ERS WAVE MISSION REPROCESSING- QC SUPPORT ENVISAT MISSION EXTENSION SUPPORT

Transcription

1 REPORT 8/2011 ISBN ISSN ERS WAVE MISSION REPROCESSING- QC SUPPORT ENVISAT MISSION EXTENSION SUPPORT - Annual Report 2010 Author (s): Harald Johnsen (Norut), Fabrice Collard (CLS)

2 Project Name: ASAR-QA Project No.: 650 Contractor(s): ESA/ESRIN Contractors Ref.: ESRIN Contract No.21334/08/I-OL Document No.: 8/2011 Document Type: Report P5 Status: Open ISBN: ISSN: No. Pages: 15 Project Manager: Harald Johnsen Date: 29 April 2011 Author (s): Harald Johnsen, Fabrice Collard (CLS) Title: ERS Wave mission reprocessing QC support - Envisat mission extension support: Assessment of WV Product Quality after Orbit Change Summary: ERS Wave mission reprocessing- QC support Envisat Mission extension support st April 2010 until 31 st March Keywords: ASAR WM, Level 2, WVW Notes: Publisher: Norut Authorization: Kjell-Arild Høgda

3 Content 1. Introduction Reference and applicable documents Reference documents Results Wave Mode Quality Monitoring Algorithm Upgrade and SAR Maintenance PF-ASAR Implementation Validation Impact on Significant Waveheight Impact on Mean Wave Period Impact on Mean Wave Direction Summary of Performance ASAR WM Exploitation ASAR WM Product Handbook Sentinel-1 Wave Mode QWG Participation ERS Wave Reprocessing Envisat Mission Extension

4 1. Introduction This is the annual report for the ESRIN/contract No as described in [R-1], [R-2] related to the work packages; WP 3100: Wave Mode Quality Monitoring WP 3200: Algorithm Upgrade and SRD Maintenance WP 3300: PF-ASAR implementation validation WP 3400: ASAR WM Exploitation WP 3500: ASAR Product Handbook WP 3600: Sentinel-1 Wave Mode WP 3700: QWG Participation and to the CCN work quotation [R-3], [R-4] related to the work packages; WP 1000: ERS Wave Reprocessing WP 2000: Envisat Mission Extension On the 7 October 2010, a new look-up table was ingested into the PF-ASAR WVW processing unit. The new look-up table was developed and validated using L2 prototype as part of Wp3200 [R- 5], [R-6]. The activities of calibrating and validating Level 1 and Level 2 products processed from the ERS ½ WM archive are described in WP1000. At the end of October 2010, the orbit change of Envisat was implemented, and the impact of orbit change on the WM products has been assessed in a separate technical report [R-8]. Summary of the achievements are described in WP2000. A validation of one month of WVW products after both look-up table and orbit changes was done by comparing global data set from November 2009 and November The results are summarized in Section 3.4, Table Reference and applicable documents 2.1. Reference documents [R-1] Statement of Work for technical support for global validation and long-term quality assessment of ASAR Wave Mode products, , ENVI-CLVL-EOPG-SW , 30 th July 2007, ESA [R-2] Global Validation and Long-Term Quality Assessment of ASAR Wave Mode Products , Descriptio IT650/5-2007, v1.4, April 2008 [R-3] Request for CCN work Quotation ESRIN/Contract No.21334, ESA/ESRIN 10/11/2009 4

5 [R-4] Global Validation and Long Term Quality Assessment of ASAR Wave Mode- ERS WM Reprocessing Cal/Val - Tender submitted in response to the request for CCN work quotation ESRIN/Contract No.21334, 27 Nov 2009 [R-5] ERS WAVE MISSION REPROCESSING- QC SUPPORT ENVISAT MISSION EXTENSION SUPPORT, Progress Report No , 5 th May [R-6] Global Validation and Long Term Quality Assessment of ASAR Wave Mode- ERS WM Reprocessing Cal/Val - Envisat Mission Extens, Annual Report , 8 th July [R-7] ASAR WM Monthly Cycle Reports [R-8] CLS-DAR-NT , v.1.0, 12/11/2010. [R-9] Global Validation and Long Term Quality Assessment of ASAR Wave Mode- ERS WM Reprocessing Cal/Val - Envisat Mission Extension Support ISSN , ISBN , Norut Report No. 17/2010, 15 Nov Results 3.1. Wave Mode Quality Monitoring The monthly cycle reports are produced regularly and made available on Internet ( In Figure 1 is shown an extract from the cycle reports from the period Jan 2007 until Jan Figure 1 shows the time evolution of the ASAR WM geophysical calibration constant, derived using CMOD in combination with global collocated ECMWF wind field. Figure 1 shows that the gain has been relatively constant (<0.5dB) since January The cycle reports show no anomaly for the parameters analyzed in the WVS and WVW products. As part of the look-up table change and the lowering of the orbit of Envisat, special Wave Mode quality monitoring were conducted and reported in [R-8]) and [R-9], respectively. 5

