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Special Issue "Ocean Surface Currents: Progress in Remote Sensing and Validation"

A special issue of Remote Sensing (ISSN 2072-4292). This special issue belongs to the section "Ocean Remote Sensing".

Deadline for manuscript submissions: closed (30 April 2018).

Special Issue Editor

Guest Editor
Prof. Mark Bourassa

Department of Earth, Ocean and Atmospheric Science & Center for Ocean-Atmospheric Prediction Studies, Florida State University, Tallahassee, FL 32306 USA
Website | E-Mail
Phone: +1 850 644 6923 or +1 850 645 4788
Interests: air/sea interaction; boundary-layer meteorology; rainfall variability; earth observing system

Special Issue Information

Dear Colleagues,

This collection will focus on satellite and HF radar (High Frequency Radar) remote sensing of ocean surface currents, and the validation of these observations. Interest in the two-way coupling between winds and currents has grown enormously over the last decade. Ocean currents appear to play an important but neglected role in coupling the ocean and atmosphere, and recent modeling efforts suggest strong surface currents are far more plentiful than expected from earlier coarse resolution models and ocean re-analyses. Currents also greatly modify vertical motion in the upper ocean, and consequently influence vertical mixing in the upper ocean and the currents in the deep ocean. Observations of surface currents is extremely challenging, with most in situ measurements taken well below the surface, high frequency radar observations likely corresponding to depths of 1 to 2 m, and altimetry providing a time-averaged geostrophic current rather than a surface current. Theory and very limited observations indicate that there is a large difference between surface currents and currents just one meter below the surface. Concepts for simultaneous measurements of winds and currents using small modifications to available technology have been put forward in China, Europe, and the United States. The feasibility of measuring surface currents from satellite has been demonstrated through these efforts. 

Papers that address the technology development towards satellite measurements of ocean surface currents are strongly encouraged, as are results for satellite and airborne campaigns. Additionally encouraged are papers on HF radar measurements of currents, and papers on the validation of either type of current measurement.

Prof. Mark Bourassa
Guest Editor

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All papers will be peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Remote Sensing is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 1800 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • Surface Current
  • Remote Sensing
  • Air/Sea Coupling
  • Current Velocity
  • Upwelling
  • Sub-mesoscale

Published Papers (13 papers)

