OSI SAF 25 km wind product (OSI-151), 50 km wind product (OSI-151) Wind Data Record Product User Manual

Ocean and Sea Ice SAF
SeaWinds wind Climate Data Record
Product User Manual
25 and 50 km wind products (OSI-151)
Version 1.3, March 2015
SAF/OSI/CDOP2/KNMI/TEC/MA/220
SeaWinds wind Climate Data Record Product User Manual
DOCUMENT SIGNATURE TABLE
Name
Date
Prepared by:
O&SI SAF Project
Team
Mar 2015
Approved by:
O&SI SAF Project
Manager
Mar 2015
Signature
DOCUMENTATION CHANGE RECORD
Issue / Revision
Date
Change
Description
Version 1.0
Nov 2014
Version 1.1
Nov 2014
Minor
Version for DRR
Version 1.2
Jan 2015
Minor
Changes according to RIDs on DRR
Version 1.3
Mar 2015
Minor
Updated results after new reprocessing
First version
KNMI, De Bilt, the Netherlands
Reference: SAF/OSI/CDOP2/KNMI/TEC/MA/220
Cover illustration: QuikSCAT wind field retrieved in the Indian Ocean at 25 km WVC spacing on 8
January 2008, approximately 3:00 UTC, overlaid on a Meteosat infrared satellite image. The orange
dots are rejected WVCs; the locations where heavy precipitation is likely to occur (bright patches in the
infrared cloud pattern) clearly correspond to locations where a large fraction of the winds are rejected.
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Contents
1.
1.1.
1.2.
1.3.
1.4.
2.
3.
3.1.
3.2.
3.3.
3.4.
4.
5.
5.1.
5.2.
6.
7.
8.
9.
10.
Introduction ..............................................................................................................................4
Overview ................................................................................................................................... 4
Disclaimer.................................................................................................................................. 4
Useful links ................................................................................................................................ 5
Limitations and remaining issues .............................................................................................. 5
The SeaWinds scatterometer ..................................................................................................6
Processing scheme .................................................................................................................8
Backscatter egg averaging........................................................................................................ 9
Backscatter calibration .............................................................................................................. 9
NWP collocation ........................................................................................................................ 9
Quality control and monitoring ................................................................................................ 10
Helpdesk and data availability ............................................................................................. 11
Data description .................................................................................................................... 12
Wind product characteristics ................................................................................................... 12
File formats.............................................................................................................................. 12
References ............................................................................................................................. 15
Abbreviations and acronyms ............................................................................................... 16
Appendix A: BUFR data descriptors ................................................................................... 17
Appendix B: NetCDF data format ........................................................................................ 20
Appendix C: Data gaps ......................................................................................................... 22
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1. Introduction
1.1.
Overview
The EUMETSAT Ocean and Sea Ice Satellite Application Facility (OSI SAF) produces a range of airsea interface products, namely: wind, sea ice characteristics, Sea Surface Temperatures (SST) and
radiative fluxes, Surface Solar Irradiance (SSI) and Downward Long wave Irradiance (DLI). The
Product Requirements Document [1] provides an overview of the committed products and their
characteristics in the current OSI SAF project phase, The Service Specification Document [2] provides
specifications and detailed information on the services committed towards the users by the OSI SAF in
a given stage of the project.
KNMI is involved in the OSI SAF as the centre where the level 1 to level 2 scatterometer wind
processing is carried out. This document is the Product User Manual to the SeaWinds wind climate
data record. More general information on the OSI SAF project is available on the OSI SAF web site:
http://www.osi-saf.org/. The user is strongly encouraged to register on this web site in order to receive
the service messages and the latest information about the OSI SAF products. More Information about
this product can also be found on http://www.knmi.nl/scatterometer/.
The scatterometer is an instrument that provides information on the wind field near the ocean surface,
and scatterometry is the knowledge of extracting this information from the instrument’s output. Spacebased scatterometry has become of great benefit to meteorology and climate in the past years. This is
extensively described in the Algorithm Theoretical Baseline Document, see [3].
KNMI has a long experience in scatterometer processing and is developing generic software for this
purpose. Processing systems have been developed for the ERS, NSCAT, SeaWinds, ASCAT,
Oceansat-2 and RapidScat scatterometers. Scatterometer processing software is developed in the
EUMETSAT Numerical Weather Prediction Satellite Application Facility (NWP SAF), whereas wind
processing is performed operationally in the Ocean and Sea Ice SAF (OSI SAF).
The complete QuikSCAT SeaWinds level 2a data record, spanning the period of 19 July 1999 to 22
November 2009 was obtained from the Physical Oceanography Distributed Active Archive Center
(PO.DAAC) at the NASA Jet Propulsion Laboratory (http://podaac.jpl.nasa.gov/). The data have been
processed using the SeaWinds Data Processor (SDP) software version 2.2, as available in the
Numerical Weather Prediction (NWP) SAF [4]. The OSI SAF Climate Data Records (CDRs) can be
obtained from the EUMETSAT Data Centre.
There are three main justifications for reprocessing the SeaWinds data record. Firstly, the archived
OSI SAF near-real time products do not span the complete 10 years of SeaWinds operations.
Secondly, the wind retrievals were improved in the sense that calibration was applied to reduce wind
speed biases w.r.t. buoys and that the processing was done using the latest knowledge on wind
processing. Thirdly, the ambiguity removal and product monitoring are done now using the ECMWF
re-analysis (ERA) Interim winds rather than the ECMWF operational winds. The ERA-Interim winds
are much more uniform over time than the operational winds.
This user manual outlines user information for the OSI SAF SeaWinds Wind CDRs on 25 km and
50 km grid spacing, OSI-151. Section 2 presents a brief description of the SeaWinds instrument, and
section 3 gives an overview of the data processing configuration. Section 4 provides details on how to
access the products. Detailed information on the file content and format is given in section 5. The
product quality is elaborated in the validation report to this CDR [5].
