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Scatterometer winds
Manager NWP SAF at KNMI
Manager OSI SAF at KNMI
PI European OSCAT Cal/Val project
Leader KNMI Satellite Winds Group
www.knmi.nl/scatterometer
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Updated @ 2013-11-04 13:03 utc
OSI SAF ASCAT Coastal product viewer ASCAT12+, status: pre-operational
Ascending passes Click in the map to zoom in
Descending passes Click in the map to zoom in
Select view
Monitoring information Buoy validations Ice maps Data from previous day
Background information
Modifications/anomalies Description of plots Access to products Acknowledgements ASCAT Product User Manual ASCAT Coastal Validation report Home OSI SAF Wind Centre
OSI SAF Wind Products
ASCAT-A 25-km winds Operational status ASCAT-A 12.5-km winds Operational status ASCAT-B 25-km winds Operational status ASCAT-B coastal winds Operational status Oceansat-2 50-km winds Operational status QuikSCAT winds Discontinued status Wind Products Processing Status Archived Wind and Stress Products
Other Wind Services at KNMI
ASCAT 25-km winds (EARS) Operational status ASCAT 12.5-km winds (EARS) Operational status ERS-2 winds (EARS) Discontinued status Scatterometer work at KNMI
Software
BUFR reader Related links
EUMETSAT Ocean and Sea Ice SAF EUMETSAT EARS system Numerical Weather Prediction SAF Description of ASCAT instrument at ESA ASCAT archived data at the EUMETSAT Data Centre ASCAT archived NetCDF data at PO.DAAC
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2013/11/01
11:32 UTC
11:00 UTC
11:30 UTC
2013/11/03
00:22 UTC
00:00 UTC
00:30 UTC
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2011/10/27
7:47 UTC
6:00 UTC
7:30 UTC
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5
Overview
• s 0 or Normalised Radar Cross Section
• Geophysics, GMF
• Swath geometry, orbit (ASCAT example)
• Accuracy, resolution
• Some limitations
• Applications
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www.howtoons.com/ toon/the-beaufort-scale/
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Backscatter modulation by surface roughness
® Z.Jelenak
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Backscatter modulation by surface roughness
® Z.Jelenak
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Backscatter modulation by surface roughness
® Z.Jelenak
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Backscatter modulation by surface roughness
® Z.Jelenak
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Backscatter as a Function of Wind Speed
and Incidence Angle At incidence angles of 0° backscatter decreases with wind
® Z.Jelenak
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Backscatter as a Function of Wind Speed
and Incidence Angle At incidence angles of 20° backscatter increases with wind
® Z.Jelenak
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Backscatter as a Function of Wind Speed
and Incidence Angle Most sensitivity to wind at moderate incidence angles 30°-60°
® Z.Jelenak
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Backscatter Sensitivity to Wind
Direction
5m/s
® Z.Jelenak
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Backscatter Sensitivity to Wind
Direction
5m/s
10m/s
15m/s
20m/s
® Z.Jelenak
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Backscatter Sensitivity to Wind
Direction
5m/s
10m/s
15m/s
20m/s 25m/s 30m/s
® Z.Jelenak
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Back Scattering Theory
• Bragg scattering – Incoming microwave radiation
in resonance with short waves (dominant for 30°< q < 70 °)
lB = l/(2sin(q)
• Specular reflection – Ocean facets normal to
incident radiation (non-negligible for q < 30°)
• Accuracy of theoretical models ~1 dB and not adequate
l ~ 2cm (Ku-band) ; l ~ 5cm (C-band)
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Geophysical Model Function
• An empirical geophysical model function (GMF) relates
ocean surface wind speed and direction to the
backscatter cross section measurements.
