Post on 09-Jan-2016
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Tolman and friends, Feb. 6, 2008 Coastal Altimetry 1/23
Altimeter wave dataDeep ocean and coastal use and issues
Hendrik L. TolmanNOAA / NWS / NCEP / EMCMarine Modeling and Analysis Branch
Hendrik.Tolman@NOAA.gov
Tolman and friends, Feb. 6, 2008 Coastal 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.
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Altimeter SWH data2
Buoys used in operational model validation
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Altimeter SWH data3
Impact of altimeter assimilation (ERS-2)
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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.
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Data quality2
Example of collocation pdf of original and error corrected Jason-1 SWH. NOTE: using offshore buoy data only.
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Data quality3
Example of collocation pdf of original and error corrected Jason-1 10 meter wind speeds. Other altimeters 20% larger errors.
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Data quality4
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.
Tolman and friends, Feb. 6, 2008 Coastal 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.
Tolman and friends, Feb. 6, 2008 Coastal 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.
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Sampling2
ENViSAT - GFO
ENViSAT - Jason-1
Jason-1 - GFO
2004-2005 mean SWH differences between Jason-1, GFO and ENVISAT from error-corrected altimeter data
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Sampling3
ENViSAT - GFO
ENViSAT - Jason-1
Jason-1 - GFO
2004-2005 mean SWH differences between Jason-1, GFO and ENVISAT synthetic data obtained by sampling NOAA’s operational wave model (NWW3)
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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.
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Data usage2
Using altimeter data to limit model biases and identify and remove biases induced by unresolved island groups.
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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.
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Tolman and friends, Feb. 6, 2008 Coastal 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.
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Katrina
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Data usage5
Scilly Islands
How to : Validate such conditions Interpret spatially averaged data
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Data usage6
Coastal Jason-1 altimeter data for Isabel illustrate biases on southern Atlantic Bight shelf, and effects of the Gulf Stream.
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Data usage7
More Jason-1 for Isabel: note sharp data transition across Bahamas, eye structure of Isabel and data drop out.
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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 2007GFO and ENVISAT – “experimental”operational May 2008
N-AWIPS Workstation displaySWH- in feet
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Jason – 16 ftNWW3 ~13 ft
Jason – 38 ftNWW3 – 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
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ChallengesData drop outs in areas of very high seasTimely NRT data deliveryNear shore applications
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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.