1.2.
Disclaimer
All intellectual property rights of the OSI SAF products belong to EUMETSAT. The use of these
products is granted to every interested user, free of charge. If you wish to use these products,
EUMETSAT's copyright credit must be shown by displaying the words "copyright (year) EUMETSAT"
on each of the products used.
The OSI SAF is much interested in receiving your feedback, would appreciate your acknowledgment
in using and publishing about the data, and like to receive a copy of any publication about the
application of the data. Your feedback helps us in maintaining the resources for the OSI SAF wind
services.
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1.3.
SeaWinds wind Climate Data Record Product User Manual
Useful links
KNMI scatterometer web site: http://www.knmi.nl/scatterometer/
Information on OSI SAF activities at KNMI: http://www.knmi.nl/scatterometer/osisaf/
OSI SAF wind product documentation on http://www.osi-saf.org/
NWP SAF website: http://nwpsaf.eu/
EUMETSAT Data Centre:
http://www.eumetsat.int/website/home/Data/DataDelivery/EUMETSATDataCentre/index.html
1.4.
Limitations and remaining issues
We plan to attach a Digital Object Identifier (DOI) and associated landing page to these data sets. Due
to delays in the signature of the DOI contract, a DOI could not yet be established for these CDRs.
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2. The SeaWinds scatterometer
The SeaWinds scatterometer is carried on-board the QuikSCAT polar satellite. It was launched on 19
June 1999 into a sun-synchronous orbit with an inclination of 98.6° and a repeat cycle of 4 days (14.25
orbits per day). The local sun time at ascending node is 6:00 AM. The QuikSCAT mission was a ‘quick
recovery’ mission from the National Aeronautics and Space Administration (NASA) to fill the gap
created by the loss of data from NSCAT, when the ADEOS-1 satellite lost power in June 1997. The
QuikSCAT nominal mission ended on 23 November 2009 due to problems with the SeaWinds
instrument antenna spinning mechanism. A similar version of the instrument (SeaWinds-2) flew on the
Japanese ADEOS-2 satellite, launched in December 2002, which was regrettably lost in October
2003. For detailed information on the instrument and data we refer to [6] and [7]. A brief description is
given below.
The SeaWinds instrument is a conically scanning pencil-beam scatterometer. It uses a 1-meter dish
antenna rotating at 18 rpm with two “spot” beams of about 25 km × 55 km size on the ground, a
horizontal polarisation beam (HH) and a vertical polarisation beam (VV) at incidence angles of 46º and
54º, respectively, that sweep the surface in a circular pattern as depicted in Figure 1. Note that the
egg-shaped beam footprints are divided into slices by applying a modulated chirp signal. Due to the
conical scanning, a WVC is generally viewed when looking forward (fore) and a second time when
looking aft. As such, up to four measurement classes (called “beam” here) emerge: HH fore, HH aft,
VV fore, and VV aft, in each wind vector cell (WVC). The 1800 km-wide swath covers 90% of the
ocean surface in 24 hours and represents a substantial improvement compared to side-looking
scatterometers like ERS, NSCAT and ASCAT.
Figure 1: Earth surface coverage of the scans of the horizontal (HH) and vertical polarisation
(VV) pencil-beams of SeaWinds. As the satellite propagates towards the top of the page the
swath (in grey) is illuminated, and three areas are discriminated:
I: Outer swath: only viewed once by the VV beam in the forward direction, and once in the aft
direction (2 views);
II: Sweet (inner) swath: Viewed both by the VV and HH beam, both in fore and aft direction (4
views);
III: Nadir (inner) swath: As II, but the azimuth view direction is close to the satellite propagation
direction, or just opposite to it.
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SeaWinds wind Climate Data Record Product User Manual
On the other hand, the wind retrieval from SeaWinds data is not trivial. In contrast with the side-looking
scatterometers, the number of measurements and the beam azimuth angles vary with the sub-satellite
cross-track location. The wind retrieval skill will therefore depend on the position in the swath. A
detailed discussion is provided in [3]. Here we only summarise some issues specific to SeaWinds.
In the outer swath (where only VV beam data are available), the individual backscatter measurements
(slices or eggs) contributing to the VV fore or aft beam in a specific WVC are re-distributed to form four
more or less independent backscatter observations. Slices or eggs are accumulated and averaged
based on their azimuth angles. The outer swath winds have slightly reduced quality but they are still
very well usable. These winds are flagged in the product and can be filtered out easily if requested,
see section 5.2.
The SeaWinds scatterometer operates at a Ku-band radar wavelength (13.5 GHz). The atmosphere is
not transparent at these wavelengths and in particular rain is detrimental for wind computation. In fact,
moderate and heavy rain cause bogus wind retrievals of 15-20 m/s wind speed which need to be
eliminated by a quality control step. Wind-rain discrimination is easiest to manage in the sweet swath,
but still performs acceptably in nadir and outer swath.
Due to the availability of VV and HH polarisation measurements, discrimination of water and ice
surfaces is generally well possible [8]. The SeaWinds Bayesian ice screening algorithm as
implemented in SDP was used when creating the CDRs.
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SeaWinds wind Climate Data Record Product User Manual
3. Processing scheme
Figure 2 shows the system architecture to generate the wind data sets. The processing environment
consists of a set of software components to do data conversions, collocate scatterometer data with
ECMWF model data, to generate the wind data and to convert the output BUFR data into level 2
(swath) NetCDF data and level 3 (gridded to a regular lat/lon grid) NetCDF data. General information
about the scatterometer wind processing algorithms can be found in the Algorithm Theoretical Basis
Document (ATBD) [3].
PO.DAAC
archive
SeaWinds L2A
ECMWF MARS
archive
ERA Interim
FTP
FTP
KNMI
workstation
h4toh5
l2a to bufr
snr
SDP
Bufr2Grid
BUFR
L3
Bufr2XmlNc
BUFR
NetCDF
KNMI
archive
External
archive(s)
Figure 2: System architecture of reprocessing chain
The following components are shown in Figure 2.
