IGARSS 2011
Vancouver, BC, Canada
July 26, 2011
All-Weather Wind Vector Measurements
from Intercalibrated Active and Passive
Microwave Satellite Sensors
Thomas Meissner
Lucrezia Ricciardulli
Frank Wentz
Outline
• Passive (radiometer: WindSat) vs active (scatterometer:
QuikSCAT) wind speed retrievals:
– Surface emissivity versus radar backscatter.
•
•
•
•
•
Ocean Surface Emissivity Model.
Overview: RSS WindSat version 7 ocean products.
WindSat all-weather wind speeds.
Improved QuikSCAT Ku2011 geophysical model function.
Validation.
– High winds.
• Rain impact study.
• Selected storm case: Hurricane Katrina.
• Conclusion: active vs passive - strength +weaknesses.
Passive vs Active Wind Speeds
• Passive (radiometer)
•
• Active (scatterometer)
Sees change in emissivity of wind
roughened sea surface compared
with specular surface
o Low winds: Polarization mixing of
large gravity waves.
o High winds: Emissivity of sea foam.
•
•
•
Sees backscatter from the Braggresonance of small capillary waves.
Geophysical Model Function (GMF)
for wind induced radar backscatter.
Radiative Transfer Model (RTM)
function for wind induced surface
emissivity.
Calibration
Ground truth:
Buoy, NWP
wind speeds
Challenge 1: High Wind Speeds (> 20 m/s)
• Passive (radiometer)
•
•
Lack of reliable ground truth.
(buoys, NWP) for calibration and
validation.
Tropical cyclones: High winds
correlated with rain (challenge 2).
• Active (scatterometer)
•
•
•
Lack of reliable ground truth.
(buoys, NWP) for calibration and
validation.
Tropical cyclones: High winds
correlated with rain (challenge 1).
Loss of sensitivity (GMF saturates).
Challenge 2: Wind Speeds in Rain
• Passive (radiometer)
•
•
•
Rainy atmosphere attenuates
signal.
Emissivity from rainy atmosphere
has similar signature than from
wind roughened surface.
Scattering from rain drops is
difficult to model.
• Active (scatterometer)
•
•
•
•
•
Rainy atmosphere attenuates
signal.
Backscatter from rainy atmosphere
has similar signature than from
wind roughened surface.
Scattering from rain drops is
difficult to model.
Splash effect on surface.
Rain flagging difficult for single
frequency sensor.
Ocean Surface Emissivity Model
•
•
•
•
Crucial part of Radiative Transfer Model (RTM).
Physical basis of passive wind retrieval algorithm.
Meissner + F. Wentz,
Dielectric constant of sea water. T.IEEE
TGRS 42(9), 2004, 1836 - 1849
Wind induced sea surface emissivity.
– Derived from WindSat and SSM/I TB measurements.
– Winds < 20 m/s:
T. Meissner + F. Wentz,
• Buoys.
IEEE TGRS, under review
• NWP.
• Scatterometer.
– Winds > 20 m/s:
• HRD wind analysis (hurricanes).
• SFMR data.
Ocean Surface Emissivity Model (cont.)
• Measured minus computed WindSat TB as function of SST
(x-axis) and wind speed (y-axis).
Overview: RSS Version 7 Ocean Products
• Intercalibrated multi-platform suite.
• 100 years of combined satellite data.
• Climate quality.
DMSP SSM/I, SSMIS
F8, F10, F11, F13, F14 ,F15, F16, F17
TRMM TMI
AMSR-E, AMSR-J
WindSat
QuikSCAT
V7 released
V7 release in progress
RSS WindSat Version 7 Ocean Products
• Optimized swath width by combining for and aft looks at
each band.
Resolution + Required Channels
Product
≥ 6.8 GHz
52 km
≥ 10.7 GHz
31 km
≥ 18.7 GHz
24 km
≥ 37.0 GHz
10 km
SST
Yes
Yes
No
No
Wind speed
no rain
Yes
Yes
Yes
No
Wind speed
through rain
Yes
No
No
No
Wind
direction
No
Yes
No
No
Water vapor
Yes
Yes
Yes
No
Liquid cloud
water
Yes
Yes
Yes
Yes
Rain rate
No
No
No
Yes
New in V7 Radiometer : Winds Through Rain
• Version 6: Rain areas needed to be blocked out.
• Version 7: Rain areas have wind speeds.
• C-band (7 GHz) required:
– WindSat, AMSR-E, GCOM
• Possible with only X-band (11 GHz): TMI, GMI.
• Residual degradation in rain.
WindSat Wind Speed Algorithms
• No-rain algorithm (≥10.7 GHz, 32 km res.)
– Physical algorithm.
– Trained from Monte Carlo simulated TB.
– Based on radiative transfer model (RTM).
• Wind speed in rain algorithms (≥ 6.8 GHz, 52 km res.)
– Statistical or hybrid algorithms
– Trained from match-ups between measured TB and ground truth
wind speeds in rainy conditions.
– Utilizes spectral difference (6.8 GHz versus 10.7 GHz) in wind/rain
response of measured brightness temperatures.
