QUALITY INFORMATION DOCUMENT
For the Global Ocean Wind Products
WIND_GLO_WIND_L4_NRT_OBSERVATIONS_012_004
Issue: 1.3
Contributors: Abderrahim Bentamy
Approval Date by Quality Assurance Review Group : under review
Ref: CMEMS-OSI-QUID-012-004
QUID for Global Ocean Wind Product
WIND_GLO_WIND_L4_NRT-OBSERVATIONS_012_004
Date
: 26 January 2016
Issue
: 1.3
CHANGE RECORD
Issue
Date
§
Description of Change
Author
Abderrahim Bentamy
1.0
2013-07-16 All
Creation of the document,
Wind QUID split up from the
SIW QUID.
1.1
1/9/2015
Change format to fit CMEMS B. Hackett
graphical rules. Introduce
all
Validated By
Abderrahim Bentamy
L. Crosnier
references to CMEMS.
1.2
05/10/2015 all
Update of the validation
results
1.3
26/01/2016 all
Update of the determination Abderrahim Bentamy
and validation of L4 V5 wind
products
© EU Copernicus Marine Service – Public
Abderrahim Bentamy
Abderrahim Bentamy
B. Hackett
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Date
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Table of contents
Executive summary ................................................................................................................................................ 4
Products covered by this document ................................................................................................................ 4
Summary of the results .................................................................................................................................... 4
Production Subsystem description ......................................................................................................................... 6
L4 Global blended ocean wind ........................................................................................................................ 6
ValidatIon framework ............................................................................................................................................ 8
Procedure .......................................................................................................................................................... 8
Result summary ................................................................................................................................................ 9
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WIND_GLO_WIND_L4_NRT-OBSERVATIONS_012_004
Date
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Issue
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EXECUTIVE SUMMARY
Products covered by this document
This document describes the quality of the newest version of the global ocean near-real-time 6hourly L4 wind product (WIND_GLO_WIND_L4_NRT_OBSERVATIONS_012_004). It is a new dataset
that contains 6-hourly blended wind fields estimated from scatterometer wind vector observations,
SSMIS wind speeds, and NWP wind forecasts. The wind variables include 6-hourly averaged wind
speed, zonal and meridional wind components and wind stress amplitude. The input (scatterometer
and retrievals) L2 scatterometer observations are made available by the operational Ocean and Sea
Ice Satellite Application Facility (OSI SAF) of EUMETSAT, SSMIS are from Remote Sensing System
(RSS), and NWP data are from European Centre of Medium Weather Forecasts (ECMWF). The
product is produced by IFREMER and distributed by the CMEMS Ocean and Sea Ice Thematic
Assembly Centre (OSI TAC). The resulting wind fields are identified as the L4 dataset CERSAT-GLOBLENDED_WIND_L4-V5-OBS_FULL_TIME_SERIE, and hereafter in this document as V5. The previous
version of the product is a blended scatterometer-based analysis; it is identified as dataset CERSATGLO-BLENDED_WIND_L4-V3-OBS_FULL_TIME_SERIE and hereafter in this document as V3; it’s
quality is documented in issue 1.1 of this document.
Product
Product description
WIND_GLO_WIND_L4_NRT
_OBSERVATIONS_012_004
Global ocean blended
wind L4
Production unit, PU Dissemination unit
DU
SIW-IFREMERBREST-FR
SST-CNR-ROME-IT
Table 1 : Product summary
Summary of the results
The investigations of the quality of the objective method, used to estimate the gridded wind
fields, the resulting blended wind estimates, and of the operational procedure are checked as
follows:

Characterization of the error relied on the objective method used to estimate global gridded
wind fields from swath observations. It is performed using synthetic data derived from
numerical model (ERA Interim) interpolated onto satellite swaths. No systematic departure is
found between ERA Interim analysis and 6-hourly averaged winds derived from the synthetic
data based on the use of the objective method. The correlations between the two sources
exceed 0.97.
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
Determination of L4 V5 wind product accuracy through comprehensive comparisons with 6hourly winds from available moored buoy data. The latter are derived from various buoy
networks: NDBC/NOAA (Atlantic, Pacific oceans), UK Met Office and Météo-France (Atlantic
and Mediterranean Sea), TAO (Tropical Pacific), PIRATA (Tropical Atlantic), and RAMA (Indian
Ocean). Comparisons are performed for November 2015. The biases for wind speed as well
as for wind direction are quite low. The associated standard deviations do not exceed
1.70m/s and 30°, respectively. The scatter and vector correlation coefficients for wind speed
and direction are higher than 0.80 and 1.80, respectively.

