Scatterometer wind processing software packages
Scatterometers are radar instruments that measure the radar backscatter from a part of the ocean surface. The radar backscatter depends on sea roughness which is linked to wind speed and wind direction. When the radar backscatter is measured from three or more directions, numerical inversion of the Geophysical Model Function (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 OSI SAF supplies and maintains three data processors for radar backscatter data: AWDP for ERS/ASCAT, PenWP for SeaWinds/OSCAT/HY-2A/RapidScat, and CWDP for the scatterometer on CFOSAT. Distribution of the software is done through the NWP SAF website: https://nwp-saf.eumetsat.int/site/software/scatterometer/.
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 and ScatSat-1, the HSCAT scatterometer on board HY-2A and the RapidScat scatterometer on board of the International Space Station. CWDP generates surface winds from data obtained by the scatterometer on board CFOSAT. CWDP is currently a beta version, we stimulate users to provide feedback, which will help us to improve the software in future releases. 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, PenWP and CWDP with accompanying documentation are available free of charge under licence, and are distributed by the NWP SAF, on https://nwp-saf.eumetsat.int/site/software/scatterometer/. A number of technical reports detailing some of the methods described in this overview can also be found there.