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  1. 1. SAR-derived Sea Surface Winds in the Seas around Korea and Air-Sea Interaction<br />Kyung-AePark and Tae-Sung Kim<br />Department of Earth Science Education / Research Institute of Oceanography, Seoul National University, KOREA<br />SEOUL(Satellite Earth/Ocean Sciences & EdUcationLab.)<br />
  2. 2. <ul><li>Air-Sea-Land interaction by sea surface wind
  3. 3. Ocean surface circulation
  4. 4. Modification of vertical thermal structure, MLD variability
  5. 5. Air-sea gas exchange through sea surface
  6. 6. Key environmental factor of Ocean Ecosystem
  7. 7. Global/Local Climate Change</li></ul>For operational Use<br /><ul><li>Weather forecasting
  8. 8. Severe weather disasters
  9. 9. Human-related activities
  10. 10. …</li></ul>Role of Sea Surface Wind<br />
  11. 11. <ul><li>Obtained easily
  12. 12. Relatively low spatial resolution : 25km × 25km
  13. 13. Unable to observe winds in the near-shore region
  14. 14. Limitation of understanding coastal phenomena</li></ul>Scatterometer Winds <br />
  15. 15. Importance of Coastal Wind Observation<br /><ul><li>Complicated coastline of Korean Peninsula
  16. 16. Insufficient in-situ wind measurements (5 ocean buoys)
  17. 17. Need for monitoring sea surface winds at the Korean coast
  18. 18. Understanding of coastal wind variability
  19. 19. Korean Coastal regions : </li></ul> Unpredictable Natural Disasters <br /> (severe fog, heavy rainfall, swell, …) <br />
  20. 20. Objectives<br /><ul><li>Retrieval of sea surface wind field along the Korean coast from SAR data
  21. 21. Comparison of the high-resolution SAR-derived wind vectors with wind measurements and other satellite winds to assess the accuracy
  22. 22. Attempted to eliminate noise problems in ALOS PALSAR data
  23. 23. Understanding air-sea interactions through wind field modification over a variety of complex oceanographic phenomena, frequently apparent along the Korean coast.
  24. 24. Understanding of the air-sea-land interactions by relationship between cold-air outbreak and along-coast wind field off Russian coast in the East/Japan Sea</li></li></ul><li>Wind Estimation for Scientific Applications<br />Operationally <br />Used ?<br />
  25. 25. L-band SAR-derivedSea Surface Wind Retrieval<br />
  26. 26. Estimation of Wind Direction<br />Wind measurement data (in situ, other satellite sensor data, reanalysis data, etc)<br />ECMWF Interim Reanalysis Winds<br />Interpolation<br />2<br />1<br />3<br />Incidence Angle<br />Wind Direction<br />Normalized Radar Cross Section(σ0)<br />Algorithms<br />L-band HH GMF<br />Distribution of interpolated near surface wind vectors from ECMWF reanalysis data on 18 Jul 2007<br />Wind Speed<br />
  27. 27. Wind Field from ALOS PALSAR<br />In spite of the noises, low winds are found along the western coast.<br />What Happened ?<br /><ul><li>High wind speed in the offsea region
  28. 28. Low values near the shore</li></li></ul><li>Wind Field from ALOS PALSAR<br />Observing Direction<br />Low Incidence Angle<br />Images of (a) NRCS and (b) retrieved wind field off the east coast of Korea from ALOS PALSAR image (2007/08/11 02:56:56 (UTC))<br /><ul><li> Noises were small but extended over the land.
