Ocean Backscatter Research Articles

On the bubble-bubbleless ocean continuum and its meaning for the LiDAR equation: LiDAR measurement of underwater bubble properties during storm conditions

This paper presents the NRL shipboard LiDAR and the first LiDAR dataset of underwater bubbles. The meaning of these LiDAR observations, the algorithms used and their current limitations are discussed. The derivation of the LiDAR multiple scattering regime is derived from the LiDAR observations and theory. The detection of the underwater bubble presence and their depth is straightforward to estimate from the depolarized laser return. This dataset strongly suggest that the whitecaps term in the LiDAR equation formalism needs to be revisited. The retrieval of the fraction of air volume within a given volume of water (void fraction) is possible and the algorithm is stable with a simple ocean backscatter LiDAR system. The accuracy of the void fraction retrieval will increase significantly with future developments.

Effects of ocean wave spectra on the polarized bidirectional reflectance distribution function matrix at Ku‐band and its implications on satellite backscattering simulations

A polarized bidirectional reflectance distribution function (pBRDF) matrix is developed from two-scale roughness theory with the aim of providing more accurate simulations of microwave emissions and scattering required for ocean–atmosphere coupled radiative transfer models. The potential of the pBRDF matrix is explored for simulating the ocean backscatter at Ku-band. The effects of ocean wave spectra including the modified Durden and Vesecky (DV2), Elfouhaily, and Kudryavtsev spectra on the pBRDF matrix backscatter simulations are investigated. Additionally, the differences in backscattering normalized radar cross-section (NRCS) simulations between the Ku-band geophysical model function and pBRDF matrix are analyzed. The results show that the pBRDF matrix can reasonably reproduce the spatial distribution of ocean surface backscattering energy, but the distribution pattern and numerical values are influenced by ocean wave spectra. The DV2 spectrum is the best one for the pBRDF matrix to simulate horizontally polarized NRCSs, with the exception of scenarios where the incidence angle is below 35°, the wind speed is less than 10 m s−1, and in the cross-wind direction. Also, the DV2 spectrum effectively characterizes the wind speed and relative azimuth angle dependence for vertically polarized NRCSs. The Elfouhaily spectrum is suitable for simulating vertically polarized NRCSs under conditions of low wind speed (below 5 m s−1) and incidence angles under 40°. The Kudryavtsev spectrum excels in simulating vertically polarized NRCSs at high incidence angles (> 40°) and horizontally polarized NRCSs at low incidence angles (< 35°).摘要为提供微波海洋-大气耦合辐射传输模型所需的精确发射和散射, 基于双尺度粗糙度理论发展了极化双向反射分布函数 (pBRDF) 矩阵. 本研究探讨了pBRDF矩阵是否能够表征ku波段洋面后向散射, 对比了三种常用海浪谱模型对pBRDF矩阵后向散射模拟的影响, 分析了ku波段地球物理模型函数 (GMF) 与pBRDF矩阵在后向散射归一化雷达截面 (NRCS) 模拟中的差异. 结果表明, pBRDF矩阵能够准确再现洋面后向散射能量的空间分布, 但后向散射能量分布受海浪谱影响较大.

Effects of Directional Wave Spectra on the Modeling of Ocean Radar Backscatter at Various Azimuth Angles by a Modified Two-Scale Method

Knowledge of the ocean backscatter at various azimuth angles is critical to the radar detection of the ocean environment. In this study, the modified two-scale model (TSM), which introduces a correction term in the conventional TSM, is improved based on the empirical model, CMOD5.n. Then, the influences of different directional wave spectra on the prediction of azimuthal behavior of ocean radar backscatter are investigated by comparing the simulated results with CMOD5.n and the Advanced Scatterometer (ASCAT) measurements. The results show that the overall performance of the single spectra of D, A, E, and H18 and the composite spectra of AH18 and AEH18 in predicting ocean backscatter are different at different wind speeds and incidence angles. Generally, the AH18 spectrum has better performance at low and moderate wind speeds, while the A spectrum works better at high wind speed. Nevertheless, the wave spectra have little effect on the prediction of the azimuthal fluctuation of scattering, which is highly dependent on the directional spreading function. The relative patterns of azimuthal undulation produced by different spreading functions are rather different at different wind speeds, but similar under different incidence angles. The Gaussian spreading function generally has better performance in predicting the azimuthal fluctuation of scattering.

