A vector radiative transfer model of coupled ocean-atmosphere system using matrix-operator method for rough sea-surface

A vector radiative transfer model termed PCOART has been developed for the coupled ocean-atmosphere system, using the matrix-operator (or adding-doubling) method, which considers the rough sea-surface. The theoretical formulations of the solution of the vector radiative transfer equation of the coupled ocean-atmosphere system, and the reflection-transmission matrices and internal radiation sources for rough sea surface are described. The model intercomparison is performed for several radiative transfer problems in the atmosphere and ocean, and the results show that PCOART can exactly predict the radiance fields for both flat and rough sea surface. Also, the polarizing remote sensing data from POLDER is used to test the capacity of PCOART to simulate the polarization radiance at the top-of-atmosphere, which shows that PCOART can perfectly reproduce the linear polarization reflectance measured by POLDER. PCOART can not only simulate the total radiance field in the coupled ocean-atmosphere system with wind-induced rough sea surface but also predict the polarization radiance field both in the atmosphere and in the ocean, which can serve as a good tool for the ocean optics and ocean color remote sensing communities.     

Doppler spectrum of polarimetric scattering field from two-dimensional time-varying nonlinear sea surfaces

Polarimetric scattering model of second-order small-slope approxi-mation combined with “choppy wave" model (CWM) for describing nonlinear hydrodynamic interactions between ocean waves is utilized in this paper to investigate the influence of sea surface nonlinearities on backscattering coefficient as well as Doppler spectrum signatures including Doppler shift and spectral bandwidth. Simulation results show that at moderate to large incidence angles the Doppler shift and spectral bandwidth of the CWM nonlinear sea surfaces are significantly larger than those of linear sea surfaces, in particular at low grazing angles. In addition, Doppler signatures show distinct polarization dependence, and most importantly the cross-polarized Doppler signatures significantly differ from the co-polarized ones. It is also indicated that co-polarized Doppler shift increases obviously with wind speed increasing, whereas the cross-polarized Doppler shift looks less sensitive to wind speed variations. The difference of Doppler signatures between co- and cross-polarization is potentially valuable for ocean remote sensing applications, especially for observing very high winds.     

海洋气溶胶多光谱偏振特性研究

为提高海洋气溶胶的反演精度,定量研究了气溶胶在可见近红外波段的多光谱偏振辐射特性。首先建立了合理的海洋气溶胶及海面模型,基于逐次散射法准确模拟了光在气溶胶中及大气—海洋交界面处的矢量辐射传输过程,接着从光谱角度定量分析了典型波段处的反射率和偏振反射率,在此基础上提出了海洋气溶胶偏振辐射的光谱分布模型,并用卫星数据对模型进行验证,最后研究了气溶胶光学厚度、观测角度、海水叶绿素a浓度和海面风速对气溶胶多光谱偏振辐射的影响。研究表明,在可见近红外波段不考虑大气吸收和耀斑效应时,气溶胶偏振辐射的光谱变化符合幂函数模型;在不同波段处,海洋气溶胶的偏振辐射受海水叶绿素a浓度和海面风速的影响不同;海洋气溶胶的多光谱偏振辐射信息有效体现了气溶胶的自身特性,且与反射率相比,偏振反射率随各因素的变化存在更明显的谱段差异性。因此,可以运用气溶胶的多光谱偏振信息反演海洋气溶胶参数,多光谱探测的加入对提高气溶胶反演精度具有重要意义。     

激光雷达数据应用于海气界面气体传输速率的估算

选取2007年1、4、7、10月4个月夜间无云的云-气溶胶激光雷达和红外探测者观测卫星(CALIPSO)星载激光雷达Version 3.01数据,提取532nm激光海面后向散射系数并计算海面波浪均方斜率。同时选用准同步AMSR-E海面风速数据,利用Wanninkhof等于2009年提出的包含线性、平方、立方项的海面风速与气体传输速率混合关系模型计算海水施密特数为660时的海气界面气体传输速率k660。通过回归分析,得到由星载激光雷达数据反演k660的关系模型。将激光雷达数据反演结果与采用4种典型海面风速关系模型的计算结果进行比较,给出由星载激光雷达数据反演的4个月平均的海气界面气体传输速率全球分布图和纬度分布图。研究表明,CALIPSO星载激光雷达532nm单脉冲测量数据可用于反演海气界面气体传输速率。     