  29. 29. Reasonable wind speed field, but still contained the erros</li></li></ul><li>Wind Field from ALOS PALSAR<br /><ul><li> Relatively Low Wind Speed near the shore
  30. 30. What mechanisms and processes are related on this? </li></li></ul><li>Backscattering Cross SectionProblems in PALSAR ImagesCauses & SolutionsHow to eliminate noises ?<br />
  31. 31. Backscattering Coefficient (σ0)<br />(a) DN<br />(b) σ0<br />Yellow Sea<br />Dark and Bright<br />Stripe- and Band-like Features<br />Irregularity<br />Animation of DN and NRCS values<br />All images reveal the same issues<br />Also found on Land !<br />
  32. 32. (a) σ0<br />(b) ▽σ0<br />Problems of ScanSAR Image<br />Gradient of NRCS<br />Yellow Sea<br /><ul><li>Land affected by stripes
  33. 33. Careful landuses are needed</li></ul>Maps of (a) NRCS(σ0) and (b) Gradient values of<br />ALOS PALSAR ScanSAR (2007. 07. 18 02:15:15 (UTC))<br />
  34. 34. Wind Field from ALOS PALSAR<br />Observing Direction<br />Low Incidence Angle<br />Images of (a) NRCS and (b) retrieved wind field off the east coast of Korea from ALOS PALSAR image (2007/08/11 02:56:56 (UTC))<br />
  35. 35. Problems of ScanSAR Image<br />Examples (PALSAR) in other regions<br /><ul><li>Not too much noises
  36. 36. The noises are not limited in the Yellow Sea
  37. 37. Non-local, but universal problem</li></li></ul><li>Problems on Other ScanSAR Images<br />Discontinuity<br />Examples (other SARs)<br />RADARSAT-1<br />(Western Lake Superior)<br />COSMO-SkyMed<br />(from Geo-Alliance)<br /><ul><li>Also apparent in other SAR data
  38. 38. Non-local, but Universal problem
  39. 39. The different noise patterns seems to depend on regional characteristics</li></ul>RADARSAT-1<br />(Southeast Alaska coast)<br />
  40. 40. Error Characteristics of SAR-derived Sea Surface Wind<br />
  41. 41. Assessment of Accuracy with QuikSCAT<br /><ul><li> Comparison of PALSAR wind with QuikSCAT wind
  42. 42. Large RMS errors (3~7m/s)
  43. 43. Interference problems from land</li></li></ul><li>Assessment of Accuracy<br />vPALSAR - vQuikSCAT<br /><ul><li> RMSE of wind speed</li></ul> 3.45 m/s (QuikSCAT)<br /> 3.74 m/s (ASCAT)<br /> Large errors along the edge<br />Comparison of wind speeds from ALOS PALSAR and QuikSCAT<br />Overestimation !<br />
  44. 44. Assessment of Accuracy<br />Comparison of wind speeds from PALSAR and QuikSCAT<br /><ul><li>General patterns of spatial distribution of wind speed showed good agreement with each other
  45. 45. No noises</li></li></ul><li>Assessment of Accuracy with QuikSCAT<br /><ul><li> Wind Field from QuikSCAT
  46. 46. Wind Field from PALSAR
  47. 47. Subsampled PALSAR winds at QuikSCAT WVC
  48. 48. Similar wind speed range (~6m/s)
  49. 49. Not too high rms errors</li></li></ul><li>Wind Field from SIR-C/X SAR Image<br />(a)<br />(b)<br /><ul><li> No noises
  50. 50. Fairly good rms errors (<2m/s)
  51. 51. Detailed patial structure of wind speeds</li></ul>(a) Location of SIR-C SAR image on the SST map and (b) distribution of wind field off the east coast of Korea from SIR-C SAR data by using CMOD4 algorithm (m/s).<br />
  52. 52. Error characteristics by wind directions<br /><ul><li>Strong dependence of wind speeds on the wind direction as input data
  53. 53. Difference of wind direction between coastal and outsea regions
  54. 54. MBL stability change-induced deflection of the wind direction</li></ul>e.g. V > 10m/s, <br />Maximum of wind speed error induced by wind direction : 6.09 m/s <br />
  55. 55. How to eliminate the noises on SAR Images ?<br />
  56. 56. DN Values<br />DN values of<br />ALOS PALSAR ScanSAR<br />2007. 07. 18 02:15:15 (UTC)<br />(Center : 125.21ºE, 35.49ºN)<br /><ul><li> Dark and Bright
  57. 57. Stripe- and Band-like Features
  58. 58. Irregularity
  59. 59. Apparent on land </li></li></ul><li>Noise Removal<br />Investigation of their characteristics- Random, Irregular, Band-like Stripes- Similarity along the range direction<br />Assumption :<br /> - Normalized Radar Cross Section (σ0) values corresponding to the noises are almost constant along the range direction<br />
  60. 60. Noise Reduction<br />Subsampling<br />FFT analysis<br />Band-pass Filtering<br />Spectrum from SAR image with noises<br />Spectrum from SAR image without any noises<br />Retrieve the image<br /><ul><li> Try & error process
  61. 61. Determination of the frequency for band-pass filtering </li></ul>Clear image after noise reduction<br />
  62. 62. Noise Reduction<br />Original<br />Noise<br />(a) - (b)<br /><ul><li>Successful filtering of band-like noises
  63. 63. Ships and Small scale oceanic features still remained</li></ul>Subimagewith band-like noises, (b) the mean image of extracted noises from the spectrum, and (c) the filtered ScanSARsubimage<br />
  64. 64. Noise Reduction<br />Noise Filtered<br />Original<br /><ul><li>Band-like noises in the original image almost removed
  65. 65. However, discontinuity between the scans were still apparent.
