OceThe OFFiciala MaganZineog OF the Oceanographyra Spocietyhy CITATION Lehner, S., A. Pleskachevsky, D. Velotto, and S. Jacobsen. 2013. Meteo-marine parameters and their variability observed by high resolution satellite radar images. Oceanography 26(2):80–91, http://dx.doi.org/10.5670/oceanog.2013.36. DOI http://dx.doi.org/10.5670/oceanog.2013.36 COPYRIGHT This article has been published inOceanography , Volume 26, Number 2, a quarterly journal of The Oceanography Society. Copyright 2013 by The Oceanography Society. All rights reserved. USAGE Permission is granted to copy this article for use in teaching and research. Republication, systematic reproduction, or collective redistribution of any portion of this article by photocopy machine, reposting, or other means is permitted only with the approval of The Oceanography Society. Send all correspondence to: [email protected] or The Oceanography Society, PO Box 1931, Rockville, MD 20849-1931, USA. doWnloaded From http://WWW.tos.org/oceanography SPECIAL IssUE ON OCEAN REMOTE SEnsING WITH SYNTHETIC APERTURE RADAR Meteo-Marine Parameters and Their Variability Observed by High-Resolution Satellite Radar Images BY SUSAnnE LEHNER, AnDREY PLESKACHEvsKY, DoMENICO VELOTTO, AND SVEN JACobsEN TerraSAR-X ScanSAR Wide image acquired over the German Bight on March 29, 2013, at 17:11 UTC. It covers an area of 400 km × 250 km with 38 m resolution. 80 Oceanography | Vol. 26, No. 2 AbsTRACT. New radar satellites image the sea surface with resolutions as high Typical incidence angles range between as 1 m. A large spectrum of ocean processes can be estimated using such Earth 20° and 55°. Coverage and resolution observation data. These data have been applied to investigations of geophysical depend on satellite mode: stripmap processes as well as to forecast model validations and near-real-time services. The covers 30 km × 50 km with a resolu- numerous processes, parameters, and features observed in high-resolution synthetic tion of about 3 m, while spotlight covers aperture radar images include winds, waves (with wavelengths as small as 30 m), oil 10 km × 10 km with resolution of about slicks, waterline changes, changes in seabed morphology in shallow waters, wakes and 1 m (Breit et al., 2010). bow waves of ships, underwater topography, wave energy flux along wave tracks from As is known, targets that are moving deep water to the coast, and breaking waves. New algorithms have been developed to the SAR sensor will not be imaged that are capable of taking into account fine-scale effects in coastal areas. in their real positions; they are shifted in flight direction. This Doppler effect, NEW SAR MIssIons Their data provide new perspectives also called “train off the rails,” plays a For OCEAnoGRAPHIC on sea state and related processes in special role in the SAR imaging of mov- APPLICATIons coastal areas, where sea surface vari- ing waves. Compared to earlier SAR Knowledge of marine and meteorologi- ability plays a significant role. A wide missions like Envisat Advanced SAR cal parameters is important for opera- range of features and signatures can be (ASAR), TerraSAR-X offers a number tional oceanographic services. In situ observed in these data, including surface of advantages in addition to its higher measurements and global, regional, and winds and gusts, individual waves and resolution. In particular, the Doppler fine-resolution forecast models provide their refraction, and effects of breaking shift of scatterers, moving with velocity information on wind, sea state, and waves. Knowledge of such background ur toward the sensor (radial velocity) related processes. Spaceborne sensors are geophysical processes and an under- at distance Ro (slant range) is reduced. especially useful because of their global standing of how they are imaged by For example, for the same incidence –1 coverage and their independence from SAR are important to successful SAR angle of 22° and ur = 1 m s , the target’s additional input data as compared to data processing and use of the results displacement in azimuth direction • in situ methods and mathematical simu- in terms of safety and security issues. Dx = (ur /Vsar) Ro (Lyzenga et al., 1985) lations. Remote-sensing data, in par- Figure 1 shows an example of the effect is ~ 73 m for TerraSAR-X but almost ticular those acquired from spaceborne of improved resolution on imaging twice as large, ~ 115 m, for Envisat due synthetic aperture radar (SAR), are an coastal features such as breaking waves. to different platform velocity Vsar and unparalleled source for model valida- The X-band TerraSAR-X satellite slant range Ro (Envisat altitude was tion and verification in the open sea and was launched in June 2007 (http:// 800 km). Thus, the smoothing of mov- in coastal zones because these data can www.dlr.de/TerraSAR-X) and its twin, ing wave crests (also