7 year scale which is similar to ENSO circle. In this sense, ENSO is a Strong signal. Quasi-ten/quasi-four yearperiods are prominent in the mid-low interaction, and quasi-if our year oscillation is uniform with the ENSO circle.Studies indicate that the mid-low interaction possess quasi-ten year oscillation beside being affected by ENSO.

Improved significance testing of wavelet power spectrum near data boundaries as applied to polar research

When one applies the wavelet transform to analyze finite-length time series, discontinuities at the data boundaries will distort its wavelet power spectrum in some regions which are defined as a wavelength-dependent cone of influence （COI）. In the COI, significance tests are unreliable. At the same time, as many time series are short and noisy, the COI is a serious limitation in wavelet analysis of time series. In this paper, we will give a method to reduce boundary effects and discover significant frequencies in the COI. After that, we will apply our method to analyze Greenland winter temperature and Baltic sea ice. The new method makes use of line removal and odd extension of the time series. This causes the derivative of the series to be continuous （unlike the case for other padding methods）. This will give the most reasonable padding methodology if the time series being analyzed has red noise characteristics.

THERMOHALINE FINESTRUCTURE AND ITS RELATION WITH THE WATER MASSES AND CURRENTS SYSTEM IN THE NORTHERN EAST CHINA SEA

Eighy-one CTD profiles gathered in springtime were used for northem East China Sea tbermohalinefinestructure studies indicating that the finestructure properties vaned with region and depth, as shown infinesructure specra, distribution of Cox numbers etc..Some results closely wiated to distribution of watermasses and Analysis of two typical profiles revealed differenes in autospectra of temperature,salihity and potential density gradients, probobility distribution of temperature finestructure gradient,Cox numberc.etc. The probability density function of vertical temperature gradients, which varied withsample interval, is given. The variances of temperatare finestructare gradient are used to estimate the lat-eral diffusivity and lateral temperatare flux, which were 10.3 （m2/s） and 5.5×10-4 （℃ m/s）,respectivly.

Bimodality and growth of the spectra of typhoon-generated waves in northern South China Sea

Buoy-based observations of wave spectra during the passage of three typhoons in the northern South China Sea are examined.Though most spectra of mature typhoon-generated waves are unimodal,double-peaked spectra account for a significant proportion during the growing and decaying stages.This is due either to the superposition of swells on local wind waves or to the mechanism of nonlinear interaction between different wave components.The growth rate of energy density is an effective way to predict spectrum variation.The dominant wave direction depends on the location of the typhoon center to the site,but the direction spread shows no regularity in distant regions.In this study,a new six-parameter spectral formula is proposed to represent doublepeaked spectra and is shown to provide a better fit than previous models.The theoretical relationship between shape parameter and spectral width is still applicable to each peak.The characteristics of the variations of spectral parameters are analyzed.It is demonstrated that the spectral parameters are not only related to the typhoon intensity and typhoon track,but also have strong intercorrelations.Moreover,the growth relation between significant wave height and significant wave period is obtained to fit the typhoon-generated waves.

Variations in Swells along Eastern Arabian Sea during the Summer Monsoon

A study was carried out to find the variation in wave characteristics along the eastern Arabian Sea and the influence of swells in the nearshore waves at 3 locations during summer monsoon in 2010. Percentage of swells in the measured waves was 75% to 79% at the locations with higher percentage of swells in the northern portion of Arabian Sea com-pared to that at the southern side. The significant wave height up to 4.7 m and maximum wave height up to 7.4 m was observed. The wave height was increasing from south to north with the average significant wave height at the northern location 20% more than that at the southern location due to the increase in the swell height. Waves having spectral peak period less than 6 s were not present during the summer monsoon period. There was no change in the average value of wave statistical parameters for data collected at 1/2, 3, 6, 12 and 24 h interval.

