OCEAN APPLICATIONS
SESSION
Morning, 4 April 2001
Chair : Benjamin Holt
Co-chair : Hiroshi Kawamura
Abstract Number : CEOS-SAR01-027
Preferences : Oral
Session : Ocean Applications
Global Wind and Ocean Wave Measurements with the ENVISAT ASAR Wave Mode - A Pre-study
using Reprocessed ERS-2 SAR Data
Johannes Schulz-Stellenfleth (1), Susanne Lehner (1), Jochen Horstmann (2), Danielle Hoja (1), Mingquan Bao (1)
(1) Institute of Remote Sensing Technology, German Aerospace Center (DLR)
Oberpfaffenhofen, 82234, Wessling, Germany, Email : Johannes.Schulz-Stellenfleth@dlr.de
(2) GKSS Research Center
Max-Planck-Str., D-21502, Geesthacht, Germany, Email : Horstmann@gkss.de
Operating in wave mode the ENVISAT satellite to be launched in 2001 will provide synthetic aperture radar
(SAR) data on a global and continuous basis. ASAR (advanced SAR) wave mode data acquired every 100 km the
track will have several advanced features compared to the SAR wave mode data so far provided by the ERS
satellites. This study presents different state of the art techniques, which make use of these features in order to
obtain measurements of ocean waves and wind fields on a global scale.
To simulate ENVISAT data in the pre launch phase, ERS-2 SAR wave mode raw data kindly provided by ESA
were reprocessed to complex 10 by 5 km SAR scenes (imagettes) using the BSAR processor of the German
Aerospace Center (DLR). The ENVISAT like imagettes acquired every 200 km along the track are basis for
global measurement of ocean wave direction, waveheight and wavelength as well as wind speed and direction.
The expected performance and the processing requirements for the ENVISAT wave mode is discussed.
A test data set of three weeks of complex imagette data (about 36000 imagettes) are used to track swell systems.
Subsequent orbits which provide imagettes about 2000 km apart with a time gap of about 100 minutes are
compared to analyze the propagation of swell. The change of swell waveheight (damping) and wavelength
(nonlinear interaction) is compared to theoretical results. Propagation along great circles as predicted by theory is
checked. Temporal swell dynamics is analyzed using pairs of imagettes less than 100 km apart with a time
separation of about 12 hours.
The imagette pairs are acquired at descending and ascending orbits with different look directions with respect to
the ocean waves. They can therefore be used to analyze the impact of the relative look direction on the SAR
swell imaging process, in particular the real aperture radar (RAR) modulation. SAR measurements are compared
with ocean wave model runs as well as with buoy and altimeter data.
ERS-2 Imagettes were radiometrically calibrated to retrieve wind speeds using C-band scatterometer models.
The required information about wind direction is taken either from wind streaks visible on the imagettes or from
atmospheric models and collocated ERS-2 scatterometer (SCAT) measurements. To verify the calibration method
and the wind retrieval algorithm a global dataset of three weeks of imagettes is evaluated and compared to
collocated atmospheric ECMWF model results and ERS-2 SCAT measurements.
Case studies including extreme events like the Hurricane Fran in 1996 are presented, demonstrating the benefit of
complex wave mode data for improving ocean wave and wind field forcast.
Abstract Number : CEOS-SAR01-031
Preferences : Oral
Session : Ocean Applications
Study on L-Band Geophysical Model Function for Ocean Surface Wind Retrieval Using JERS-1/SAR
Hiroshi Kawamura (1), Teruhisa Shimada (1), and Masanobu Shimada (2)
(1) Center for Atomospheric and Oceanic Studies (CAOS), Tohoku University
Aramaki Aza Aoba, Aoba-ku, Sendai, Miyagi 980-8578, Japan, Email : kamu@ocean.caos.tohoku.ac.jp
(2) Earth Observation Research Center, National Space Development Agency of Japan (NASDA / EORC)
Roppongi First Bldg. 13F, 1-9-9, Roppongi, Minato, Tokyo 106-0032, Japan, E-mail : shimada@eorc.nasda.go.jp
Geophysical model functions to retrieve the ocean surface winds for the global oceans have been investigated
and established using satellite scatterometers, i. e., ERS1/AMI for C-band and Seasat/SASS for Ku-band. The
C-Band model function (CMOD-4) is used for the ERS1/SAR scattrometry to retrieve high-spatial resolution
winds. In the present study, we present a L-Band Geophysical Model function (LMOD-1) for the SAR
scattrometery using JERS-1/SAR data.