•
SDP is the wind processing software for SeaWinds data. It is publicly available in the NWP SAF,
see [4].
•
h4toh5 is a tool available from the HDF Group which converts the SeaWinds level 2a data from
HDF4 into HDF5 format.
•
l2a_to_bufr is a tool which was built to convert SeaWinds level 2a data from HDF5 into BUFR
format. SDP cannot handle HDF5 data, only BUFR.
•
snr (SeaWinds NWP Replacer) is a program to collocate ECMWF model winds with scatterometer
data. This functionality is not available in SDP. snr was used in the operational near-real time OSI
SAF wind processing until the end of the QuikSCAT mission in 2009.
•
Bufr2XmlNc is a program to convert BUFR scatterometer data into level 2 NetCDF data. It is
currently used in the near-real time OSI SAF processing.
•
Bufr2Grid is a program to convert BUFR scatterometer data into level 3 NetCDF data. Two daily
files are produced containing the ascending and descending parts of the orbits, respectively. It is
currently used in the near-real time data processing for MyOcean.
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3.1.
SeaWinds wind Climate Data Record Product User Manual
Backscatter egg averaging
The level 2a backscatter data from JPL are organised in eggs, each σ is based on the sum of the
echo energies measured among the eight centre high resolution slices in a single scatterometer pulse.
The eggs are beam-wise accumulated to a WVC level by program l2a_to_bufr before wind inversion is
done. The egg weights are proportional to the estimated transmitted power contained in an egg, i.e.,
inversely proportional to the Kp value. The Sigma0 Quality Flag present in the level 2a data is
evaluated and egg data with one of the following flags set are skipped:
0
•
Bit 0: Measurement not usable
•
Bit 1: Signal to Noise Ratio level low
•
Bit 3: Data outside acceptable range
•
Bit 4: Pulse quality unacceptable
•
Bit 5: Location algorithm does not converge
•
Bit 6: Frequency shift beyond range
•
Bit 7: Temperature outside range
•
Bit 8: No applicable attitude records
•
Bit 9: Ephemeris data unacceptable
3.2.
Backscatter calibration
No absolute instrumental instrument calibration exists for Ku-band pencil-beam scatterometers. Kuband pencil-beam backscatter distributions should however be matched to achieve wind
intercalibration of all space-borne scatterometer instruments. We thus developed methods that
calibrate the winds of each scatterometer effectively to the mean winds at collocated moored buoys.
No signs of azimuth (or WVC) dependent instrument biases have been found for QuikSCAT. Also the
beam incidence angles are constant and hence we have chosen to apply backscatter corrections that
are only dependent on the beam polarisation. The goal of applying backscatter corrections was to
remove any wind speed biases between scatterometer winds and buoy winds over the year 2008. The
year 2008 was chosen as reference year since in 2008 both SeaWinds and ASCAT data were
available and cross calibration can easily be done. With this in mind, the calibration corrections have
been obtained such that we get calibration coefficients close to 1.0 from the triple collocation analysis
over 2008 [5]. The ratio between HH and VV corrections was obtained such that we get uniform wind
speed biases against ERA-Interim winds over the swath both in the regions with HH and VV
backscatter data as well as in the regions with VV-only backscatter data (outer swath). In this simple
way, within a few iterations the two calibration coefficients could be obtained:
•
For HH, 0.25 dB has been added in the 25 km products and 0.18 dB in the 50 km products
•
For VV, 0.22 dB has been added in the 25 km products and 0.15 dB in the 50 km products
The constant (WVC independent) σ correction used, yields an average wind speed bias of -0.16 m/s
against the buoy winds over the year 2008 for the 25 km product and -0.13 m/s for the 50 km product.
The wind speed biases against ERA-Interim winds are 0.0 m/s over the year 2008 both for the 25 km
and the 50 km products [5]. Note that the calibrated backscatter values are only available within the
0
wind processing software; the σ data in the BUFR wind product are uncorrected values.
0
3.3.
NWP collocation
NWP forecast wind data are necessary in the ambiguity removal step of the processing. The
scatterometer winds have been collocated with ERA-Interim wind data from ECMWF [9]. Equivalent
neutral (U10N) winds have been computed from the real ERA-Interim forecast 10m winds, sea surface
temperature, air temperature, Charnock parameter and specific humidity, using a stand-alone
implementation of the ECMWF model surface layer physics [10]. Wind forecasts are available twice a
day (00 and 12 GMT analysis time) with forecast time steps of +3h, +6h, …, +18h. The model wind
vector component data have been quadratically interpolated with respect to time and linearly
interpolated with respect to location and put into the level 2 information part of each WVC.
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3.4.
SeaWinds wind Climate Data Record Product User Manual
Quality control and monitoring
In each WVC, the σ data is checked for quality and completeness and the inversion residual [3] is
checked. Degraded WVCs are flagged; see section 5.2 for more details.
An information file is made for each product. The content of the file is identical whatever the product
and results from a compilation of all the global information concerning this product. From these files,
various graphs have been produced to visually display the confidence levels of the products and their
evolution with time. Any deviations from nominal behaviour would be immediately visible as steps in
these graphs. Data quality is also available to the users within the products; see section 5 for a
description of quality flags.
0
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4. Helpdesk and data availability
For a swift response management procedure, user requests on the OSI SAF data products should be
issued at the Ocean and Sea Ice SAF website (http://www.osi-saf.org/). You can also send an email to
[email protected].
A BUFR reader is available at www.knmi.nl/scatterometer/bufr_reader/.
The products are available (after registration) from the EUMETSAT Data Centre,
The
http://www.eumetsat.int/website/home/Data/DataDelivery/EUMETSATDataCentre/index.html.
data sizes for the entire data set and per orbit file are listed in the table below. There are 14.25
QuikSCAT orbits per day, the repeat cycle is 4 days (57 orbits).
