OSI-SAF Wind Product

Summary

Provider: Ocean and Sea Ice SAF

Host institute : Météo-France - Centre de Météorologie Spatiale - Avenue de Lorraine - BP 50747 - 22307 LANNION CEDEX - France

Provider : KNMI scatterometer team, De Bilt, the Netherlands

Contact person: MF-CMS (send mail)

Contact for production: OSI-SAF Manager (send mail) or KNMI scatterometer team (send mail)

The ASCAT Wind Product contains measurements of the wind direction and wind speed at 10 m above the sea surface. The measurements are obtained through the processing of scatterometer data originating from the ASCAT instrument on EUMETSAT's Metop. Used for meteorology and oceanography by environmental, marine, meteorological and climate organizations and research institutions.

Wind speed is measured in m/s. Values range between 0 and 50 m/s.

Data Portal: OSI-SAF

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Platform & processing

Satellite: MetOp. Instrument: ASCAT. 12.5 km resolution. ASCAT is a real aperture radar using vertically polarised antennas.

KNMI has a processing chain running in near real-time with ASCAT data. This processor is based on the NWP-SAF software and runs in the KNMI operational environment. The processing software (AWDP) is obtainable through the software request form on the NWP-SAF scatterometer SAF: (http://research.metoffice.gov.uk/research/interproj/nwpsaf/request_forms/request_awdp_1.html). See terms and conditions for use of AWDP.

Algorithm

A scatterometer measurement relates to the ocean surface roughness while the scatterometer product represents the wind at 10 meters height over the wind vector cell. Any effect (air stability, long or gravity waves, etc.) can be fitted by a geographical model function (GMF), and is incorporated in the backscatter to wind relationship. OSI-SAF wind product uses the CMOD5.n GMF. ASCAT level 1b data from EUMETSAT. The radar beams (from two sets of three antennas) illuminate approximately 550 km-wide swaths (separated by about 700 km) as the satellite moves along its orbit, and each provides measurements of radar backscatter from the sea surface.

Backscatter values calibration (the calibration table was obtained by fitting the measurements to the theoretical GMF function).

NWP collocation (by using ECMWF Numerical Weather Prediction model → land-sea mask data, forecast windsare necessary in the ambiguity removal step of the processing).

Wind computation by GMF inversion

References are available on http://www.knmi.nl/scatterometer/publications/

Product uncertainties

Although the Level 1 product is fully calibrated now using three transponders on the ground, the backscatter values are corrected using a calibration table, derived from a geophysical wind model. The backscatter values in the Level 2 wind product are however plain copies of those in Level 1 product. The corrections are very small and expected to be incorporated into a new version of the Geophysical Model Function.

The Sigma-0 usability flag in the Level 1 product is neglected in the wind processing.

Product dissemination

The OSI-SAF data are archived in the EUMETSAT Data Centre in BUFR and NetCDF:

(http://www.eumetsat.int/Home/Main/DataAccess/EUMETSATDataCentre/index.htm).

KNMI also keeps an off line archive of the global products

ASCAT winds at 25 km and 12.5 km swath grid (L2) are distributed via GEONETCast in BUFR format.

ASCAT winds at 25 km and 12.5 km swath grid (L2) are available in BUFR and NetCDF on a password-protected FTP site at KNMI and in NetCDF format on PO.DAAC FTP site: (http://podaac.jpl.nasa.gov/DATA_CATALOG/ascatinfo.html)

100 to 180 minutes elapses between the acquisition of the data and the availability of the global product for users.

Product validation

Product is peer reviewed. Each step of the processing is validated separately by a quality control and monitoring scheme. The product validation step is controlled by visual inspection, and a statistical analysis is performed to control the validation steps.

The accuracy is validated against in-situ wind measurements from buoys, platforms, or ship, and against NWP data. The performance of the OSI-SAF products is characterised by a wind component RMS error smaller than 2 m/s and a bias of less than 0.5 m/s in wind speed

Data characteristics

Wind direction and speed for each Wind Vector Cell above the ocean

An overview of the versions are available in the ASCAT Wind Product User Manual (http://www.osi-saf.org/biblio/docs/ss3_pm_ascat_1_8.pdf). And see the latest information at http://www.osi-saf.org/

In the BUFR products, the wind direction is in meteorological (WMO) convention relative to North: 0 degree corresponds to a wind flowing to the South with a clockwise increment. In the NetCDF products, the wind direction is in oceanographic convention: 0 degree corresponds to a wind flowing to the North with a clockwise increment.

The product contains geo-located measurements triplets on a satellite swath WVC grid of 25 km or 12.5 km size.

Flags for bad data are used

ASCAT winds at 25 km and 12.5 km swath grid (L2). Spatial coverage : global. Temporal resolution : ~ twice a day for most of the locations.

Wind products are in BUFR Edition 4 or in NetCDF format. A complete description of BUFR can be found in WMO publication n°306, Manual on Codes.For NetCDF files, see the netCDF Climate and Forecast Metadata Convention version 1.4.

100 to 180 minutes for the global product, between the acquisition of the data and the availability for the users.

Data are only available above the ocean. An information is made for each product, data quality is available to the users within the products.

Notes

Useful references:

Algorithm development: KNMI scatterometer team (send mail) & EUMETSAT NWP SAF

ASCAT Wind product User Manual : http://www.osi-saf.org/biblio/docs/ss3_pm_ascat_1_8.pdf