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Since the 1960’s, meteorological satellites have been able to monitor tropical cyclones and typhoons. Their images have been acquired by passive remote sensing instruments that operate in the visible and infrared bands, where they only display the cloud-top structure of tropical cyclones and make it a challenge to study the air-sea interaction near the sea surface. On the other hand, active remote sensors, such as spaceborne microwave scatterometers and synthetic aperture radars (SARs), can “see” through clouds and facilitate observations of the air-sea interaction processes. However, SAR acquires images and provides the wind field at a much higher resolution, where the eye of a tropical cyclone at surface level can be identified. The backscattered signals received by the SAR can be processed into a high-resolution image and calibrated to represent the normalized radar cross-section (NRCS) of the sea surface. In this study, 33 RADARSAT-2 and 102 Sentinel-1 SAR images of Atlantic and Indian Ocean tropical cyclones and Pacific typhoons from 2016-2021, which display eye structure, have been statistically analyzed with ancillary tropical cyclone intensity information. To measure the size of the eye, a 34-kt contour is defined around it and the amount and size of pixels within the eye is utilized to provide its area in km2. Additionally, an azimuthal wavenumber for each shape of the eye was assigned. Results showed that eye areas increase with decreasing wind speed and increasing wavenumber and demonstrate that SAR-derived data is useful for studying tropical cyclones at the air-sea interface and provide results of these behaviors closely to data derived from best-track archives.

One of the challenges in ocean surface current retrieval from synthetic aperture radar (SAR) data is the estimation and removal of the wave-induced Doppler centroid (DC). This article demonstrates empirically the relationship between the dc derived from spaceborne X-band InSAR data and the ocean surface wind and waves. In this study, we analyzed over 300 TanDEM-X image pairs. It is found that the general characteristics of the estimated dc follow the theoretically expected variation with incidence angle, wind speed, and wind direction. An empirical geophysical model function (GMF) is fit to the estimated dc and compared to existing models and previous experiments. Our GMF is in good agreement (within 0.2 m/s) with other models and data sets. It is found that the wind-induced Doppler velocity contributes to the total Doppler velocity with about 15% of the radial wind speed. This is much larger than the sum of the contributions from the Bragg waves (~0.2 m/s) and the wind-induced drift current (~3% of wind speed). This indicates a significant (dominant) contribution of the long wind waves to the SAR dc. Moreover, analysis of dual-polarized data shows that the backscatter polarization ratio (PR=σ⁰VV/σ⁰HH) and the dc polarization difference (PD=|dcVV|-|dcHH|) are systematically larger than 1 and smaller than 0 Hz, respectively, and both increase in magnitude with incidence angle. The estimated PR and PD are compared to other theoretical and empirical models. The Bragg scattering theory-based (pure Bragg and composite surface) models overestimate both PR and PD, suggesting that other scattering mechanisms, e.g., wave breaking, are involved. In general, it is found that empirical models are more consistent with both backscatter and Doppler data than theory-based models. This motivates a further improvement of SAR dc GMFs.

17 USC 105 interim-entered record; under review. The article of record as published may be found at http://dx.doi.org/10.1175/BAMS-D-19-0281.1 The inner shelf, the transition zone between the surfzone and the midshelf, is a dynamically complex region with the evolution of circulation and stratification driven by multiple physical processes. Cross-shelf exchange through the inner shelf has important implications for coastal water quality, ecological connectivity, and lateral movement of sediment and heat. The Inner-Shelf Dynamics Experiment (ISDE) was an intensive, coordinated, multi-institution field experiment from September–October 2017, conducted from the midshelf, through the inner shelf, and into the surfzone near Point Sal, California. Satellite, airborne, shore- and ship-based remote sensing, in-water moorings and ship-based sampling, and numerical ocean circulation models forced by winds, waves, and tides were used to investigate the dynamics governing the circulation and transport in the inner shelf and the role of coastline variability on regional circulation dynamics. Here, the following physical processes are highlighted: internal wave dynamics from the midshelf to the inner shelf; flow separation and eddy shedding off Point Sal; offshore ejection of surfzone waters from rip currents; and wind-driven subtidal circulation dynamics. The extensive dataset from ISDE allows for unprecedented investigations into the role of physical processes in creating spatial heterogeneity, and nonlinear interactions between various inner-shelf physical processes. Overall, the highly spatially and temporally resolved oceanographic measurements and numerical simulations of ISDE provide a central framework for studies exploring this complex and fascinating region of the ocean. U.S. Office of Naval Research (ONR) ONR Departmental Research Initiative (DRI) Inner-Shelf Dynamics Experiment (ISDE)

The Synthetic Aperture Radar (SAR) carried on-board satellites yields invaluable data of global wave spectra since the early 1990s, with several satellites in orbit at present and more launches sch...