Product
25 km BUFR
25 km NetCDF (g-zipped)
50 km BUFR
50 km NetCDF (g-zipped)
Size of one orbit file
11 MB
1.1 MB
1.8 MB
0.3 MB
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Size of 10 years data record
600 GB
60 GB
100GB
17 GB
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SeaWinds wind Climate Data Record Product User Manual
5. Data description
5.1.
Wind product characteristics
Physical definition
Horizontal equivalent neutral wind vector at 10 m height, obtained using the NSCAT-4 GMF, see [3].
Units and range
Wind speed is measured in m/s. The wind speed range is from 0-50 m/s, but wind speeds over 25 m/s
are generally less reliable [3]. In the BUFR products, the wind direction is in meteorological (World
Meteorological Organisation, WMO) convention relative to North: 0 degrees corresponds to a wind
flowing to the South with a clockwise increment. In the NetCDF products, the wind direction is in
oceanographic convention: 0 degrees corresponds to a wind flowing to the North with a clockwise
increment.
Input satellite data
The QuikSCAT level 2a input data [7] were obtained from PO.DAAC. These ‘Surface Flagged
0
Sigma0s and Attenuations in 25 km Swath Grid’ data are so-called egg data, each σ is based on the
sum of the echo energies measured among the eight centre high resolution slices in a single
scatterometer pulse. The product contains geo-located backscatter measurements on a satellite swath
WVC grid of 25 km size.
Geographical definition
The QuikSCAT satellite flies in a near-polar sun synchronous orbit at 98 degrees inclination at
approximately 800 km orbit height. Swath width is 76 25 km size WVCs or 38 50 km size WVCs.
Products are organised in files containing one orbit, starting at the South Pole.
Output product
The input product in HDF5 is processed into a BUFR output product including a unique wind solution
(chosen), its corresponding ambiguous wind solutions and quality information (distance to cone,
quality flag). The products are also available in NetCDF format; see section 9 for more details.
Expected accuracy
The expected accuracy is defined as the expected bias and standard deviation of the primary
calculations. The accuracy is validated against in situ wind measurements from buoys, and against
NWP data. Even better, the errors of all NWP model winds, in situ data, and scatterometer winds are
computed in a triple collocation exercise [14]. The performance is pretty constant over the globe and
depends mainly on the sub footprint wind variability. The performance of the products issued by the
OSI SAF is characterised by a wind component standard deviation smaller than 2 m/s and a bias of
less than 0.5 m/s in wind speed. More validation information is available in [5], showing that the actual
products are much more accurate.
5.2.
File formats
Wind products are in BUFR Edition 4 or in NetCDF format. A complete description of BUFR can be
found in WMO publication No 306, Manual on Codes.
The OSI SAF wind product is stored in exactly the same BUFR format as described in the SeaWinds
BUFR manual from NOAA [13], a list of descriptors (fields) contained in each WVC is provided in
section 8. Data are organised in files containing approximately one orbit (100 minutes) of data.
The file name convention for the level 2 BUFR product is
seawi_YYYYMMDD_HHMMSS_qscat__ORBIT_T_SMPL_CONT_l2.bufr
•
YYYYMMDD denotes the acquisition date (year, month and day) of the first data in the file
•
HHMMSS denotes the acquisition time (hour, minute and second) of the first data in the file
•
ORBIT is the orbit number of the first data in the file (00000-99999)
•
T is the processing type (o for operational, t for test)
•
SMPL is the WVC sampling (cell spacing): 250 for the 25 km and 500 for the 50 km product
•
CONT refers to the product contents: always ovw for a product containing Ocean Vector Winds
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An example of a file name is
seawi_20081231_094032_qscat__49654_o_250_ovw_l2.bufr.
In each node or wind vector cell (WVC) 118 data descriptors are defined. In addition some extra
information/alterations have been put in place:
•
In the BUFR header the value for “generating centre” is set to 99, representing KNMI.
•
The products contain up to four ambiguous wind solutions, with an index to the selected wind
solution. After the wind inversion step, we initially store the up to four solutions corresponding to
the inversion residual (Maximum Likelihood Estimator, MLE) relative minima. However,
subsequently the wind speed and wind direction of the after 2DVAR-selected MSS wind solution is
put at the index of the selected wind solution. This index is set to the initial wind vector solution
which is closest to the MSS wind vector selection obtained after 2DVAR. Thus, the former wind
vector is not provided in the product, but rather the MSS selected wind vector. The ‘Formal
Uncertainty in Wind Direction’ does not contain the uncertainty, but the normalised inversion
residual (referred to as Rn in [15]).
•
The Wind Vector Cell Quality Flag (table 021109) is redefined and now has the following
definitions:
Description
BUFR bit
Fortran bit
Not enough good sigma-0 available for wind retrieval
1
15
Not used
2
14
VV polarised data in more than two beams
3
13
Product monitoring not used
4
12
Product monitoring flag
5
11
KNMI Quality Control (including rain) data rejection
6
10
Variational QC data rejection
7
9
Land presence
8
8
Ice presence
9
7
Not used
10
6
Reported wind speed is greater than 30 m/s
11
5
Reported wind speed is less than or equal to 3 m/s
12
4
Not used
13
3
Rain flag algorithm detects rain
14
2
Data from at least one of the four possible beam/view
combinations are not available
15
1
Not used
16
0
Missing value
All 17 set
All 17 set
In Fortran, if the Wind Vector Cell Quality Flag is stored in an integer I then use BTEST(I,NDW-NB-1)
to test BUFR bit NB, where NDW=17 is the width in bits of the data element in BUFR.
The flag indicating that more than two beams contain VV polarised data, BUFR bit 3, is active in the
outer part of the swath (WVCs 1-8 and 69-76 at 25 km, WVCs 1-4 and 35-38 at 50 km). It indicates
0
that outer beam data is used to obtain four independent σ values, contrary to the middle part of the
swath where two beams contain VV (outer beam) data and two beams contain HH (inner beam) data.
