altimeter wave data

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

[email protected]


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


Altimeter SWH data3

Impact of altimeter assimilation (ERS-2)


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.


Data quality3

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


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.


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.


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


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)


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.


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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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


Data usage5

Scilly Islands

How to : Validate such conditions Interpret spatially averaged data


Data usage6

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


Data usage7

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


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


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


ChallengesData drop outs in areas of very high seasTimely NRT data deliveryNear shore applications


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.

altimeter wave data

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