Ocean Vector Wind as Essential Climate Variable.

Transcription

1 Ocean Vector Wind as Essential Climate Variable

2 Context WCRP, World Climate Research Program WOAP, WCRP Observation Assimilation Panel GCOS, Global Climate Observing System Essential Climate Variable Ocean Vector Wind Fundamental Climate Data Records FCDR inventory

3 Satelite Ocean Winds NRT ASCAT, QSCAT, OSCAT: NOAA, ISRO, OSI SAF L2/L3/L4 archive, all scatterometers, PO.DAAC, Ifremer, RSS, FSU, EUMETSAT, WindSat (Bettenhausen, NRL) NRT / archive Wind speeds from radiometers (RSS, HOAPS) archive (SMMR, SSMI(S), AMSR, MIS,.. ) Altimeter wind speeds (GeoSat, Topex, GFO, ERS, Jason, ENVISAT, CryoSat, HY2 (2011), SARAL (2012), Jason3 (2013), S3 (2013)) SAR (off-line) (ERS, RadarSat ($), ENVISAT, S1 (2012),.. ) Variables Wind vector or speed, stress vector, curl, divergence, see also white paper OceanObs 09 paper by Bourassa et al.

4 Sustainability Radiometer missions, wind speed swath SSM/I, WindSat (vector > 8 m/s), MIS Altimeter missions, wind speed track Historic scatterometer missions, wind vector swath SeaSat, 3 months in 1978 ERS1&ERS2 AMI (ERS2 is regional since) NSCAT and SeaWinds1, each 9 months QuikScat, ASCAT, 2007OceanSat-2, Continuous wind vector coverage since 1991

5 GLOBAL SCATTEROMETER MISSIONS (CEOS VC) Launch Date C-band 10/06 EPS SG Europe Ku-band 6/99 QuikSCAT USA Global availability uncertain Availability?? Combined C- and Ku-band FY-3E with 2FS China Operational Series with 2FS India Operatin IOVWST, Extended MayApproved 2011 sw 24feb11

6 GCOS needs 1. Full description of all steps taken in the generation of FCDRs and ECV products, including algorithms used, specific FCDRs used, and characteristics and outcomes of validation activities 2. Application of appropriate calibration/validation activities 3. Statement of expected accuracy, stability and resolution (time, space) of the product, including, where possible, a comparison with the GCOS requirements 4. Assessment of long-term stability and homogeneity of the product 5. Information on the scientific review process related to FCDR/product construction (including algorithm selection), FCDR/product quality and applications 6. Global coverage of FCDRs and products where possible 7. Version management of FCDRs and products, particularly in connection with improved algorithms and reprocessing 8. Arrangements for access to the FCDRs, products and all documentation 9. Timeliness of data release to the user community to enable monitoring activities 10.Facility for user feedback 11.Application of a quantitative maturity index if possible 12.Publication of a summary (a webpage or a peer-reviewed article) documenting point-by-point the extent to which this guideline has been followed These are NRT ops. production needs too (planned up to L2/L3) GCOS-143 (WMO/TD No. 1530)

7 Assessment of ocean ECVs Geophysical parameter and related ECV Existing and/or potential users History and outlook; sustainability Availability and DOI registration Maturity (Bates & Barkstrom maturity index or others) Point-by-point description of how the effort adheres to the GCOS guidelines Strengths and weaknesses or limitations Uncertainty estimates, possibly as a function of time Dataset details, such as time period, spatial resolution, data formats From FCDR inventory; I need your help!

8 Wind stress ECV Radiometers/scatterometers measure ocean roughness Ocean roughness consists in small (cm) waves generated by air impact and subsequent wave breaking processes; depends on water mass density ρsea= 1024±4 kg m-3 and e.m. sea properties (assumed constant) Air-sea momentum exchange is described by τ = ρair u* u*, the stress vector; depends on air mass density ρair, friction velocity vector u* Surface layer winds (e.g., u10) depend on u*, atmospheric stability, surface roughness and the presence of ocean currents Equivalent neutral winds, u10n, depend only on u*, surface roughness and the presence of ocean currents and is currently used for backscatter geophysical model functions (GMFs) ρair. u10n is suggested to be a better input for backscatter GMFs (under evaluation by IOVWST)

9 Users FCDR/ECV stress Oceanography, eddy scale winds (MyOcean) Re-analyses (data assimilation uses wind) IOVWST; process studies (air-sea momentum exchange, cyclones, extreme winds, convection, tropical circulation,...) Climate, fluxes (incl. carbon) Design, policy-makers, wind energy, adaptation,..

10 Strengths / Limitations Scatterometer / Passive Excellent precision, mature algorithms, complete coverage Small scales (25 km), order better than NWP Since 1991 vector winds Intercalibration, accuracy assessment speed scale Calibration above 30 m/s (truth?) Rain (bias) for Ku band and passive systems Temporal coverage does not match scales (yet) Low spatial resolution (physical processes) Ambiguous direction retrieval

11 Strengths/Limitations Altimeter Continuity/sustainability Potentially low uncertainty Limited speed range (25 m/s) Instrument anomalies, calibration,.. Very low coverage (track) Sea state

12 L1 Calibration Transponder procedure in development for ASCAT Rain forest (stable points) Sea ice / snow /desert (stable points) Geographically limited, while some errors may be orbit phase dependent Need to combine all methods of calibration, including ocean calibration Calibration procedures and GMFs need to be shared between producers to achieve intercalibrated NRCS WOAP workshop, April 2011 IOVWST, May 2011