– Same method is used by NOAA aircraft step frequency microwave
radiometers (SFMR) to measure wind speeds in hurricanes.
Radiometer winds in rain:
T. Meissner + F. Wentz,
IEEE TGRS 47(9), 2009, 3065 - 3083
WindSat All-Weather Wind Speeds
• Blending between
,
and
algorithms.
• Depends on SST, wind speed and cloud water.
• Smooth transitions between zones.
L=0.2 mm
W=15 m/s
H-Wind Algo
(tropical cyclones)
No-Rain Algo
SST=28oC
SST
Global Rain Algo
Liquid Water
Wind Speed
WindSat Wind Speed Validation
• 2-dimensional PDF: WindSat versus CCMP (crosscalibrated multi-platform) wind speed.
• Rain free and with rain.
WindSat Wind Validation at High Winds (1)
• Renfrew et al. QJRMS 135, 2009, 2046 – 2066
– Aircraft observations taken during the
Greenland Flow Distortion Experiment,
Feb + Mar 2007.
– 150 measurements during 5 missions.
– Wind vectors measured by turbulence probe.
– Adjusted to 10m above surface.
Improved QuikSCAT Ku2011 GMF: Purpose
• Improvement at high wind speeds.
– When RSS Ku2001 was developed (Wentz and Smith, 1999),
validation data at high winds were limited.
– GMF at high winds had to be extrapolated.
– Analyses showed Ku2001 overestimated high winds.
• WindSat wind speeds have been validated.
– Confident up to 30 – 35 m/s.
– Emissivity does not saturate at high winds. Good sensitivity.
– Excellent validation at low and moderate wind speeds < 20 m/s
(Buoys, SSM/I, CCMP, NCEP,…), > 20 m/s: Aircraft flights.
– WindSat can be used as ground truth to calibrate new Ku-band
scatterometer GMF.
• Produce a climate data record of ocean vector winds.
– Combining QuikSCAT with other sensors using consistent
methodology.
Improved QuikSCAT Ku2011 GMF: Development
• The GMF relates the observed
backscatter ratio σ0 to
wind speed w and direction φ
at the ocean’s surface.
N 5
 0   An ( w)  cos(n  R )
n 0
• To develop the new GMF we used 7 years of QuikSCAT σ0
collocated with WindSat wind speeds (90 min) and CCMP
(Atlas et al, 2009) wind direction.
– WindSat also measures rain rate, used to flag QuikSCAT σ0 when
developing GMF.
– We had hundreds of millions of reliable rain-free collocations, with
about 0.2% at winds greater than 20 m/s.
Ku2001 versus Ku2011
Greenland
Aircraft
Flights
Ku2001 Ku2011
A0
Ku2001 Ku2011
A2
Rain Impact: WindSat/QuikSCAT vs Buoys
• Table shows WindSat/QuikSCAT – Buoy wind speed as
function of rain rate (5 years of data)
Rain Rate
no rain
light rain
0 – 3 mm/h
moderate rain
3 – 8 mm/h
heavy rain
> 8 mm/h
WindSat All-Weather
QuikSCAT 2011
Bias
Std. Dev.
Bias
Std. Dev.
0.04
0.9
0.01
0.9
0.70
1.6
1.7
2.3
0.02
2.0
4.8
3.6
-0.05
2.5
7.1
4.5
Rain Impact: WindSat/QuikSCAT/CCMP
• Figures show WindSat – CCMP and QuikSCAT – WindSat wind
speeds as function of wind speed and rain rate.
– 5 years of data.
• No rain correction for scatterometer has been applied yet.
• With only single frequency (SF) scatterometer (QuikSCAT,
ASCAT) it is very difficult to
– Reliably flag rain events
– Retrieve rain rate which is needed to perform rain correction
Rain Impact on Scatterometer: Caveat
• Rain impact depends on rain rate + wind speed:
– At low wind speeds: QuikSCAT wind speeds too high in rain.
– At high wind speeds: QuikSCAT wind speeds too low in rain.
• Important: Correct GMF at high wind speeds.
– Ku2001 wind speeds too high at high wind speeds.
– Accidental error cancellation possible in certain cases.
WindSat all-weather wind
HRD analysis wind
QuikSCAT Ku 2011 wind
Hurricane Katrina
08/29/2005 0:00 Z
WindSat rain rate
Active vs Passive - Strength + Weaknesses
Condition
Wind
speed
Wind
direction
Rain
detection
Passive
WindSat V7
Active
QuikSCAT
Ku2011 GMF
no rain
low – moderate winds
++
++
no rain
high winds
++
+
rain
+
moderate – high
winds
no - moderate rain
++
++
low winds
+
high rain
+
++
WindSat and
QuikSCAT V7 Data
Sets available on
www.remss.com
++
very good
+
slightly
degraded
strongly
degraded /
impossible
• Assessment based on operating instruments:
– Polarimetric radiometer (WindSat).
– Single frequency scatterometer (QuikSCAT, ASCAT, Oceansat).