In the outer parts of the swath, the VV backscatter data present in the level 2a product are distributed
to two WVC beams based on their azimuth angle such that maximum azimuth dispersion is obtained.
This generally results in slightly less optimal wind retrieval; users assimilating the data into NWP
models may consider to reject WVCs for which this flag is set.
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If the ‘product monitoring not used’ bit is set to zero, the product is monitored. If the product is
monitored and the ‘product monitoring flag’ bit is set to zero, the product is valid; otherwise it is
rejected by the product monitoring [3]. This is based on a statistical check of the number of WVC QC
rejections, the wind speed bias with respect to the NWP background, and the wind vector RMS
difference with respect to the NWP background. The product monitoring bits have the same value for
all WVCs in one BUFR output file. Since all problematic data due to instrument issues already have
been removed from the PO.DAAC input data set, product monitoring rejection does not occur in the
wind CDRs.
If the KNMI QC flag is set in a WVC, then the backscatter information is not useable for various
geophysical reasons like rain, confused sea-state etc, resulting in a too large inversion residual. WVCs
in which the KNMI QC flag is set, are not used in the calculation of the analysis field in the ambiguity
removal step. However, after the ambiguity removal the wind solution closest to the analysis field is
chosen (if wind solutions are present in the WVC). This means that such a WVC may contain a
selected wind solution, but it is suspect.
The land presence flag is set if a land fraction (see section 3.3) larger than zero is calculated for the
WVC. As long as the land fraction is below the limit value, a reliable wind solution may however still be
present so there is normally no reason to reject WVCs with the land flag set.
The ice presence flag is set if the Bayesian sea ice screening algorithm calculates ice for the WVC [3].
Note that the products contain wind solutions also over sea ice regions. These bogus winds are
flagged both by the KNMI quality control flag and by the ice flag. Hence it is important to reject any
winds with the KNMI quality control flag set when ingesting the products. Note that WVCs that are
rejected due to a large inversion residual (e.g., in case of rain), only have the KNMI quality control flag
set. On the other hand, WVCs that are rejected due to sea ice, have both the KNMI quality control flag
and the ice flag set.
If the variational QC flag is set, the wind vector in the WVC is rejected during ambiguity removal due to
spatial inconsistency. A wind solution is present, but it may be suspect.
It is recommended not to use WVCs with the KNMI quality control flag or the variational quality control
flag set. See [3] for more information on product reliability.
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6. References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
OSI SAF,
Product Requirements Document,
SAF/OSI/CDOP2/M-F/MGT/PL/2-001, 2015
OSI SAF,
Service Specification Document,
SAF/OSI/CDOP2/M-F/MGT/PL/2-003, 2015
OSI SAF,
Algorithm Theoretical Basis Document for the OSI SAF wind products,
SAF/OSI/CDOP2/KNMI/SCI/MA/197, 2014 (*)
Vogelzang, J., A. Verhoef, J. Verspeek, J. de Kloe and A. Stoffelen,
SDP User Manual and Reference Guide, version 2.2,
NWPSAF-KN-UD-002, 2014 (*)
Verhoef, A., J. Vogelzang and A. Stoffelen,
SeaWinds wind Climate Data Record validation report,
SAF/OSI/CDOP2/KNMI/TEC/RP/221, 2015 (*)
Spencer, M.W., Wu, C., and Long, D.G.,
Tradeoffs in the design of a spaceborn scanning pencil beam scatterometer: application to
SeaWinds,
IEEE Transactions on Geoscience and Remote Sensing, 35, 1, 115-126, 1997,
doi:10.1109/36.551940
JPL,
QuikSCAT Science Data Product User’s Manual, version 3.0,
Jet Propulsion Laboratory D-18053-Rev A, September 2006
Belmonte Rivas, M. and A. Stoffelen,
New Bayesian algorithm for sea ice detection with QuikSCAT,
IEEE Transactions on Geoscience and Remote Sensing, I, 49, 6, 1894-1901, 2011,
doi:10.1109/TGRS.2010.2101608
Dee, D. et al.,
The ERA-Interim reanalysis: configuration and performance of the data assimilation system
Quarterly Journal of the Royal Meteorological Society, 137: 553–597, 2011, doi:10.1002/qj.828
Hersbach, H.,
Assimilation of scatterometer data as equivalent-neutral wind
ECMWF Technical Memorandum 629, 2010
Verhoef, A. and A. Stoffelen,
Quality Control of Ku-band scatterometer winds,
OSI SAF report SAF/OSI/CDOP2/KNMI/TEC/RP/194, 2012 (*)
Verspeek, J., A. Stoffelen, A. Verhoef and M. Portabella,
Improved ASCAT Wind Retrieval Using NWP Ocean Calibration,
IEEE Transactions on Geoscience and Remote Sensing, 50, 7, 2488-2494, 2012,
doi:10.1109/TGRS.2011.2180730
Leidner, M., R. Hoffman, and J. Augenbaum,
SeaWinds scatterometer real-time BUFR geophysical data product, version 2.2.0,
NOAA/NESDIS, February 2000, available on ftp://www.scp.byu.edu/data/qscat/docs/bufr.pdf
Thesis Scatterometry by Ad Stoffelen, 1998 (*)
Thesis Wind Field Retrieval from Satellite radar systems by Marcos Portabella, 2002 (*)
References marked with a (*) are available on http://www.knmi.nl/scatterometer/publications/.