Effective Surface Roughness in Radar Ocean Backscattering

Effective Surface Roughness in Radar Ocean Backscattering

A Study of the Sensitivity of SAR Ocean Backscatter to Oil Slick Properties Using an Electromagnetic Scattering Model

A Study of the Sensitivity of SAR Ocean Backscatter to Oil Slick Properties Using an Electromagnetic Scattering Model

The Role of Phytoplankton Biomacromolecules in Controlling Ocean Surface Roughness

Satellite altimetric data routinely map sea surface topography by measuring the ocean return signal. One source of altimeter measurement contamination occurs when the radar ocean backscatter becomes unusually large, a situation termed a Sigma-0 bloom. Past research suggests Sigma-0 blooms are associated with weak wind and natural surface slick conditions where capillary waves at the air–sea interface are suppressed. To date, no explicit connection between these conditions and Sigma-0 bloom presence has been provided. Using a series of simplified equations, our reduced model determines capillary wave heights from estimates of planktonic carbon concentrations and regional wind speed. Our results suggest that the radar signal reflection increases as capillary wave height decreases. This relationship depends on surfactant concentration, surfactant composition, and wind speed. Model sensitivity analysis indicates that the interface reflectivity depends on biological activity and wind speed. Our proposed simplified model provides a method to identify potential Sigma-0 bloom regions. We conclude that because of the demonstrated impact of biological surfactants on ocean roughness, it is necessary to consider the biological activity, i.e., phytoplankton bloom events, when interpreting signals from radar altimetry and when developing ocean hydrology models.

A Ka-Band Wind Geophysical Model Function Using Doppler Scatterometer Measurements from the Air-Sea Interaction Tower Experiment

Physical understanding and modeling of Ka-band ocean surface backscatter is challenging due to a lack of measurements. In the framework of the NASA Earth Ventures Suborbital-3 Submesoscale Ocean Dynamics Experiment (S-MODE) mission, a Ka-Band Ocean continuous wave Doppler Scatterometer (KaBODS) built by the University of Massachusetts, Amherst (UMass) was installed on the Woods Hole Oceanographic Institution (WHOI) Air-Sea Interaction Tower. Together with ASIT anemometers, a new data set of Ka-band ocean surface backscatter measurements along with surface wind/wave and weather parameters was collected. In this work, we present the KaBODS instrument and an empirical Ka-band wind Geophysical Model Function (GMF), the so-called ASIT GMF, based on the KaBODS data collected over a period of three months, from October 2019 to January 2020, for incidence angles ranging between 40° and 68°. The ASIT GMF results are compared with an existing Ka-band wind GMF developed from data collected during a tower experiment conducted over the Black Sea. The two GMFs show differences in terms of wind speed and wind direction sensitivity. However, they are consistent in the values of the standard deviation of the model residuals. This suggests an intrinsic geophysical variability characterizing the Ka-band surface backscatter. The observed variability does not significantly change when filtering out swell-dominated data, indicating that the long-wave induced backscatter modulation is not the primary source of the KaBODS backscatter variability. We observe evidence of wave breaking events, which increase the skewness of the backscatter distribution in linear space, consistent with previous studies. Interestingly, a better agreement is seen between the GMFs and the actual data at an incidence angle of 60° for both GMFs, and the statistical analysis of the model residuals shows a reduced backscatter variability at this incidence angle. This study shows that the ASIT data set is a valuable reference for studies of Ka-band backscatter. Further investigations are on-going to fully characterize the observed variability and its implication in the wind GMF development.