The main challenging problems in ocean current retrieval from along-track interferometric (ATI)-synthetic aperture radar (SAR) are phase calibration and wave bias removal. In this paper, a method based on differential InSAR (DInSAR) technique for correcting the phase offset and its variation is proposed. The wave bias removal is assessed using two different Doppler models and two different wind sources. In addition to the wind provided by an atmospheric model, the wind speed used for wave correction in this work is extracted from the calibrated SAR backscatter. This demonstrates that current retrieval from ATI-SAR can be completed independently of atmospheric models. The retrieved currents, from four TanDEM-X (TDX) acquisitions over the Oresund channel in the Baltic Sea, are compared to a regional ocean circulation model. It is shown that by applying the proposed phase correction and wave bias removal, a good agreement in spatial variation and current direction is achieved. The residual bias , between the ocean model and the current retrievals, varies between 0.013 and 0.3 m/s depending on the Doppler model and wind source used for wave correction. This paper shows that using SAR as a source of wind speed reduces the bias and root-mean-squared-error ( RMSE ) of the retrieved currents by 20% and 15%, respectively. Finally, the sensitivity of the sea current retrieval to Doppler model and wind errors are discussed.

Off the central California coast near Pt. Sal, a large-amplitude internal bore was observed for 20 h over 10 km cross shore, or 100–10-m water depth (D), and 30 km along coast by remote sensing, 39 in situ moorings, ship surveys, and drifters. The bore is associated with steep isotherm displacements representing a significant fraction of D. Observations were used to estimate bore arrival time tB, thickness h, and bore and nonbore (ambient) temperature difference ΔT, leading to reduced gravity g′. Bore speeds c, estimated from mapped tB, varied from 0.25 to 0.1 m s−1 from D = 50 to 10 m. The h varied from 5 to 35 m, generally decreased with D, and varied regionally along isobath. The bore ΔT varied from 0.75° to 2.15°C. Bore evolution was interpreted from the perspective of a two-layer gravity current. Gravity current speeds U, estimated from the local bore h and g′, compared well to observed bore speeds throughout its cross-shore propagation. Linear internal wave speeds based on various stratification estimates result in larger errors. On average bore thickness h = D/2, with regional variation, suggesting energy saturation. From 50- to 10-m depths, observed bore speeds compared well to saturated gravity current speeds and energetics that depend only on water depth and shelf-wide mean g′. This suggests that this internal bore is the internal wave analog to a saturated surfzone surface gravity bore. Along-coast variations in prebore stratification explain variations in bore properties. Near Pt. Sal, bore Doppler shifting by barotropic currents is observed.

International audience; not required. (Capsule Summary) Workshop on Doppler Oceanography from Space. What: This workshop brought together oceanographers and radar experts to discuss how new radar technology can be used in existing and future satellite missions to directly measure the motions at the ocean surface, namely currents and waves, and their relation to ocean vector winds, for a wide range of applications from sub-kilometer scales to the global ocean. Satellite remote sensing has revolutionized oceanogra-phy, starting from sea surface temperature, ocean color, sea level, winds, waves, and the recent addition of sea surface salinity, providing a global view of upper ocean processes. The possible addition of a direct measurement of surface velocities related to currents, winds and waves opens great opportunities for research and applications. Velocity can be measured using Doppler radar, using along-track interferometry with two synthetic aperture radars (InSAR) or the Doppler centroid (DC) from a single radar. Both techniques measure the same surface motions (Romeiser et al. 2014), with different resolving and revisit capabilities, summarized in Figure 1. InSAR is uniquely able to resolve kilometer-scale patterns in ocean dynam

Along Track Interferometric Synthetic Aperture Radar (ATI-SAR) is a remote sensing technique intended for measuring surface currents. As presented in [1] and [2] the current vector measurements are biased by the presence of surface winds and waves. It was shown in [3] that these biases are due to the contribution of the orbital velocity of the gravity waves (V o ) to the measured Doppler velocity by the ATI-SAR.

