VALIDATION OF THE ASAR WAVE MODE LEVEL 2 PRODUCT USING WAM AND BUOY SPECTRA

Transcription

1 VALIDATION OF THE ASAR WAVE MODE LEVEL 2 PRODUCT USING WAM AND BUOY SPECTRA Harald Johnen (1), Geir Engen (1), Bertrand Chapron (2) (1) Norut Informajonteknologi AS, PP 6463 Forkningparken, N-9294 Tromø, harald.johnen@itek.norut.no (2) IFREMER, PP 70, Plouzane, France, bchapron@ifremer.fr ABSTRACT More than 70 thouand of ASAR Wave Mode Level 2 product are validated globally uing co-located ECMWF WAM pectra and wind peed data. A large number of co-located ASAR, WAM and buoy pectra are alo ued in the validation. The data ued in the validation cover all ASAR Wave Mode acquiition in the period April and March Globally, the geophyical validation of wind and wave parameter how reaonably agreement for the mean wave period, ignificant waveheight and mean wave direction, epecially for the long wavelength part of the pectra. However, for certain coatal and heltered area larger deviation are oberved, probably due to fetch limitation and/or impact of other coatal urface feature on the SAR ignature. On average the ASAR tend to meaure a lightly longer mean well period ( 0.8) than predicted by WAM. Comparing ASAR, WAM and buoy pectra, example are hown where ASAR conitently add information on the well ytem beyond what i predicted by WAM. For the ASAR Level2 ignificant waveheight, aturation effect at high ea-tate are oberved, increaing with increaing wind peed. Le aturation i oberved for the long wavelength part of the pectrum. The aturation i motly due to the well-known effect of azimuth cut-off, which increae with increaing ea tate. At low wind peed an overetimation of waveheight i oberved compared to WAM. Thi i caued by the ue of CMOD-IF2 baed MTF [5] which underetimate the backcatter modulation at low wind peed a compared to SAR due to the high patial variability of SAR cro ection at low wind peed. Statitic and global map comparion are alo preented, howing the capabilitie of ASAR Wave Mode to provide global coverage of wave pectral parameter. The ASAR Wave Mode Level 2 pectra are given on log-polar grid in wavenumber and direction domain, k, ϕ) [4]. Note that the ASAR pectrum i in general not the total ocean wave pectrum, but only the wave pectrum within the SAR imaging domain [3]. The ize of thi domain i again dependent on the ea tate. The frequency-, f ) and directional pectra, ψ ( ϕ) are then derived from the Level 2 pectra, F ( k, ϕ) according to the formula: (1) F ( f ) = k, ϕ) k dkdf dϕ (3) (2) ψ ( ϕ) = F ( k, ϕ) dk φ( f ) = tan 1 k, ϕ)inϕdϕ k, ϕ) coϕdϕ where dkdf = 4π k / g. The ignificant waveheight, H, mean period, T,T, and mean wave H p p direction, Φ are then computed a: fmax 1/ (4) H = 4 f ) df, H = 4 f ) df (5) T p fmax f fmin f ) f 1 1/ fmin 1 min min =, T = f p max 1/ f ) df f ) df fmin df f f ) f fmin df 1. VALIDATION APPROACH Following the approach of validation of ASAR Wave Mode given in [1], more than two month of global coverage of ASAR wave mode data i validated againt co-located ECMWF model data, and where available alo locally againt buoy data from the NDBC and MEDS network. Statitic of ASAR- WAM deviation are preented together with global coverage comparion. (6) fmax f )in( φ( f )) df min Φ = tan 1 f f max f ) co( φ( f )) df fmin

2 where f, f min max are the lowet and highet frequencie in the pectrum to be computed over. Similar parameter can be derived from the co-located WAM and buoy pectra. Spectral peak period, T and direction, Φ peak peak are alo extracted from the pectra and compared. The wave direction are alway clock-wie from north toward the direction the wave propagate. patial cale are very different (ASAR 5 10km, 2 WAM50 50km ) VALIDATION RESULTS In the following, reult from the geophyical validation of wave pectra parameter, wind field and radar cro ection are hown and dicued. 2.1 Wave Spectra Parameter Validation of wave parameter i performed globally againt ECMWF WAM pectra and locally againt buoy data from the network of buoy outide US, Canada and Hawaii. The location of the buoy ued are hown in Fig. 1. The data ued in the validation cover all ASAR Wave Mode acquiition in the period April and March Fig. 2. Example of triple co-location of ASAR, WAM and buoy outide Hawaii. In the validation proce we have retricted the ASAR data et to thoe with normalized image variance 2 between σ 2 [ 1.0,1. 4]. Thi i done in order to µ avoid data corrupted by lick, current and other coatal urface feature. Fig. 3 how f ) and φ( f ) pectra from ASAR, ECMWF WAM, and buoy computed according to equation (1) and (2). Note that only f ) pectra are available from buoy, except for buoy Fig. 1. Map howing the location of buoy ued in validation of ASAR WM Level2 wave pectra. The co-location of the data i done at IFREMER/Cerat. A typical output of the co-location i hown in Fig. 2. The patial ditance between ASAR, buoy and WAM hould be taken into account when interpreting the plot in Fig. 3. The WAM and ASAR are alway well co-located patially, while the ASAR and buoy are eldom co-located better than 50km. In the comparion between WAM and ASAR parameter, we hould alo keep in mind that the

