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THE IMPACT OF OCEAN SURFACE FEATURES ON THE WIND RETRIEVAL FROM SAR Xiaofeng Yang, Ziwei Li Institute of Remote Sensing Applications, CAS , Beijing 100101, China Xiaofeng Li, William Pichel NOAA/NESDIS

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OUTLINE <ul><li>Introduction </li></ul><ul><li>SAR Wind Speed Validation </li></ul><ul><li>Boundary Layer Stability Effect </li></ul><ul><li>Other Impacts </li></ul><ul><li>Summary </li></ul>

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SAR wind retrieval <ul><li>NRCS (Normalized ) increases with wind speed. </li></ul><ul><li>Given a specific NRCS, it can not obtain a unique wind speed and direction pair. </li></ul><ul><li>A single NRCS measurement is not sufficient to infer wind speed. (unlike QuikSCAT, multi-angle measurements). </li></ul><ul><li>However, if wind direction is known a priori, it is possible to use the equation to estimate wind speed. </li></ul><ul><ul><li>(a) from atmospheric models (NOGAPS, GFS...) or other observation data; </li></ul></ul><ul><ul><li>(b) using linear features in SAR image itself. </li></ul></ul>ASAR WSM wind speed retrieval (02:13:50 UTC, Feb. 21, 2010) GMF: CMOD5 /Pol: VV/ Incid: 35 °

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NOAA Operational SAR Winds Product <ul><li>From 1990s, NOAA has sponsored several projects to develop and demonstrate the ability to use satellite SAR imagery to produce high resolution wind speed estimates. </li></ul><ul><li>All SAR wind data used in this study are from NOAA NESDIS SAR wind system. </li></ul>ALOS Winds – PNG Wind Image December 8, 2007 18:47 UTC RADARSAT-1 Winds – Google Earth kmz Image 03/14/2007 03:29 UT

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<ul><li>SAR Wind Speed Validation </li></ul>

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Comparison ASAR with buoy <ul><li>17 months of ASAR VV imagery off U.S. Coast is used. </li></ul><ul><li>SAR winds are compared with NDSC buoy wind observations. </li></ul><ul><li>SAR winds within 5 km area centered on buoy location are averaged. </li></ul><ul><li>Time difference between buoy and SAR is no more than 20 min. </li></ul><ul><li>Comparison results are slightly better than published results between Radarsat-1 SAR and buoy ( Frank Monaldo...,2001): </li></ul><ul><ul><li>Bias=0.85m/s </li></ul></ul><ul><ul><li>STD=1.76m/s </li></ul></ul>X. Yang et al , IEEE TGRS, 2011

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Comparison SAR with M odel <ul><li>ASAR VV wind retrievals are compared with NOGAPS model winds . </li></ul><ul><li>Also better than R1 and NOGAPS comparisons (Monaldo, 2001): </li></ul><ul><ul><li>Bias=0.05 m/s </li></ul></ul><ul><ul><li>STD=4.07 m/s </li></ul></ul>X. Yang et al , IEEE TGRS, 2011

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Comparison SAR with scatterometer 234 R-1 SAR images collected off the U.S. West Coast are reprocessed to winds and compared with QuikSCAT . 111 ASAR VV images collected off the U.S. Coast are reprocessed to winds and compared with ASCAT . X. Yang et al , IEEE TGRS, 2011

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Polarization Ratio Issue <ul><li>Sea surface wind is retireved from 18 ASAR AP(alternating polarization) data. </li></ul><ul><li>use Thompson (1998) function with polarization ratio α =0.6 to process HH data. </li></ul><ul><li>Then compared the HH retrievals with VV retrievals. </li></ul><ul><li>HH Wind is higher: </li></ul><ul><ul><li>Bias: -1.44m/s </li></ul></ul><ul><ul><li>STD: 2.08m/s </li></ul></ul>Match number: 7,742,525 Bias (VV-HH): -1.44 m/s STD: 2.08 m/s

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Calibration Effect <ul><li>Wind retrieval depends upon the absolute NRCS calibration accuracy; </li></ul><ul><li>For low-to-moderate winds that are under 15 m/s, a 0.5– 1.0 dB calibration error will not have a big impact on SAR wind retrievals for moderate incidence angles. </li></ul><ul><li>For high winds over 20 m/s, in storm or hurricane conditions, the 0.5– 1.0 dB SAR calibration error will induce 3–8-m/s errors using the CMOD5 algorithm, due to the saturation of the GMF. </li></ul><ul><li>In general, a 0.5-dB calibration accuracy requirement will help to improve the GMF wind retrieval accuracy under hurricane conditions. </li></ul>X. Yang et al , IEEE TGRS, 2011