Trend analysis of relative sea level rise or fall of the tide gauge stations in the Pacific

On the basis of the analytical results of the period components of monthly mean sea level of 236 stationsin the Pacific, the period components plus linear trend are ed to fit the monthly mean sea level series. The statisticalresults of linear trend Coefficients of these stations indicate that, if the abnormal values of sea-level rise and fall are neglected, the average rise rate of relative sea level in the Pacific is 1. 16 mm/a. Affected by nonuniformity Of land subsidence and other factors, the regional change of relative sea level rise or fall in the Pacific is greater. In the light of thepositive or negative values of linear trend coefficients as well as the geographical position of the sea area, zoning is madeof the sea level rise or fall in the Pacific including the coastal areas of China and Southeast Asia to obtain the averagerate of rise or fall in each sea area. The rise or fall trends of relative sea level obtained for the entire Pacific Ocean,west coast of North America, the northern and central South America, the greater part of the tropical Pacific and thecoastal Islands of Japan are basically in keeping with the other relevant results. The regional average estimated result ofthe relative sea level in the coast of East Asia is on the rise while the estimated results provided by Barnett tend todrop; the main cause of this nonuniformity is the number of stations selected and the distributional density.

DISTRIBUTION FEATURES OF GIANT SEA－SALT NUCLEI IN ATMOSPHERE OVER YONGXING ISLAND， XISHA ISLANDS，DURING NORTHEAST WINTER MONSOON

In this paper, the chlorine ions nuclei data, observed at Yongxing (Xisha Islands ) during November 1987, are analyzed. The main results are: 1) The average concentration of chiorine ions nuclei with dry diameters larger than 2 μm is 878 per litre and the average salt content is 105. 4μg/m3, in which special giant nuclei with dry diameters larger than 4 μm are 19O per litre. The largest dry diameter of chloride ions observed reaches 57 μm, which are larger than that observed there during the southwest summer monsoon. The spetrum type shows quasi-unimodal and exponential lapse. 2) The salt content decreases with the height over sea surface. 3) The concentrations of chlorine ions nuclei reach the maximum value at 0800 (Local time, same below ). and minimum at 0200.

Microwave Doppler spectra of sea return at small incidence angles:specular point scattering contribution

It is well known that the sea return echo contains contributions from at least two scattering mechanisms. In addition to the resonant Bragg scattering, the specular point scattering plays an important role as the incidence angle becomes smaller （≥ 20°）. Here, in combination with the Kirchhoff integral equation of scattering field and the stationary phase approximation, analytical expressions for Doppler shift and spectral bandwidth of specular point scattering, which are insensitive to the polarization state, are derived theoretically. For comparison, the simulated results related to the two-scale method （TSM） and the method of moment （MOM） are also presented. It is found that the Doppler shift and the spectral bandwidth given by TSM are insufficient at small incidence angles. However, a comparison between the analytical results and the numerical simulations by MOM in the backscatter configuration shows that our proposed formulas are valid for the specular point scattering case. In this work, the dependences of the predicted results on incidence angle, radar frequency, and wind speed are also discussed. The obtained conclusions seem promising for a better understanding of the Doppler spectra of the specular point scattering fields from time-varying sea surfaces.

中法海洋卫星产品体系及风浪数据精度综述

中法海洋卫星(China-France Oceanography SATellite,CFOSAT)由中国和法国联合研制,于2018年发射并已在轨运行5年多,星上的载荷包括SWIM(Surface Waves Investigation and Monitoring)波谱仪和扇形波束旋转扫描微波散射计(CFOSAT SCATterometer,CSCAT),提供了全球海面风场和海浪谱的大面积联合遥感观测资料。中法海洋卫星1—4级数据产品包括后向散射系数、沿轨海面风场和海浪谱、全球网格化风场和海浪谱、宽刈幅海浪、极地海冰、多星融合风场和有效波高等,为海洋环境预报、海洋灾害监测、全球气候变化研究等提供了新的数据源。文中具体介绍了中法海洋卫星产品体系以及国内外学者和机构对海风、海浪数据精度的评估结果。

