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Geophysical Model Functions for the Retrieval of Ocean Surface Winds

Jochen Horstmann Institute for Coastal Research GKSS Research Center Max-Planck-Str. 1, D-21502 Geesthacht, Germany jochen.horstmann@gkss.de. Merete Bruun Christiansen National Laboratory for Sustainable Energy Technical University of Denmark Frederiksborgvej 399, 4000 Roskilde, Denmark

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Geophysical Model Functions for the Retrieval of Ocean Surface Winds

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  1. Jochen Horstmann Institute for Coastal Research GKSS Research Center Max-Planck-Str. 1, D-21502 Geesthacht, Germany jochen.horstmann@gkss.de Merete Bruun Christiansen National Laboratory for Sustainable Energy Technical University of Denmark Frederiksborgvej 399, 4000 Roskilde, Denmark merete.bruun.christiansen@risoe.dk Geophysical Model Functions for the Retrieval of Ocean Surface Winds Donald R. Thompson and Frank M. Monaldo Johns Hopkins University Applied Physics Laboratory 11100 Johns Hopkins Road, Laurel, MD 20708 USA donald.r.thompson@jhuapl.edu frank.monaldo@jhuapl.edu

  2. Outline • Motivation • Recent Availability of ALOS and TerraSAR-X Imagery • Utility of L-, C-, and X-Band for High Wind Remote Sensing • Description of Models • Based on Wind-Dependent Surface Wave Spectral Models • Simple Composite-Model Scattering Physics • Issues • Discussion of Model Behavior • Polarization Dependence • Comparisons with Airborne SAR Data • Simultaneous Dual-Polarization C- and L-Band • Recent ALOS and TerraSAR-X Wind Inversions • Issues • Summary and Future Plans Thompson, Monaldo, Horstmann, and Christiansen

  3. Simple Model Physics andConstruction of GMFs Thompson, Monaldo, Horstmann, and Christiansen

  4. The Bragg Scattering Cross Section is: The Specular Scattering Cross Section is: where the surface slope probability density function W(s) has the form and M is the (long wave) slope covariance matrix. With these definitions, a (schematic) representation of the composite model becomes: Components of Composite Scattering Model L-band Separation wavenumber, kc, taken from Thompson, et al., TGRS, 43, vol 12, 2810-2821, 2005 Thompson, Monaldo, Horstmann, and Christiansen

  5. For a fixed look direction and polarization, the composite model NRCS is expanded as a 3rd order polynomial in wind speed where the expansion parameters an are themselves polynomial functions of the incident angle . Typical fits for V-Pol upwind and H-Pol crosswind as a function of wind speed for incident angles between 20 (highest curve) and 60 (lowest curve) are shown below Wind Speed (m/s) Wind Speed (m/s) NRCS (dB) NRCS (dB) H-Pol; Cross Wind V-Pol; Up-Wind Fitting Composite Model Predictions Thompson, Monaldo, Horstmann, and Christiansen

  6. Construction of the Geophysical Model Function Using the procedure outlined above, we can compute the relevant best-fit parameters for the up-wind, cross-wind, and down-wind NRCS at both V- and H-polarization. With this assumption, the full geophysical model function may be written as: where f is the angle between the radar look-direction and the wind direction and the coefficients a(u,q, p), b(u,q, p) and c(u,q, p) are found by inverting the above equation at up-wind, cross-wind, and down-wind looks. Note that for the results to shown in this presentation, the up-/down-wind ratio is assumed to be unity; i.e. b(u,q, p) is assumed to be zero. Thompson, Monaldo, Horstmann, and Christiansen

  7. Behavior of L-Band GMFs Thompson, Monaldo, Horstmann, and Christiansen

  8. Curvature Spectra vs Wavenumber Curvature Spectral Density k (rad/m) Surface Wave Curvature Spectra; k4(k) • Differences in curvature spectra are quite large in the vicinity of the L- and C-band wavenumbers • Scattering model based on Elfouhaily’s spectrum shows agreement with the empirical CMOD models • Elfouhaily, et al., JGR102 C7, 15,781-15,796, 1997 • Romeiser, et al., JGR, 102, 25,237-25,250, 1997 kC kX kKa kL Thompson, Monaldo, Horstmann, and Christiansen

  9. L-Band Cross Section vs Azimuth Angle L-Band Cross Section vs Wind Speed NRCS (dB) NRCS (dB) Wind Speed (m/s) Azimuth Angle (deg) Wind Speed and Azimuth Dependence of L-Band GMF Thompson, Monaldo, Horstmann, and Christiansen

