Altimeter wave data

1. 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. 1 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.

3. 2 Altimeter SWH data Buoys used in operational model validation

4. 3 Altimeter SWH data Impact of altimeter assimilation (ERS-2)

5. 1 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.

6. 2 Data quality Example of collocation pdf of original and error corrected Jason-1 SWH. NOTE: using offshore buoy data only.

7. 3 Data quality Example of collocation pdf of original and error corrected Jason-1 10 meter wind speeds. Other altimeters 20% larger errors.

8. 4 Data quality 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. 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. 1 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.

11. 2 Sampling ENViSAT - GFO 2004-2005 mean SWH differences between Jason-1, GFO and ENVISAT from error-corrected altimeter data ENViSAT - Jason-1 Jason-1 - GFO

12. 3 Sampling 2004-2005 mean SWH differences between Jason-1, GFO and ENVISAT synthetic data obtained by sampling NOAA’s operational wave model (NWW3) ENViSAT - GFO ENViSAT - Jason-1 Jason-1 - GFO

13. 1 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.

14. 2 Data usage Using altimeter data to limit model biases and identify and remove biases induced by unresolved island groups.

15. 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 Data usage

16. 4 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. Katrina

17. 5 Data usage How to : Validate such conditions Interpret spatially averaged data Scilly Islands

18. 6 Data usage Coastal Jason-1 altimeter data for Isabel illustrate biases on southern Atlantic Bight shelf, and effects of the Gulf Stream.

19. 7 Data usage More Jason-1 for Isabel: note sharp data transition across Bahamas, eye structure of Isabel and data drop out.

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

21. 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. Challenges Data drop outs in areas of very high seas Timely NRT data delivery Near shore applications

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.