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Côte d'Azur (France)

The 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 infrared channel.
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, atmospheric chemistry