1. Tolman and friends, Feb. 6, 2008Coastal Altimetry 1/23 Altimeter wave data Deep ocean and coastal use and issues Hendrik L. Tolman NOAA / NWS / NCEP / EMC Marine Modeling and Analysis Branch Hendrik.Tolman@NOAA.gov

2. Tolman and friends, Feb. 6, 2008Coastal Altimetry 2/23 Altimeter SWH data Altimeters provide the only true global wave height observations available. Bulk observation of wave height only (no spectrum):  In principle, this would limit the usefulness of these data.  In practice, these data are very useful because:  In global validation accurate local swells can only be obtained with good spectral wave representation at generation areas.  Sparse spatial sampling adds to temporal sampling of buoys at even fewer locations.  Collocated wind data potentially useful to isolate forcing errors from model errors. 1

3. Tolman and friends, Feb. 6, 2008Coastal Altimetry 3/23 Altimeter SWH data 2 Buoys used in operational model validation

4. Tolman and friends, Feb. 6, 2008Coastal Altimetry 4/23 Altimeter SWH data 3 Impact of altimeter assimilation (ERS-2)

5. Tolman and friends, Feb. 6, 2008Coastal Altimetry 5/23 Data quality Validate and/or bias correct with buoy data. Using 10s averaging to remove some sampling variability and to get scales comparable to ocean wave models.  Bias corrected SWH data better than buoy data; remaining random error dominated by buoy sampling and collocation errors.  Cannot (?) validate with coastal buoys. Winds much less accurate;  Larger scatter.  Swell mistaken for small scale roughness and hence wind; removal process mathematically poorly posed.  Only Jason-1 algorithm for wind shows wind that is independent of background wave field. 1

6. Tolman and friends, Feb. 6, 2008Coastal Altimetry 6/23 Data quality 2 Example of collocation pdf of original and error corrected Jason-1 SWH. NOTE: using offshore buoy data only.

7. Tolman and friends, Feb. 6, 2008Coastal Altimetry 7/23 Data quality 3 Example of collocation pdf of original and error corrected Jason-1 10 meter wind speeds. Other altimeters 20% larger errors.

8. Tolman and friends, Feb. 6, 2008Coastal Altimetry 8/23 Data quality 4 Wind speed regression lines for collocation data stratified by non- dimensional wave height from buoy data (wind sea through old swell). Only Jason-1 data is independent of background wave field.

9. Tolman and friends, Feb. 6, 2008Coastal Altimetry 9/23 Other altimeters Structure mounted downward looking altimeters used instead of buoys as in-situ observations (oil and navigation platforms). Scanning Radar Altimeter (SRA, Ed Walsh, NASA) provides a 3D surface map of the ocean at order of 10m resolution in a swath along a flight track of an airplane  5,000 to 15,000 feet flight from P3 platforms.  Will be operational on hurricane hunter flights.  Full 2D wave spectrum available.  All weather capability.

10. Tolman and friends, Feb. 6, 2008Coastal Altimetry 10/23 Sampling Altimeter wave data are invaluable due to global coverage, but represent a very sparse sampling pattern.  One altimeter can give seasonal model assessment at 100km scales.  Three altimeters give reasonable monthly model assessment.  Order of magnitude(s) more data needed for data- only analyses in deep ocean. Scales at the coast are much smaller, needs much higher data density. Does sparse sampling influence climatologies?  Work with Degui Cao and Vera Gerald.  Compare ENVISAT, GFO and Jason-1.  Corresponding synthetic data from WAVEWATCH III. 1

11. Tolman and friends, Feb. 6, 2008Coastal Altimetry 11/23 Sampling 2 ENViSAT - GFO ENViSAT - Jason-1Jason-1 - GFO 2004-2005 mean SWH differences between Jason-1, GFO and ENVISAT from error-corrected altimeter data

12. Tolman and friends, Feb. 6, 2008Coastal Altimetry 12/23 Sampling 3 ENViSAT - GFO ENViSAT - Jason-1Jason-1 - GFO 2004-2005 mean SWH differences between Jason-1, GFO and ENVISAT synthetic data obtained by sampling NOAA’s operational wave model (NWW3)

13. Tolman and friends, Feb. 6, 2008Coastal Altimetry 13/23 Data usage Due to lack of confidence in wind data, only SWH data are used consistently.  Validation and tuning of wave models.  Assimilation (see previous slide).  Case studies.  Operational forecast use  Examples courtesy of Joe Sienkiewicz. 1

14. Tolman and friends, Feb. 6, 2008Coastal Altimetry 14/23 Data usage 2 Using altimeter data to limit model biases and identify and remove biases induced by unresolved island groups.

15. Tolman and friends, Feb. 6, 2008Coastal Altimetry 15/23 Data usage Regional seasonal model validations (bias and norm. rms error = SI)  2001 ERS2 data  0.5 degree resolution due to track spacing.  At this scale no apparent coastal issues. 3

16. Tolman and friends, Feb. 6, 2008Coastal Altimetry 16/23 Data usage Operational wave modeling at NCEP now provides 7km resolution model guidance four times per day  Reasonable coastal (shelf resolving) resolution.  Model and spatial altimeter resolution are near identical.  For this time: four buoys in this map area, no altimeter data.  Less than 10 altimeter tracks close to Katrina in life cycle. 4 Katrina

17. Tolman and friends, Feb. 6, 2008Coastal Altimetry 17/23 Data usage 5 Scilly Islands How to : Validate such conditions Interpret spatially averaged data

18. Tolman and friends, Feb. 6, 2008Coastal Altimetry 18/23 Data usage 6 Coastal Jason-1 altimeter data for Isabel illustrate biases on southern Atlantic Bight shelf, and effects of the Gulf Stream.

19. Tolman and friends, Feb. 6, 2008Coastal Altimetry 19/23 Data usage 7 More Jason-1 for Isabel: note sharp data transition across Bahamas, eye structure of Isabel and data drop out.

20. Tolman and friends, Feb. 6, 2008Coastal Altimetry 20/23 Operational Application of Altimeter based Significant Wave Heights at the NOAA Ocean Prediction Center Jason GFO ENVISAT Have built on Scatterometer experience ( delivering data to operations ) Jason – “operational” - early 2007 GFO and ENVISAT – “experimental” operational May 2008 N-AWIPS Workstation display SWH- in feet

21. Tolman and friends, Feb. 6, 2008Coastal Altimetry 21/23 Jason – 16 ft NWW3 ~13 ft Jason – 38 ft NWW3 – 30 ft Operational use Augment in situ observations Validate NWP Wave Model Output  Forecaster “on the fly” assessment and correction  post analysis Small scale features  Wind - current interactions SWH in feet

22. Tolman and friends, Feb. 6, 2008Coastal Altimetry 22/23 Challenges Data drop outs in areas of very high seas Timely NRT data delivery Near shore applications

23. Tolman and friends, Feb. 6, 2008Coastal Altimetry 23/23 Issues Data dropout in most interesting conditions. Effects of waves on winds, except in Jason-1. Coastal biases in climatologies appear to be due to sampling. Would be of major importance if we could get spatial rather than line observations (wide-swath SWH).  Need much more data.