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
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
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
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
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,