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
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 °
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
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
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
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
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
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
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
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
SST Pattern in SAR wind R1. 11:05 UTC May, 5, 2006
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>
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:
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 ).
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>
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>
Reprocess SAR Wind Retrieval Wind Speed Before SST Correction cloud AVHRR SST Wind Speed After SST Correction +
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>
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>
<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>
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>
Notas do Editor
As more and more multi-polarization data become available from SAR satellites, it will become increasingly important to measure consistent winds from all polarization modes. Below, HH and VV winds are compared taken from the same ENVISAT ASAR Alternating Polarization image. The Thompson polarization ratio has been used here. Further work is underway to improve the HH/VV ratio relationship used in the wind Geophysical model Function.
Fig. shows the wind retrieval error for 0.5- and 1.0-dB calibration errors, which are comparable to the current ENVISAT ASAR instrument NRCS calibration errors.
We scatter plot the matchup data in this near shore region (< 100 km) in Fig.
There is clear SST pattern on SAR winds images. The SAR wind speed image is combined by two consecutive Radarsat-1 WS retrieval results. NSS.R1.AF.D06125.T110503.P36N075.P35N074.A.h0.6.CMOD5.aplwind2.hdf NSS.R1.AF.D06125.T110456.P36N075.P35N074.A.h0.6.CMOD5.aplwind2.hdf
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