6 Figure 1 : Evolution of ASAR WM gain (i.e. absolute calibration constant) as function of time since January Algorithm Upgrade and SAR Maintenance The PF-ASAR algorithm for Level 2 processing has been upgraded with a new look-up table. Based on the long-term validation of ASAR WVW product, it was last year decided to upgrade the look-up table for the ASAR WM Level 2 processing in order to improve the estimates of the significant waveheight. The upgrade consisted of modification of the amplitude of the RAR Modulation Transfer Function (MTF) as function of wind speed. The modification implemented in the new look-up table is shown in Figure 2. The empirical relation of RAR MTF amplitude was derived using 100K of ASAR WM imagettes collocated with WAM data. Figure 2 : RAR MTF amplitudes as function of wind speed for HH and VV polarization and incidence angle corresponding to swath S2. The dotted red line shows the wind dependency of the old RAR MTF amplitude while the full red line shows the wind dependency of the updated RAR MTF amplitude for VV polarization. 6

7 3.3. PF-ASAR Implementation Validation The impact of new look-up table on the ASAR WVW, using the WVW prototype, was shown at the SeaSAR2010 conference and also presented in reports [R-5], [R-6]. The new look-up table was ingested into the PF-ASAR L2 processing unit the 7 th October. As of part of this work package the WVW product is evaluated before an after the ingestion of the new look-up table, and before the orbit change started. Data from 1 th 30 th September and data from 7 th 21 th October are validated independently and performances are compared. The WVW products are validated against collocated WAM data from ECMWF. The impact on key wave parameters is presented in the following Impact on Significant Waveheight The main impact/improvement is expected for the significant waveheight, since the look-up table upgrade consisted of updating the RAR MTF amplitude as function of wind speed. The figures denoted are from September 2010 and are thus processed with the old look-up table, while those denoted are from between 7 th 21 th October 2010 and are thus processed with the new look-up table and before orbit change. In Figure 3 is shown the histogram of difference in significant waveheights between WVW and WAM for the September and the October data set. Figure 3 : Histogram of difference in significant between WVW and WAM computed over different wavelength regions H s, H 12 Az s, H s. Left: Data from September 2010, Right: Data between 7 th -21 th October Figure 3 shows a significant improvement of the significant waveheight of the long wavelength part of the wave spectra ( H 12 s ). Both bias and RMSe are significantly reduced. The percentage of WVW 12 with H s 0.5m increases from 78% to 86%. Note also the reduction in RMSe (from 0.73m to 0.60m) and the increase in bias of the total significant waveheight ( H s ), which is also as expected and more correct. The wind dependency in the significant waveheight bias was the main error observed with the previous look-up table. Figure 4 shows the difference in significant waveheight between WVW and WAM for the long wavelength part of the wave spectra (i.e. H 12 s, H Az s ) as function of wind speed before and after the look-up table change. 7

8 Figure 4 : Difference in significant waveheight between WVW and WAM as function of wind speed. Upper: H s 12 (U 10 ), Lower: H s Az (U 10 ). Figure 4 shows that the decreasing trend in waveheight difference (bias) for the long wave part of the spectra as function of wind speed is almost completely removed with the new look-up table Impact on Mean Wave Period The impact on mean wave period is expected to be minor since we only modified the RAR MTF amplitude. The performance results before and after change of look-up table is shown in Figure 5. 8

9 Figure 5 : Histogram of the difference in mean wave period (WVW WAM) for different wavelength regions. Figure 5 shows that the impact on wave period is minimal, but a small positive impact (from 87% to 90%) is observed on the percentage of WVW data with T p sec Impact on Mean Wave Direction As for the mean wave period, the impact on mean wave direction is expected to be minor. The performance results before and after change of look-up table is shown in Figure 6. Figure 6 : Histogram of the difference in mean wave direction (WVW WAM) for different wavelength regions. Figure 6 shows that the impact on mean wave direction is negligible. 9

10 3.4. Summary of Performance In order to test the performance of WVW product before and after orbit change and look-up table change on a larger dataset acquired under same period of year (same climate conditions), the dataset from November 2009 and November 2010 were compared. A summary of the performance for key wave spectral parameters of the WVW product from these datasets is listed in Table 1, and in Figure 7 is shown the waveheight bias (WVW-WAM of detected waves) versus wind speed for the and data set. The conclusion is that the impact of using a new look-up table (and possibly also orbit change), significantly improves the geophysical content of the WVW product. As expected, the largest improvement is observed for the significant waveheight, but significant improvement is also observed for mean wave direction and mean wave period. Table 1 : Performance of old versus new ASAR WVW products Parameters Nov Nov.2010 BIAS RMSe % BIAS RMSe % H s [m] H s 12 [m] H s [m] p( H s m) T p [sec] T p 12 [sec] T p Az [sec] p( T p sec) [deg] [deg] [deg] p( o )