U10N: equivalent neutral wind speed
f : wind direction w.r.t. beam pointing
q : incidence angle
p : radar beam polarization
l : microwave wavelength
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EUMETRAIN Marine Course, 2013
Equivalent neutral wind U10N
U10 depends on air stability y
while s 0 is a sea property
Surface roughness z0 relates to
s 0 and depends on friction
velocity u* and thus on stress
U10N is computed from u* by
setting y = 0 and is available
from NWP models and buoys
GMF fits s 0 and collocated U10N
So, s 0 = GMF ( U10N, f, q, p, l )
NWP models usually ignore
current (Us = 0), but a
scatterometer does measure
relative to ocean motion
19
)k
zuU 0*
N10
/10ln=
) SU
k
LzuU
=
)/10(/10ln 0*10
y
Portabella & Stoffelen, 2009
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EUMETRAIN Marine Course, 2013
Fixed fan beam C-band (5 cm)
VV-pol
Sampling 12.5-25 km
Static geometry
ASCAT, double swath
ERS2, single swath
Current scatterometer instruments
Rotating pencil beam
Ku-band (2 cm)
Dual polarization
Sampling 25 km, 50 km
Rotating antenna
OSCAT, QuikScat, HY2A
RAIN
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21
ASCAT scatterometer
Three ASCAT arms
Fan beams
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22
ASCAT scatterometer
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23
550 km
45
Left swath
ASCAT observation geometry
550 km
Right swath
90
135
• Real aperture radar, 5.255 GHz
(C-band), VV polarisation
• All weather measuring capability
• Measuring geometry: 3 fan-beam
antennas, double swath,
incidence angles between 25 and
65 deg
• Measurement: normalised radar
cross-section (NRCS, backscatter,
s0)
• Swath gridded into nodes (25 km
and 12.5 km spacing), one triplet
of averaged backscatter
measurements per node
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24
550 km
Left swath
ASCAT observation geometry
550 km
Right swath
• Each swath is divided into 21
Wind Vector Cells (WVCs) for
the 25 km product
• For the 12.5-km product 43
WVCs exist on each side
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25
Spatial representation
• We estimate area-mean (WVC) winds using the empirical GMFs
• 25-km areal winds are less extreme than 10-minute sustained in
situ winds (e.g., from buoys)
• So, extreme buoy winds should be higher than extreme
scatterometer winds (allow for gustiness factor)
• Extreme NWP winds are again somewhat lower due to lacking
resolution
Wind scales
0
10
20
30
40
0 25 50 75 100 125 150 175 200
Distance (km)
Win
d s
peed
(m
/s)
Buoy
ASCAT
ECMWF
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26
Buoy Verification
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27
Buoy
verification
November 2012
www.knmi.nl/scatterometer
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28
Buoy verification
ASCAT @ 12.5 compares best to buoys
SeaWinds @25 is slightly noisier than ASCAT @12.5 and @25
October 2008 ASCAT 12.5 ASCAT 25 SeaWinds 25
KNMI
SeaWinds 25
USA
su
[m/s]
sv
[m/s]
su
[m/s]
sv
[m/s]
su
[m/s]
sv
[m/s]
su
[m/s]
sv
[m/s]
1.67 1.65 1.70 1.64 1.76 1.83 2.19 1.99
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Triple collocation result
12.5 km coastal product u v
Bias ASCAT (m/s)
Bias ECMWF (m/s)
0.15
0.28
-0.02
0.08
Trend ASCAT
Trend ECMWF
1.01
1.03
1.01
1.04
s ASCAT (m/s)
s ECMWF (m/s)
s buoy (m/s)
0.69
1.50
1.21
0.81
1.52
1.23
On scatterometer scale (25 km)
OSI SAF NRT req. 2 m/s, WMO in speed/dir.
See also Vogelzang et al., JGR, 2011
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30
Operational
25 - km
product
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31
Operational
12.5-km
product
Convective
systems
SST
Currents
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32
ASCAT
25 km
12.5 km
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Quality Control (QC)
• Scatterometers provide good quality sea surface
winds except for:
– Sea ice or land contamination
– Large spatial wind variability (e.g., vicinity of fronts
and low-pressure centres, downbursts)
– Rain (especially in Ku-band systems,
e.g.,OceanSat-2)
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Rain Effects
• The radar signal is attenuated by the rain as it
travels to and from the Earth’s surface σ0
• Retrieved wind speed
• The radar signal is scattered by the raindrops.
Some of this scattered energy returns to the
instrument σ0
• Retrieved wind speed ( to ~ 15 m/s)
• Directional information can be lost
34
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Rain Effects
• The radar signal is attenuated by the rain as it
travels to and from the Earth’s surface σ0
• Retrieved wind speed
• The radar signal is scattered by the raindrops.