大气-海浪-海流耦合模式的建立和一次台风过程的初步试验

利用Linux系统中共享内存及进程间通信技术,将广泛应用于大气、海洋、 海浪研究的数值模式耦合,综合考虑了大气对海流和海浪的风力驱动、 海洋对大气下垫面的改变及海浪对海洋的浪致混合作用, 建立起高分辨率的中尺度台风模式.以一次台风过程为模拟背景, 对2006年的"格美"台风进行模拟,利用可得到的观测资料,对模拟结果进行验证. 结果表明:模式经过spin-up阶段后,基本能够模拟出实况台风强度的变化趋势, 中心最低气压、风速和路径的模拟与实况较为一致. 同时,对台风强风条件下的海面降温及海浪高度都有较为合理的再现. 模式误差的出现,与当前模式微物理过程参数化方案、大尺度初始场、 Bogus台风的构造及海洋模式中海洋上层物理参数化方案等相关.     

水中油对水体上行辐亮度光谱的影响

目前,学者主要关注利用遥感技术探测海面油膜。然而,经海洋物理过程或人为喷洒化学分散剂处理形成的水中油对海洋生境也具有危害作用。水体上行辐亮度是水色传感器的重要信号源,通过分析含油水体的上行辐亮度光谱特征,探索快速有效地遥测水中油的方法对保护海洋生境具有重要意义。基于大连港海域现场实测数据及Hydrolight模拟含油水体水下光场,通过分析上行辐亮度随波长、水深及太阳天顶角的变化特征,剖析水中油对上行辐亮度光谱的影响及水中油的敏感光谱特性。结果表明水中油的主要波谱响应区间位于可见光波段（380~760 nm）。随着水中油浓度的增加,上行辐亮度光谱峰值有逐渐向长波方向移动及蓝光波段辐亮度量值逐渐降低的趋势,这些变化处于水色遥感的探测光谱范畴,为利用水色遥感技术探测水中油提供了光谱依据。其次,上行辐亮度随水深逐级递减,并在接近水体下界面前不降反升的现象说明刚好在水面之上的上行辐亮度由各深度水体组分的后向散射及下界面的反射共同贡献,再经水汽界面上行透射而得,属于水体辐射传输的核心机理。这与水面油膜通过油类物质改变海表反射率而产生与自然海表不同反射光谱的探测机理具有本质上的差别。再者,与含水中油水体后向散射产生的上行辐亮度相比,海表对太阳光的反射属于强信号,会掩盖水体组分信息。水色卫星搭载的水色传感器具有一定的侧摆能力,能避开太阳辐射反射信号并接收到含水中油水体的上行辐亮度;水色卫星的当地过境时间一般为10至14点,且水色传感器具有高信噪比特征,满足含水中油水体的暗像元探测要求。该研究揭示了水色遥感探测含水中油水体的光谱和机理依据,表明可以视水中油为一种新的水体组分,基于光在水体中的辐射传输过程,开展含水中油水体的水色遥感反演研究。     

Anomalous sea surface reverberation scale model experiments

Low frequency sea surface sound backscattering from approximately 100 Hz to a few kHz observed from the 1960s broadband measurements using explosive charges to the Critical Sea Test measurements conducted in the 1990 s is substantially higher than explained by rough sea surface scattering theory. Alternative theories for explaining this difference range from scattering by bubble plumes/clouds formed by breaking waves to stochastic scattering from fluctuating bubble layers near the sea surface. In each case, theories focus on reverberation in the absence of the large-scale surface wave height fluctuations that are characteristic of a sea that produces bubble clouds and plumes. At shallow grazing angles, shadowing of bubble plumes and clouds caused by surface wave height fluctuations may induce first order changes in the backscattered signal strength. To understand the magnitude of shadowing effects under controlled and repeatable conditions, scale model experiments were performed in a 3 m x 1.5 m x 1.5 m tank at the Technical University of Denmark. The experiments used a 1 MHz transducer as the source and receiver, a computer controlled data acquisition system, a scale model target, and a surface wave generator. The scattered signal strength fluctuations observed at shallow angles are characteristic of the predicted ocean environment. These experiments demonstrate that shadowing has a first order impact on bubble plume and cloud scattering strength and emphasize the usefulness of model scale experiments for studying underwater acoustic events under controlled conditions.     