  10. Up-Wind Cross Section vs Wind Speed Cross-Wind Cross Section vs Wind Speed NRCS (dB) NRCS (dB) Wind Speed (m/s) Wind Speed (m/s) Wind sensitivity of Cmod4 is greater than L-band GMFs based on spectral models. Wind sensitivity of Isoguchi (PalSAR) L-band GMF is comparable to Cmod4 for wind  15 m/s. Comparison of L-Band GMFs with Cmod4 Thompson, Monaldo, Horstmann, and Christiansen

  11. Up-Wind Cross Section vs Wind Speed Cross-Wind Cross Section vs Wind Speed NRCS (dB) NRCS (dB) Wind Speed (m/s) Wind Speed (m/s) Wind sensitivity of Cmod4 is similar to that of all the X-band GMFs, but the Masuko model is significantly lower than spectrally based GMFs at lower wind speeds. Comparison of X-Band GMFs with Cmod4 Thompson, Monaldo, Horstmann, and Christiansen

  12. Comparisons of GMFs with L-Band Measurements Thompson, Monaldo, Horstmann, and Christiansen

  13. L-Band VV-Pol Cross Section vs Incident Angle NRCS (dB) Incident Angle (deg) Comparison with the L-Band Data of Guinard and Daley (early 1970s)† L-Band HH-Pol Cross Section vs Incident Angle NRCS (dB) Incident Angle (deg) †Guinard and Daley, Proc. IEEE 58, 543-550, 1970; Daley, Ransone, Burkett, NRL Report 7268, 1971 Thompson, Monaldo, Horstmann, and Christiansen

  14. Validation Using Airborne Dual Polarization SAR Systems DLR 4-Frequency E-SAR System DTU L- / C-Band EMISAR System • The fully-polarimetric L- C-band EMISAR system is operated by the Electromagnetics Institute (EMI) of the Danish Technical University • Simultaneous L- C-band dual-pol SAR imagery were collected at both frequencies near the Great Belt Simultaneous L- / C-band SAR Imagery • The fully-polarimetric 4-frequency (P-, L-, C-, and X-band) E-SAR system is operated by the SAR-Technology Institute of the German Space Agency (DLR) • Dual-polarization (L- and C-band) SAR imagery were collected in the Horns Rev campaign http://www.dlr.de/hr/en/desktopdefault.aspx/tabid-2326/ http://www.oersted.dtu.dk/English/research/drc/rs/sensors/ Thompson, Monaldo, Horstmann, and Christiansen

  15. Invert EMISAR C-Band V-Pol Image to Wind Speed Using Cmod4 C-Band V-Pol Cross Section vs Incident Angle NRCS (dB) Inverted C-Band Wind Speed vs Incident Angle C-VV Image near Great Belt; 16.06.98 Wind Speed (m/s) Flight direction: 249ºT Wind; from 297ºT Incident Angle (deg) Thompson, Monaldo, Horstmann, and Christiansen

  16. Use Inverted Wind to to Compare Predictions of L-Band GMF with Measurements • Mean Wind Speed: 5.7 m/s; Direction (relative to radar look): 138º • GMF using Elfouhaily’s spectrum shows good agreement with the data at V-pol and is ~2-3 dB low at H-pol. • GMFs using the Romeiser spectrum are too high at V-pol over the entire range of incident angles and also too high at H-pol for angles < 40º or so. • Isoguchi GMF (H-pol only) agrees well with Romeiser result, but is several dB larger than data. L-Band Cross Section vs Incident Angle NRCS (dB) Incident Angle (deg) Thompson, Monaldo, Horstmann, and Christiansen

  17. L-VV Image near Horns Rev; 12.10.03 Comparisons with E-SAR Imagery from the Horns-Rev Campaign Wind Direction Thompson, Monaldo, Horstmann, and Christiansen

  18. C-Band (V-Pol) Cross Section vs Incident Angle Inverted Wind Speed (C-Band, V-Pol) vs Incident Angle NRCS (dB) Wind Speed (m/s) Incident Angle (deg) Incident Angle (deg) Invert E-SAR C-Band V-Pol Images to Wind Speed Using Cmod4 Thompson, Monaldo, Horstmann, and Christiansen

  19. L-Band Cross Section vs Incident Angle NRCS (dB) Incident Angle (deg) Compare Predictions of GMF Using Inverted Winds with E-SAR Up-Wind Measurements • GMF using Elfouhaily’s spectrum shows good agreement with the data at V-pol for angles > 35º or so and is ~4 dB low at H-pol for angles > 35º. • GMF using the Romeiser spectrum is too high over the entire range of incident angles; significantly at V-pol and between ~2-5 dB at H-pol. • Isoguchi (PalSAR) GMF is also too high out to 43º (suggested maximum). Thompson, Monaldo, Horstmann, and Christiansen