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Open AccessArticle
Measurement Characteristics of Near-Surface Currents from Ultra-Thin Drifters, Drogued Drifters, and HF Radar
Remote Sens. 2018, 10(10), 1633; https://doi.org/10.3390/rs10101633
Received: 8 September 2018 / Revised: 2 October 2018 / Accepted: 9 October 2018 / Published: 14 October 2018
Cited by 2 | PDF Full-text (3017 KB) | HTML Full-text | XML Full-text
Abstract
Concurrent measurements by satellite tracked drifters of different hull and drogue configurations and coastal high-frequency radar reveal substantial differences in estimates of the near-surface velocity. These measurements are important for understanding and predicting material transport on the ocean surface as well as the [...] Read more.
Concurrent measurements by satellite tracked drifters of different hull and drogue configurations and coastal high-frequency radar reveal substantial differences in estimates of the near-surface velocity. These measurements are important for understanding and predicting material transport on the ocean surface as well as the vertical structure of the near-surface currents. These near-surface current observations were obtained during a field experiment in the northern Gulf of Mexico intended to test a new ultra-thin drifter design. During the experiment, thirty small cylindrical drifters with 5 cm height, twenty-eight similar drifters with 10 cm hull height, and fourteen drifters with 91 cm tall drogues centered at 100 cm depth were deployed within the footprint of coastal High-Frequency (HF) radar. Comparison of collocated velocity measurements reveals systematic differences in surface velocity estimates obtained from the different measurement techniques, as well as provides information on properties of the drifter behavior and near-surface shear. Results show that the HF radar velocity estimates had magnitudes significantly lower than the 5 cm and 10 cm drifter velocity of approximately 45% and 35%, respectively. The HF radar velocity magnitudes were similar to the drogued drifter velocity. Analysis of wave directional spectra measurements reveals that surface Stokes drift accounts for much of the velocity difference between the drogued drifters and the thin surface drifters except during times of wave breaking. Full article
(This article belongs to the Special Issue Ocean Surface Currents: Progress in Remote Sensing and Validation)
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Open AccessArticle
Airborne Remote Sensing of the Upper Ocean Turbulence during CASPER-East
Remote Sens. 2018, 10(8), 1224; https://doi.org/10.3390/rs10081224
Received: 2 July 2018 / Revised: 31 July 2018 / Accepted: 1 August 2018 / Published: 4 August 2018
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Abstract
This study takes on the challenge of resolving upper ocean surface currents with a suite of airborne remote sensing methodologies, simultaneously imaging the ocean surface in visible, infrared, and microwave bands. A series of flights were conducted over an air-sea interaction supersite established [...] Read more.
This study takes on the challenge of resolving upper ocean surface currents with a suite of airborne remote sensing methodologies, simultaneously imaging the ocean surface in visible, infrared, and microwave bands. A series of flights were conducted over an air-sea interaction supersite established 63 km offshore by a large multi-platform CASPER-East experiment. The supersite was equipped with a range of in situ instruments resolving air-sea interface and underwater properties, of which a bottom-mounted acoustic Doppler current profiler was used extensively in this paper for the purposes of airborne current retrieval validation and interpretation. A series of water-tracing dye releases took place in coordination with aircraft overpasses, enabling dye plume velocimetry over 100 m to 10 km spatial scales. Similar scales were resolved by a Multichannel Synthetic Aperture Radar, which resolved a swath of instantaneous surface velocities (wave and current) with 10 m resolution and 5 cm/s accuracy. Details of the skin temperature variability imprinted by the upper ocean turbulence were revealed in 1–14,000 m range of spatial scales by a mid-wave infrared camera. Combined, these methodologies provide a unique insight into the complex spatial structure of the upper ocean turbulence on a previously under-resolved range of spatial scales from meters to kilometers. However, much attention in this paper is dedicated to quantifying and understanding uncertainties and ambiguities associated with these remote sensing methodologies, especially regarding the smallest resolvable turbulent scales and reference depths of retrieved currents. Full article
(This article belongs to the Special Issue Ocean Surface Currents: Progress in Remote Sensing and Validation)
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Open AccessArticle
Examining the Accuracy of GlobCurrent Upper Ocean Velocity Data Products on the Northwestern Atlantic Shelf
Remote Sens. 2018, 10(8), 1205; https://doi.org/10.3390/rs10081205
