Mechanisms for the SAR Imaging of the Ocean

We perform a comparative analysis of two mechanisms for the formation of a synthetic aperture radar (SAR) image of the ocean, which are related to orbital velocities on the surface. It is shown that the mechanism of fluctuations of the effective density of scatterers, which is commonly accepted at present as the physical basis for the SAR diagnostics of roughness, actually works only in the case of a sufficiently flat swell. In the presence of wind roughness, the dominant role is played by another mechanism, namely, fluctuations of the number of surface elements whose images are randomly shifted and superimposed in the image plane due to orbital velocities. In the case of a developed wind roughness propagated in the azimuthal direction, the power contributions of two above-mentioned mechanisms to the SAR imaging of the ocean differ by about two orders of magnitude.     

Spectral characteristics of a microwave radar signal backscattered by the sea surface at small incidence angles

We present a theoretical analysis of the Doppler-spectrum properties of a microwave radar signal scattered by the sea surface at small incidence angles. The dependences of Doppler-spectrum width and shift on the wind velocity and wave development stage and their azimuthal dependence are analyzed. The case of mixed sea (wind wave plus swell) is also considered. The JONSWAP spectrum model is used to describe sea waves. The study shows that Doppler-spectrum parameters are sensitive to variation of sea-surface state; for example, for the case of developed sea waves, an increase in wind velocity by 1 m/sec leads to increases in the Doppler-spectrum width and shift by 15 Hz and 3 Hz, respectively. It is shown that for the case of a moving radar the Doppler spectrum remains sensitive to variation of sea- surface state with a sufficiently narrow radar directivity pattern. Estimates show that in the case of a single sea- wave system on the surface, using Doppler-spectrum parameters we can, in principle, determine wave type (developing wind wave, developed wind wave, or swell), dominant wavelength, wave propagation direction, and wave height; wind velocity, direction, and acceleration distance can be determined for wind waves.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 39, No. 5, pp. 517–526, May, 1996.This paper was supported by the Russian Foundation for Fundamental Research (project 93-02-15892).     

Modeling the spectrum of infrasonic hydroacoustic radiation generated by the sea surface under storm conditions

Generation of infrasonic radiation into a water medium by sea surface waves is analyzed. The analysis is carried out for the situation in which the infrasound is generated by surface waves with frequencies close to those of dominant waves. The presence of two wave systems on the sea surface is assumed: swell and wind waves. It is shown that if the frequencies of spectral peaks of wind waves and swell diverge by 20%, the maximum value of the radiation spectrum decreases by approximately 40% (if the general directions of the two wave systems are oriented strictly towards each other). A deviation of the general directions of the two wave systems from the opposite direction by 45° leads to a decrease in the maximum value of the radiation spectrum by more than two times.     

Atmospheric stability of surface boundary layer in coastal region of the Wol–Ryong site

In order to provide statistically reliable information of a wind energy site, accurate analysis on the atmospheric stability and climate characteristics in a certain area is a prerequisite. Two 2-D ultrasonic anemometers and one cup anemometer, located perpendicular to the prevailing wind direction, were used to measure the atmospheric wind environment at a height of 4.5 m in coastal region of the Wol–Ryong, Jeju, South Korea. The study is aiming to understand the atmospheric stability about a coastal region, and the effect of roughness length. We calculate the Monin–Obukhov length for division of atmospheric stability about unstable regime, neutral regime and stable regime. The distribution of diurnal Monin–Obukhov length is highly sporadic in the coastal region due to the effect of radiant heat from the surface or other environmental effects. In order to calculate the roughness length in coastal region, three different methods are applied in terms of the surface roughness, flow fluctuation and gust wind, which are called logarithmic profile, standard deviation and gust factor methods. In the study, the atmospheric stability was insignificant when applying these three methods. In the results, three different roughness length scales sufficiently showed the effect of obstacle and surface conditions around the measurement position. On the basis of an overall analysis of the short-term data measured in the Wol–Ryong area, Jeju Island, it is concluded that for the development of future wind energy resources, the Wol–Ryong site could be a good candidate for a future wind energy site.     

