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2. Results and publications
a) to affirm that the
SAR
signal attenuation in presence of sea surface films is in
tight connection to the measured damping of the wave components in Bragg condition
[4, 5, 6, 7] and then, that the presence of sea surface films can be detected by
means of
SAR
imagery.
b) to obtain wind stress spatial distributions. It has been possible, by studying
the radar backscattering structures, to infer the wind direction and evaluate the
lifetime of the wind stress bursts that create them. Furthermore, by analyzing the
tail of the backscattering structures (connected to the Bragg wave damping), we derived the
translation velocity of the wind stress structures. Finally we designed a model for
the wind vector extraction from
SAR
imagery. This model is an improvement of a previous
one [8].
The
AIRSAR
campaign data processing demonstrated that the role of the backscattering
due to Bragg resonance in the
SAR
signal is prevalent over other backscattering mechanisms
for incidence angles between 16 and 61 degrees with
VV
polarization. From the P, L, C bands images of this campaign, by extracting the corresponding Bragg components
for each incidence angle, we obtained a wide portion of the high frequency sea spectra
inside and outside the film covered areas. We observed the same spectral trends obtained with the wave gauge. Spectral ratios obtained from remote sensing and by means
of sea truth measurements are almost coincident. This demonstrates the possibility
of detection and characterization of surface films by means of multifrequency
SAR
images [9].
These results are reported in the references 3, 7, and 9.
3. Future activity
While waiting for the 1997
SIR-C
mission we plan to perform a preliminary measurement
campaign in the Mediterranean Sea.
Aims of the campaign are the following:
a) evaluation of the amount of wind energy transferred to the sea inside and outside
the film;
b) investigation of the aspects of the energy transfer from shorter waves, directly
wind coupled, to longer ones (in Bragg condition) which are responsible for the
SAR
backscattering.
c) investigation about the damping effects in the marine environment caused by concentration
and breaking for adsorption and spreading films respectively. We also plan to make
some artificial slick and surface measurements with ground based scatterometers, high frequency wave meters and to collect surface water samples for physico-chemical
analysis.
d) improving of the performances of the L, S, C scatterometer with a new single multiband
antenna and testing of the new Ku band scatterometer. Tune-up of the new multi wire
interferential microwave probe (10 GHz) for the measurement of bi-dimensional sea
spectra in the gravito-capillary region useful for the understanding of the relation
between backscattering and wave azimuth and possible spectral frequency shifts due
to sea currents.
Such a campaign will be conducted in collaboration with several research institutions
(Univ. Torino, Univ. Firenze, CNR-Torino, CNR-Venezia).
Finally we plan a second campaign in the Mediterranean Sea during the
SIR-C
flight
with collection of all the needed sea truth data.
The use of the three-band images will also permit us to extend the research on the
wind stress. It will be possible to extract the wind speed from the comparison between
the backscattering intensities, and, thanks to the Bragg condition, to retrieve the
pattern of the high frequency part of the wave spectrum. A study about the tail of the
backscattering structures at the three bands will permit the evaluation of the translation
velocity of the atmospheric structures. The planned experimental campaign will be useful in order to match the information of the time structure with that of the
spatial structure. This study will permit us to obtain from
SAR
data the spatial
picture of the marine surface layer as well as an estimate of the fraction of area
covered by the wind stress structures at different atmospheric regimes.
References
[1] Fiscella, B., P. P. Lombardini, P. Trivero, and R., "Ripple damping on water surface
covered by a spreading film: theory and experiment," Il Nuovo Cimento, vol. 8C, no.
5, p. 491 (1985) .
[2] Lombardini, P. P., B.Fiscella, P. Trivero, C.Cappa, and W. D.Garrett, "Modulation
of the spectra of short gravity waves by sea surface films: slick detection and characterization
with a microwave probe," Journal of Atmospheric and Oceanic Technology, vol. 6, no.6, p. 882, (1989).
[3] Zecchetto, S., P. Trivero, B.Fiscella, and P.Pavese, "The turbulent structures in
the unstable marine surface layer detected by
SAR,
" submitted to Boundary Layer Meteorology.
[4] S.Zecchetto, P.Trivero, "Experiment and results of the italian activity in the field
of ocean microwave backscattering", Pacific Ocean Remote Sensing, Okinawa Japan,
Vol. I, 347, (1992).
[5] Zecchetto, S., P. Trivero, "Experimental ocean active microwave remote sensing," Satellite
Remote Sensing of the Oceanic Environment, Ed. by I.S.F.Jones, Y. Sugimori and R.
W. Stewart, Publ.by Seibutsu Kenkyusha Co Ltd, Tokyo Japan, Chap. 4, p. 115, (1993).
[6] Fiscella, B., F. Gomez, P. Pavese, P. Trivero, S.Curiotto, G. Umlieber, and S. Zecchetto,
"The Venice SAR-580 Experiment," Rapp. Int. ICG-CNR, 242/91, (1991).
[7] Trivero, P., B.Fiscella, F. Gomez, P. Pavese, and S. Zecchetto, "Microwave Remote
Sensing of Marine Surface Films from Multiband Radar," in progress.
[8] Trivero, P., B. Fiscella, F. Gomez, and P. Pavese, "Wind field deduced from marine
SAR
images," Il Nuovo Cimento C, vol. 17, no. 5, p. 689, (1994).
[9] Trivero, P., B.Fiscella, F. Gomez, and P. Pavese, "Detection and characterization
of sea surface films by means of multi-frequency
SAR,
" in progress.
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