  20. L-Band Cross Section vs Incident Angle NRCS (dB) Incident Angle (deg) Compare Predictions of GMF Using Inverted Winds with E-SAR Cross-Wind Measurements • Elfouhaily’s spectrum again shows good agreement with the data at V-pol and is ~3-4 dB low at H-pol for angles > 35º. • V-pol GMF using the Romeiser spectrum again is significantly too high over the entire range of incident angles. It shows reasonable agreement at H-pol for angles > 45º or so. • The H-pol Isoguchi (PalSAR) GMF is again too high over its range of validity. Thompson, Monaldo, Horstmann, and Christiansen

  21. Gibraltar TerraSAR-X NRCS North Sea PALSAR NRCS Wind Inversion from PALSAR and TerraSAR-X Thompson, Monaldo, Horstmann, and Christiansen

  22. 10 m Winds; 2006 Nov. 1 10:00:00 UT Comparison with High-Resolution (WRF†) Meteorological Model Wind Inversion; PALSAR GMF †Weather Research and Forecasting Model; http://www.wrf-model.org/ Thompson, Monaldo, Horstmann, and Christiansen

  23. 10 m Winds; 2006 Nov. 1 10:00:00 UT Comparison with High-Resolution (WRF†) Meteorological Model Wind Inversion; PALSAR GMF †Weather Research and Forecasting Model; http://www.wrf-model.org/ Thompson, Monaldo, Horstmann, and Christiansen

  24. Comparison with High-Resolution (WRF†) Meteorological Model Wind Inversion; Elfouhaily GMF WRF Output; 2007 July 9 06:30:00 UT †Weather Research and Forecasting Model; http://www.wrf-model.org/ Thompson, Monaldo, Horstmann, and Christiansen

  25. Summary /Future Plans • New (first cut) L-band geophysical model functions based on the wind dependent spectral models of Elfouhaily et al., and Romeiser et al. and simple composite scattering physics (no up-wind/down-wind asymmetry). • GMF predictions at L-band were compared against simultaneous dual-polarization SAR data from the Danish EMISAR and German E-SAR systems. • The GMF using the Elfouhaily spectrum shows generally good agreement with the V-Pol SAR data, but is ~2-4 dB low for H-Pol. The GMFs using the Romeiser spectral model is somewhat better at H-Pol, but significantly higher than the measurements at V-Pol. • The empirically-based ALOS GMF has stronger wind sensitivity than either of the spectrally-based models and differs in magnitude from them, especially at higher wind speeds. The spectrally based L-band GMFs (at both V- and H-Pol) are less sensitive to wind than the C-band Cmod4; X-band wind sensitivity is similar to C-band. • IDL codes for all the GMFs are available. • Verify the sensor calibration accuracy for both ALOS and TerraSAR-X. • Continue comparisons with ALOS and TerraSAR-X as data become available. • Ocean surface NRCS values from ALOS wide-swath H-Pol imagery are somewhat higher than predictions from the spectrally-based GMFs. • Refine L- and X-band GMFs. (Improve scattering physics, up- / down-wind dependence, approximately account for nonlinear waves, …..) Thompson, Monaldo, Horstmann, and Christiansen

  26. Backup Slides Thompson, Monaldo, Horstmann, and Christiansen

  27. Up-Wind Cross Section vs Wind Speed Up-Wind Cross Section vs Wind Speed NRCS (dB) NRCS (dB) Wind Speed (m/s) Wind Speed (m/s) Wind sensitivity of Cmod4 is greater than L-band GMFs based on spectral models. Wind sensitivity of Isoguchi (PalSAR) L-band GMF is comparable to Cmod4 for wind  15 m/s. Comparison of L- and X-Band GMFs with Cmod4 Thompson, Monaldo, Horstmann, and Christiansen

  28. C-Band Cross Section vs Incident Angle NRCS (dB) Incident Angle (deg) Consistency Check at C-Band • Radar look direction (relative to up wind) is 138º. • The red (V-Pol) curve is consistent with the data as expected. • The H-Pol curves were computed using CMOD4 and the polarization-ratio models of Thompson et al., Proc. Igarss98, 1671– 1673,1998 (green curve). and Mouche, et al. Trans. Geosci. Remote Sensing, 43 #4, 753-769, 2005 (blue curve).  = 0.6 Thompson, Monaldo, Horstmann, and Christiansen

  29. PALSAR NRCS Image PALSAR Wind Inversion Extracted Wind Image Isoguchi and Shimada GMF Thompson, Monaldo, Horstmann, and Christiansen

  30. TerraSAR-X Wind Inversion Raw TSX Image • Extracted Wind Image • GMF uses Elfouhaily spectrum • Calibration from met model Thompson, Monaldo, Horstmann, and Christiansen

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