Côte d'Azur (France)
image shows the Côte d'Azur between Monaco in the north and Bormes in
the south. The cities are brightly imaged. well visible is Monte Carlo in the
northernmost bay. To be noted is the airport of Nice, situated on a peninsula
just south of Monaco, appearing as a distinctive dark zone. The land-part of
the image consists of the Alpes Maritimes, with some very step slopes shown
in black, because they fall into the shadow of the radar. A good example for
this are the Grand Cañon du Verdon, to be found towards the upper left
corner of the image.
The multitemporal ERS-1 SAR image is composed of the following 3 acquisitions:
- 13 Sept 1991, shown in blue,
- 19 Sept 1991 shown in green and
- 25 Sept 1991 shown in red.
All images were acquired at the same time of the day, at 10:20 GMT.
The colors in the image are linked to changes in backscatter within this period
of time. Although it is a very short period, backscatter on the Sea may change
rapidly because of the wind regime, while on the land changes of that magnitude
can not be explained by vegetation growth.
In fact the colors are explained by the change in ground humidity caused by
rain. The overall bluish color tone is due to a bad weather zone (low pressure)
including south-eastern France, Switzerland and southern Germany bringing rain
on a large scale.
This can be well observed on the METEOSAT
image of the 13 Sept 91.
However the well visible red stripes were caused on the 25 Sept 1991 by a cold
front just start to pass over the area at the time of data acquisition. The
two reddish/magenta bands (as well as a third shorter one near the image bottom)
are clear signs of a trail of showers produced shortly before ERS-1 passed.
This showers rendered the affected area much brighter, since water on the plant
augments the di-electric property and hence the backscatter.
Since the SAR data can be calibrated the actual increase of backscatter due
to rain can be quantified. However as this depends from many unknown factors
such as the amount of rain and the type of vegetation only a rough estimate
can be given. The overall appearing bluish color caused by rain on Sept 13 represent
an increase between 0.8 and 2.4 dB, and the increase within the shower-bands
was in the same order of quantities (0.8 to 2.3 dB). It seems that rougher surfaces
respond more in wet circumstances.
The METEOSAT image
shows the situation on 25 Sept 1991 between 07:00 and 10:30. Two animations
of METEOSAT are provided, the data from the visible
and from the thermal
The resolution of METEOSAT does not allow to resolve smaller clouds and thus
the individual rain cells can not be identified. However the pre-frontal weather
activity also produces a wind-front in line with the advancing clouds. It can
be seen very well off the coast, analysing the black and white SAR
image of 25 Sept.
The front divides the sea-part of the image into two, a northern part with higher
wind and a southern part with the typical texture of a unstable air-layer: the
cold air of the frontal system.
Note also the circular feature on the front-line. It is a footprint of a cumulus
or small cumulonimbus. The bright part is produced by wind down-draft and the
dark patches shower-impacts on the sea-surface smoothing the small wind waves.
One can speculate whether this is actually the rain cell that has caused some
or all of the rain strips on land; and at the moment of data acquisition was
already positioned over the sea.
(ESA/ESRIN ERS Utilization Section)
Keywords: ESA European
Space Agency - Agence spatiale europeenne,
observation de la terre, earth observation,
satellite remote sensing,
teledetection, geophysique, altimetrie, radar,
chimique atmospherique, geophysics, altimetry, radar,