SOLS: An Open-Source Spaceborne Oceanic Lidar Simulator

In recent years, oceanic lidar has seen a wide range of oceanic applications, such as optical profiling and detecting bathymetry. Furthermore, spaceborne lidars, CALIOP and ICESat-2, designed for atmospheric and ice science applications, have been used for ocean backscattering retrievals, but, until now, there has been no spaceborne lidar specifically designed for ocean detection. There is a demand for an effective lidar simulator to study the detection potential capability of spaceborne oceanic lidar. In this study, an open-source spaceborne oceanic lidar simulator named SOLS was developed, which is available freely. Moreover, the maximum detectable depth and corresponding optimal wavelength for spaceborne lidar were analyzed at a global scale by using SOLS. The factors controlling detection limits of a spaceborne ocean profiling lidar in different cases were discussed. Then, the maximum detectable depths with different relative measurement errors and the influence of solar background radiance were estimated. Subsequently, the effects of laser and detector parameters on maximum detectable depths were studied. The relationship between the lidar detectable depth and the ocean mixed layer depth was also discussed. Preliminary results show that the maximum detectable depth could reach deeper than 120 m in the oligotrophic sea at low latitudes. We found that 490 nm is the optimal wavelength for most of the open seawater. For coastal water, 532 nm is a more suitable choice considering both the technical maturity and geophysical parameters. If possible, a lidar equipped with 440 nm could achieve the greatest depth in oligotrophic seawater in subtropical gyres north and south of the equator. The upper mixed layer vertical structure in most of the global open ocean is within the lidar maximum detectable depth. These results show that SOLS can help the design of future spaceborne oceanic lidar systems a lot.

On the Convolutions of Sea Wave Spectrum in Radar Backscattering From Ocean Surfaces

By expanding the first-order small slope approximation scattering model in the polynomial terms containing convolutions of the sea wave spectrum, this letter quantitatively investigated how the convolutions of the sea wave spectrum affect the normalized ocean backscattering cross-section (NBCS) under various wind speeds, radar frequencies, and incident angles. First, numerical results show that only a portion of convolution powers of the sea wave spectrum contribute significantly to the NBCS, and such a portion of convolution powers are defined as the “effective convolutions”. Based on this, an empirical model related to the radar frequency, incident angle, and wind speed is established to determine the effective convolutions to reduce the computation cost. Finally, the NBCSs using the effective convolutions by the empirical model and the reference data calculated by the direct numerical integration are compared. The comparison results indicate that compared to the direct numerical integration, implementing the first-order small slope approximation with the effective convolutions has high accuracy and relatively low computational cost, notably with small Rayleigh parameters.

Validation of the High-Altitude Wind and Rain Airborne Profiler during the Tampa Bay Rain Experiment

This paper deals with the validation of rain rate and wind speed measurements from the High-Altitude Wind and Rain Airborne Profiler (HIWRAP), which occurred in September 2013 when the NASA Global Hawk unmanned aerial vehicle passed over an ocean rain squall line in the Gulf of Mexico near the North Florida coast. The three-dimensional atmospheric rain distribution and the associated ocean surface wind vector field were simultaneously measured by two independent remote sensing and two in situ systems, namely the ground-based National Weather Service Next-Generation Weather Radar (NEXRAD); the European Space Agency satellite Advanced Scatterometer (ASCAT), and two instrumented weather buoys. These independent measurements provided the necessary data to calibrate the HIWRAP radar using the measured ocean radar backscatter and to validate the HIWRAP rain and wind vector retrievals against NEXRAD, ASCAT and ocean buoys observations. In addition, this paper presents data processing procedures for the HIWRAP instrument, including the development of a geometric model to collocate time-morphed rain rates from the NEXRAD radar with HIWRAP atmospheric rain profiles. Results of the rain rate intercomparison are presented, and they demonstrate excellent agreement with the NEXRAD time-interpolated rain volume scans. In our analysis, we find that HIWRAP produces wind and rain rates that are consistent with the supporting ground and satellite estimates, thereby providing validation of the geolocation, the calibration, and the geophysical retrieval algorithms for the HIWRAP instrument.

