Radial velocity of ocean surface current estimated from SAR Doppler frequency measurements—a case study of Kuroshio in the East China Sea
Abstract: Ocean currents are a key element in ocean processes and in meteorology, affecting material transport and modulating climate change patterns. The Doppler frequency shift information of the synthetic aperture radar (SAR) echo signal can reflect the dynamic characteristics of the sea surface, and has become an essential sea surface dynamic remote sensing parameter. Studies have verified that the instantaneous Doppler frequency shift can realize the SAR detection of the sea surface current. However, the validation of SAR-derived ocean current data and a thorough analysis of the errors associated with them remain lacking. In this study, we derive high spatial resolution flow measurements for the Kuroshio in the East China Sea from SAR data using a theoretical model of shifts in Doppler frequency driven by ocean surface current. Global ocean multi observation (MOB) products and global surface Lagrangian drifter (GLD) data are used to validate the Kuroshio flow retrieved from the SAR data. Results show that the central flow velocity for the Kuroshio derived from the SAR is 0.4–1.5 m/s. The error distribution between SAR ocean currents and MOB products is an approximate standard normal distribution, with the 90% confidence interval concentrated between –0.1 m/s and 0.1 m/ s. Comparative analysis of SAR ocean current and GLD products, the correlation coefficient is 0.803, which shows to be significant at a confidence level of 99%. The cross-validation of different ocean current dataset illustrate that the SAR radial current captures the positions and dynamics of the Kuroshio central flow and the Kuroshio Counter Current, and has the capability to monitor current velocity over a wide range of values.
Figure 2. Location of advanced synthetic aperture radar (ASAR) images and surface drifter data. The rectangles show the areas covered by the ASAR images: green and black denote images from ascending and descending orbits, respectively. The dots represent drifter data: red and blue denote data 1-h and 6-h resolutions, respectively.
Figure 8. The radial Doppler velocities from the SAR scenes on July 26, 2007 (a), and on May 28, 2008 (b) with the corresponding MOB products superimposed as arrows (arrow length indicates strength); and transects of SAR range velocity and MOB velocity on July 26, 2007 and May 28, 2008 for the Kuroshio central flow (c), and for the Kuroshio Counter Current (d).
Table 1. ENVISAT advanced synthetic aperture radar (ASAR) wide swath mode (WSM) products description
Acquisition time (UTC) Polarization Doppler centroid numbers (range by azimuth) Orbit Track Pass 2007-07-26 13:29:03 VV 100×115 28249 139 ascending 2007-11-13 01:22:38 VV 100×70 29816 203 descending 2007-11-29 01:19:42 VV 100×70 30045 432 descending 2007-12-02 01:25:27 VV 100×70 30088 475 descending 2007-12-18 01:22:36 VV 100×70 30317 203 descending 2008-01-31 01:39:52 VV 100×70 30947 332 descending 2008-05-28 01:31:19 HH 100×54 32636 017 descending 2011-08-02 13:19:29 VV 100×67 49277 125 ascending
Table 2. The RMS variation of Doppler frequency
Acquisition date for SAR scene fDc RMS for the Doppler frequency anomaly/Hz fDca fDca–fw–ferr fg July 26, 2007 97.56 17.70 16.42 14.32 November 13, 2007 109.55 15.80 14.80 13.65 November 29, 2007 104.56 10.72 10.12 9.26 December 2, 2007 110.13 15.53 14.49 13.41 December 18, 2007 113.64 16.27 14.97 13.03 January 31, 2008 112.58 9.22 8.51 5.91 May 28, 2008 101.89 12.25 11.80 10.82 August 2, 2011 89.44 18.42 16.48 14.22
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