During three research cruises in the Pacific in 2016 and 2017, a Helmholtz-Zentrum Geesthacht rotating vertically-polarized (VV) X-band Doppler Marine Radar (DMR) was installed onboard the R/V Roger Revelle and the R/V Oceanus. Ocean currents with marine radar are typically acquired with the use of the three-dimensional fast Fourier transform (3D FFT) on a radar image sequence in order to measure the velocity of the ocean waves in comparison to the theoretical wave velocity in the absence of a current. However, dispersion shell currents cannot resolve very fine-scale current variations, such as at the interface of a current front or an internal wave. To fill this gap, the marine radar data were used in a different way. The raw data include the phase of the backscattered signal of each radar pulse, which can be converted to radial Doppler velocities. The Doppler velocity itself includes contributions from a multitude of ocean surface processes. A linear regression was set up to determine a functional relationship between Doppler velocity and surface current. The Doppler Velocity Synthesis algorithm (DoVeS) was developed to synthesize radial Doppler velocity information from a moving ship in order to obtain a two-dimensional vector surface current field around the ship track. Finally, to showcase the fine-scale capabilities of this method, an example is shown where DoVeS is able determine the small-scale variations of the Doppler velocity associated with an internal wave.

As shown at several conferences in 2018, we have developed a novel ocean wave retrieval technique based on spot-light-mode SAR data, which takes advantage of the long SAR integration time to identify moving wave patterns. In the first step, a complex spotlight-mode image is reprocessed into a series of subaperture images, which reveals motions during a time interval of a few seconds. Then individual Fourier transforms of parts of the subaperture images are computed and combined into three-dimensional wavenumber-frequency spectra, in which most of the spectral energy is found near a dispersion shell given by the theoretical dispersion relation of ocean waves. From this point, techniques known from the analysis of sequences of marine radar images or videos of wave fields can be applied to derive the most likely water depth (in coastal areas) and surface current vector from the shape of the dispersion shell. This conference presentation will focus on first examples of results and discuss the achievable accuracy and effective spatial resolution and the potential of the technique for various applications.

The near-surface response to underwater topography changes is of great importance for navigational safety near regions of strong bathymetry gradients, such as around the islands of the Republic of Palau. There, mean underwater inclines can be as much as 25% of the horizontal distance, leading to a depth change of up to 85% within 26 km. Many processes associated with oceanic flows produce surface manifestations that are readily imaged from afar by synthetic aperture radar (SAR) or ship-based X-band Doppler marine radar (DMR). SAR and DMR imagery complement each other, with SAR providing a large-scale snapshot of the region on the order of 50 km or more and the DMR offering a mobile smaller-scale look at a particular area on the order of about 6 km with the ability to measure near-surface currents (1–5 m depth) in the vicinity of the ship. In this paper, we discuss the results from an analysis of thousands of DMR images around the islands of Palau. Three types of ocean surface features were identified: internal waves, surface slicks, and convergent fronts. Internal waves and convergent fronts are directly influenced by abrupt topography, and surface slicks can aid in the surface feature imaging process if their shapes are modulated by spatially varying surface currents. These ocean surface features are examined with respect to their associations with changes in the seafloor thousands of meters beneath them.

Tidal current power is a renewable and predictable energy source that can be converted into a significant amount of useful energy. However, as tidal currents vary from site to site, it is necessary to map the tidal energy density over the chosen region, which is associated with economic feasibility. A promising technique called along-track interferometry (ATI) based in space-borne SAR is evaluated to be considered for mapping surface tidal currents and, this way, for assessment of its tidal energy resource. Under modeling environment, different conditions of ATI-SAR imaging were simulated to retrieve a known tidal current field. Environmental conditions such as ebb and flood tides, wind speeds and directions and SAR sensor parameters as incidence angle, ATI baseline between the antennas, and postprocessing filters were considered in the scenarios presented here. The retrieved current field obtained through ATI-SAR simulations were compared statistically with the initial current field. The results indicate that ATI-SAR could be used as a technique for investigating the spatial and temporal variability of surface currents. Once ATI-SAR is a remote technique, it can be relevant to identify suitable sites in the first steps of tidal energy resource assessment.