Comparison of the application of co- and cross-polarized sentinel-1 synthetic aperture radar data to tropical cyclone evaluation

ABSTRACT Spaceborne synthetic aperture radar (SAR) is considered one of the most powerful sensors for tropical cyclone assessment. In this study, 14 tropical cyclone (TC) events captured by the Sentinel-1A/B satellite are collected to compare the similarities, differences and quantitative relationships between co- and cross-polarization SAR applications for tropical cyclone evaluation. This study uses multiple co-polarized C-band geophysical model functions (GMFs) and cross-polarized ocean backscatter models, including a C-band Cross-Polarized Ocean Backscatter (C-2PO) model and a C-band Cross-Polarization Coupled-Parameters Ocean (C-3PO) model to estimate the sea surface tropical cyclone wind field. The results reveal that the structure values of the wind circle radius by co-polarization SAR wind inversion are similar to those of cross-polarization. The combination of the two polarization modes can get a more accurate estimation on tropical cyclone rating of the intensity level and hazard of the potential tropical cyclone damages. In addition, a corresponding formula with a correlation coefficient of 0.677 between co- and cross-polarization at 10-km resolution is obtained.

Ocean Vector Wind Retrievals From TRMM Using a Novel Combined Active/Passive Algorithm

This article presents a novel ocean vector wind (OVW) retrieval algorithm for remote sensing of ocean winds, using the precipitation radar (PR) and tropical rainfall measurement mission (TRMM) microwave imager (TMI) that operated for >17 years on the TRMM observatory. In this study, the wind speed and anisotropic wind direction (WD) characteristics of the ocean backscatter and brightness temperature (Tb) were empirically characterized to produce geophysical model functions for 49 PR beam positions and three TMI frequency channels (10, 19, and 37 GHz). For passive TMI measurements, the modified second stokes parameter (weighted difference of V and H-pol Tb's) was used to cancel the atmosphere contribution, which enhances the WD signal. Thus, combining linearly polarized Tb measurements with the anisotropic backscatter signature of the Ku-band PR, the feasibility of retrieving ocean surface wind vector is demonstrated. The OVW retrieval algorithm is validated by comparing retrievals with collocated numerical weather models wind vectors, and the statistics of OVW retrieval accuracy is presented.

C-Band Simulations of Melting Icebergs Using GRECOSAR and an EM Model: Varying Wind Conditions at Lower Beam Mode

This article presents an electromagnetic backscatter model of iceberg and compares the modeled scattering behavior with C-band RADARSAT-2 synthetic aperture radar (SAR) images. It also explores iceberg SAR signature variability over various ocean parameters. Three-dimensional (3-D) profiles of icebergs were captured in a field study off the coast of Bonavista, NL, Canada, in June 2017 at the time of an SAR satellite overpass. The 3-D profiles were captured from a vessel, using a LiDAR and multibeam sonar. The SAR image and 3-D profiles were captured within hours of one another. Simulated SAR images of the icebergs were generated in a simulator called GRECOSAR with the satellite, target orientation, and ocean parameters that closely mimic the real SAR scene. A new ocean model was introduced to model an ocean backscatter at satellites' lower incidence angle beam mode. Comparison between real and simulated SAR images of the icebergs shows good agreement in terms of SAR signature, total radar cross section, and polarimetric decomposition. Wind direction was varied over 90° extent to observe icebergs' backscatter variability in the simulator. Furthermore, simulated SAR images were generated for low and high wind conditions. Our study finds that the macrostructure of the melt iceberg dominates its polarimetric behavior of its backscatter. Large variability of iceberg SAR signature over varying ocean parameters was also observed. A mathematical model that considers the melting condition of iceberg suggested that significant backscattering can reflect from top surface when the melt water layer could be as little as 0.1 mm.

Calibration of an airborne oceanographic lidar using ocean backscattering measurements from space.