Assessment of L4 V5 wind product quality based on comparisons with spatially and
temporally collocated ASCAT retrievals. The former are performed over global ocean to
characterize L4 and ASCAT wind speed and direction agreements. The comparisons indicate
that blended wind speed and wind components are in good agreement with the remotely
sensed ones. The main aim of such comparisons is to highlight how the blended analysis
retrieves the remotely sensed wind observations derived mainly from ASCAT. Collocated
data are used for comparisons purposes. The overall statistics characterizing ASCAT and
blended collocated data comparisons indicate that the biases are close to zero and the
standard deviation (std) values are less than 1 m/s. The correlation coefficients exceed 0.95.
It is noticeable that zonal and meridional components have similar behaviours. Furthermore,
ASCAT and blended exhibit better agreements than those drawn from ASCAT and ECMWF
forecast comparisons.

Quality control of each netcdf file geophysical content is performed based on the calculation
of the statistical parameters characterizing the difference between L4 wind speed and
direction data and the related ECMWF forecast. Only files such as differences between L4
and ECMWF data indicate significant discrepancies are checked. The statistical parameter
time variabilities are quite steady. For instance, wind speed bias, standard deviation, and
correlation are of 0.20m/s, 1.10m/s, and 0.97, respectively, along the study period.
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PRODUCTION SUBSYSTEM DESCRIPTION
L4 V5 Global blended ocean wind
The regular (in space and time) wind fields are estimated from near real time scatterometer
and radiometer data in combination with ECMWF analysis. The remotely sensed winds are derived
from available ASCAT scatterometers onboard Metop-A and Metop-B satellites. The radiometer
winds are from the special sensor microwave imager sensor (SSMIS) onboard defense meteorological
polar satellites (DMSP) F16 and F17. The scatterometer as well as radiometer are provided in near
real time by SAF OSI (KNMI) and RSS, respectively. They are extracted based on ftp procedures. The
scatterometer data are provided as L2b products including backscatter coefficients measurements
and the associated radar parameters as well as wind retrievals. SSMIS winds are provided as L2b
products. Both L2b products are quality controlled prior any analysis. Based on the objective method
used to estimate daily ASCAT and QuikSCAT gridded wind fields
(ftp://ftp.ifremer.fr/ifremer/cersat/products/gridded/MWF/L3/ ), L4 winds are calculated from l2b
products in combination with ECMWF forecasts. The analysis is performed for each synoptic time
(00h:00; 06h:00; 12h:00; 18h:00 UTC) and with a spatial resolution of 0.25° in longitude and latitude
over global ocean. The method details may be found in (Bentamy and Croizé-Fillon, 2011).1 Figure 1
shows zooms of L4 V5 wind products estimated over the Arabian Sea and occurring during
November, 1st 2015.
1
Bentamy A., D. Croizé. Fillon, 2011: Gridded Surface Wind Fields from Metop/ASCAT
Measurements. Inter. Journal of Remote Sensing, 33, pp 1729-1754.
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Figure1 : Examples of L4 wind products estimated from ASCAT-A, ASCAT-B, SSMIS F16, SSMIS F17 L2b
retrievals for four epochs of November , 1st 2015: 00H:00 UTC (top/left), 06h:00 UTC (top/right),
12h:00 UTC (bottom/left), and 18h:00 UTC (bottom/right). Color indicates wind speed amplitude in
m/s. Panels show tropical cyclone “Chapala” occuring in the Arabian sea.
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VALIDATION FRAMEWORK
Validation is a continuous on going activity to characterize accuracy and quality of the
delivered sea ice and wind products. It is mainly be based on operational data, but can be supported
by campaign data
Each PU is responsible for validation of their products. The OSI TAC Validation activities are for the
most based on what is already implemented at the partners’ institutes and has shown to be useful.
Description of validation data and procedures and link to validation results for each product are given
in the next sections.
Procedure
The subsystem performance and associated product quality are scientifically assessed in the
following way:

