92
W. Alpers
At this stage of the development of the eddy, the biological activity was still low such
that no or very little surface active material was secreted by the biota within the eddy
which could give rise to biogenic surface films.
4.4 Summary and Conclusions
Scatterometers and SARs are active microwave instruments which have been flown
since 1978 on several satellites. Both instruments measure over the ocean the smallscale roughness, which can be used to measure near-surface winds. While the nearsurface wind fields derived from scatterometer data have a resolution of typically
25 km, the ones derived from SAR data have a resolution of typically 200–500 m.
Examples of SAR images showing an atmospheric front, atmospheric gravity waves,
and a wind jet over African coastal waters have been presented. While the SAR can
resolve the near-surface wind field with a much higher spatial high resolution than
the scatterometer, it has the disadvantage that, from a single satellite, it can provide
near-surface wind information from a given area only with a time separation of
several days. On the other hand, one can obtain wind field maps of most areas of the
World’s ocean from scatterometer data every day or every second day.
Not only atmospheric phenomena in the marine boundary layer are detectable by
SAR, but also oceanic phenomena, like internal waves, tidal flow over shallow underwater topography, oceanic fronts, upwelling, oceanic eddies, and oil or biogenic
surface films. These oceanic phenomena are primarily encountered in coastal areas.
The atmospheric phenomena most often observed on SAR images of African coastal
waters are land/sea breeze, katabatic winds, gap winds, wind jets, and atmospheric
gravity waves. The oceanic phenomena observed most often are upwelling phenomena, internal waves, oceanic eddies, and surface film coverage (mineral oil films or
biogenic surface films). The interpretation of features visible on SAR images of the
sea surface is not always straightforward and requires some knowledge of the SAR
imaging mechanism. E.g., it is often not easy to decide whether a features visible on
a SAR image results from an atmospheric or an oceanic phenomenon. This applies
in particular to atmospheric/oceanic fronts and atmospheric/oceanic gravity waves
(Alpers and Huang 2011). Therefore, in order make optimum use of SAR data acquired over the ocean, one should combine them with other satellite data acquired in
the infrared, visible or ultraviolet bands, with in -situ data, and with results of model
calculation.
References
Abdalla S, Janssen PAFM, Bidlot JR (2010) Status of Envisat fast delivery wind wave products.
Proceedings of the ESA Living Planet Symposium held in Bergen, Norway from 28 June to 2
July 2010, ESA publication SP-686
Alpers W (1985) Theory of radar imaging of internal waves. Nature 314:245–247
W. Alpers
At this stage of the development of the eddy, the biological activity was still low such
that no or very little surface active material was secreted by the biota within the eddy
which could give rise to biogenic surface films.
4.4 Summary and Conclusions
Scatterometers and SARs are active microwave instruments which have been flown
since 1978 on several satellites. Both instruments measure over the ocean the smallscale roughness, which can be used to measure near-surface winds. While the nearsurface wind fields derived from scatterometer data have a resolution of typically
25 km, the ones derived from SAR data have a resolution of typically 200–500 m.
Examples of SAR images showing an atmospheric front, atmospheric gravity waves,
and a wind jet over African coastal waters have been presented. While the SAR can
resolve the near-surface wind field with a much higher spatial high resolution than
the scatterometer, it has the disadvantage that, from a single satellite, it can provide
near-surface wind information from a given area only with a time separation of
several days. On the other hand, one can obtain wind field maps of most areas of the
World’s ocean from scatterometer data every day or every second day.
Not only atmospheric phenomena in the marine boundary layer are detectable by
SAR, but also oceanic phenomena, like internal waves, tidal flow over shallow underwater topography, oceanic fronts, upwelling, oceanic eddies, and oil or biogenic
surface films. These oceanic phenomena are primarily encountered in coastal areas.
The atmospheric phenomena most often observed on SAR images of African coastal
waters are land/sea breeze, katabatic winds, gap winds, wind jets, and atmospheric
gravity waves. The oceanic phenomena observed most often are upwelling phenomena, internal waves, oceanic eddies, and surface film coverage (mineral oil films or
biogenic surface films). The interpretation of features visible on SAR images of the
sea surface is not always straightforward and requires some knowledge of the SAR
imaging mechanism. E.g., it is often not easy to decide whether a features visible on
a SAR image results from an atmospheric or an oceanic phenomenon. This applies
in particular to atmospheric/oceanic fronts and atmospheric/oceanic gravity waves
(Alpers and Huang 2011). Therefore, in order make optimum use of SAR data acquired over the ocean, one should combine them with other satellite data acquired in
the infrared, visible or ultraviolet bands, with in -situ data, and with results of model
calculation.
References
Abdalla S, Janssen PAFM, Bidlot JR (2010) Status of Envisat fast delivery wind wave products.
Proceedings of the ESA Living Planet Symposium held in Bergen, Norway from 28 June to 2
July 2010, ESA publication SP-686
Alpers W (1985) Theory of radar imaging of internal waves. Nature 314:245–247