Wave Spectral Patterns during a Historical Cyclone: A Numerical Model for Cyclone Gonu in the Northern Oman Sea

The third generation wind-wave model Mike21-SW was used to study spectral characteristics of waves generated by the historical Cyclone Gonu in June 2007 along and off the Iranian coasts on the northern Oman Sea. The model was forced with the cyclone wind field generated using a Holland (1980) model based on cyclone data obtained from the Joint Typhon Warning Center (JTWC). The wave model was calibrated for the northern Oman Sea using bulk and spectral wave data at a station out of the Chabahar Bay. Evolution of directional-frequency spectra during the cyclone was investigated for two locations near the entrance and off the Chabahar Bay. At the offshore station, energy was contributed to the spectrum over an approximately 180 degree directional span that included different local and remotely generated waves. As the cyclone proceeded northwestward, all spectral directions continuously rotated in the clockwise direction at both locations. Frequency spectra at these locations were investigated for four different times corresponding to different locations of Cyclone’s eye and were justified using the sea growth parameter of the Joint North Sea Wave Project (JONSWAP) experiment. Using the modified JONSWAP parameters for hurricane conditions resulted in a frequency spectrum consistent with simulation results.

Application of ESMD Method to Air-Sea Flux Investigation

The ESMD method can be seen as a new alternate of the well-known Hilbert-Huang transform (HHT) for non-steady data processing. It is good at finding the optimal adaptive global mean fitting curve, which is superior to the common least-square method and running-mean approach. Take the air-sea momentum flux investigation as an example, only when the non-turbulent wind components is well extracted, can the remainder signal be seen as actual oscillations caused by turbulence. With the aid of —5/3 power law for the turbulence, a mode-filtering approach based on ESMD decomposition is developed here. The test on observational data indicates that this approach is very feasible and it may greatly reduce the error caused by the non-turbulent components.

Wave Pressure Acting on V-Shaped Floating Breakwater in Random Seas

Wave pressure on the wet surface of a V-shaped floating breakwater in random seas is investigated. Considering the diffraction effect, the unit velocity potential caused by the single regular waves around the breakwater is solved using the finite-depth Green function and boundary element method, in which the Green function is solved by integral method. The Response-Amplitude Operator(RAO) of wave pressure is acquired according to the Longuet-Higgins' wave model and the linear Bernoulli equation. Furthermore, the wave pressure's response spectrum is calculated according to the wave spectrum by discretizing the frequency domain. The wave pressure's characteristic value corresponding to certain cumulative probability is determined according to the Rayleigh distribution of wave heights. The numerical results and field test results are compared, which indicates that the wave pressure calculated in random seas agrees with that of field measurements. It is found that the bigger angle between legs will cause the bigger pressure response, while the increase in leg length does not influence the pressure significantly. The pressure at the side of head sea is larger than that of back waves. When the incident wave angle changes from 0? to 90?, the pressure at the side of back waves decreases clearly, while at the side of head sea, the situation is more complicated and there seems no obvious tendency. The concentration of wave energy around low frequency(long wavelength) will induce bigger wave pressure, and more attention should be paid to this situation for the structure safety.

海浪谱法模拟海面场景仿真参数选取准则

为提升海面场景模拟的物理真实感,本文探讨了仿真参数对海洋粗糙面模拟精确性的影响,并建立了海表模拟中仿真参数的选取准则。依据海浪谱和随机过程统计理论,采用自相关函数研究了海面相关长度随风速变化规律。通过建立海浪蒙特卡罗仿真模型,结合随机粗糙面误差分析,提出了不同风速条件下海面场景仿真参数的选取准则。研究表明:将海表模拟的采样间隔限定在顺风向相关长度的0.1~0.5倍,可显著提高海面场景模拟的精确度;而当海面仿真参数超出上述范围时,可能导致海面场景模拟失真,进而影响其视觉真实感。本文所提出的仿真参数选取标准可为开发航海模拟器提供技术支撑。