called the bunching also be collected independent of sunlight TanDEM-X, in June 2010. TerraSAR-X effect; Alpers and Rufenach, 1979) is and cloud coverage. and TanDEM-X operate from an altitude noticeably reduced. As a result, imag- Spaceborne SAR is a unique sensor of 514 km in sun-synchronous orbits, ing of the ocean surface is more stable, that provides two-dimensional informa- with ground speeds of 7 km s–1 (15 orbits and the shortest waves imaged have tion about the ocean surface. The latest- per day). The two satellites orbit in close wavelengths of ~ 25 to 30 m. generation of high-resolution SARs is formation with typical distances between particularly suitable for many ocean them of 250 to 500 m. They operate with Susanne Lehner ([email protected]), and coastal applications. In the last a wavelength of 31 mm and frequency Andrey Pleskachevsky, Domenico few years, a number of high-resolution of 9.6 GHz. The repeat cycle is 11 days, Velotto, and Sven Jacobsen are all on X-band radar satellites have been but the same region can be imaged with the staff of the German Aerospace Center launched, for example, TerraSAR-X, different incidence angles after three (DLR), Remote Sensing Technology Institute, TanDEM-X, and COSMO-SkyMed. days, depending on image latitude. Bremen, Germany. Oceanography | June 2013 81 ProCEssES AND FEATURES where storms can change the soft sea- Sea State: Individual Long ObsErvED IN SAR HIGH- bed relatively rapidly, plays a key role in Wave Refraction and REsoLUTION IMAGES erosion and transformation of the sea- Underwater Topography Knowledge of basic geophysical pro- bed and the shoreline. For example, in In coastal areas, underwater topography cesses and what remote-sensing mecha- Figure 2, a TerraSAR-X stripmap scene influences the refraction of long-period nism was used to collect the data is acquired at low tide over Elbe Estuary swells at water depths shallower than necessary for successful processing of in the North Sea on November 11, 70 to 50 m. Ocean surface wave proper- images and for use in near-real-time 2008, showing sandbanks that have ties change when water depths become services. The images contain information been partially eroded and split near less than about half of their wavelength. on wind and on sea state-related pro- tidal inlets is compared with smoother When a long-period ocean swell propa- cesses that must be properly extracted bathymetry in the same area processed gates toward shore, its wavelength short- and assessed. by BAW (German Federal Waterways ens and its wave height increases due to Engineering and Research Institute) in conservation of energy. Morphodynamic Developments 2006. These bathymetry changes can be The algorithm used to obtain in Coastal Areas observed in the SAR image as a result of swell wavelength and direction from Changes in seabed morphology in shal- the way waves propagate and disperse TerraSAR-X images is based on FFT low waters can be mapped using SAR in the estuary, as well as in the flow (Fast Fourier Transform) analy- images. Wave action in coastal areas, of local currents. sis of subscenes with dimensions of 800 m × 800 m. By computing the FFT for the selected subimage, a two- dimensional image spectrum in wave number space is retrieved. The peak in the two-dimensional spectrum deter- mines peak wavelength and peak wave direction of all waves in the subimage. The retrieved wave directions have an ambiguity of 180° due to the static nature of a SAR image. In coastal areas where wave shoaling and refraction are recog- nized, propagation direction toward the coast is unambiguous. Starting in the open sea, the box for the FFT is moved along with the wave, and a new FFT is computed. This procedure is repeated until the corners of the FFT box reach the shoreline. In this way, a wave can be tracked from the open sea to the shoreline, and changes in its wavelength and direction can be measured. Wind streaks and ocean wind patterns are removed from the spectra by filtering for analyzed wavelengths between about Figure 1. Effect of the improved resolution of the new synthetic aperture radar (SAR) satellites. Details of images from ERS-2 (1995–2011), Envisat ASAR (2002–2012), and TerraSAR-X (launched in 2007) 50 m and 300 m (background values acquired over Norderney Island in the North Sea. must be checked for every scene). The 82 Oceanography | Vol. 26, No. 2 translation of the FFT box to the next forecast by the US National Oceanic and equation 1 is obtained using a com- point in the swell propagation direction Atmospheric Administration (NOAA) bination of first guess and analysis varies in range by ± 15° in order to avoid WAVEWATCH III model (http://polar. of the tracks (Tp = 13.25 sec). The switching to another wave system in the ncep.noaa.gov/waves) was used to longest observed wave in the image is case of cross seas.