Received: 29 June 2018 / Revised: 26 July 2018 / Accepted: 30 July 2018 / Published: 1 August 2018
Cited by 2 | PDF Full-text (2421 KB) | HTML Full-text | XML Full-text
Abstract
This study provides a regional coastal ocean assessment of global upper ocean current data developed by the GlobCurrent (GC) project. These gridded data synthesize multiple satellite altimeter and wind model inputs to estimate both Geostrophic and Ekman-layer velocities. While the GC product was [...] Read more.
This study provides a regional coastal ocean assessment of global upper ocean current data developed by the GlobCurrent (GC) project. These gridded data synthesize multiple satellite altimeter and wind model inputs to estimate both Geostrophic and Ekman-layer velocities. While the GC product was mostly devised and intended for open ocean studies, the present objective is to assess whether its data quality nearer the coast is suitable for other applications. The key ground truth sources are long-term mean and time series observations on the Northwestern Atlantic (NWA) shelf derived from Acoustic Doppler Current Profilers (ADCP) and high frequency (HF) radar networks in both the Mid-Atlantic Bight (MAB) and the Gulf of Maine (GoM). Results indicate that mean geostrophic currents across the MAB and the offshore GoM agree to roughly 10% in speed and 10 degree in direction with the in situ depth-averaged currents, with correlation levels of 0.5–0.8 at seasonal and longer time scales. Interior GoM comparisons at 5 coastal buoys show much less agreement. One likely source of GoM error is shown to be the GC mean dynamic topography near the coast. Comparison to near-surface MAB HF radar current measurements on the MAB shelf shows significant GC data improvement when including the surface Ekman term. Overall, the study results imply that application of GlobCurrent data may prove useful in coastal seas with broad continental shelves such as the MAB or Scotian shelf, but that large inaccuracies inside the GoM diminish its utility there. Full article
(This article belongs to the Special Issue Ocean Surface Currents: Progress in Remote Sensing and Validation)
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Open AccessArticle
Perspective of a Ku-Ka Dual-Frequency Scatterometer for Simultaneous Wide-Swath Ocean Surface Wind and Current Measurement
Remote Sens. 2018, 10(7), 1042; https://doi.org/10.3390/rs10071042
Received: 2 May 2018 / Revised: 19 June 2018 / Accepted: 24 June 2018 / Published: 2 July 2018
Cited by 2 | PDF Full-text (5653 KB) | HTML Full-text | XML Full-text
Abstract
Ocean surface wind and current are essential ocean dynamic environment and climate variables, and simultaneous observations at high resolution have attracted considerable interest. The study on surface wind and current will improve our knowledge of energy transfer between the atmosphere and the ocean, [...] Read more.
Ocean surface wind and current are essential ocean dynamic environment and climate variables, and simultaneous observations at high resolution have attracted considerable interest. The study on surface wind and current will improve our knowledge of energy transfer between the atmosphere and the ocean, as well as the advection of heat, nutrients, and pollutants in the ocean. Since ocean surface wind and current are tightly coupled, we discuss the wind speed and direction errors effects on current, and find that current velocity is sensitive to wind speed and direction errors. Thus, it is necessary to obtain simultaneous wind field information to mitigate the effects of the wind on current retrieval. In this study, we present a Ka-Ku dual-frequency pencil-beam Doppler Scatterometer (DopScat), keeping the Ku-band for wind measurement and the Ka-band for current measurement. We establish an end-to-end simulation model to analyze the performance of the dual-frequency DopScat, and discuss the effects of satellite attitude and velocity determinations on current retrieval. The system parameters were optimized based on the simulation model. The simulation results show that the Kpc of the Ku-band DopScat is better than that of the traditional fan-beam and pencil-beam scatterometer, which is beneficial to improve wind measurement accuracy. In the Ka-band DopScat, the standard deviations (Stds) of current velocity in both along-track and cross-track directions could be smaller than 0.05 m/s, when the wind speed is larger than 5 m/s. When the wind speed is 7 m/s, the current field effective swath is 660 km, accounting for 63% of the DopScat swath, with an accuracy of 0.05 m/s. Our results indicate that the use of a Ku-Ka dual-frequency DopScat could be a feasible method for wide-swath and high-accuracy simultaneous measurements of ocean surface wind and current. Full article
(This article belongs to the Special Issue Ocean Surface Currents: Progress in Remote Sensing and Validation)
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Open AccessArticle
Low-Frequency Sea Surface Radar Doppler Echo
Remote Sens. 2018, 10(6), 870; https://doi.org/10.3390/rs10060870
