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Manaus (Amazonas)

This multitemporal SAR image is centered some 30 km north of Manaus, on 2º48'S, 59º57'W. The city of Manaus is characterized by the accumulation of bright spots (see overlay, showing its extension along the river as well as the location of the neighboring towns.
This multitemporal image is made from 3 ERS-1 SAR passes of:
- 24-Jul-92 (displayed in blue);
- 11-Dec-1992 (displayed in green); and
- 15-May-1992 (displayed in red).
The image is particularly interesting for 2 reasons:
1. The weather dependence of C-band SAR
The forest appears mainly grey, this means that there is no change in backscatter between the seasons, however a greenish tone can be recognized, suggesting that the weather on 11 December was much wetter compared with the other acquisition dates.
The magenta spots must be interpreted as areas of major attenuation of the microwave in the atmosphere due to extreme downpour of tropical rains. It seems that just north of Manaus a system made of several cumulonimbus arranged in a line are advancing towards west leaving behind the wet (greenish) tropical forest. The attenuation in the magenta area is about 2.5 dB which is in line with findings in literature, stating 1.3 dB attenuation/km in 200mm/h tropical downpoor.
On the contrary, the wetness augments the backscatter for about 0.6 dB. For comparison the changes of the backscatter elsewhere in the scene were found to be 0.15 dB, which is within radiometric stability of the ERS-1 SAR system (0.2 dB).
The SAR images were acquired at all dates at the same time: 14:17 hours. To confirm the weather situation on 11 December, the NOAA AVHRR image
can be used. The image has been acquired at 18:41 hours GMT and shows a huge thunderstorm above the Manaus area.
The weather records of Manaus Meteorological Station were as follows:
21/22-July-1992: a total of 17.5 mm rain
24 July-1992: no rain.
8/9-Dec-1992: a total of 15.8 mm rain,
10 Dec-1992: no rain,
11 Dec-1992: 31.3 mm rain.
12/15-May-1992: no rain.
2. The affluence of two rivers
Manaus lies at the affluence of the (clear water) Rio Negro which appears black and the Solimoes (colorful on the images) - sediment laden and perhaps more turbulent, where both meet a bright front is visible . This front cannot be followed downstream (as it is the case on optical images and for the eye) however the Amazon gets very bright towards the right edge of the image. It is well possible that the mixing mechanism of both waters may cause this, as a similar phenomena has been observed in SAR images from the floods of the Ganges in India.
Other features such as the colors on the "Ilha do Careiro", the island east of Manaus, might well be explained more by the water level of the river at the different dates than by seasonal changes. The green spots on Rio Negro are probably gusts from nearby thunderstorms, roughening the water locally. The linear feature on that river could be a wake of a ship on that day (11 December).
On an optical spaceborne image the forest would appear homogeneous, while observed with the SAR the geomorphology is perfectly worked out. The geologist will find different surfacetypes and landscapes and link it to soil and underlying rock type.
Due to the radar illumination from the east the north/south running highway cut into the forest can be traced. In a few places it seems to be split up in two branches.

ERS Data Utilization Section, ESA/ESRIN

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