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     

风成海面的极化辐射

采用几何光学模型和统计的方法研究了风成海面的极化辐射特性.本文假设海面由一系列随 机分布的小镜面组成，海面对电磁波的散射可看作镜面的反射，风成海面的辐射是小镜面辐 射的统计平均.计算了前三个Stokes 参数，结果表明辐射亮温随方位的变化关系为余弦函数 ，周期为π；辐射亮温随方位角的变化幅度为几个Κ.风成海面的极化辐射特性提供了利用 被动微波遥感手段反演海洋风场的途径. 关键词： 极化 辐射 亮温 风成海面     

油膜覆盖的非线性海面电磁散射多普勒谱特性研究

当海面上方漂浮油膜时,海面的毛细波成分将因油膜的阻尼作用而被破坏.本文采用PM谱,基于Marangoni阻尼效应,建立油膜覆盖的一维Creamer非线性海面模型,并简单分析了油膜的阻尼作用对海面轮廓的影响.在此基础上,利用迭代物理光学方法研究了L波段下该模型的后向散射回波的多普勒谱特性,通过与基于线性模型的海面散射回波多普勒谱对比发现,在大中入射角下,非线性海面散射回波与线性海面多普勒谱的差异不可忽略,说明采用Creamer非线性理论建立海面几何模型的必要性.研究发现,油膜覆盖海面的散射回波的多普勒频移及展宽与干净海面雷达回波的多普勒特性具有明显差异,这表明海面上漂浮的油膜对雷达散射回波的多普勒特性具有显著的影响.数值结果重点分析了入射角、油膜参数以及风速对油膜覆盖海面散射回波多普勒谱展宽和频移的影响规律.     

高风速下海洋飞沫水滴对拖曳系数的影响

海面飞沫水滴改变着海气动量通量的分布, 从而在相当程度上影响着海面拖曳系数. 为了能够精确估计海面飞沫水滴对海面拖曳系数的影响, 推导出新的依赖于风速和海浪状态的海洋飞沫生成函数, 将该飞沫生成函数用在改进的飞沫动量通量计算公式中进行飞沫动量通量的计算, 发现本文提出的飞沫动量通量计算公式对海浪状态具有较强的敏感性, 能够清晰地表达海浪状态对飞沫动量通量的影响. 海面总动量通量包含飞沫动量通量和海气界面动量通量, 基于此理论, 得到高风速下受飞沫水滴影响的海面拖曳系数关系式, 从关系式的理论值可知, 在高风速下, 海面拖曳系数开始衰减, 说明高风速下海面飞沫水滴能够抑制拖曳系数值的增加. 将理论结果与实验室和外海测量值进行对比, 发现海面拖曳系数的理论值变化范围覆盖了测量值, 同时将该海面拖曳系数代入海浪模式进行台风浪模拟, 发现海浪模式能够较好地模拟出有效波高, 说明本文提出的新的海面拖曳系数公式能够合理地用在高风速条件下.     

Acoustic propagation from a spiral wave front source in an ocean environment

A spiral wave front source generates a pressure field that has a phase that depends linearly on the azimuthal angle at which it is measured. This differs from a point source that has a phase that is constant with direction. The spiral wave front source has been developed for use in navigation; however, very little work has been done to model this source in an ocean environment. To this end, the spiral wave front analogue of the acoustic point source is developed and is shown to be related to the point source through a simple transformation. This makes it possible to transform the point source solution in a particular ocean environment into the solution for a spiral source in the same environment. Applications of this transformation are presented for a spiral source near the ocean surface and seafloor as well as for the more general case of propagation in a horizontally stratified waveguide.     

Numerical study of electromagnetic scattering from one-dimensional nonlinear fractal sea surface

In recent years, linear fractal sea surface models have been developed for the sea surface in order to establish an electromagnetic backscattering model. Unfortunately, the sea surface is always nonlinear, particularly at high sea states. We present a nonlinear fractal sea surface model and derive an electromagnetic backscattering model. Using this model, we numerically calculate the normalized radar cross section (NRCS) of a nonlinear sea surface. Comparing the averaged NRCS between linear and nonlinear fractal models, we show that the NRCS of a linear fractal sea surface underestimates the NRCS of the real sea surface, especially for sea states with high fractal dimensions, and for dominant ocean surface gravity waves that are either very short or extremely long.     