Received: 20 April 2018 / Revised: 30 May 2018 / Accepted: 1 June 2018 / Published: 4 June 2018
Cited by 4 | PDF Full-text (5780 KB) | HTML Full-text | XML Full-text
Abstract
The sea surface normalized radar backscatter cross-section (NRCS) and Doppler velocity (DV) exhibit energy at low frequencies (LF) below the surface wave peak. These NRCS and DV variations are coherent and thus may produce a bias in the DV averaged over large footprints, [...] Read more.
The sea surface normalized radar backscatter cross-section (NRCS) and Doppler velocity (DV) exhibit energy at low frequencies (LF) below the surface wave peak. These NRCS and DV variations are coherent and thus may produce a bias in the DV averaged over large footprints, which is important for interpretation of Doppler scatterometer measurements. To understand the origin of LF variations, the platform-borne Ka-band radar measurements with well-pronounced LF variations at frequencies below wave peak (0.19 Hz) are analyzed. These data show that the LF NRCS is coherent with wind speed at 21 m height while the LF DV is not. The NRCS-wind correlation is significant only at frequencies below 0.01 Hz indicating either differences between near-surface wind (affecting radar signal) and 21-m height wind (actually measured) or contributions of other mechanisms of LF radar signal variations. It is shown that non-linearity in NRCS-wave slope Modulation Transfer Function (MTF) and inherent averaging within radar footprint account for NRCS and DV LF variance, with the exception of VV NRCS for which almost half of the LF variance is unexplainable by these mechanisms and perhaps attributable to wind fluctuations. Although the distribution of radar DV is quasi-Gaussian, suggesting virtually little impact of non-linearity, the LF DV variations arise due to footprint averaging of correlated local DV and non-linear NRCS. Numerical simulations demonstrate that MTF non-linearity weakly affects traditional linear MTF estimate (less than 10% for typical MTF magnitudes less than 20). Thus the linear MTF is a good approximation to evaluate the DV averaged over large footprints typical of satellite observations. Full article
(This article belongs to the Special Issue Ocean Surface Currents: Progress in Remote Sensing and Validation)
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Open AccessArticle
Estimating Ocean Vector Winds and Currents Using a Ka-Band Pencil-Beam Doppler Scatterometer
Remote Sens. 2018, 10(4), 576; https://doi.org/10.3390/rs10040576
Received: 14 March 2018 / Revised: 31 March 2018 / Accepted: 4 April 2018 / Published: 9 April 2018
Cited by 17 | PDF Full-text (31554 KB) | HTML Full-text | XML Full-text
Abstract
Ocean surface currents and winds are tightly coupled essential climate variables, and, given their short time scales, observing them at the same time and resolution is of great interest. DopplerScatt is an airborne Ka-band scatterometer that has been developed under NASA’s Instrument Incubator [...] Read more.
Ocean surface currents and winds are tightly coupled essential climate variables, and, given their short time scales, observing them at the same time and resolution is of great interest. DopplerScatt is an airborne Ka-band scatterometer that has been developed under NASA’s Instrument Incubator Program (IIP) to provide a proof of concept of the feasability of measuring these variables using pencil-beam scanning Doppler scatterometry. In the first half of this paper, we present the Doppler scatterometer measurement and processing principles, paying particular attention to deriving a complete measurement error budget. Although Doppler radars have been used for the estimation of surface currents, pencil-beam Doppler Scatterometry offers challenges and opportunities that require separate treatment. The calibration of the Doppler measurement to remove platform and instrument biases has been a traditional challenge for Doppler systems, and we introduce several new techniques to mitigate these errors when conical scanning is used. The use of Ka-band for airborne Doppler scatterometry measurements is also new, and, in the second half of the paper, we examine the phenomenology of the mapping from radar cross section and radial velocity measurements to winds and surface currents. To this end, we present new Ka-band Geophysical Model Functions (GMFs) for winds and surface currents obtained from multiple airborne campaigns. We find that the wind Ka-band GMF exhibits similar dependence on wind speed as that for Ku-band scatterometers, such as QuikSCAT, albeit with much greater upwind-crosswind modulation. The surface current GMF at Ka-band is significantly different from that at C-band, and, above 4.5 m/s has a weak dependence on wind speed, although still dependent on wind direction. We examine the effects of Bragg-wave modulation by long waves through a Modululation Transfer Function (MTF), and show that the observed surface current dependence on winds is consistent with past Ka-band MTF observations. Finally, we provide a preliminary validation of our geophysical retrievals, which will be expanded in subsequent publications. Our results indicate that Ka-band Doppler scatterometry could be a feasible method for wide-swath simultaneous measurements of winds and currents from space. Full article