13 ASCAT stability - Ocean calibration Trends of 0.1 m/s just visible (10 year req.) Sampling error to be accounted for (buoy) WOAP workshop, April 2011 IOVWST, May 2011

14 Precision, accuracy: triple collocation u v Bias ASCAT (m/s) Bias ECMWF (m/s) Trend ASCAT Trend ECMWF σ ASCAT (m/s) σ ECMWF (m/s) Representation error *) (m/s) Representation error is part of ECMWF error OSI SAF NRT req. 2 m/s, WMO in speed/dir. See also Vogelzang et al., JGR, 2011 WOAP workshop, April 2011 IOVWST, May 2011 Spatial representation error from spectrum difference integrated from scales from 25 km to 800 km

15 Spatial resolution Spectral analysis of collocated fields Comparison to in situ spectra in time / space to check energy cascade behaviour Verification of variances and resolution by averaging products (e.g., QSCAT 100km vs 25km, ASCAT 25km vs 12.5km) WOAP workshop, April 2011 IOVWST, May 2011

16 Define Uncertainty, Stability, Resolution Users have little clue how different products compare and whether they use the product most fit for their purpose Standardization of methods (software?) to assess uncertainty, resolution and stability to be discussed in the IOVWST NWP ocean calibration, triple collocation, CDF matching The resulting speed scale standard would be applicable to scatterometers, radiometers, altimeters and SAR Accuracy of speed scale TBD (speed dependent) Producers to share match-up data bases / independent cal/val Publish / post results for users (in central place(s)?) WOAP workshop, April 2011 IOVWST, May 2011

17 Maturity Index

18 Summary Several producers provide OVW FCDRs, which are defensible by their own verification metric These products cannot be easily understood nor combined by the user community Mature (5) stable products exist over long times, but not reprocessed according to GCOS guidelines; some uncoordinated RP plans exist Matchup data bases exist too, but by producer Moored buoys are the main reference, but lacking in open ocean Quality metrics and assessment standards (software) exist too by producer, but resolution, wind scale, wind quality to be coordinated/agreed An IOVWST has been set up last year, which could address ECV coordinated needs when mandated as such CEOS Virtual Constellation coordinates satellites/products

19 Suggested actions Obtain data set details from producers and make ECV inventory (me) Reprocessing of all satellite winds following GCOS guidelines Share matchup data bases Extend moored buoy network in open ocean Coordinate quality metrics and assessment standards (software) on resolution, wind scale, wind quality IOVWST to be mandated to address wind ECV coordinated needs (by satellite agencies) (incl. altimeters) CEOS VC to promote satellite coordination and intercalibration Maintain L1 reprocessing facilities (e.g., ESA ERS) Complete efforts in ASCAT and OSCAT calibration Perform ASCAT-ERS and QSCAT-OSCAT NRCS intercalibration Finally, develop a reference wind scale (intercalibration) for all satellite winds, scatterometer, radiometer, altimeter, SAR

20 Discussion!

21 WMO 21 ISRO 15May Dec IOVWST,

22 Triple collocation for winds wbuoy = t + δbuoy w = wind component wscat = ascatt + bscat + δscat wback = abackt + bback + δback Calculate first and second (mixed) moments Test assumptions on errors and derive spatial representation error Eliminate <t> and <t 2> (get rid of the truth) Solve for calibration coefficients and error variances <δ 2> Apply CDF matching for higher order calibration (beyond linear) For derivation: Ad Stoffelen, 1998, Toward the true near-surface wind speed: error modeling and calibration using triple collocation. J. Geophys. Res. 103C4, WOAP workshop, April 2011 IOVWST, May 2011

23 Ocean momentum The atmospheric stress forces wave motion by momentum transfer Wikipedia Wave momentum depends on water mass density Varies by.5% WOAP workshop, April 2011 IOVWST, May 2011

24 Air-density effect on satellite ocean wind Microwave roughness relates to stress Lower air density (Tropics) relates to higher winds 10% change pole vs Tropics, gives 5%, or ~0.4 m/s IOVWST meeting, May (c) ECMWF, Hans Hersbach

25 Air-sea interaction ECMWF weak Chelton et al., Science ECMWF model ECMWF model

26 Lack of ageostrophic flow ECMWF, Hans Hersbach; A. Brown et al., 2005

27 6-hourly EC wind change (4 cycles) Forcing is dominated by transient or temporal effects - temporal wind variance larger than in small scales

28 ECMWF increments ECMWF analysis increments modest wrt spatial deficit (1.2 m 2s-2) Most mesoscale scatterometer information remains unexploited Develop MyOcean L3/L4 products

29 GCOS guidelines 1) Full description of all steps taken in the generation of FCDRs and ECV products, including algorithms used, specific FCDRs used, and characteristics and outcomes of validation activities 2) Application of appropriate calibration/validation activities 3) Statement of expected accuracy, stability and resolution (time, space) of the product, including, where possible, a comparison with the GCOS requirements 4) Assessment of long-term stability and homogeneity of the product 5) Information on the scientific review process related to FCDR/product construction (including algorithm selection), FCDR/product quality and applications 6) Global coverage of FCDRs and products where possible 7) Version management of FCDRs and products, particularly in connection with improved algorithms and reprocessing 8) Arrangements for access to the FCDRs, products and all documentation 9) Timeliness of data release to the user community to enable monitoring activities 10) Facility for user feedback 11) Application of a quantitative maturity index if possible 12) Publication of a summary (a webpage or a peer-reviewed article) documenting point-by-point the extent to which this guideline has been followed