In order to examine backscattering characteristics of the L-band microwave at the sea surface, JERS1/SAR data
was examined together with surface winds obtained by ocean buoys and NASA Scattrometer (NSCAT) on board
ADEOS-1. Before the analyses, systematic noises of the JERS-1/SAR Normalized Radar Cross Section (NRCS)
images are eliminated. The match-up data is generated coupling the SAR NRCS/incidence angle, the relative
azimuth angle between the satellite pass and wind direction and wind speed. Seven thousands match-ups of the
SAR and buoy/NSCAT winds were collected to examine NRCS dependence on the incidence angle, the wind
speed and the relative azimuth angle.
It is found that NRCS does not depend on the incidence angle in the range of small SAR swath. The NRCS
dependence on the relative azimuth angle is clearly seen when the wind speed becomes greater than 8m/s. In the
higher wind range, the peak at the downward direction dominates the backscattering intensity. NRCS increase
with the wind speed exponentially. Using these quantitative characteristics revealed in the present study, the
L-Band Geophysical Model Function for the wind-speed retrieval is developed. It is indicated that the wind speed
obtained through this model function has an RMS error of 2.06m/s against the truth wind speed.
Abstract Number : CEOS-SAR01-041
Preferences : Oral
Session : Ocean Applications
High Resolution Wind Fields Retrieved from RADARSAT-1 ScanSAR
Jochen Horstmann (1), Wolfgang Koch (1), Susanne Lehner (2), Rasmus Tonboe (3), and Richard Bamler (2)
(1) GKSS Research Center
Max-Planck Street, D-21502, Geesthacht, Germany, Email : horstmann@gkss.de
(2) Deutsches Zentrum fur Luft und Raumfahrt (DLR)
Oberpfaffenhofen, 82234, Wessling, Germany, Email : Susanne.Lehner@dlr.de
(3) Danish Meteorological Institute
Lyngbyvej 100, DK-2001, Copenhagen, Denmark
An algorithm is presented to retrieve mesoscale wind fields from the ScanSAR data acquired aboard the
Canadian satellite RADARSAT-1, which operates at C-band (5.3 GHz) with horizontal polarization in transmit
and receive. The algorithm consists of two modules, one for wind direction and another for wind speed retrieval.
Wind directions are extracted from wind induced streaks e.g. from boundary layer rolls, Langmuir cells, or wind
shadowing, which are approximately in line with the mean wind direction. The algorithm defines wind direction
as normal to the gradient of the amplitude image, which is approximated by finite differences over an appropriate
length. The resulting wind direction is normal to the best and most frequent values of the retrieved local gradients.
Wind speeds are derived from the normalized radar cross section (NRCS) and image geometry of the calibrated
ScanSAR images, together with the local wind direction. Therefore semi empirical C-band models, e.g. CMOD4
or CMOD_IFR2, which de To improve and verify the algorithm, wind speeds were computed from several
RADARSAT-1 ScanSAR wide swath A images and compared to collocated measurements from the scatterometer
aboard ERS-2 and to results of the Danish high resolution limited area model (HIRLAM). Comparison of wind
speeds retrieved from RADARSAT ScanSAR to results of HIRLAM show the applicability of SAR images
especially in coastal regions with high wind variability. To estimate relative errors of wind speed due to
uncertainties in wind direction and NRCS, sensitivity studies were performed with respect to various settings,
including settings of the future mission of the ENVISAT ASAR. From these studies recommendations are given
concerning the best settings for C-band SARs for wind field retrieval.