(This article belongs to the Special Issue Ocean Surface Currents: Progress in Remote Sensing and Validation)
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Open AccessArticle
Proof of Feasibility of the Sea State Monitoring from Data Collected in Medium Pulse Mode by a X-Band Wave Radar System
Remote Sens. 2018, 10(3), 459; https://doi.org/10.3390/rs10030459
Received: 16 January 2018 / Revised: 27 February 2018 / Accepted: 13 March 2018 / Published: 15 March 2018
Cited by 1 | PDF Full-text (8570 KB) | HTML Full-text | XML Full-text
Abstract
X-band marine radars can be exploited to estimate the sea state parameters and surface current. However, to pursue this aim, they are set in such a way as to radiate a very short pulse to exploit the maximum spatial resolution. However, this condition [...] Read more.
X-band marine radars can be exploited to estimate the sea state parameters and surface current. However, to pursue this aim, they are set in such a way as to radiate a very short pulse to exploit the maximum spatial resolution. However, this condition strongly limits the use of radar as an anti-collision system during navigation. Consequently, a continuous change of radar scale is needed to perform both the operations of waves and current estimations and target tracking activities. The goal of this manuscript is to investigate the possibility of using marine radar working in a medium pulse mode to estimate the sea state parameters and surface current, while assuring suitable anti-collision performance. Specifically, we compare the capabilities of the X-band radar for sea state monitoring when it works in short and medium pulse modes and we present the results of a comparison based on data collected during two experimental campaigns. The provided results show that there is good agreement about the estimation of wave parameters and the surface current field that make us hopeful that, in principle, it is possible to use the medium pulse mode to achieve information about sea state with a reasonable degradation. Full article
(This article belongs to the Special Issue Ocean Surface Currents: Progress in Remote Sensing and Validation)
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Open AccessArticle
Reconstructing Large- and Mesoscale Dynamics in the Black Sea Region from Satellite Imagery and Altimetry Data—A Comparison of Two Methods
Remote Sens. 2018, 10(2), 239; https://doi.org/10.3390/rs10020239
Received: 27 December 2017 / Revised: 29 January 2018 / Accepted: 30 January 2018 / Published: 5 February 2018
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Abstract
Two remote sensing methods, satellite altimetry and 4D-Var assimilation of satellite imagery, are used to compute surface velocity fields in the Black Sea region. Surface currents derived from the two methods are compared for several cases with intense mesoscale and large-scale dynamics during [...] Read more.
Two remote sensing methods, satellite altimetry and 4D-Var assimilation of satellite imagery, are used to compute surface velocity fields in the Black Sea region. Surface currents derived from the two methods are compared for several cases with intense mesoscale and large-scale dynamics during low wind conditions. Comparison shows that the obtained results coincide well quantitatively and qualitatively. However, satellite imagery provides more reasonable results on the spatial variability of coastal dynamics than altimetry data. In particular, this is related to the reconstruction of eddy coastal dynamics, such as Black Sea near-shore anticyclones. Current streamlines in these eddies are not closed near the coast in altimetry data, which we relate to the extrapolation during mapping procedure in the absence of coastal along-track measurements. On the other hand, in offshore areas, imagery-derived currents can be underestimated due to the absence of thermal contrasts and smoothing during the procedure of the 4D-Var assimilation. Wind drift currents are another source of inconsistency, as their impact is directly observed in satellite imagery but absent in altimetry data. The advantage of the 4D-Var method for reconstructing coastal dynamics is used to compute surface currents in the Marmara Sea on the base of 250 m resolution Modis optical data. The results reveal the very complex dynamics of the basin, with a large number of mesoscale and sub-mesoscale eddies. 4D-Var assimilation of Modis imagery is used to obtain information about dynamic characteristics of these small eddies with radiuses of 4–10 km. Full article
(This article belongs to the Special Issue Ocean Surface Currents: Progress in Remote Sensing and Validation)
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Open AccessArticle
Validation of Sensing Ocean Surface Currents Using Multi-Frequency HF Radar Based on a Circular Receiving Array
Remote Sens. 2018, 10(2), 184; https://doi.org/10.3390/rs10020184