Page 15 of 22
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SeaWinds wind Climate Data Record Product User Manual
7. Abbreviations and acronyms
2DVAR
ATBD
AR
ASCAT
BUFR
CDR
DLI
ECMWF
ERA
ERS
EUMETSAT
GMF
HDF
HH
KNMI
MLE
MSS
NASA
NetCDF
NOAA
NSCAT
NWP
OSI SAF
QC
QuikSCAT
SAF
SDP
SeaWinds
snr
SSI
SST
u
v
VV
WMO
WVC
Two-dimensional Variational Ambiguity Removal
Algorithm Theoretical Basis Document
Ambiguity Removal
Advanced Scatterometer
Binary Universal Format Representation
Climate Data Record
Downward Long wave Irradiance
European Centre for Medium-Range Weather Forecasts
ECMWF re-analysis
European Remote-Sensing Satellite
European Organisation for the Exploitation of Meteorological Satellites
Geophysical Model Function
Hierarchical Data Format
Horizontal polarisation of sending and receiving radar antennas
Royal Netherlands Meteorological Institute
Maximum Likelihood Estimator
Multiple Solution Scheme
National Aeronautics and Space Administration (USA)
Network Common Data Form
National Oceanic and Atmospheric Administration (USA)
NASA Scatterometer
Numerical Weather Prediction
Ocean and Sea Ice SAF
Quality Control
US Quick Scatterometer mission carrying the SeaWinds scatterometer
Satellite Application Facility
SeaWinds Data Processor
Scatterometer on-board QuikSCAT platform (USA)
SeaWinds NWP Replacer
Surface Solar Irradiance
Sea Surface Temperature
West-to-east (zonal) wind component
South-to-north (meridional) wind component
Vertical polarisation of sending and receiving radar antennas
World Meteorological Organisation
Wind Vector Cell
Page 16 of 22
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SeaWinds wind Climate Data Record Product User Manual
8. Appendix A: BUFR data descriptors
Number
001
002
003
004
005
006
007
008
009
010
011
012
013
014
015
016
017
018
019
020
021
022
023
024
025
026
027
028
029
030
031
032
033
034
035
036
037
038
039
040
041
042
043
044
045
046
047
048
049
050
051
Descriptor
(01007)
(01012)
(02048)
(21119)
(25060)
(02026)
(02027)
(05040)
(04001)
(04002)
(04003)
(04004)
(04005)
(04006)
(05002)
(06002)
(08025)
(04001)
(05034)
(06034)
(21109)
(11081)
(11082)
(21101)
(21102)
(21103)
(21120)
(21121)
(13055)
(21122)
(11012)
(11052)
(11011)
(11053)
(21104)
(11012)
(11052)
(11011)
(11053)
(21104)
(11012)
(11052)
(11011)
(11053)
(21104)
(11012)
(11052)
(11011)
(11053)
(21104)
(02104)
Parameter
Satellite Identifier
Direction of Flight
Satellite Instrument Identifier
Wind Scatterometer GMF
Software Identification
Cross Track Resolution
Along Track Resolution
Orbit Number
Year
Month
Day
Hour
Minute
Second
Latitude (Coarse Accuracy)
Longitude (Coarse Accuracy)
Time Difference Qualifier
Time to Edge
Along Track Row Number
Cross Track Cell Number
Seawinds Wind Vector Cell Quality Flag
Model Wind Direction At 10 M
Model Wind Speed At 10 M
Number of Vector Ambiguities
Index of Selected Wind Vector
Total Number of Sigma0 Measurements
Seawinds Probability of Rain
Seawinds NOF Rain Index
Intensity Of Precipitation
Attenuation Correction On Sigma-0 (from Tb)
Wind Speed At 10 M
Formal Uncertainty In Wind Speed
Wind Direction At 10 M
Formal Uncertainty In Wind Direction
Likelihood Computed for Wind Solution
Wind Speed At 10 M
Formal Uncertainty In Wind Speed
Wind Direction At 10 M
Formal Uncertainty In Wind Direction
Likelihood Computed for Wind Solution
Wind Speed At 10 M
Formal Uncertainty In Wind Speed
Wind Direction At 10 M
Formal Uncertainty In Wind Direction
Likelihood Computed for Wind Solution
Wind Speed At 10 M
Formal Uncertainty In Wind Speed
Wind Direction At 10 M
Formal Uncertainty In Wind Direction
Likelihood Computed for Wind Solution
Antenna Polarisation
Page 17 of 22
Unit
Code Table
Degree True
Code Table
Code Table
Numeric
m
m
Numeric
Year
Month
Day
Hour
Minute
Second
Degree
Degree
Code Table
Second
Numeric
Numeric
Flag Table
Degree True
m/s
Numeric
Numeric
Numeric
Numeric
Numeric
kg/m**2/sec
dB
m/s
m/s
Degree True
Degree True
Numeric
m/s
m/s
Degree True
Degree True
Numeric
m/s
m/s
Degree True
Degree True
Numeric
m/s
m/s
Degree True
Degree True
Numeric
Code Table
SAF/OSI/CDOP2/KNMI/TEC/MA/220
Number
052
053
054
055
056
057
058
059
060
061
062
063
064
065
066
067
068
069
070
071
072
073
074
075
076
077
078
079
080
081
082
083
084
085
086
087
088
089
090
091
092
093
094
095
096
097
098
099
100
101
102
103
104
105
106
Descriptor
(08022)
(12063)
(12065)
(02104)
(08022)
(12063)
(12065)
(21110)
(05002)
(06002)
(21118)
(02112)
(02111)
(02104)
(21105)
(21106)
(21107)
(21114)
(21115)
(21116)
(08018)
(21117)
(21111)
(05002)
(06002)
(21118)
(02112)
(02111)
(02104)
(21105)
(21106)
(21107)
(21114)
(21115)
(21116)
(08018)
(21117)
(21112)
(05002)
(06002)
(21118)
(02112)
(02111)
(02104)
(21105)
(21106)
(21107)
(21114)
(21115)
(21116)
(08018)
(21117)
(21113)
(05002)
(06002)
SeaWinds wind Climate Data Record Product User Manual
Parameter
Total Number w.r.t. accumulation or average
Brightness Temperature
Standard Deviation Brightness Temperature
Antenna Polarisation
Total Number w.r.t. accumulation or average
Brightness Temperature
Standard Deviation Brightness Temperature
Number of Inner-Beam Sigma0 (fwd of sat.)