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Near Shoal SAR Wind Accuracy <ul><li>We performed a SAR VS QuikSCAT wind as a function of distance to the coastline. </li></ul><ul><li>the bias reduces as the matchup points are located further away from the coast. </li></ul><ul><ul><li><75 km, RMS error is as high as 2.8 m/s, and STD is about 2.3 m/s. </li></ul></ul><ul><ul><li>>100km, The STD reduces to about 1.4 m/s, and becomes stable. </li></ul></ul>X. Yang et al , IEEE GRSL, 2011

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Near Shoal SAR Wind Accuracy-2 QuikSCAT winds are inconsistent with the SAR winds within 100 km from the coastline. An empirical exponential relationship to corrected coastal QuikSCAT winds is proposed as follow: X. Yang et al , IEEE GRSL, 2011

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<ul><li>Boundary Layer Stability Effect </li></ul>

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SST Pattern in SAR wind R1. 11:05 UTC May, 5, 2006

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How can SST affect SAR winds retrieval <ul><li>SAR winds retrieval is based on the GMFs, which describe the relationship of NRCS and wind field. </li></ul><ul><li>Several observations has been reported to present results on radar NRCS across the SST front. </li></ul><ul><li>They ( Hayes 1981, Babin 2000, Pablo Clemente-colon 2001… ) found that the NRCS difference is caused by changes in atmospheric boundary layer stability conditions. </li></ul><ul><ul><li>Stability changes produce changes in the turbulent flow over the sea surface. </li></ul></ul><ul><ul><li>Air-sea stability is influenced by the sea surface wind and the air-sea temperature difference. </li></ul></ul>

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SST-NRCS Modeling <ul><li>Some models were developed to quantitatively analyze the SST effect on radar backscatter: </li></ul>Quanan Zheng （ 1997 ）： Pablo Clemente-colon （ 2001 ）： Hayes （ 1981 ）： Based on theories, we propose a radar backscatter correction model which takes into account the relationship between NRCS and air-sea boundary stability for wind retrieval:

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Model Coefficients Determination warm cold △ SST data obtained from NOAA AVHRR △ NCRS extracted from SAR images Assuming wind speed of low SST area is the actual wind speed. We use 116 RADARSAT-1 SAR, AVHRR SST and NDBC buoy wind data to calculate function coefficients (α1andα2 ).

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Model Validation <ul><li>Using buoy wind speed data to test the new NRCS-SST model: </li></ul><ul><ul><li>SST data come from AVHRR </li></ul></ul><ul><ul><li>NRCS extracted from radarsat-1 images: average of 10*10 pixels at both sides of GS boundary </li></ul></ul><ul><ul><li>Mean wind speed of low SST pixels is used as initial U10 </li></ul></ul><ul><ul><li>Using cmod5 to convert corrected NRCS to wind speed </li></ul></ul><ul><ul><li>Compare the corrected wind speed to buoy . Only Buoys located at high temperature area are used. </li></ul></ul>

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Wind Speed Comparison <ul><li>Total number of match up data is 68; </li></ul><ul><li>After correction the wind speed RMS error can reduce to less than 2m/s. </li></ul>

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Reprocess SAR Wind Retrieval Wind Speed Before SST Correction cloud AVHRR SST Wind Speed After SST Correction +

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IV. Other Impacts

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Middle latitude sea ice ENVISAT ASAR （ 02:07:16 UTC, Dec 30, 2009 ） <ul><li>In mid-latitude area, some coastal regions will covered by sea ice during the winter. </li></ul><ul><li>Sea ice coverage changes every day, but the land mask process used in SAR wind retrieval don’t consider this so far. </li></ul>

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Ships and platforms RADARSAT-1 (11:04:51 UTC, Jan 29, 2006) <ul><li>High resolution imaging radar is used to detect ships and platforms. </li></ul><ul><li>These artificial objects will cause significant retrieval error of local wind speed. </li></ul>

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Internal waves RADARSAT-1 image (10:07:55 UTC, Apr 7, 2001)

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<ul><li>Summary </li></ul><ul><li>SAR wind retrieval is as accurate as operational model and scatterometer wind product in low to moderate wind speed, but have higher resolution and near coast observation ability. </li></ul><ul><li>Calibration accuracy and Multi-Pol GMFs are very important for SAR wind retrieval. </li></ul><ul><li>Many oceanic and atmospheric phenomena change the surface roughness and affect SAR backscatter signatures. Thus, introducing additional preprocessing to remove and/or compensate those effects can greatly improve the accuracy of SAR sea surface wind retrieval. </li></ul>

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Thanks! <ul><li>Acknowledgment </li></ul><ul><li>RADARSAT-1 SAR data are provided by CSA. </li></ul><ul><li>ENVISAT ASAR data are provided by ESA . </li></ul><ul><li>QuikSCAT and ASCAT wind products are provided by the NASA/JPL Physical Oceanography Distributed Active Archive Center (PO.DAAC) data distribution site. </li></ul><ul><li>AVHRR SST data are provided by the NOAA CoastWatch. </li></ul><ul><li>Buoy data are provided by NOAA National Data Buoy Center (NDBC). </li></ul>