Received: 29 November 2017 / Revised: 20 January 2018 / Accepted: 23 January 2018 / Published: 26 January 2018
Cited by 2 | PDF Full-text (52141 KB) | HTML Full-text | XML Full-text
Abstract
To reduce the floor space of receiving antenna arrays, the Radio Ocean Remote SEnsing (RORSE) laboratory of Wuhan University developed a circular receiving array for a multi-frequency high frequency (MHF) radar system in 2014, consisting of seven uniformly spaced antenna elements positioned on [...] Read more.
To reduce the floor space of receiving antenna arrays, the Radio Ocean Remote SEnsing (RORSE) laboratory of Wuhan University developed a circular receiving array for a multi-frequency high frequency (MHF) radar system in 2014, consisting of seven uniformly spaced antenna elements positioned on a circle with a diameter of 5 m. The new system, which is abbreviated MHF-C radar, adopts frequency modulated interrupted continuous wave (FMICW) chirps and is capable of simultaneously operating at a maximum of four frequencies in the band of 7.5–25 MHz, and providing current, wave and wind maps every ten minutes. The phase direction-finding method is utilized to estimate the directions of the current signals, and array phase uncertainties are also taken into consideration in the signal model. This paper introduces the system in detail and investigates the performance of current measurements using MHF-C radars installed at Shengshan and Zhujiajian along the coast of the East China Sea. Radial current measurements derived from 8.27 MHz and 19.20 MHz at the same range are compared. Observations and comparisons between MHF-C radars and acoustic Doppler current profilers (ADCPs) are also presented in this paper. The results preliminarily demonstrate that the MHF-C radar system is capable of maintaining the same performance for current measurements whenever it steers to any other azimuth in the coverage and has a good ability to measure currents. Full article
(This article belongs to the Special Issue Ocean Surface Currents: Progress in Remote Sensing and Validation)
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Open AccessArticle
Sea Surface Current Estimation Using Airborne Circular Scanning SAR with a Medium Grazing Angle
Remote Sens. 2018, 10(2), 178; https://doi.org/10.3390/rs10020178
Received: 19 November 2017 / Revised: 15 January 2018 / Accepted: 15 January 2018 / Published: 26 January 2018
Cited by 1 | PDF Full-text (4186 KB) | HTML Full-text | XML Full-text
Abstract
Circular scanning synthetic aperture radar (SAR) is a novel imaging mode wherein the radar antenna rotates from 0 degrees to 360 degrees along the platform flight direction, providing us with a potentially effective technique to estimate the sea surface current velocity. In this [...] Read more.
Circular scanning synthetic aperture radar (SAR) is a novel imaging mode wherein the radar antenna rotates from 0 degrees to 360 degrees along the platform flight direction, providing us with a potentially effective technique to estimate the sea surface current velocity. In this paper, we propose a novel method to estimate the sea surface current velocity utilizing the Doppler centroid shifts of different scan angles over 360 degrees after the airborne platform motion compensation. In this method, the Doppler centroid shifts of the sea clutter at different scan angles are first extracted, and the corresponding compensation errors caused by the azimuth pointing and the incidence angle of the radar beam are considered. Finally, the least squares (LS) technique is applied to estimate the along-track velocity component and the cross-track velocity component of the sea surface current. The effectiveness of the proposed method is verified by the real data recorded by an airborne circular scanning SAR system. Full article
(This article belongs to the Special Issue Ocean Surface Currents: Progress in Remote Sensing and Validation)
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Open AccessArticle
Ocean Wind and Current Retrievals Based on Satellite SAR Measurements in Conjunction with Buoy and HF Radar Data
Remote Sens. 2017, 9(12), 1321; https://doi.org/10.3390/rs9121321
Received: 22 September 2017 / Revised: 28 November 2017 / Accepted: 13 December 2017 / Published: 15 December 2017
Cited by 3 | PDF Full-text (13086 KB) | HTML Full-text | XML Full-text
Abstract
A total of 168 fully polarimetric synthetic-aperture radar (SAR) images are selected together with the buoy measurements of ocean surface wind fields and high-frequency radar measurements of ocean surface currents. Our objective is to investigate the effect of the ocean currents on the [...] Read more.
