Italian experiments

Study of marine and atmospheric events
in the Mediterranean Sea using SAR

Ice
Courtesy of NASA/JPL

Department of Advanced Science and Technology ,
Amedeo Avogadro University of Eastern Piedmont, Alessandria
Research Group Coordinator Prof. Paolo Trivero

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The sea as seen by SAR

For the last quarter century, meteorology and oceanography have been making use of microwave remote-survey data from aeroplanes and satellites. As we know, the advantage of using Synthetic Aperture Radar (SAR) is the ability to work night and day in all meteorological conditions and with high spatial resolution. The Mediterranean Sea, surrounded by islands and indented coastlines, has seen advantageous use of such techniques, especially for small-scale (< 10 km) observation of marine and atmospheric phenomena and for detecting and quantifying surface pollutants.

Since SAR sends a radar beam with an oblique angle of incidence, there is a return signal only when the sea surface is rich in gravito-capillary waves (centimeter wavelengths) produced by the wind, which back-diffuse sufficient energy to produce images.

Use of SAR is particularly useful for:

a. observing and measuring wave motion and in general marine phenomena influencing sea surface roughness (currents, vortices, internal waves etc.);

b. visualizing the spatial characteristics of the boundary layer of air on the sea surface;

c. extracting the wind field on the sea at high spatial resolution;

d. detecting deliberate or accidental crude-oil spillages in the water.

Given the complexity of the phenomena arising at the sea surface, mainly of an impulsive, intermittent nature, we are still far from fully understanding the phenomena associated with the air-sea interaction. Thus the use, possible today, of SAR images of the sea cannot be separated from knowledge of the mechanisms influencing the images themselves. Resources still need to be earmarked for scientific research in this sector.

Our project

This project consists of two separate but complementary aspects.

1 Basic research on physical mechanisms influencing the radar echo

2 Application of the SAR images to meteorological and oceanographical problems like:

a. coastal meteorology (study of orographical effects on the wind);

b. atmospheric phenomena of turbulent or convective vertical transport;

c. study of the directional properties of wave motion;

d. pollution.

To reach the objectives listed above, an open-sea experiment is to be done during the SRTM mission.

BarbaraC

Agip "Barbara C" oil platform, located over an approx.
100m seabed in the central Adriatic off Ancona.

The air-sea-microwave interaction experiment will be done on board the AGIP "Barbara C" oil platform in the Adriatic Sea where scatterometric radars operating in bands S, L, C  e Ku, ), ultrasonic triaxial anemometers and Sodar Doppler phased arrays will be installed.

LSC

Coherent impulse scatterometer operating
in bands L, S e C mod. ITS-600,
specially designed to measure backscattering from the sea

KU

Coherent impulse scatterometer operating in band Ku
mod. ITS-800,
specially designed to measure backscattering from the sea.

During the Shuttle mission, simultaneous measurements will be taken of the radar backscatter from satellite and platform, wind direction, wind stress and sea and air temperatures.

The data obtained will supply time series of radar echoes, radar Doppler frequencies, wind, wind stress and heat flows. The radar Doppler frequencies will yield frequency spectra for the waves, which will be compared with those obtained from the SAR images. Directional properties of the wave motion off the island of Lampedusa will also be gathered, to study the properties of the transfer functions modulating the SAR radar echo as a function of polarization and frequency.

SAR

Detail if SAR image in band C:
1024 x 1024 bytes with approx. 4.3 m interpixel. Whiteness intensity is proportional
to radar cross-section. The centre shows the Acqua Alta platform (light area);
the two dark spots are artificial slicks (two liters of oleic alcohol poured out).

time

Examples of time series obtained from measures from platform.
Top figure: time series of radar backscatter in band C.
Middle figure: Doppler radar frequency. Bottom figure: wind speed. (ISDGM-CNR).

