Coastal Altimetry Workshop February 5-7, Supported by NOAA (Stan Wilson) NASA (Eric Lindstrom, Lee Fu)

Transcription

1 Coastal Altimetry Workshop February 5-7, 2008 Organized by: Laury Miller, Walter Smith: NOAA/NESDIS Ted Strub, Amy Vandehey: CIOSS/COAS/OSU With help from many of you! Supported by NOAA (Stan Wilson) NASA (Eric Lindstrom, Lee Fu)

2 Fundamental Simple (?) Question: How does a nadir altimeter s alongtrack estimates of range and corrections change as it moves from water to land or land to water (or passes near land)? Answers get complicated fast. In general, gridding is not the focus, but this brings in the question: What are the time and space scales that need to be resolved in the coastal ocean and of the range and correction terms?

3 Further Issues and Questions What time and space scales are associated with priority applications in the coastal ocean leads to a discussion of applications and the Programs that are supporting this research. To resolve these space scales, Dynamical models are needed to produce continuous fields that assimilate the altimeter and other data. This leads to a discussion of Integrated Ocean Observing Systems (remote sensing, in situ data and models).

4 Meeting Objectives Develop a new error budget specifically for the coastal ocean for the alongtrack signals. Identify specific problems where progress can be made in improving the alongtrack estimates of range and corrections in the region next to the coast. Form partnerships and collaborations to work on these. How do we do this? Through the new OST Science Teams? Through the various Europeans initiatives? In doing this, provide information and recommendations to the agencies and programs which will be reprocessing the historical data for better coastal coverage.

5 Error budget slides for intro/overview By Walter Smith for Ted Strub

6 Altimeter corrections

7 Absolute & Relative Errors The error budget for an absolute height measurement must include all the error sources at left. Are there applications (surface geostrophic currents, detecting fronts, transport) where only a height gradient, or relative change in height, is required? If the spatial correlation scale of a height error is longer than the width of a height gradient signal, then that error is not significant for detecting the gradient of interest. Height gradients are much more accurate than heights.

8 Example: wet delay correction Maximum IWV Gradient: 4 mm IWV per 40 km There are 6.36 mm of Ku-band radar correction for each 1 mm of IWV. Correction is 6 to 19 cm, but max. gradient in correction is 0.6 mm per km (μrad). For comparison, 0.6 μrad of dynamic height gradient is 6 mm/sec geostrophic velocity at mid-latitudes.

9 Example: sea state bias The sea state bias correction is ~2% of SWH. If the SWH correlation scale is such that a 1 meter change in SWH is spread over 20 km or more, then the gradient of the SSB correction is 1 μrad or less. SWH values separated by 20 km are expected to differ by no more than 0.2 m [F. Monaldo, JGR, 93 C(3), 1988, at page 2292]. Thus the expected gradient in the SSB correction is ~0.2 mm/km (2 mm/sec equivalent geostrophic velocity at mid lat.). We need not care about the details of the SSB correction if the SWH field is smooth and we are only interested in height changes, not absolute heights.

10 Example: tides 15 cm of error in semi-diurnal tide height may give us only 1 μrad error of height gradient in 50 meters depth, where a simple theory holds.

11 Example: Geoid or MSS The geoid gradient over trenches can be 300 μrad. Thus near the coasts of Central and South America, Japan, the Philippines and Indonesia, a 1 km shift in an altimeter track can introduce a geoid change of 30 cm. In these areas one must account for the displacement of each altimeter track from the mean repeat track, and the errors in the geoid or MSS may be the dominant term in the height error budget.

12 Error budget for height and slope Signal or Error Source Half-Wavelength Scale (km) Height (cm) Slope (μrad) Single 6-Year Measurement Average ABYSS signal 6 to to to to 300 Height error sources Orbit height errors, 1-σ a 10,000 to 20, <0.2 <0.1 Ionosphere (Imel, 1994; Yale, 1997) > <0.22 <0.1 SWH-induced precision error >20 (See F.2.4.2, F.2.4.3, and Table F-4) *Wet troposphere (Chelton et al., 2001) >100 3 to 6 <0.6 <0.3 *Sea-state bias (Monaldo, 1988) >20 <0.6 <0.3 <0.1 Inverse barometer (Ponte, 1994; Gill, 1982) >1000 <14 <0.14 <0.07 Oceanographic error sources Basin-scale circulation (steady) (Fu & Chelton, 2001) > <1 <1 El Ni o (Picaut & Busalacchi, 2001), other b > <0.2 <0.1 Deep ocean tide model errors (Chelton et al., 2001; > <0.03 <0.01 Shum et al., 2001) Coastal tide model errors (Shum et al., 2001; Yale, 50 to 100 <13 <2.6 < ) Eddies, Mesoscale Variability (Le Traon & Morrow, 60 to to to 5 1 to ) Meandering jet (Gulf Stream) (Fu & Chelton, 2001; 100 to to to 10 2 to 4 Smith & Sandwell, 1995) Steady jet (Florida Current) (Fu & Chelton, 2001; to to 10 5 to 10 Smith & Sandwell, 1995) a At the ABYSS antenna phase center (see section G.4.1.2). Interannual variability, planetary waves. *Bold type indicates height error constituents that are pertinent to ABYSS (see section F.2.4). From ABYSS proposal (peer-reviewed by NASA in 2001)

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20 What do real altimeters see in the region next to the coast? Jason-1 data on tracks 247 and 28 are being extended closer to the coast by substituting ECMWF water vapor for the microwave radiometer wet tropo correction.

21 Jason-1 Pass 28, Cycle 130: July 18, 2005

22 Extending the Altimeter Alongtrack SSH to the Coast 2005 ~ July 18 ~ July 28 ~ August 27

23 Wet Troposphere Corrections from Jason Microwave, NCEP & ECMWF Pass 28, Cycle 130 Pass 28, Cycle 131 Wet tropo (ECMWF) Dry Tropo (ECMWF) Wet tropo (ECMWF) Dry Tropo (ECMWF) cm cm cm cm E longitude E longitude E longitude E longitude 0 Wet tropo (NCEP) -220 Dry Tropo (NCEP) -5 Wet tropo (NCEP) -220 Dry Tropo (NCEP) cm cm cm -15 cm E longitude E longitude pass 28 cycle 130 Jul corrected FES04 tide Oct E longitude E longitude pass 28 cycle 131 Jul corrected FES04 tide