Current versions: AWDP v3.1, May 2017; PenWP v2.1, February 2017; OWDP v1.1, February 2013; SDP v2.2, September 2014
Scatterometers are radar instruments that measure the radar backscatter from a part of the ocean surface. The radar backscatter depends on wind speed, wind direction, and observations (known as the geophysical model function or GMF). When the radar backscatter is measured from three or more directions, numerical inversion of the GMF yields the wind speed and direction at the ocean surface.
In general, such a procedure does not have a unique solution locally. Sophisticated software is needed to find the possible solutions by local inversion, control the quality of these solutions, and find the best unique solution for the spatial wind field.
The Ocean and Sea Ice SAF (OSI SAF) supplies and maintains two data processors for radar backscatter data. Distribution of the software packages with accompanying documentation is done through the NWP SAF. Available packages are AWDP for ERS/ASCAT and PenWP for SeaWinds/OSCAT/HY-2A/RapidScat.
These packages process radar backscatter data to wind vector field solutions featuring:
- Local representation of the full wind vector PDF based on the radar backscatter measurement input (Multiple Solution Scheme);
- State of the art quality control procedures;
- Two-dimensional variational ambiguity removal.
The wind processing packages are written in Fortran 90. AWDP generates surface winds from data obtained by the Advanced Scatterometer (ASCAT) and European Remote Sensing satellite (ERS) scatterometer instruments. PenWP generates surface winds from data obtained by the SeaWinds scatterometer on board QuikSCAT, the OSCAT scatterometer on board Oceansat-2, the HSCAT scatterometer on board HY-2A and the RapidScat scatterometer on board of the International Space Station. All processors allow the use of ambiguity removal with the Two-dimensional Variational Ambiguity Removal (2DVAR) method and they support the Multiple Solution Scheme (MSS). Their output consists of wind vectors which represent surface winds within the ground swath of the scatterometer. Normalized Radar Cross Section (NRCS) data is input by all wind processors.
AWDP and PenWP are available free of charge under licence, and are distributed by the NWPSAF, on this website.
A number of technical reports detailing some of the methods described in this overview can be found here.
In April 2011, the NWPSAF ran a Scatterometer Data Assimilation Workshop at EUMETSAT; the goal of the workshop was to provide NWP data assimilation colleagues insight in the main relevant characteristics of the scatterometer data, provide guidance for data assimilation and collect NWP user requirements. Slides, posters and training material have been made available here.