Depending on the environmental conditions found during the experiment, the following topics will be studied:

a. spatial structure of convective atmospheric systems over the sea (resolution > 25 m);
b. spatial structure of wind stress in any wind system (resolution > 25 m);
c. spatial properties of the wind, derived from SAR measurements at approx. 1 km resolution: atmospheric vorticity field and atmospheric vertical velocity fields (Ekman pumping);
d. validation of electromagnetic and statistical models able to describe the radar echo from the sea surface;
e. influence of wave age on wind stress and on radar echo, in deeper or shallower waters;
f. reconstruction of two-dimensional sea spectrum from SAR data, using inversion techniques implemented by our research teams.

In relation to this last point, we have tackled the problem of retrieval of wave spectra using the method of cross-spectra applied to the images in band X. The different wavelength used to acquire the images and the different acquisition geometry of the sensor are in fact parameters to be taken into account when analyzing a SAR image. It is therefore important to understand how the cross-spectra method is related to these parameters and how the environmental parameters, primarily wind velocity and direction, affect the radar response.

ERS2 - SAR
Figure 1: ERS-2 SAR image taken on 13 November 1977 over Strait of Sicily, of an area round the island of Lampedusa. This image was used to test the procedure of comparison between the classical inversion method (Hasselmann & Hasselmann) and the cross-spectra method.

With an eye to the SAR mission, a procedure has been developed to draw a comparison between the results obtained using a classical inversion method (Hasselmann & Hasselmann) and those obtained using the cross-spectra method.

Fig. 2

Figure 2: Wave height for an area of 2 x 2 degrees around the island of Lampedusa. This dataset, derived from the ECMWF as the output of a wave model (WAM), is the first of the three parameters used for the inversion procedure.

Fig. 3

Figure 3: Wind speed trend on 13 November 1997 (the direction remained constant at 300-310 ). The data were supplied by the Italian Air Force Meteorological Service.

For the figures shown here, K=n*d k was taken with d k=p /8*10-2 rad m-1. In particular, fig. 5a shows the SAR spectrum observed; fig. 5b the one obtained using the HH inversion method (with the WAM spectrum from fig. 2 used for initialization); fig. 5c shows the result obtained by applying the cross-spectra method.

Good agreement can be noted between the result obtained with the HH method, which needs a first-guess spectrum for initialization, and the one obtained with the cross-spectra method, which by contrast requires no additional information. In both cases it emerges that, by contrast with the WAM spectrum, the peak energy is such as to mask the wind system distribution. This might be due to a different value from the theoretical one to be allocated to the RAR transfer function in relation to wind speed. Study of the relationship between MTF RAR and wind speed and direction will be one of the objectives to pursue using the data collected in situ. Finally, it may be noted that the swell system is not present in the sea spectrum recovered. The reason might be the fact that the significant height (some 0.4 m in the WAM spectrum) is too low and therefore cannot be picked up by the SAR aboard the ERS-2.

Fig. 4

Figure 4: Wave spectrum for the node at 35.75N, 12.5E, including both a wind sea system and a swell moving SW. This dataset, derived from the ECMWF as the output of a wave model (WAM), is used as the third input parameter for the inversion procedure.

Fig. 5a

Figure 5a

Fig. 5b

Figure 5b

Fig. 5c

Figure 5c

Activities are also scheduled to check models for describing electromagnetic scattering from the sea surface. The actual in situ data acquired at the same time as the SAR data are of fundamental importance here. The electromagnetic models will be used to develop, verify and interpret the results obtained using inversion algorithms, to recover the sea spectrum from the one in the SAR image.

Plans are to gather satellite data useful for describing the state of the atmosphere, with particular reference to temperature, water vapor and liquid water content, the presence and characteristics of cloud systems, and, possibly, the intensity of precipitation at the surface. Data to be taken into consideration will be those from Meteosat (visible and infrared radiometer, resolution approx. 2 km), DMSP-SSM/I (7-channel microwave radiometer in the 19-85 GHz interval, resolution some 15 kilometers at the frequency of 85 GHz), DMSP-SSM/T (microwave radiometer operating in the oxygen band). The data from the radiometers will be analyzed to generate products on the state of the atmosphere using the sea data (wind, surface temperature, waves) to model the effect of the surface on the radiometric signal.

Bibliografia