Determination of L4 V5 wind products accuracy through comprehensive comparisons with 6hourly winds from available and valid moored buoy data. The latter are derived from various
buoy networks: NDBC/NOAA (Atlantic, Pacific oceans), UK Met Office and Météo-France
(Atlantic and Mediterranean Sea), TAO (Tropical Pacific), PIRATA (Tropical Atlantic), and
RAMA (Indian Ocean). More than 170 buoy raw data are routinely collected, investigated,
and collocated in space and time with monthly satellite estimates. The main statistical
parameters, including the first four conventional moments and the linear regression
parameters, will be estimated and provided. The differences between buoy and V5 wind
products are investigated according to geographical locations (e.g. off-shore, coastal, highlatitudes, mid-latitudes and tropical areas). For further assessment of V5 product accuracy,
statistics related to buoy and V3 comparisons are provided too.

Scatterplots and the related statistical parameters (bias, rms, correlation, linear regression
coefficients) illustrating the comparison between L4 V5 global ocean wind and available and
validated buoy wind speeds and directions.

At global scale the quality of each L4 V5 blended wind product is monthly assessed based on
comparisons with spatially and temporally collocated ASCAT (when available) retrievals. The
former are performed over global ocean to characterize V3 and scatterometer wind speed
and direction agreements.

Determination of global maps of differences between 6-hourly satellite wind product and the
associated ECMWF analysis. Maps illustrate the bias, rms difference and correlation
coefficient spatial patterns for wind speed, zonal, and meridional components.
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
Time series of differences between 6-hourly satellite wind product and the associated
ECMWF analysis. They deal with bias, rms difference and correlation coefficient for wind
speed, zonal, and meridional components.

Summary of the results characterising L4 V5 wind product (wind speed, zonal and meridional
components) and ECMWF analysis.