Some of this scattered energy returns to the
instrument σ0
• Retrieved wind speed ( to ~ 15 m/s)
• Directional information can be lost
• The roughness of the sea surface is increased
because of the splashing due to raindrops
σ0
• Retrieved wind speed (at low winds)
• Directional information can be lost
• Variable roughness due to wind downbursts • Confused sea state, speed/direction unclear
35
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Typical Rain
Patterns Rain effects: Cross swath vectors Higher wind speeds
Some intense rain not flagged by RSS
® RSS slide
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Quality Control
• Inversion residual value (MLE)
– low = good quality wind
– high = low quality wind
• A uniform metric is derived (Rn)
• A Rn threshold is derived to optimize
– rejection of low quality
– accept good quality SeaWinds ECMWF
Portabella and Stoffelen, 2001
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Quality Control
• Areas with significant Rain (large squares) effectively detected
• Frontal and low-pressure centre areas effectively removed
• Vast majority of spatially consitent winds are accepted (green arrows)
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39
Discrimination of land, water and ice
Detached sea ice field of
400kmx400km at South Pole
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40
April 2008 October 2008
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41
6 km s 0 grid 12.5 km grid size 6.25 km grid size
• Further development ongoing since release of new full-res EUMETSAT product
In development
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Generic Scatterometer
Data Processor
Observations Inversion Ambiguity
Removal Wind
Field
INPUT OUTPUT
Observations Inversion Ambiguity
Removal
Quality
Control
Quality
Control
Wind
Field
INPUT OUTPUT
Quality
Monitor
AWDP: ASCAT Wind Data Processor, also ERS
SDP: SeaWinds Data Processor
OWDP: OSCAT Wind Data Processor, also HY2A
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43
Scatterometer winds
• Represent the mean WVC wind
• Are provided as equivalent neutral winds
• Verify very well with NWP model
• Verify very well with buoys
• Show spectra close to that expected for 3D turbulence for scales < 500 km
• Spatial plots show small-scale features in line with these three features
• Can be contaminated by land, sea ice and rain
• Winds > 30 m/s are difficult to measure/calibrate
• Are ambiguous
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10 11 12 13 14 15 16 17 18 19 20 21 22
Ku-band
Combined C- and Ku-band
HY-2B China
09 08
C-band
Launch Date
10/06
6/99
Design Life Extended Life Proposed Designed
Operating
CFOSat China/France
Meteor-M3 Russia
Oceansat-2 India
ScatSat India
QuikSCAT USA
sw 24feb11
GLOBAL SCATTEROMETER MISSIONS (CEOS VC)
FY-3E with 2FS China
Operational Series with 2FS India
METOP-A Europe METOP-C Europe
METOP-B Europe EPS SG Europe
Extended
Approved
Extended
No NRT global availability
Availability ?
HY-2A China
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ASCAT-A and ASCAT-B come together
Convective downbursts
Lin et al. manuscripts on rain and wind variability effects on ASCAT
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NWP SAF software users
Global constellation users
OSI SAF Message ListSouth America
Oceania
Europe
Other
Canada
China
Hong Kong
India
Japan
Korea
Russia
Taiwan
USA
KNMI products are used at all major NWP centres (e.g., ECMWF, NCEP,
BoM, JMA, NRL, UKMet, DWD, MF, ..), nowcasting centres (e.g., NHC, ..),
marine services, wind energy branche, universities, …
All EUMETSAT SAF wind product service messages are popular world wide
(service messages also through EUMETCAST and JPL PODAAC)
Both data products and wind processing software are popular
NWP SAF software users
Africa
China
Europa
India
Other Asia
Russia
South America
USA
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Scatterometer winds for tropical storms
• ASCAT has smaller rain effect; splash remains
Japan Meteorological Agency
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Underpredicted surge Delfzijl by 0.5 m
31/10/’6 18Z 1/11/’06 4Z
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Norwegian Meteorological Institute met.no
Polar Low
20.11.2008
ASCAT 10m wind OK
Wind overestimated
NOAA-17 AVHRR
20.11.2008, 18 utc
ASCAT 10m wind
Hirlam8 10m wind
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ERS-2 scatterometer wave train; missed by NWP models
NWP models
miss wave;
Next day
forecast bust
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Missed wave
train in
QuikScat
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Tropical
variability 1. Dry areas reasonable
2. NWP models lack air-
sea interaction in
rainy areas
3. ASCAT scatterometer
does a good job near
rain
4. QuikScat, OSCAT and
radiometers are
affected by rain
droplets
Portabella et al., TGRS, 2012
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Further references • [email protected]
– Registration for data, software, service messages
– Help desk
• EUMETCAST, RMDCN, KNMI FTP
• www.knmi.nl/scatterometer
– Viewer
– Status, monitoring, validation
– User Manual
• EUMETrain, forecasters forum
• NWP SAF monitoring
www.metoffice.gov.uk/research
/interproj/nwpsaf/monitoring.html
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Ocean references
• MyOcean (Copernicus), www.myocean.eu/
• PODAAC, podaac.jpl.nasa.gov/
• eSurge (ESA), www.storm-surge.info/
• MyWave (EU)
• 2011 scatterometer conference, www.eumetsat.int/Home/Main/Satellites/Metop/index.htm?l=en
• IOVWST meetings, coaps.fsu.edu/scatterometry/meeting/
June 2014, IFREMER, Brest, France
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Thanks !