11 Figure 7 : Waveheight bias (WVW-WAM) as function of wind speed for the and data set. Only waves resolved by the ASAR are considered here ASAR WM Exploitation ASAR wave mode exploitation has been promoted through two main initiatives : A close collaboration with GLOBWAVE project has helped to define L2P product format including some additional information on wave partitions and bring a good understanding of the product for the user guide. ASAR wave mode L2 products are now processed into L2P products and available in near real time through GLOBWAVE portal (ftp eftp.ifremer.fr user:w1f612 pass: tempo2) Investigation of the use of Wave mode level 2 products for swell tracking and short term swell forecast is being investigated by a PHD student at CLS. A first demonstration of swell forecast bulletin has been setup to help warn long swell event at La Reunion Island and are delivered twice a day since March 2011 to EDF. This is currently under evaluation by comparison with local sea state models ASAR WM Product Handbook Input has been provided on request from task leader Sentinel-1 Wave Mode The partners have contributed to the development of a new ocean state product (OCN) in the framework of the S-1 IPF developments. This new product is a merging of the ocean swell wave spectra (OSW product), the ocean wind field (OWI product), and the ocean radial velocity (RVL product). In this work package the team has also investigated the capabilities of deriving the OSW component from the S-1 IWS mode, since this is initially not defined for the S-1 IPF Level2 processing. A particular interesting option to utilize is the ability of S-1 TOPS to create partial overlapping burst images with time separation much larger than any Stripmap or standard ScanSAR system can do. This again will significantly improve the resolving of the ocean wave propagation direction due to the increased phase signal of the cross-spectra. The preliminary study 11

12 was based on simulations. Comparison of cross-spectra simulated using S-1 IWS TOPS and ASAR WM specifications is shown in Figure 8. Figure 8 : Simulated cross-spectra for ASAR WM (left) and Sentinel-1 IWS TOPS (right). Imaginary part of cross-spectra in upper plots, and amplitude of cross-spectra in lower plots. Noise level set equal to the SNR threshold (ambiguity) used in ASAR WM Level 2 processing. The overall improvements of S-1 IWS versus ASAR WM in terms of resolving the wave propagation direction is shown in Figure 9. Figure 9 : Relative performance between S-1 IWS and ASAR WM. Resolved swell propagation direction versus swell waveheight for ASAR WM and S-1 IWS modes. With dedicated processing techniques of S-1 IWS TOPS mode data, a new and improved product can be achieved for wave imaging with very high range resolution, moderate azimuth resolution, and ideal look separation time QWG Participation The partners have participated in QWG meetings with presentations of results on request from ESA. Slides from presentation are provided to ESA. 12

13 3.9. ERS Wave Reprocessing A limited number of ERS-2 WVS and WVW products were evaluated and validated using the ASAR WM WVW and WVS prototype software in combination with collocated model wind field and wave spectra (WAM). The model wind field and WAM spectra were provided by ECMWF. Evaluation of the operational products versus the prototype generated products shows very good agreement. The ERS-2 data were recalibrated using collocated wind field from ECMWF (see Figure 10). The new calibration constant is (or 46.82), which is around 0.2 db lower than the external ASAR WM calibration constant used in MDA/ESA Level 2 processor. Figure 10 : Derivation of ERS-2 WM calibration constant for Level 2 reprocessing (left) and comparison between calibrated and modelled NRCS in db after the new derived calibration constant has been applied to ERS-2 WM data (right). The standard deviation between ERS and CMOD NRCS is around 0.9dB. After recalibrating the data, the Level 2 processing was done using the prototype software. The performance of the ERS-2 WVW products was then validated using WAM data. The conclusion is that the overall quality of the ERS-2 WVW product is similar to the ASAR WVW product. The small decrease in performance for some of the wave parameters (wave direction, waveheight) may come from the fact that the data sets are different as well due to higher noise level in the ERS-2 data. Finally, the recent improvements in RAR MTF developed for ASAR WM was tested on ERS-2 WVW processing, and positive impact was demonstrated. Detail results of the ERS Wave Reprocessing validation can be found in [R-5] Envisat Mission Extension The ENVISAT orbit change took place between Oct. 22 to Oct. 26, 2010, with the effect of lowering the orbit of 17.4 km. The ASAR wave mode level2 product quality after ENVISAT orbit change was investigated by using ASAR Level 2 products from Oct 8 to 21, 2010 as reference of the period before orbit change and from Oct 28 to Nov 8, 2010 for the period after the orbit change. The dataset before orbit change is limited to data after Oct 7 since a new version of the wave mode lookup tables has been used after this date and we wanted to dissociate possible changes coming from the processor update from those coming from orbit change. 13

14 The conclusion is that the impact on the quality of the ASAR WVW product is insignificant. A small change is azimuth cut-off due to lower R/V ratio results in a small improvement in the SWH estimates. The change in mean backscatter is around 0.08dB, which is within the expected accuracy over a period of 10 days (see Figure 11). A detailed analysis can be found in [R-8]. Figure 11 : Backscatter cross section versus ECMWF wind speed before (left) and after (right) orbit change. Solid lines are the CMOD limits for upwind (upper line) and crosswind (lower line) 14