3 Fig. 3. Example of one-dimenional frequency (left) and directional pectra (right) extracted from ASAR Level 2 wave pectra (full line), WAM wave pectra (dahed line), buoy pectra (dotted line). Fig.3 how that the ASAR predict the well part of the wave pectrum well, both in term of energy and hape for thee data. We alo ee example where ASAR and buoy agree well while the WAM differ ignificantly. For the wind ea part of the pectrum, we ee that ASAR fail to meaure it properly. Bet correlation with buoy data i oberved in open ocean area.

4 Global tatitic of the comparion of ignificant waveheight, mean and peak wave period, and mean and peak wave direction from ASAR and WAM are hown in Fig. 4 to Fig. 7. Fig. 4 and Fig. 5 how the reult for the ignificant waveheight. Fig.6 how the comparion between ASAR and WAM mean and peak wave period, and Fig. 7 how the ame for the wave direction. a) b) Fig. 4. Scatter plot of ignificant waveheight derived from co-located WAM and ASAR Wave Mode pectra globally, a) : H, b) : H. a) b) Fig. 6. Scatter plot of mean and peak wave period derived from co-located WAM and ASAR Wave Mode pectra globally, a) : T p p, b): T, c) : T. peak c) Fig. 5. Waveheight difference between ASAR and WAM a function of wind peed (left plot : H, right plot: ). H Fig.4 and Fig.5 how that ASAR Level 2 product predict the ignificant waveheight well, epecially at wind peed between 5 m/ and 10 m/. At higher wind peed the azimuth cut-off will highly affect the ASAR pectrum by filtering out the contribution from the wind ea part of the pectrum. At lower wind peed the SAR tend to overetimate the waveheight. The latter i mot likely due to the ue of catterometer baed CMOD-IF2 in the SAR wave retrieval. The lowreolution CMOD-IF2 derived backcatter tend to be biaed high for low wind peed when compared to high-reolution SAR backcatter (ee Fig. 10a). Thi will affect not only the wind peed retrieval but alo the wave retrieval ince the RAR MTF ued i baed on the CMOD-IF2 propertie [5]. a) b) c) Fig. 7. Scatter plot of mean and peak wave direction derived from co-located WAM and ASAR Wave Mode pectra globally, a): Φ, b) : Φ, c) : Φ. peak Fig.6 how that the SAR lightly meaure a larger value than the WAM for both the mean and the peak period, which for ome cae can be explained by the

5 azimuth mearing effect. However, comparion with buoy (ee Fig. 3) how that WAM ometime underetimate the well. If we retrict the computation of the mean period to wave with period larger than, both the bia and the RMS error are reduced, but the bia i till ignificant (0.8). From the global plot hown in Fig. 9, we will ee that the overetimation of mean wave period dominate at certain coatal and heltered area. For the wave direction we ee from Fig.7 that the ASAR and WAM mean wave direction agree well taken into account that part of the ASAR pectra may be rotated toward range due to the azimuth cut-off effect [2]. The tandard deviation i around 57 o while the bia i between 0 o and 17 o. The global waveheight difference map of Fig.8 how that the larget deviation between ASAR and WAM occur in the low preure region. Thi i the area with highet wind peed (wind ea) a een from Fig. cauing evere azimuth cut-off problem in the ASAR wave pectra a hown in Fig.. In the low wind area around Equator, better agreement in waveheight i oberved. We alo ee from Fig. 8b that conidering only wave with period larger than, reduce the deviation between WAM and ASAR WVW, epecially in the wind ea area. In Fig.9 i hown the global map of the ASAR and WAM difference in mean wave period. In the next figure are hown global map of waveheight, mean wave period, and wind peed for one month of data (March 2003). We ee the ame global feature in both the ASAR and WAM map for all the parameter conidered here. a) a) b) Fig. 9. Global map of one month (March 2003) difference between ASAR WVW and WAM mean wave period, a) for, b) for. T p T p b) Fig. 8. Global map of one month (March 2003) average difference between ASAR WVW and WAM ignificant waveheight, a) for H, b) for H. For the mean wave period hown in Fig.9 we ee a very good agreement globally. However, for certain area around coat of Africa and ome heltered region at Eat Coat of US and between Autralia and Aia, we ee that ASAR tend to meaure a longer mean wave period than WAM for the well part of the pectrum. For the heltered area thi can be due to effect from fetch-limitation and/or urface feature interpreted a long wavelength ytem in the ASAR