We used satellite measurements of the optical backscattering coefficient to calibrate the signal from an airborne oceanographic lidar. This technique provided the radiometric calibration for the lidar signal and a local estimate of the ratio of the particulate backscattering coefficient, bbp, to the volume scattering function at the scattering angle of 180°, βp(180). Results using an ordinary regression, a reduced major axis regression, and a least squares bisector suggest that either of the latter two provided a better result than an ordinary regression. The statistical errors in the two recommended regressions, the difference in calibrations factors between them, and the difference between these and a laboratory calibration were all less than 5%.

A Perspective on the Performance of the CFOSAT Rotating Fan-Beam Scatterometer

The China-France Oceanography Satellite (CFOSAT) to be launched in October 2018 will carry two innovative payloads, i.e., the surface wave investigation and monitoring instrument and the rotating fan-beam scatterometer [CFOSAT scatterometer (CFOSCAT)]. Both instruments, operated in Ku-band microwave frequency, are dedicated to the measurement of sea surface wave spectra and wind vectors, respectively. This paper provides an overview of the system definition and characteristics of the CFOSCAT instrument. A prelaunch analysis is carried out to estimate the scatterometer backscatter and wind quality based on the developed CFOSCAT simulator prototype. The overall simulation includes two parts: first, a forward model is developed to simulate the ocean backscatter signals, accounting for both instrument and geophysical noise. Second, a wind inversion processor is used to retrieve wind vectors from the outputs of the forward model. The benefits and challenges of the novel observing geometries are addressed in terms of the CFOSCAT wind retrieval. The simulations show that the backscatter accuracy and the retrieved wind quality of CFOSCAT are quite promising and meet the CFOSAT mission requirements.

Ocean Backscatter Profiling Using High-Spectral-Resolution Lidar and a Perturbation Retrieval

Ocean lidar attenuation and scattering parameters were derived from a high-spectral-resolution lidar (HSRL) using two different retrieval techniques. The first used the standard HSRL retrieval, and the second used only the total backscatter channel and a perturbation retrieval (PR). The motivation is to evaluate differences between the two techniques that would affect the decision of whether to use a simple backscatter lidar or a more complex HSRL in future applications. For the data set investigated, the attenuation coefficient from the PR was an average of 11% lower than that from the HSRL. The PR estimate of the scattering parameter decreased with depth relative to the HSRL estimate, although the overall bias was zero as a result of the calibration procedure. Near the surface, the coefficient of variability in both estimates of attenuation and in HSRL estimates of scattering were around 5%, but that in the PR estimate of scattering was over 10%. At greater depths, the variability increases for all of the profile parameters. The correlation between the two estimates of attenuation coefficient was 0.7. The correlation between scattering parameters was &gt; 0.8 near the surface, but decreased to 0.4 at a depth of around 20 m. Overall, the PR performed better relative to the HSRL in offshore waters than in nearshore waters.

Comparison of C-Band Quad-Polarization Synthetic Aperture Radar Wind Retrieval Models

This work discusses the accuracy of C-2PO (C-band cross-polarized ocean backscatter) and CMOD4 (C-band model) geophysical model functions (GMF) for sea surface wind speed retrieval from satellite-born Synthetic Aperture Radar (SAR) images over in the Northwest Pacific off the coast of China. In situ observations are used for comparison of the retrieved wind speed using two established wind retrieval models: C-2PO model and CMOD4 GMF. Using 439 samples from 92 RADARSAT-2 fine quad-polarization SAR images and corresponding reference winds, we created two subset wind speed databases: the training and testing subsets. From the training data subset, we retrieve ocean surface wind speeds (OSWSs) from different models at each polarization and compare with reference wind speeds. The RMSEs of SAR-retrieved wind speeds are: 2.5 m/s: 2.11 m/s (VH-polarized), 2.13 m/s (HV-polarized), 1.86 m/s (VV-polarized) and 2.26 m/s (HH-polarized) and the correlation coefficients are 0.86 (VH-polarized), 0.85(HV-polarized), 0.87(VV-polarized) and 0.83 (HH-polarized), which are statistically significant at the 99.9% significance level. Moreover, we found that OSWSs retrieved using C-2PO model at VH-polarized are most suitable for moderate-to-high winds while CMOD4 GMF at VV-polarized tend to be best for low-to-moderate winds. A hybrid wind retrieval model is put forward composed of the two models, C-2PO and CMOD4 and sets of SAR test data are used in order to establish an appropriate wind speed threshold, to differentiate the wind speed range appropriate for one model from that of the other. The results show that the OSWSs retrieved using our hybrid method has RMSE of 1.66 m/s and the correlation coefficient are 0.9, thereby significantly outperforming both the C-2PO and CMOD4 models.