Ocean circumstance effects on the sound scattering of thermocline

I.IntroductionTheformationofoccanthermoc1ineistheresultfromasea-atmosphereintcraction,whichdependsonvariousintensitiesanddepthsofthethermocline,andthedynamictransactionofthermoenergybetweenlayers.Thercfore,thetcmporalandspacialchangcofthethcrmoc1inebecomesanimportantparameterforrcsearchonuppcrboundary1ayerofsea.Thetempera-tUre,salinityandprcssurevarywithdcpth,whichhasformedthcvcrticalprofileofsoundvclooityintheuppcrboundarylaycrsofsca.A1ongthcshallowwatcrareaofChina,thereareobvi-ousnon-unifo…     

机载C波段全极化SAR海面风矢量反演理论研究及实验验证

机载全极化SAR海面风矢量反演研究对于近海岸复杂气象条件下风矢量探测具有重要意义. 本文从极化散射理论出发,通过分析全极化SAR探测数据与海面风矢量的关系, 设计了全极化SAR海面风矢量反演方案.依据机载SAR高机动性和全极化两个探测特点, 针对VV极化探测数据,提出了基于最大似然估计的海面风矢量反演方法,并设计了飞行实验方案; 针对VH极化探测数据,提出了通过带约束最优拟合的VH极化海面散射模型反演风速, 再利用CMOD5地球物理模型函数计算风向的海面风矢量反演方法. 利用机载全极化SAR探测的台风'海葵'边缘数据,开展了海面风矢量反演实验研究. 研究结果表明,两种风矢量反演方法均可不借助辅助信息,反演复杂气象条件下的海面风矢量. 前者反演风向、风速的均方根误差分别为18.0°, 1.8 m/s, 后者反演风向、风速的均方根误差分别为9.3°, 1.2 m/s,后者的反演精度优于前者. 这是因为VH极化归一化雷达截面与风向和雷达入射角无关,仅与风速密切相关, 更适合复杂气象条件下的海面风矢量反演. 关键词： 机载全极化SAR 海面风矢量 理论研究 实验验证     

Microwave subsurface remote sensing in the Negev Desert: monitoring of soil water-content

We report the use of the microwave remote sensing as a technique with great potential for the mapping of subsurface properties including the monitoring of soil water conditions. Remote sensing experiments with microwave instrumentation were conducted in the Negev desert in Israel. The remote sensors used were a P-band (68 cm, 441 MHz) scatterometer and an ERS-2 C-band (5.3 cm, 5.7 GHz) SAR (synthetic aperture radar) along with the collection of ground truth data such as volumetric and gravimetric soil water-content, surface roughness and dielectric measurements. Corner reflectors in the field were used for calibration and geo-rectification of the SAR data. The results of the microwave experiments are in a good agreement with the developed theoretical models that take into account the effects of the random surface roughness. The optical modelling of microwave processes is presented as a tool for developing the physical basis for empirical studies. This practice simplifies testing theoretical predictions and reduces the immense cost of running field and laboratory measurements in the microwave range.     

On the Influence of Asymmetry of Short Wind Waves on Statistical Characteristics of the Sea-Surface Image in Diffuse Sky Light

We develop an optical model of imaging of wind roughness on a sea surface in diffuse sky light with allowance for bound waves (parasitic capillary ripple) on the front slope of decimeter gravity waves. It is found that in the case of oblique observation, free and bound waves give different contribution to the statistical characteristics of the image of a rough sea surface.     

基于小波的多极化机载合成孔径雷达海面风向反演

为了实现从合成孔径雷达(SAR)图像本身提取高精度的海面风向信息, 提高SAR海面风场反演精度,研究了多极化机载SAR海面风向反演技术, 借助小波分析相对傅里叶分析和局部梯度更精细的时-频分析能力, 将二维连续小波变换与快速傅里叶变换(FFT)相结合,提出一种新的机载SAR海面风向反演方法. 为验证反演方法的有效性,通过海上同步飞行试验获取多极化机载SAR数据及同步调查船实测风向数据,用于反演试验的数据比对.采用本文提出的方法, 利用多种小波基对机载C波段SAR的同极化和交叉极化数据进行风向反演, 将反演结果与美国国家环境预报中心再分析资料以及调查船实测风向进行比对. 结果表明,本文提出的基于小波分析的海面风向反演方法适用于机载SAR探测数据, 反演精度优于二维FFT法和局部梯度方法;小波基的选择对反演结果影响较大, Mexican-Hat小波基是机载SAR海面风向反演的最优小波基, 且同极化与交叉极化机载SAR数据均可用于海面风向的反演.     