基于光谱处理方法的棉花叶绿素含量高光谱反演研究

【目的】叶绿素含量可以用来评价棉花的长势情况,快速、准确和大面积监测棉花叶绿素含量,有助于实现精准农业。【方法】分别用0~2阶(步长为0.2)的分数阶微分处理和1~10尺度下的小波变换对田间测定的陆地棉和海岛棉等2种棉花的高光谱反射率进行处理,提高棉花叶绿素含量反演精度。通过分析不同处理方式的光谱与叶绿素含量之间的相关性,筛选得出敏感波段;并运用支持向量机回归和随机森林回归模型分别构建棉花叶绿素含量高光谱估算模型。【结果】(1)在全波段范围内,2种棉花325~1075 nm光谱反射率曲线整体变化趋势基本相同,其反射率均随着叶绿素含量的增加而增大。(2)经连续小波变换和分数阶微分变换后,2种棉花高光谱数据和叶绿素含量的相关性有所增强。使用随机森林回归和小波能量系数7对陆地棉叶绿素含量的反演效果最好,建模集决定系数(coefficient of determination,R^(2))为0.931,均方根误差(root mean square error,RMSE)为0.782,剩余预测偏差(residual prediction deviation,RPD)为2.162;使用随机森林回归和小波能量系数6对海岛棉叶绿素含量的反演效果最佳,建模集R^(2)为0.932,RMSE为1.198,RPD为2.687。【结论】本研究可为棉花叶绿素含量遥感估算提供技术参考。

随机海况下浮式结构物时域响应分析

为快速准确地获得随机海况下浮式结构物的时域响应,提出一种快速计算结构时域响应的方法。建立有限元应力模式的动力学方程,论证节点在周期荷载作用下的稳态解形式;推导节点应力响应函数的计算公式;将海况波浪谱分解,通过叠加应力响应方式计算随机海况下节点应力的时域响应。通过数值仿真验证及具体的工程项目应用,证明了该方法的可行性和优越性,为浮式平台时域疲劳分析提供了一种新思路。

基于江苏海域定点观测资料的双峰谱谱型研究

江苏海域作为全国海上风电重点建设海域,走向远海是未来发展的主趋势,外海开敞海域涌浪成分较多,海浪谱往往以双峰谱的形式出现,因此双峰谱海浪的谱型特征需要进一步深入研究,为海上施工提供参考依据。基于浮标测站2018年全年观测数据,经过异常值检验、双峰谱识别后得到1 223笔双峰谱数据,进而研究江苏海域波浪双峰谱谱型特征,对比不同典型双峰谱谱型并展开实测双峰谱的拟合,提出了修正谱宽参量,探究了修正系数、峰升高因子、谱宽参量间的依赖关系,得到了双峰谱拟合谱型表达式。结果表明:基于北大西洋海域提出Ochi-Hubble谱系及基于挪威海域提出的Torsethaugen谱并不适用于江苏海域双峰谱谱型,本文提出的双峰JONSWAP拟合谱具有自适应性,可以科学合理地描述江苏海域双峰谱谱型,并推广应用于不同风场、地形条件的海域中。

多途混响环境中声扩频信号线性与相关性分析

为分析浅海多途信道中声扩频信号的相关性和线性,建立了计入频率衰减与海底损耗的浅海Pekeris波导虚源法计算模型;对浅海中的脉冲信号与扩频信号进行了仿真计算,通过混响室试验初步研究了多途混响环境中扩频信号的线性。仿真结果显示:虚源法和简正波法的频域解一致,得到了与简正波理论相同的低频模式频散现象,验证了虚源法适用于浅海理想波导、Pekeris波导中的低频问题;当发生模式频散,低频扩频信号的相关性能强于同扩频参数的高频信号。试验结果表明:低频脉冲的时域波形发生畸变、展宽时,同频率的扩频信号能准确测出反射信号的几何时延,证明了低频多途效果仍可看作每个“路径”信号的线性叠加。