Latitude (Coarse Accuracy)
Longitude (Coarse Accuracy)
Attenuation Correction On Sigma-0
Radar Look (Azimuth) Angle
Radar Incidence Angle
Antenna Polarisation
Normalized Radar Cross Section
Kp Variance Coefficient (Alpha)
Kp Variance Coefficient (Beta)
Kp Variance Coefficient (Gamma)
Seawinds Sigma-0 Quality Flag
Seawinds Sigma-0 Mode Flag
Seawinds Land/Ice Surface Flag
Sigma-0 Variance Quality Control
Number of Outer-Beam Sigma0 (fwd of sat.)
Latitude (Coarse Accuracy)
Longitude (Coarse Accuracy)
Attenuation Correction On Sigma-0
Radar Look (Azimuth) Angle
Radar Incidence Angle
Antenna Polarisation
Normalized Radar Cross Section
Kp Variance Coefficient (Alpha)
Kp Variance Coefficient (Beta)
Kp Variance Coefficient (Gamma)
Seawinds Sigma-0 Quality Flag
Seawinds Sigma-0 Mode Flag
Seawinds Land/Ice Surface Flag
Sigma-0 Variance Quality Control
Number of Inner-Beam Sigma0 (aft of sat.)
Latitude (Coarse Accuracy)
Longitude (Coarse Accuracy)
Attenuation Correction On Sigma-0
Radar Look (Azimuth) Angle
Radar Incidence Angle
Antenna Polarisation
Normalized Radar Cross Section
Kp Variance Coefficient (Alpha)
Kp Variance Coefficient (Beta)
Kp Variance Coefficient (Gamma)
Seawinds Sigma-0 Quality Flag
Seawinds Sigma-0 Mode Flag
Seawinds Land/Ice Surface Flag
Sigma-0 Variance Quality Control
Number of Outer-Beam Sigma0 (aft of sat.)
Latitude (Coarse Accuracy)
Longitude (Coarse Accuracy)
Page 18 of 22
Unit
Numeric
K
K
Code Table
Numeric
K
K
Numeric
Degree
Degree
dB
Degree
Degree
Code Table
dB
Numeric
Numeric
dB
Flag Table
Flag Table
Flag Table
Numeric
Numeric
Degree
Degree
dB
Degree
Degree
Code Table
dB
Numeric
Numeric
dB
Flag Table
Flag Table
Flag Table
Numeric
Numeric
Degree
Degree
dB
Degree
Degree
Code Table
dB
Numeric
Numeric
dB
Flag Table
Flag Table
Flag Table
Numeric
Numeric
Degree
Degree
SAF/OSI/CDOP2/KNMI/TEC/MA/220
Number
107
108
109
110
111
112
113
114
115
116
117
118
Descriptor
(21118)
(02112)
(02111)
(02104)
(21105)
(21106)
(21107)
(21114)
(21115)
(21116)
(08018)
(21117)
SeaWinds wind Climate Data Record Product User Manual
Parameter
Attenuation Correction On Sigma-0
Radar Look (Azimuth) Angle
Radar Incidence Angle
Antenna Polarisation
Normalized Radar Cross Section
Kp Variance Coefficient (Alpha)
Kp Variance Coefficient (Beta)
Kp Variance Coefficient (Gamma)
Seawinds Sigma-0 Quality Flag
Seawinds Sigma-0 Mode Flag
Seawinds Land/Ice Surface Flag
Sigma-0 Variance Quality Control
Page 19 of 22
Unit
dB
Degree
Degree
Code Table
dB
Numeric
Numeric
dB
Flag Table
Flag Table
Flag Table
Numeric
SAF/OSI/CDOP2/KNMI/TEC/MA/220
SeaWinds wind Climate Data Record Product User Manual
9. Appendix B: NetCDF data format
The wind products are also available in the NetCDF format, with the following characteristics:
•
The data format meets the NetCDF Climate and Forecast Metadata Convention version 1.4
(http://cf-pcmdi.llnl.gov/).
•
The data contain, contrary to the BUFR data, only level 2 wind and sea ice information, no sigma0
nor soil moisture information. The aim was to create a compact and easy to handle product for
oceanographic and climatological users.
•
The data contain only the selected wind solutions, no ambiguity information.
•
The wind directions are in oceanographic rather than meteorological convention (see section 5.1)
•
The format is identical for SeaWinds, ASCAT and any other scatterometer data.
•
The data has file sizes somewhat smaller than those of the corresponding BUFR data (e.g., one
orbit file of 25 km wind data is 11 MB in BUFR and 4 MB in NetCDF). When compressed with
gzip, the size of one file in NetCDF reduces to 1.1 MB.
The file name convention for the gzipped NetCDF product is
seawi_YYYYMMDD_HHMMSS_qscat__ORBIT_T_SMPL_VERS_CONT_l2.nc.gz where the meaning
of the fields is identical to those in the BUFR file names (see section 5.2). The VERS part of the file
name denotes the SDP software version (2202 for this data record). A file name example is:
seawi_20080101_005132_qscat__44444_o_250_2202_ovw_l2.nc.gz.