Quality control of each netcdf file geophysical content is performed based on the calculation
of the statistical parameters characterizing the difference between L4 V3 wind speed and
direction data and the related ECMWF analysis. Only files such as the difference frequency
exceeding three times of standard deviations is higher than 10% will be checked. According
to the finding, the blended wind field could be reprocessed.
Result summary
L4 V5 and NDBC buoy wind Comparisons
The quality of the resulting wind fields is investigated trough comprehensive comparisons with
6-hourly averaged wind estimated derived from buoy measurements. Comparisons are performed
for coastal buoy (located less than 50km from coastline) and for offshore buoys. Figure 2 illustrates
the results obtained from collocated, in space and time, offshore buoy and L4 V5 wind data occurring
during November 2015.
Table1 and Figure 2 indicate that for offshore comparisons, no systematic departures are
depicted and for most wind variable bins, the collocated data are close to the perfect line. The
associated errors are lower than 1m/s and 20° for wind speed and direction, respectively. The
significant bias is found for buoy wind speeds less than 4m/s. Indeed, L4 V5 wind speeds tend to be
slightly overestimated compared to in-situ data due to differences in spatial representation and
binning. Table 1 summarizes the related statistical parameters: mean (Bias) and standard deviation
(Std) of buoy minus blended data, scalar and vector correlation coefficients (Cor) for wind speed and
direction, respectively. One should notice that for wind direction vector correlation is provided in
Table 1. The overall statistics indicate that the 6-hourly satellite wind fields compare well to averaged
buoy data. The rms differences do not exceed the scatterometer specifications, for wind speed and
direction, respectively. For in-situ and scatterometer winds higher than 3 m/s no significant bias
trend is found. The wind direction bias is relatively small. The results obtained from comparisons
performed based on the use of collocated coastal buoys are lower than those found for offshore
buoys. Therefore, L4 V5 winds should be used with caution in near shore areas.
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Table1: Statistical parameters characterizing differences between 6-hourly averaged NDBC buoy and
blended wind speed and direction estimates for November 2015. Std and Cor stand for standard
deviation and correlation, respectively. Statistics are provided for L4 V3 and V5 products.
Wind Speed
Wind Direction
Length
Bias
(m/s)
Std
(m/s)
Cor
(scalar)
Bias
(deg)
Std
(deg)
Cor
(Vector)
L4 V5
5573
-0.06
1.26
0.94
-2
15
1.88
L4 V3
5125
-0.03
1.16
0.95
-1
16
1.87
L4 V5
1500
0.23
1.30
0.93
-2
16
1.86
L4 V3
1065
0.22
1.38
0.91
-3
17
1.86
OffShore(>=50km)
Coastal(<50km)
Figure 2 : Comparison of 6-hourly wind speeds (left column) and wind direction (right column) from L4 V3
winds versus offshore NDBC buoy winds. panels on top show comparison results for the buoy wind speed
binned in 1m/s and 20° bins. The dashed blue line indicates the results obtained for the 1m/s binned
blended wind speeds. One standard deviation values estimated for each bin are also shown.
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L4 V5 and ASCAT L2b wind Comparisons
Surface winds retrieved from scatterometer measurements represent the state of art in global
wind vector estimations. Therefore, ASCAT retrievals are used to assess and evaluate the blended
wind vector estimates at global scales. The main aim of such comparisons is to highlight how the
blended analysis retrieves the remotely sensed wind observations derived mainly from ASCAT.
Collocated data are used for comparisons purposes. The bias and rms differences between collocated
ASCAT wind observations and the blended wind analyses of wind speed, zonal and meridional
components, estimated during November 2015, are shown in Figure 3. The overall statistics
characterizing ASCAT and blended collocated data comparisons indicate that the biases are close to
zero and the standard deviation (std) values are less than 1 m/s. The correlation coefficients exceed
0.95. It is noticeable that zonal and meridional components have similar behaviours. More
specifically, the std differences of the three variables (wind speed, zonal, and meridional
components) exhibit low values (lower than 1m/s) over the Atlantic, Pacific and Indian trade wind
regions, where the wind is quite steady. The highest std values are found are at high latitude and
especially north and south 40° where surface wind is more variable. At these mid and high latitudes
the std values are about 1-2m/s for wind speed, while for zonal and meridional components they are
about 1.5–3m/s. Through the three std patterns, we can notice a band of high rms values located
north of the equator in the Atlantic and Pacific oceans and in the Gulf of Guinea. This may be related
to a misplacement of the intertropical convergence zone (ITCZ) in the blended wind fields. However,
the sampling length of collocated data from ASCAT and blended winds is minimum over this band.
Indeed, between the equator and 10°N, the sampling length is about 30 whereas is more than 70
elsewhere. Some high std value regions are depicted too and especially for both wind components.
For instance, over the northwest Atlantic Ocean (Gulf Stream current) the std values may exceed
2m/s for both wind components. This is mainly related to storm tracks characterising surface wind
condition during north hemisphere winter.
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Figure 3 : Mean (left) and standard deviation (right) wind differences between collocated ASCAT
retrievals and L4 V5 estimates during November 2015. The left panels indicate the wind speed
(a), zonal wind component (b), and meridional wind component (c) biases. The corresponding
L4 V3 andare
ECMWF
(Longrespectively
term quality
std distributions
shownwind
in d),Comparisons
e), and f) panels,
. control)
The quality control of geophysical content of each L4 V5 wind file is performed based on the
calculation of the statistical parameters characterizing the difference between V3 wind speed, zonal,
and meridional wind component 6-hourly data and the related ECMWF analysis. Figure 4 shows time
series of difference (blended – ECMWF) biases (top) and the related standard deviations (middle),
and of correlation coefficients (bottom). Times series are shown for wind speeds (red colour), zonal
component (blue colour), and for meridional component (black colour) for November 2015. The
statistical parameter time variabilities are quite steady. For instance, wind speed bias, standard
deviation, and correlation are of 0.40m/s, 1.40m/s, and 0.93, respectively, along the study period.
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Figure 4 : Time series (1st - 30th November 2015) of statistical parameters characterizing Blended
and ECMWF wind comparisons over global ocean. Top through bottom panels show time series of
biases (Blended –ECMWF in m/s), standard deviations (m/s), and correlation coefficients,
respectively.
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