Geophysical Model Functions for the Retrieval of Ocean Surface Winds

Transcription

1 Geophysical Model Functions for the Retrieval of Ocean Surface Winds Donald R. Thompson and Frank M. Monaldo Johns Hopkins University Applied Physics Laboratory Johns Hopkins Road, Laurel, MD USA Jochen Horstmann Institute for Coastal Research GKSS Research Center Max-Planck-Str. 1, D Geesthacht, Germany Merete Bruun Christiansen National Laboratory for Sustainable Energy Technical University of Denmark Frederiksborgvej 399, 4000 Roskilde, Denmark

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 SeaSAR 2008; Frascati Italy; January

3 Simple Model Physics and Construction of GMFs SeaSAR 2008; Frascati Italy; January

4 Components of Composite Scattering Model The Bragg Scattering Cross Section is: σ B = 8πk r 4 cosθ I R θ I,ε, p ( ) 2 ψ k B { ( )+ ψ( k B )}, with k B = 4π λ sin θ I The Specular Scattering Cross Section is: σ S = 2π R( θ I,ε, p) 2 W k H k z where the surface slope probability density function W(s) has the form W()= s 1 1 2π M exp 1 2 st M 1 s and M is the (long wave) slope covariance matrix. With these definitions, a (schematic) representation of the composite model becomes: 2k B σ comp σ S + W k k B σ B ()dk k 2k r cosθ k c L-band Separation wavenumber, k c, taken from Thompson, et al., TGRS, 43, vol 12, , 2005 SeaSAR 2008; Frascati Italy; January

5 Fitting Composite Model Predictions For a fixed look direction and polarization, the composite model NRCS is expanded as a 3rd order polynomial in wind speed σ comp = 3 a n n=0 ()u θ n where the expansion parameters a n are themselves polynomial functions of the incident angle θ. 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) V-Pol; Up-Wind H-Pol; Cross Wind SeaSAR 2008; Frascati Italy; January

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: [ ] σ( u,θ,φ,p)= au,θ,p ( )+ bu,θ,p ( )cos()+ φ cu,θ,p ( )cos( 2φ) 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. SeaSAR 2008; Frascati Italy; January

7 Behavior of L-Band GMFs SeaSAR 2008; Frascati Italy; January

8 Surface Wave Curvature Spectra; k 4 ψ(k) Curvature Spectra vs Wavenumber Curvature Spectral Density k L k C k X k Ka 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., JGR 102 C7, 15,781-15,796, 1997 Romeiser, et al., JGR, 102, 25,237-25,250, 1997 k (rad/m) SeaSAR 2008; Frascati Italy; January

9 Wind Speed and Azimuth Dependence of L-Band GMF L-Band Cross Section vs Wind Speed L-Band Cross Section vs Azimuth Angle Wind Speed (m/s) Azimuth Angle (deg) SeaSAR 2008; Frascati Italy; January

10 Comparison of L-Band GMFs with Cmod4 Up-Wind Cross Section vs Wind Speed Cross-Wind Cross Section vs Wind Speed 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. SeaSAR 2008; Frascati Italy; January

11 Comparison of X-Band GMFs with Cmod4 Up-Wind Cross Section vs Wind Speed Cross-Wind Cross Section vs Wind Speed 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. SeaSAR 2008; Frascati Italy; January

12 Comparisons of GMFs with L-Band Measurements SeaSAR 2008; Frascati Italy; January

13 Comparison with the L-Band Data of Guinard and Daley (early 1970s) L-Band VV-Pol Cross Section vs Incident Angle L-Band HH-Pol Cross Section vs Incident Angle Incident Angle (deg) Incident Angle (deg) Guinard and Daley, Proc. IEEE 58, , 1970; Daley, Ransone, Burkett, NRL Report 7268, 1971 SeaSAR 2008; Frascati Italy; January

14 Validation Using Airborne Dual Polarization SAR Systems DTU L- / C-Band EMISAR System DLR 4-Frequency E-SAR System Simultaneous L- / C-band SAR Imagery 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 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 SeaSAR 2008; Frascati Italy; January

15 Invert EMISAR C-Band V-Pol Image to Wind Speed Using Cmod4 C-Band V-Pol Cross Section vs Incident Angle Inverted C-Band Wind Speed vs Incident Angle C-VV Image near Great Belt; Wind Speed (m/s) Wind; from 297ºT Flight direction: 249ºT Incident Angle (deg) SeaSAR 2008; Frascati Italy; January

16 Use Inverted Wind to to Compare Predictions of L-Band GMF with Measurements L-Band Cross Section vs Incident Angle Incident Angle (deg) 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. SeaSAR 2008; Frascati Italy; January

17 Comparisons with E-SAR Imagery from the Horns-Rev Campaign L-VV Image near Horns Rev; Wind Direction SeaSAR 2008; Frascati Italy; January

18 Invert E-SAR C-Band V-Pol Images to Wind Speed Using Cmod4 C-Band (V-Pol) Cross Section vs Incident Angle Inverted Wind Speed (C-Band, V-Pol) vs Incident Angle Wind Speed (m/s) Incident Angle (deg) Incident Angle (deg) SeaSAR 2008; Frascati Italy; January

19 Compare Predictions of GMF Using Inverted Winds with E-SAR Up-Wind Measurements L-Band Cross Section vs Incident Angle 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). Incident Angle (deg) SeaSAR 2008; Frascati Italy; January

20 Compare Predictions of GMF Using Inverted Winds with E-SAR Cross-Wind Measurements L-Band Cross Section vs Incident Angle Incident Angle (deg) 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. SeaSAR 2008; Frascati Italy; January

21 Wind Inversion from PALSAR and TerraSAR-X Gibraltar TerraSAR-X NRCS North Sea PALSAR NRCS SeaSAR 2008; Frascati Italy; January

22 Comparison with High-Resolution (WRF ) Meteorological Model Wind Inversion; PALSAR GMF 10 m Winds; 2006 Nov. 1 10:00:00 UT Weather Research and Forecasting Model; SeaSAR 2008; Frascati Italy; January

23 Comparison with High-Resolution (WRF ) Meteorological Model Wind Inversion; PALSAR GMF 10 m Winds; 2006 Nov. 1 10:00:00 UT Weather Research and Forecasting Model; SeaSAR 2008; Frascati Italy; January

24 Comparison with High-Resolution (WRF ) Meteorological Model WRF Output; 2007 July 9 06:30:00 UT Wind Inversion; Elfouhaily GMF Weather Research and Forecasting Model; SeaSAR 2008; Frascati Italy; January

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 dualpolarization 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,..) SeaSAR 2008; Frascati Italy; January

26 Backup Slides SeaSAR 2008; Frascati Italy; January

27 Comparison of L- and X-Band GMFs with Cmod4 Up-Wind Cross Section vs Wind Speed Up-Wind Cross Section vs Wind Speed 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. SeaSAR 2008; Frascati Italy; January

28 Consistency Check at C-Band C-Band Cross Section vs Incident Angle α = 0.6 Incident Angle (deg) 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, ,1998 (green curve). and Mouche, et al. Trans. Geosci. Remote Sensing, 43 #4, , 2005 (blue curve). SeaSAR 2008; Frascati Italy; January

29 PALSAR Wind Inversion PALSAR NRCS Image Extracted Wind Image Isoguchi and Shimada GMF SeaSAR 2008; Frascati Italy; January

30 TerraSAR-X Wind Inversion Raw TSX Image Extracted Wind Image GMF uses Elfouhaily spectrum Calibration from met model SeaSAR 2008; Frascati Italy; January