A total of 168 fully polarimetric synthetic-aperture radar (SAR) images are selected together with the buoy measurements of ocean surface wind fields and high-frequency radar measurements of ocean surface currents. Our objective is to investigate the effect of the ocean currents on the retrieved SAR ocean surface wind fields. The results show that, compared to SAR wind fields that are retrieved without taking into account the ocean currents, the accuracy of the winds obtained when ocean currents are taken into account is increased by 0.2–0.3 m/s; the accuracy of the wind direction is improved by 3 4 ° . Based on these results, a semi-empirical formula for the errors in the winds and the ocean currents is derived. Verification is achieved by analysis of 52 SAR images, buoy measurements of the corresponding ocean surface winds, and high-frequency radar measurements of ocean currents. Results of the comparisons between data obtained by the semi-empirical formula and data measured by the high-frequency radar show that the root-mean-square error in the ocean current speed is 12.32 cm/s and the error in the current direction is 6.32°. Full article
(This article belongs to the Special Issue Ocean Surface Currents: Progress in Remote Sensing and Validation)
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Open AccessArticle
Computing Coastal Ocean Surface Currents from MODIS and VIIRS Satellite Imagery
Remote Sens. 2017, 9(10), 1083; https://doi.org/10.3390/rs9101083
Received: 26 August 2017 / Revised: 1 October 2017 / Accepted: 5 October 2017 / Published: 24 October 2017
Cited by 1 | PDF Full-text (8571 KB) | HTML Full-text | XML Full-text
Abstract
We explore the potential of computing coastal ocean surface currents from Moderate-Resolution Imaging Spectroradiometer (MODIS) and Visible Infrared Imaging Radiometer Suite (VIIRS) satellite imagery using the maximum cross-correlation (MCC) method. To improve on past versions of this method, we evaluate combining MODIS and [...] Read more.
We explore the potential of computing coastal ocean surface currents from Moderate-Resolution Imaging Spectroradiometer (MODIS) and Visible Infrared Imaging Radiometer Suite (VIIRS) satellite imagery using the maximum cross-correlation (MCC) method. To improve on past versions of this method, we evaluate combining MODIS and VIIRS thermal infrared (IR) and ocean color (OC) imagery to map the coastal surface currents and discuss the benefits of this combination of sensors and optical channels. By combining these two sensors, the total number of vectors increases by 58.3 % . In addition, we also make use of the different surface patterns of IR and OC imagery to improve the tracking performance of the MCC method. By merging the MCC velocity fields inferred from IR and OC products, the spatial coverage of each individual MCC field is increased by 65.8 % relative to the vectors derived from OC images. The root mean square (RMS) error of the merged currents is 18 cm · s 1 compared with coincident HF radar surface currents. A 5-year long time serious of merged MCC computed currents was used to investigate the current structure of the California Current (CC). Weekly, seasonal, and 5-year mean flows provide a unique space-time picture of the oceanographic variability of the CC. Full article
(This article belongs to the Special Issue Ocean Surface Currents: Progress in Remote Sensing and Validation)
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Open AccessLetter
Dual-Polarized Backscatter Features of Surface Currents in the Open Ocean during Typhoon Lan (2017)
Remote Sens. 2018, 10(6), 875; https://doi.org/10.3390/rs10060875
Received: 26 April 2018 / Revised: 26 May 2018 / Accepted: 31 May 2018 / Published: 5 June 2018
Cited by 1 | PDF Full-text (4707 KB) | HTML Full-text | XML Full-text
Abstract
Ocean surface current measurements from satellites have historically been limited. We propose a new approach to detect ocean surface currents as observed by dual-polarized (VV and VH) spaceborne synthetic aperture radar (SAR). This approach is based on the assumptions that the VH-polarized SAR [...] Read more.
Ocean surface current measurements from satellites have historically been limited. We propose a new approach to detect ocean surface currents as observed by dual-polarized (VV and VH) spaceborne synthetic aperture radar (SAR). This approach is based on the assumptions that the VH-polarized SAR signal is only generated by the effects of ocean winds creating surface waves, whereas the VV-polarization data are due to the effects of both ocean winds and surface currents. Therefore, the surface currents features may be extracted after retrieving the ocean winds from VH-polarized backscatter and inputting signal due to the wind to the VV-polarized backscatter data. To investigate the performance of this approach under extreme wind conditions, we consider a scene of C-band RADARSAT-2 dual-polarized ScanSAR images over Typhoon Lan (2017) in the open ocean, and we verify our results with current estimates from altimeter data. The ocean current features extracted from the backscatter data that were collected from the SAR images are shown to correspond to an area of strong currents and an oceanic front observed by altimeters. We suggest that the proposed method has the potential capacity to provide information about ocean surface currents from high-resolution dual-polarized ScanSAR images. Full article
(This article belongs to the Special Issue Ocean Surface Currents: Progress in Remote Sensing and Validation)
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