NWP SAF: Scatterometers

Introduction

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 observation (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.

Scatterometer data processors for ERS/ASCAT (AWDP) and SeaWinds/OSCAT/HY-2A/RapidScat (PenWP) are supplied and maintained by the NWP SAF. 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.

The SDP processor for SeaWinds and the OWDP processor for OSCAT are obsolete now, they are replaced by PenWP. Support is still available for existing users through the NWP SAF helpdesk.

The input files of AWDP are in BUFR or Product Format Specification (PFS, native MetOp) format. BUFR input may be provided using the BUFR templates for ERS or ASCAT; output is always written using the ASCAT BUFR template. Moreover, AWDP needs Numerical Weather Prediction (NWP) model winds as a first guess for the Ambiguity Removal step. These data must be provided in GRIB format.

The input files of PenWP are BUFR Level 2 data or HDF5 Level 2A data. BUFR input may be provided from PenWP output files in case of reprocessing. The output is written in BUFR using the SeaWinds and /or the KNMI BUFR templates. Moreover, PenWP needs Numerical Weather Prediction (NWP) model winds as a first guess for the Ambiguity Removal step. These data must be provided in GRIB format.

AWDP and PenWP are available free of charge under licence, and are distributed by the NWPSAF, on this website.

The current version of PenWP is v2.0 and it is available using the link at the bottom of this page. It is the first publicly released version.

The current version of AWDP is v2.3 and it is available using the link at the bottom of this page. Compared to the previous version 2.2, this version can handle the latest Metop-A ASCAT calibration (level 1 version 9.02).

See the links at the bottom of the page for instructions on how to request a copy of AWDP or PenWP. Copies of the user manuals can be downloaded below.