Sea Surface Radar Scattering at L-Band Based on Numerical Solution of Maxwell’s Equations in 3-D (NMM3D)

Radar scattering from ocean surfaces is investigated by 3-D numerical solution of Maxwell’s equations [numerical Maxwell’s model in 3-D (NMM3D)] using the ocean surface profiles stochastically generated from a 3-D Durden–Vesecky ocean spectrum. The surface integral equations (SIEs) are formulated for dielectric surfaces using Green’s functions of the air and the ocean permittivities with the surface tangential electric and magnetic fields as the unknowns. In solving the SIEs using the method of moment, a fast matrix solver of the sparse matrix canonical grid is used in conjunction with Rao–Wilton–Glisson basis functions. The computation has been implemented on a high-performance parallel computing cluster for problems with up to six million surface unknowns. Unlike the two-scale model (TSM) approximation, NMM3D does not require division of the surface spectrum into large- and small-scale ocean waves. The results of backscattering simulations are compared to Aquarius satellite radar measurements for wind speeds of 5, 8, and 10 m/s and for incidence angles of 29°, 39°, and 46°. The results show that NMM3D ocean backscattering solutions at L-band are in good agreement with Aquarius satellite radar data for co-polarized VV, HH, and cross-polarized VH returns as well as for the VV/HH ratio. The azimuthal dependence of L-band backscatter is also assessed. Finally, NMM3D results are compared to TSM solutions and are shown to lie close to Aquarius data in observed VV/HH ratio, and their azimuthal dependencies.

Motion compensation for high‐frequency surface wave radar on a floating platform

A method for mitigating antenna motion effects in high-frequency radar Doppler spectra developed from ocean backscatter is proposed. On the basis of the established radar cross-section models for a fixed antenna and for an antenna on a floating platform, the relationship between these models is examined. Through this relationship, motion compensation can be achieved by deconvolving the radar cross-section data with the derived transfer function. Four different deconvolution methods are illustrated and discussed in this study. Calculations involving a radar cross-section model, incorporating external noise, for an antenna on a floating platform are conducted in order to simulate field data and to examine this motion compensation method. The external noise is characterised as a white Gaussian zero-mean process. By using this newly developed radar cross-section model with external noise, motion compensation results under different sea states are examined. The outcomes indicate that an iterative Tikhonov regularisation deconvolution technique is superior to the other compensation methods implemented in this study.

An Improved Spectrum Model for Sea Surface Radar Backscattering at L-Band

L-band active microwave remote sensing is one of the most important technical methods of ocean environmental monitoring and dynamic parameter retrieval. Recently, a unique negative upwind-crosswind (NUC) asymmetry of L-band ocean backscatter over a low wind speed range was observed. To study the directional features of L-band ocean surface backscattering, a new directional spectrum model is proposed and built into the advanced integral equation method (AIEM). This spectrum combines Apel’s omnidirectional spectrum and an improved empirical angular spreading function (ASF). The coefficients in the ASF were determined by the fitting of radar observations so that it provides a better description of wave directionality, especially over wavenumber ranges from short-gravity waves to capillary waves. Based on the improved spectrum and the AIEM scattering model, L-band NUC asymmetry at low wind speeds and positive upwind-crosswind (PUC) asymmetry at higher wind speeds are simulated successfully. The model outputs are validated against Aquarius/SAC-D observations under different incidence angles, azimuth angles and wind speed conditions.