Coherence effects in surface roughness induced by vacuum ultraviolet F2 laser ablation

We analyze statistical fluctuations in the pulse-to-pulse spatial fluence distribution of a highly multimode F2 laser beam and the influence that coherence effects have on the attainable smoothness of an ablated surface. The magnitude of fluctuations is predicted to be a few percent, with associated roughness increasing as m1/2, where m is the number of ablation pulses. By use of a white-light optical interferometer, measurements have been made of roughness induced on N-BK7 glass surfaces ablated with a 157-nm laser, and reasonable agreement with predictions based on this roughening mechanism has been found.     

New expressions for the surface roughness length and displacement height in the atmospheric boundary layer

An alternative model for the prediction of surface roughness length is developed. In the model a new factor is introduced to compensate for the effects of wake diffusion and interactions between the wake and roughness obstacles. The experiments are carried out by the use of the hot wire anemometry in the simulated atmospheric boundary layer in a wind tunnel. Based on the experimental data, a new expression for the zero-plane displacement height is proposed for the square arrays of roughness elements, which highlights the influence of free-stream speed on the roughness length. It appears that the displacement height increases with the wind speed while the surface roughness length decreases with Reynolds number increasing. It is shown that the calculation results based on the new expressions are in reasonable agreement with the experimental data.     

Highly time-resolved investigation of the electric wind caused by surface DBD at various ac frequencies

Surface plasma discharges find relevant applications for manipulating laminar or turbulent flows. The control mechanism can be related to time-averaged momentum transfer or be associated to favorable interactions between the fluctuations imposed by the discharge and the natural instabilities developing in the incoming flow. In the present paper, the electric wind produced by a non-thermal surface dielectric barrier discharge is investigated by a non-intrusive measurement technique having a high temporal resolution. The measurements are carried out for an actuator in quiescent flow condition, but also for an actuator placed in the test section of a wind tunnel with external flow speed from 5 to 30 m/s. The results confirm that a surface DBD actuator has a low control authority on the mean characteristics of the velocity profiles developing above the plasma region. However, it is also demonstrated that the plasma actuator imposes periodic fluctuations at the electrical frequency driving the gas ionization, this over the whole range of tested speeds.     

随机粗糙度对风力机翼型气动性能影响

建立人工神经网络、径向基函数网络和支持向量回归机三种近似模型,结合蒙特卡洛方法与表征粗糙度参数随机特性的概率模型对风力机翼型气动性能进行不确定性分析。结果表明,支持向量回归机具有最佳预测精度。对于风力机翼型FX 63-137,最大升力系数对吸力面前缘粗糙度的敏感性明显高于压力面;对于吸力面或压力面,前缘粗糙带厚度对最大升力系数的影响稍大于其覆盖长度的影响。研究工作为风力机翼型的鲁棒性设计优化奠定了理论基础。     

Microwave subsurface remote sensing in the Negev Desert: monitoring of soil water-content

We report the use of the microwave remote sensing as a technique with great potential for the mapping of subsurface properties including the monitoring of soil water conditions. Remote sensing experiments with microwave instrumentation were conducted in the Negev desert in Israel. The remote sensors used were a P-band (68 cm, 441 MHz) scatterometer and an ERS-2 C-band (5.3 cm, 5.7 GHz) SAR (synthetic aperture radar) along with the collection of ground truth data such as volumetric and gravimetric soil water-content, surface roughness and dielectric measurements. Corner reflectors in the field were used for calibration and geo-rectification of the SAR data. The results of the microwave experiments are in a good agreement with the developed theoretical models that take into account the effects of the random surface roughness. The optical modelling of microwave processes is presented as a tool for developing the physical basis for empirical studies. This practice simplifies testing theoretical predictions and reduces the immense cost of running field and laboratory measurements in the microwave range.     