  66. 66. Further researches for more accurate field</li></li></ul><li>Hypothesis High-resolution SAR wind field Might provide us with Low Wind near the western coast of Korea in summer Particularly at 3 dominant places MBL Dynamics associated with stability changes Sea surface Temperature Change Tidal Currents and Mixing Strong thermal stratification<br />
  67. 67. Coastal Wind Field<br />ERROR!<br />Real Feature ?<br />Nevertheless,<br />Still <br />Meaningful !<br /><ul><li>The hypothesis seems to be correct</li></li></ul><li>Wind Field from ALOS PALSAR<br />Asiana Air OZ733<br />July 26, 1993<br />Gimpo to Mokpo Airport<br />68 /100 died<br /><ul><li>induced by heavy sea fog
  68. 68. Seafog: SST difference ,strong tidal current, Tidal mixing</li></li></ul><li>Wind Field from ALOS PALSAR<br />Oil Spill by ENVISAT<br />Dec 14, 2007<br /><ul><li>Such a disastrous oil spill event occurred near Taean</li></li></ul><li>Effects of Tidal Front<br />(b)<br />(a)<br /><ul><li>Low resolution </li></ul> High-resolution under development<br /> Expectation of detail structure coincident with wind field<br />(a) Wind field from ALOS PALSAR (2007. 07. 18 02:15:15 (UTC)) and (b) Contours of log(H/U3) in the spring tide<br />
  69. 69. Land-Air-Sea Interactionsfrom Scatterometer and SARIntensification of Winds throughLand OrographyCharacteristic Ekman Pumping Structure<br />
  70. 70. Schematic of air flow over coastal ranges cold-air outbreak in winter<br />Role of Wind Field has been paid no attention <br />Beardsley et al.(2000),<br />The mountain ranges along the North Korean and Russian coasts reach heights of 1000m or higher, and are separated by an orographic gap at Vladivostok<br />The wintertime synoptic weather pattern is dominated by the Siberian high, which forces northerly or northwesterly winds(cold-air outbreak) from the Asian continent over the JES <br />36<br />
  71. 71. Ekman Pumping Variability<br />Miniature of Global Ocean<br />The East/Japan Sea<br /><ul><li> Zero line of EKP coincides with maximum wind speed.
  72. 72. Characteristic EKP structure
  73. 73. Hypothesis on air-sea-land interaction</li></ul> Fig. (a) Land topography,and (b) mean of Ekman pumping. Contour overlaid in (a) is mean of EKP. Contours with zero EKP corresponds to the enter of strong winds along the wind direction, which seem to be affected by intensified winds through shallow trough of land orography.<br />37<br />
  74. 74. Evidence of Orography-Induced Ekman Pumping Structure<br />VladivostokGap<br /> Fig. Land topography, wind speed, and EKP along the Russian and North Korean coast. Zero value of EKP corresponds to the center of strong winds along the wind direction, which is produced by intensified winds through a shallow trough of land orography.<br />38<br />
  75. 75. Effects of Topography on SAR Image<br />Wind vectors from ECMWF reanalysis data off Primorye coast<br />Wind field from ALOS PALSAR (2008. 01. 29 01:26:07 (UTC))<br /><ul><li>Strong wind blows to the offshore in winter.
  76. 76. SAR wind field : detailed spatial structure and elongated wind speed field.</li></li></ul><li>Wind field from PALSAR / QuikSCAT<br />Wind field from QuikSCAT<br />Wind field from PALSAR<br /><ul><li>PALSAR winds : Small-scale, detail features
  77. 77. QuikSCAT winds : Similar to SAR Wind, low resolution</li></ul> No observation along the coast.<br />
  78. 78. Influence of Land Orography on Winds<br />Relationship along-coast wind speed and land elevation (< 50 km)<br />R = -0.7395<br /><ul><li> Subset : along-coast topography, mean of wind speeds along offshore direction
  79. 79. Negative Relationship
  80. 80. As land elevations are lower, so the wind speeds tend to increase
  81. 81. Confirmed the potential impact of land orography on the EKP structure.</li></li></ul><li>Summary & Conclusion<br />In this study, we retrieved the high-resolution wind fields along the coast of Korea from SAR images and assessed the accuracy of wind vectors.<br />We found some noisy signals, but still PALSAR ScanSAR images provide us some meaningful ocean signal.<br />Winds at the coastal region were significantly low relative to those in the outsea. This supports our hypothesis on winds, SSTs, tides, and MBL dynamics.<br />We could find an evidence on Land-Air-Sea Interactions through the relationship between land orography and characteristic EKP structure by the intensification of wind speed change through the orographic gaps. <br />In order to use SAR wind field for diverse purposes, the wind errors and causes should be beforehand specified and understood regionally and globally.<br />
  82. 82. Thank You !<br />