6 WVW product. Thee regional effect are reproduced alo in the April 2003 data. Of particular interet i the localized deviation oberved outide coat of Wet- Africa. Thee area are known a well pool [6], and will be invetigated in more detail when the correponding WVI product become available. 2.2 Wind Speed and Radar Cro Section In the following a validation of backcatter and wind peed i done uing CMOD-IF2 backcatter model and the co-located wind field from ECMWF. Backcatter comparion i hown in Fig. 10. The wind peed retrieval in the ASAR WM Level 2 product i baed on uing CMOD-IF2 with a fixed o wind direction of 45 relative to range, and calibration contant derived from co-located WAM and ASAR data from the Commiioning Phae. The reult are hown in Fig. 11. a) b) a) b) Fig. 11. a) Scatter plot of wind peed derived from colocated ECWMF atmopheric model and ASAR Wave Mode Level 2 product. b) Scatter plot of CMOD-IF2 intenity veru ASAR WM Level 1b image intenity. The tandard deviation for the wind peed comparion i around 2.2 m/ with a bia of 0.5 m/. The effect of patial variability at low wind peed i again oberved to increae the deviation between ASAR and WAM. The abolute calibration contant achieved i 48.05dB, which i around 0.5dB higher than what wa achieved from tranponder. c) d) Fig. 10. a) ASAR meaured radar cro-ection veru ECMWF wind peed. Full line how the dynamic range of CMOD-IF2 predicted cro ection. b) CMOD-IF2 radar cro ection veru ASAR WM cro-ection all wind direction c) CMOD-IF2 radar cro ection veru ASAR WM croection cro wind direction. d) CMOD-IF2 radar cro ection veru ASAR WM cro-ection up/down wind direction. Fig. how the global map of ASAR and WAM wind peed from March The ame global feature are oberved, but the ASAR ha larger variability than the WAM, which i expected due to the different patial cale. Fig.13 how the azimuth cut-off wavelength of the ASAR wave pectra for March Comparing the plot with Fig. we ee, a expected, a trong correlation between the wind peed and the azimuth cut-off. From Fig. 10a we ee that at low wind peed the SAR tend to meaure higher radar cro-ection than predicted by CMOD-IF2. Thi i mot likely to caue the overetimation of waveheight at low wind peed hown in Fig. 5. Another obervation (Fig 10 c and d) i the difference in predicted and oberved radar cro ection for cro wind veru up/down wind. The up/down wind data eem to fit better to CMOD-IF2 model than the cro wind data. No difference wa oberved between up and down wind data.

7 a) b) Fig.. Global map of ASAR WVW (a) and ECMWF (b) wind peed from March domain. Cro comparion of dara from WAM, ASAR and buoy how example where ASAR conitently add information on ocean well beyond what WAM predict. Significant bia i oberved for the wave period while for the waveheight deviation increae with wind peed. An overetimation of waveheight i oberved at low wind peed which can be due to the effect of uing low-reolution catterometer baed CMOD-IF2 in the wave retrieval. The deviation at higher wind peed i due to the azimuth cut-off increaing with increaing wind peed/ea tate. Global comparion how that the bia in wave period i larget in certain coatal and heltered area, while the deviation in waveheight i larget in the low preure (high wind) area. In the table below a ummary of difference between WAM and ASAR Level 2 product i hown for ome key wave and wind parameter. H RMS 0.6, H [ m] Bia Φ,Φ RMS 0.2 [ rad] Bia T, T [] p p RMS Bia T peak RMS [ ] Bia rad U m / Φ [ ] [ ] peak 10 RMS Bia Table 1: Performance of key ASAR Level 2 parameter a compared with ECMWF WAM data. ACKNOWLEDGEMENT: The work wa co-funded by the Reearch DG of the European Commiion under contract EVG1-CT , EnviWave, and by ESA under the AO-799 project, and the ESRIN/Contract No /03/I-OL. REFERENCES Fig. 13. Global map of ASAR WVW azimuth cut-off wavelength from March SUMMARY Validation of ASAR Wave Mode how that the Level 2 product perform well in term of providing a 2-d ambiguity free wave pectra within the SAR imaging 1. Johnen H., Engen G., Chapron B., Walker N., and Cloa J., Validation of ASAR Wave Mode Level 2 Product, Proc. of Enviat Cal/Val Workhop, ESA SP- 531, 2. Rufenach C., Johnen H., Høgda K.A., An approximative analytical method for correcting ditortion in Synthetic Aperture Radar ocean wave pectral peak, IEEE Tran. Rem. Sen., Vol. 33, No.2, pp , Engen G., Johnen H., Høgda K.A., Chapron B., Enviat ASAR Level 2 Wave Mode Product Algorithm Specification Software Requirement Document, NORUT IT Doc. No.: 650/1-01,v2.2.5, Oct., Enviat Data Product Handbook,

8 5. Johnen H., Engen G., Chapron B., Enviat ASAR Wind&Wave Meaurement from Level1 product wind and wave retrieval methodologie, Norut IT report No. IT650/2-01, Verion 1.2, Chen G., Chapron B., Ezraty R., Vandemark D., A Global View of Swell and Wind Sea Climate in the Ocean by Satellite Altimeter and Scatterometer, J. of Atm. & Ocean Techn., Vol. 19, pp , 2002