Satellite SAR Remote Sensing of Ocean Internal Waves
Zhou Changbao, Yang Jingsong, Huang Weigen, Fu Bin,
Shi Aiqin, Li Donglin
Second Institute of Oceanography, State Oceanic Administration
Hangzhou, P.R.China
Abstract
Internal wave is one of important ocean
mesoscale phenomena. SAR imaging
mechanism of internal wave is very complex
and its time-space distribution is of random
features. Many scientists are of great
interesting in the world on studies of SAR
detecting internal wave. In the paper, the
present situation and development of
internal wave, SAR imaging mechanism and
model and so on is introduced briefly at first.
The detection and retrieval technology of
internal wave is studied in detail. At final
the examples of internal wave from SAR
images are shown.
Introduction
Internal wave is one of important ocean
mesoscale phenomena and of large energies.
They are of the great number of destructive
power to the buildings on ocean, for
examples, marine petroleum platform. The
safety of ocean navigation is obstructed, and
acoustic propagation courses are disturbed.
Produce serious impact to the various
marine activities of population. So the
happening, growing and change of internal
waves are widely followed with interests by
scientists of different application department
in the world.
In the past, instruments deployed in the
ocean internal wave fields have been
measured. Like temperature and salinity
sensors or current meters. Or by acoustic
instruments like sonar, and then, the
manifestation of internal wave can be
captured by a variety of remote sensing
instruments, e.g., by ship radar, ground-based
radar and photographic cameral and
imaging radar of airborne. Recently, internal
wave has been detected by synthetic
aperture radar (SAR) from spaceborne for
example. ERS-1, Radarsat, SIR-C and so on.
The detection of internal wave is very
difficult by SAR due to its imaging
mechanism complexity and the random
features of time-space distribution. That the
ability of SAR to detect internal wave at any
weather (cloud cover, storm) and any faces
(day and night) as well as high resolution
provides the most advanced technique.
Alpers et al (1985, 1994), Lyzenga et al
(1988), Wandy and Chambers et al (1997),
da Silva et al (1998), Liu et al (1998) have
carried out a number of studies on SAR
internal wave imaging mechanism,
backscattering features, models, main
influence factors, measurement technique
and typical internal wave images, and so on.
Zhao jun sheng et al (1991) have studies
internal wave features of China Yellow Sea
with routine observation. Changbao Zhou
and Weigen Huang (1994, 1996, 1998,
1999) have explored them and all the
research make new considerable progresses.
SAR imaging mechanisms and detection
model of internal wave
The research results indicate that SAR is an excellent fool to detect internal wave and to
estimate solution wavelengths with a good
degree of precisian.
Ocean internal waves are often visible in
SAR images. The manifestations of internal
waves can be summarized as: (a) Their
propagation in wave groups or packets with
four to ten crests per-groups and forward
offshore direction. (b) The crests and trough
are often parallel to the bottom topography
or else radiate out as if from a source point
or region. Wavelength between right and
dark bands is about 200m to 1600m. (c) The
separate groups of wave are typically tens to
a hundred kilometers apart. (d) The crests
(or surface manifestation of a constant-phase
line) are usually tens to hundreds of
kilometers long and very often the lengths of
crests (as revealed on image) decrease
forward the rear of wave group. (e) These
internal waves appear either as dark in a
right background (presumably under rough-sea
conditions). As right in a dark
background (calmer conditions), or as dark
and right bands in the intermediate case,
suggesting the internal wave can be imaged
over a broad range of wind conditions.
It is known, internal waves come in many
sizes, shapes, and frequencies, and it is only
small subset of these, which appear to be
imaged.
SAR imaging of internal wave is attributed
to variation in the short-scale surface
roughness induced interaction of ocean
currents or tidal flow with abrupt
topographic features.
SAR is a sensitive surface roughness sensor.
The higher the roughness, the higher is the
radar return and the brighter is the image
intensity. Internal wave forms are associated
with rough and smooth bands and usually
appears as bright and dark bands in the
images.
According to the first order radar imaging
theory (Alpers, 1985), relative variation of
the normalized radar cross section (NRCS)
associated with internal waves,
D s/ s0, is
linearly related to the gradient of the surface
velocity of the surface convergence:
Where A denotes a positive function that
depends on radar wavelength, incidence
angle and surface wind velocity. For a linear
SAR system,
D s/ s0 is equal to the
relative variation of the SAR image intensity
DI/I
0. Thus the variation of SAR image
intensity as proportional to gradient of the
surface wind velocity