Electromagnetic backscattering from one-dimensional drifting fractal sea surface Ⅱ:Electromagnetic backscattering model

Sea surface current has a significant influence on electromagnetic(EM) backscattering signals and may constitute a dominant synthetic aperture radar(SAR) imaging mechanism. An effective EM backscattering model for a one-dimensional drifting fractal sea surface is presented in this paper. This model is used to simulate EM backscattering signals from the drifting sea surface. Numerical results show that ocean currents have a significant influence on EM backscattering signals from the sea surface. The normalized radar cross section(NRCS) discrepancies between the model for a coupled wavecurrent fractal sea surface and the model for an uncoupled fractal sea surface increase with the increase of incidence angle,as well as with increasing ocean currents. Ocean currents that are parallel to the direction of the wave can weaken the EM backscattering signal intensity, while the EM backscattering signal is intensified by ocean currents propagating oppositely to the wave direction. The model presented in this paper can be used to study the SAR imaging mechanism for a drifting sea surface.     

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).     

The effect of anisotropy of a rough sea surface on the generation of acoustic radiation

A model that relates the frequency-angular characteristics of wind waves on the sea surface to the frequency spectrum of acoustic radiation generated by them is constructed. Based on empirical wave energy distribution functions, it is shown that, in the vicinity of the peak of the wind wave spectrum, the intensity of acoustic radiation strongly depends on the model chosen for the angular distribution function ?(α) and on its parameters. At high frequencies, four to five times higher than the dominant wave frequency, it is possible to assume that, to a first approximation, ?(α) = const. In the intermediate frequency range, between the dominant frequency and the high frequencies, the predominant contribution to the sound radiation is made by the waves that travel in the direction close to orthogonal with respect to the wind.     

一个L波段海表盐度遥感反演的新经验模式

海表盐度是描述海洋的一个重要参量，海表盐度的微波探测是海洋遥感研究的重要内容.利用黄海实测盐度数据和海表亮温资料，建立了一个适用于黄海近岸的盐度反演模式.在没有海浪破碎情况下，利用L波段的亮温资料反演海洋表层盐度.本模式充分考虑了海面粗糙度的影响，从海水亮温和海水的介电常数反演的表层盐度和实测的表层海水盐度基本一致，其平均绝对误差为0.288psu，与同类模式相比具有较高的精度. 关键词： 盐度 亮温 辐射率 介电常数     

空间多普勒激光雷达海表面散射信号分析

为了验证受海表面风速驱动的海表面反射率模型和计算海表面信号进行零风速校准的精度,开展了激光雷达海表面散射信号强度和速度特征的分析。在强度特征信息方面,进行了海表面反射率仿真模型和机载激光雷达海面散射信号实测数据的对比,考虑激光水下散射,参考QuikSCAT的海表面风速大小,测量结果和模型曲线基本吻合。在速度特征方面,通过海表面运动波形的仿真和海表面速度信息的提取,考虑波面的垂直和水平运动,采用时间积分的海表面探测信号零风速校准误差小于0.2 m/s。     

A new approach to the retrieval of surface properties from earthshine measurements

Instruments such as the MODIS and MISR radiometers on EOS AM-1, and POLDER on ADEOS have been deployed for the remote sensing retrieval of surface properties. Typically, retrieval algorithms use linear combinations of semi-empirical bidirectional reflectance distribution function (BRDF) kernels to model surface reflectance. The retrieval proceeds in two steps; first, an atmospheric correction relates surface BRDF to top-of-atmosphere (TOA) reflectances, then regression is used to establish the linear coefficients used in the kernel combination. BRDF kernels may also depend on a number of physical or empirical non-linear parameters (e.g. ocean wind speed for a specular BRDF); such parameters are usually assumed known. A major source of error in this retrieval comes from lack of knowledge of planetary boundary layer (PBL) aerosol properties.In this paper, we present a different approach to surface property retrieval. For the radiative transfer simulations, we use the discrete ordinate LIDORT model, which has the capability to generate simultaneous fields of radiances and weighting functions in a multiply scattering multi-layer atmosphere. Surface-atmosphere coupling due to multiple scattering and reflection effects is treated in full; the use of an atmospheric correction is not required. Further, it is shown that sensitivities of TOA reflectances to both linear and non-linear surface BRDF parameters may be established directly by explicit analytic differentiation of the discrete ordinate radiative transfer equations. Surface properties may thus be retrieved directly and conveniently from satellite measurements using standard non-linear fitting methods. In the fitting for BRDF parameters, lower-boundary aerosol properties can either be retrieved as auxiliary parameters, or they can be regarded as forward model parameter errors. We present examples of simulated radiances and surface/aerosol weighting functions for combinations of multi-angle measurements at several different wavelengths, and we perform some examples of self-consistent non-linear fitting to demonstrate feasibility for this kind of surface property retrieval.