Below are some meta data contained in the NetCDF data files:
dimensions:
NUMROWS = 1624 ;
NUMCELLS = 76 ;
variables:
int time(NUMROWS, NUMCELLS) ;
time:long_name = "time" ;
time:units = "seconds since 1990-01-01 00:00:00" ;
int lat(NUMROWS, NUMCELLS) ;
lat:long_name = "latitude" ;
lat:units = "degrees_north" ;
int lon(NUMROWS, NUMCELLS) ;
lon:long_name = "longitude" ;
lon:units = "degrees_east" ;
short wvc_index(NUMROWS, NUMCELLS) ;
wvc_index:long_name = "cross track wind vector cell number" ;
wvc_index:units = "1" ;
short model_speed(NUMROWS, NUMCELLS) ;
model_speed:long_name = "model wind speed at 10 m" ;
model_speed:units = "m s-1" ;
short model_dir(NUMROWS, NUMCELLS) ;
model_dir:long_name = "model wind direction at 10 m" ;
model_dir:units = "degree" ;
short ice_prob(NUMROWS, NUMCELLS) ;
ice_prob:long_name = "ice probability" ;
ice_prob:units = "1" ;
short ice_age(NUMROWS, NUMCELLS) ;
ice_age:long_name = "ice age (a-parameter)" ;
ice_age:units = "dB" ;
int wvc_quality_flag(NUMROWS, NUMCELLS) ;
wvc_quality_flag:long_name = "wind vector cell quality" ;
wvc_quality_flag:flag_masks = 64, 128, 256, 512, 1024, 2048, 4096,
8192, 16384, 32768, 65536, 131072, 262144, 524288, 1048576, 2097152, 4194304 ;
Page 20 of 22
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SeaWinds wind Climate Data Record Product User Manual
wvc_quality_flag:flag_meanings = "distance_to_gmf_too_large
data_are_redundant no_meteorological_background_used rain_detected
rain_flag_not_usable small_wind_less_than_or_equal_to_3_m_s
large_wind_greater_than_30_m_s wind_inversion_not_successful
some_portion_of_wvc_is_over_ice some_portion_of_wvc_is_over_land
variational_quality_control_fails knmi_quality_control_fails
product_monitoring_event_flag product_monitoring_not_used
any_beam_noise_content_above_threshold poor_azimuth_diversity
not_enough_good_sigma0_for_wind_retrieval" ;
short wind_speed(NUMROWS, NUMCELLS) ;
wind_speed:long_name = "wind speed at 10 m" ;
wind_speed:units = "m s-1" ;
short wind_dir(NUMROWS, NUMCELLS) ;
wind_dir:long_name = "wind direction at 10 m" ;
wind_dir:units = "degree" ;
short bs_distance(NUMROWS, NUMCELLS) ;
bs_distance:long_name = "backscatter distance" ;
bs_distance:units = "1" ;
// global attributes:
:title = "QuikSCAT SeaWinds Level 2 25.0 km Ocean Surface Wind
Vector Product" ;
:title_short_name = "SeaWinds-L2-25km" ;
:Conventions = "CF-1.4" ;
:institution = "EUMETSAT/OSI SAF/KNMI" ;
:source = "QuikSCAT SeaWinds" ;
:software_identification_level_1 = 2202 ;
:instrument_calibration_version = 0 ;
:software_identification_wind = 2202 ;
:pixel_size_on_horizontal = "25.0 km" ;
:service_type = "N/A" ;
:processing_type = "O" ;
:contents = "ovw" ;
:granule_name =
"seawi_20080101_005132_qscat__44444_o_250_2202_ovw_l2.nc" ;
:processing_level = "L2" ;
:orbit_number = 44444 ;
:start_date = "2008-01-01" ;
:start_time = "00:51:32" ;
:stop_date = "2008-01-01" ;
:stop_time = "02:32:27" ;
:equator_crossing_longitude = " 97.082" ;
:equator_crossing_date = "2007-12-31" ;
:equator_crossing_time = "23:35:45" ;
:rev_orbit_period = "6059.1" ;
:orbit_inclination = "98.6" ;
:history = "N/A" ;
:references = "SeaWinds Product Manual, http://www.osi-saf.org/,
http://www.knmi.nl/scatterometer/" ;
:comment = "Orbit period and inclination are constant values. All
wind directions in oceanographic convention (0 deg. flowing North)" ;
:creation_date = "2014-10-27" ;
:creation_time = "09:57:40" ;
Page 21 of 22
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SeaWinds wind Climate Data Record Product User Manual
10. Appendix C: Data gaps
The table below shows the gaps with a length of at least one orbit in the SeaWinds Data Record.
Start date
End date
19-Nov-1999
Last orbit
before gap
2144
First orbit
after gap
2172
Number of
missing orbits
27
17-Nov-1999
1-Jan-2000
2-Jan-2000
2792
2811
18
20-Jan-2000
21-Jan-2000
3067
3071
3
18-Jul-2000
19-Jul-2000
5626
5637
10
28-Aug-2000
29-Aug-2000
6216
6219
2
16-Nov-2000
18-Nov-2000
7355
7386
30
11-May-2001
14-May-2001
9858
9908
49
7-Jul-2001
9-Jul-2001
10666
10695
28
17-Nov-2001
19-Nov-2001
12571
12600
28
19-Mar-2002
21-Mar-2002
14305
14330
24
4-Jul-2002
4-Jul-2002
15830
15837
6
19-Aug-2002
20-Aug-2002
16491
16505
13
18-Nov-2002
20-Nov-2002
17787
17808
20
19-May-2003
20-May-2003
20384
20390
5
11-Sep-2003
12-Sep-2003
22027
22030
2
17-Dec-2003
18-Dec-2003
23410
23424
13
6-Aug-2004
6-Aug-2004
26721
26730
8
17-Jun-2006
17-Jun-2006
36417
36427
9
15-Jul-2006
17-Jul-2006
36820
36856
35
11-Nov-2006
12-Nov-2006
38523
38528
4
3-Mar-2007
3-Mar-2007
40112
40116
3
7-Apr-2007
8-Apr-2007
40618
40622
3
6-Dec-2007
7-Dec-2007
44085
44097
11
24-Jun-2008
25-Jun-2008
46942
46963
20
25-Nov-2008
28-Nov-2008
49148
49191
42
6-Dec-2008
6-Dec-2008
49295
49303
7
3-Sep-2009
5-Sep-2009
53167
53197
29
Page 22 of 22
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