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).