Acoustic propagation in the subsurface sound channel

Experimental data obtained in the kilohertz frequency band for the sound propagation in the subsurface channel formed by the wind-caused mixing of subsurface waters are discussed. The data were obtained in different years in the northeastern region of the Atlantic Ocean, where the subsurface waters down to the depths of 40–70 m are mixed by both wind waves and the swell that arrives from distant ocean areas. The hydrological conditions in the subsurface waters of this region are characterized by a good reproducibility. The spatial structure of the sound field and the attenuation of sound propagating in the subsurface channel are analyzed. The origin of the additional attenuation (in comparison with the absorption in sea water) is discussed. The data of our experiments are compared with those obtained by other experimenters and with the calculations performed using the computer code by Avilov. The necessity of improving the computer codes to allow for the scattering of sound beyond the channel under the influence of the swell, whose parameters are unrelated to the wind regime at the experimental site, is emphasized.     

Monte Carlo sampling of energy-constrained quantum superpositions in high-dimensional Hilbert spaces

We derive scaling laws for the steady spectrum of wind excited waves, neglecting surface tension and taking air and water as inviscid, an approximation valid at large wind speed. Independently of the wind speed, there exists an unique (small) dimensionless parameter ϵ, the ratio of the mass densities of the two fluids (air and water). The smallness of ϵ allows to derive some important average properties of the wave system. The average square slope of the waves is, as observed, a small but not very small quantity, because it is of order |ln(ϵ ²)|^-1. This supports the often used assumption of small nonlinearity in the wave-wave interaction. We introduce an equation to be satisfied by the two-point correlation of the height fluctuations. Lastly we reconsider the formation of swell, that is the relationship between the randomness of waves and the observation of quasi monochromatic water waves.     

分形粗糙面合成孔径雷达成像研究

研究了分形粗糙面的成像问题. 分形粗糙面能够较好的逼近真实环境, 采用带限形式的Weierstrass-Mandelbrot函数建立了分形粗糙面几何模型, 对分形粗糙面参数的选取进行了讨论. 对大尺度粗糙面散射问题提出了一种基于大面元的Kirchhoff近似方法, 采用面元模型来计算粗糙面总的后向散射场以及每一个面元的后向散射场, 并对面元的尺寸选取进行了研究, 通过与解析解进行对比证明了该方法的正确性. 在分形理论建立的确定性粗糙面几何模型与面元的Kirchhoff方法获取的散射场的基础上, 采用正侧视条带式成像模式, 并选用距离多普勒算法对不同分形参数的粗糙面进行合成孔径雷达(SAR) 成像模拟, 结果显示从SAR像中可以清晰地观察到不同分形参数对粗糙面几何轮廓的影响. 该研究包括了从环境模型、电磁模型到SAR成像技术在内的完整的分形环境SAR像模拟过程, 仿真结果显示出分形环境的SAR像特点, 这对基于分形理论的自然环境的遥感探测以及参数反演具有一定的理论支撑作用.     

随机散射屏的原子力显微镜形貌分析及其光散射特性

用原子力显微镜对三种不同粗糙度的随机散射屏的表面形貌进行了测量分析,发现它们在短程范围内具有明显的分形特征.对于粗糙度较大和较小的散射屏,分形特征分别以无规则的 高度调制和无特征大小的小颗粒的形式存在.用自仿射分形表面模型对散射屏的统计特性进 行了描述和拟合.光散射测量发现,散射光强在远轴区域按负幂函数下降,理论分析证明这 源于表面的分形结构;在近轴区域有散射亮环存在,用自仿射分形表面模型尚不能给出理论解释. 关键词： 随机散射屏 光散射 自仿射分形表面     

Ovalling oscillations of thin circular cylindrical shells in cross flow—An experimental study

Experiments have been made to study the effect of a splitter-plate on the oscillation amplitudes, wake fluctuations and unsteady surface pressures of a shell undergoing ovalling oscillations in a cross flow in a wind tunnel. The results suggest that there is no major effect of the splitter-plate on these quantities and that periodic vortex shedding is probably the case of the ovalling.