Synergy within MARSAIS
MARSAIS combines data from SAR sensors with those acquired by either optical or thermal infrared sensors. The aim of this is a better understanding of physical and bio-physical processes in the coastal zone than could be achieved with the help of SAR analysis alone.
Upwelling - Example 1: Iberian Shelf
The figure below shows two images of the Iberian Shelf (37°N, 9°W) that were acquired on September 3, 1999, at 11:19 UTC and 04:24 UTC, respectively. Size of the images are approximately 75 x 100 square km.
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| ERS-2 SAR image acquired at 11:19 UTC. © 1999 ESA | AVHRR derived sea surface temperature (SST) image acquired at 04:24 UTC. © 1999 NOAA | |
The ERS-2 SAR image on the left-hand side shows two pronounced frontal signatures oriented parallel to the coast. The first one is located approximately 20 km offshore, the second approximately 40 km offshore. The radar backscatter in the region between the coast and these signatures is low compared to that of the surrounding waters which could be due to either a low wind speed in this region or to the presence of surface films. The increased radar backscatter towards the bottom right of the image suggests a higher wind speed in this region which gives rise to the assumption that an easterly wind direction might have caused land shadowing of the wind and thus low wind speed in the region of low backscatter.
The image on the right-hand side was acquired by the Advanced Very High Resolution Radiometer (AVHRR) and shows derived sea surface temperature (SST). While the offshore waters have a relatively uniform temperature of approximately 22°C, the SST drops to around 17°C near the coast over a distance of 30 km. This temperature drop coincides with the decrease in radar backscatter observed in the SAR image. A temperature gradient of this strength is usually maintained by a dynamic process. The combined information from both images suggests that in this case, this process is coastal upwelling which could be caused by either offshore winds (as indicated by the SAR image) and/or the general current patterns in the region. Thus, the frontal signatures in the SAR image are likely to be caused by an offshore transport of surface waters in the region up to 40 km away from the coast resulting in a convergent flow at its boundary. The low SAR backscatter within this region is likely to be caused by a combination of three factors: (1) the described wind-shadowing effect, (2) surface films due to increased biological activity in the nutrient-rich upwelled waters near the coast, and (3) decreased sea surface roughness due to the lower SST in the area.
Algorithm:
The UoS Synergy Toolkit enables joint display and analysis of SAR and other satellite data.