Scatterometer
by Mary Forsythe — last modified Sep 01, 2016 08:55 AM
Contents
Derivation
A scatterometer is a satellite radar-instrument which provides a measure of wind speed and direction near the sea surface. Scatterometers emit a microwave beam to the ocean surface at an angle. The radiation is scattered by the wind-roughened ocean surface; a small part of the emitted power is sent back in the direction of the scatterometer antenna. This is known as backscatter. By using radiation with a wavelength of a few centimetres the backscatter will be related to the size of small-scale ocean waves, which in turn are largely determined by the surface wind. The windier it is, the rougher the ocean surface and the more backscattered power is received. Moreover, the wind-generated roughness elements are aligned perpendicular to the wind and thus backscatter response depends on wind direction. By sampling the ocean surface from several azimuth directions, the collocated backscatter power measurements are used to locally retrieve the wind vector by inverting an empirical Geophysical Model Function, GMF. Scatterometer winds are collected in swaths and processed into spatial ocean wind fields.
Complications that are addressed in scatterometer wind processing include the local wind vector solution ambiguity and the sensitivity to other geophysical effects than vector winds, particularly sensitivity to rain for so-called Ku band scatterometer systems.
Further information on scatterometry can be found in Portabella, M., 2002: Wind field retrieval from satellite radar systems and Stoffelen, A.,et al., 1998: Scatterometry
http://igitur-archive.library.uu.nl/dissertations/01840669/inhoud.htm
Advice for research users
Software to produce scatterometer winds is provided by the NWP SAF - see http://research.metoffice.gov.uk/research/interproj/nwpsaf/scatterometer/index.html. The Seawinds Data Processor (SDP) and ASCAT Wind Data Processor (AWDP) have been fully operational since January 2009. SDP and AWDP are available free of charge under licence, and are distributed by the Met Office.
Real-time plots
Plots of scatterometer winds are produced in real-time. These can be useful for forecasters and as a tool for better understanding the scatterometer wind data.
Monitoring
- NWP SAF scatterometer and WindSat monitoring - real time and monthly monitoring information.
Web-based wind processing notifications
EUMETSAT OSI SAF scatterometer wind processing notifications are distributed through service messages by the Service Desk at scat@knmi.nl. Anomalies and modifications are provided by the wind product. Links are provided here:
A similar ScatSat (https://www.wmo-sat.info/oscar/satellites/view/605) notification service will be added in December 2016. All EUMETSAT user notification messages are kept on-line at the UNS (http://oiswww.eumetsat.int/uns/webapps/index.html).
NWP usage and impact
For information on NWP usage see the NWP SAF AMV web pages at http://research.metoffice.gov.uk/research/interproj/nwpsaf/scatter_report/nwp[[http://research.metoffice.gov.uk/research/interproj/nwpsaf/satwind_report/amvinfo|]]
For information on NWP impact see the winds impact study under the IWWG activities section.
Other users
- Scatterometer winds are used directly by forecasters and can be helpful in meteorologically interesting areas (e.g. tropical cyclones....) or in cases of increased NWP model uncertainty.