Development of coastal altimetry and perspectives for data assimilation in

coastal domain

Matthieu Le Hénaff(1)

and many more…

(1) RSMAS, Miami, FL

Outline

• Historical perspective from a user point of view

• Example of the Florida Keys area as a potential test-bed for coastal altimetry

• Perspectives for data assimilation and conclusion

• Improving 2D maps • by using tide gauges along the coast (Saraceno et al., 2008, Wang et al., 2011)

Recent developments in coastal altimetry

(right) Example of interpolation of (left) AVISO SLA and

currents based on the use of tide gauges data (middle).

From Saraceno et al., 2008. Improved along-shore currents

• Improving 2D maps • by using shorter correlation scales, in combination with multi-mission dataset (Dussurget et al., 2011): mesoscale

Recent developments in coastal altimetry

Example of optimal interpolation of HF along-track data to access mesoscale

dynamics. From Dussurget et al., 2011.

• Improving 2D maps Limitation of such approach: interpolation leads to smoothing of the signal, with final scales that may not be adapted to the study zone, esp. near the coast (anisotropy)

Recent developments in coastal altimetry

• Improving along-track data• First developed in the Mediterranean Sea, in the Capraia Island area (bwn. Italy and Corsica)• use of raw data with improved data flagging (Vignudelli et al., 2000): retrieve steric signal in SLA

Location of the Capraia Island. From Vignudelli et al., 2005.Improvement of data

coverage in the Capraia Island area. From Bouffard et al.,

2008.

Estimation of the geostrophic velocities anomalies. From

Bouffard et al., 2008.

• improved tidal and atmospheric corrections (Vignudelli et al., 2005): currents at seasonal scale• use of high resolution data (10, 20 Hz) (Bouffard et al., 2008)

• Improving along-track data• “recipes” tested in other areas:

Recent developments in coastal altimetry

Signature of the Navidad Current in

along-track geostrophic current anomaly and SST. From Le Hénaff

et al., 2011.

Structure of the EICC. Adapted from Durand et

al., 2009.

HF radar radial velocity with satellite tracks. From

Liu et al., 2011.

• Bay of Biscay (NE Atlantic): characterization of the Navidad Current (North of Spain) (Le Hénaff et al., 2011, Herbert et al., 2011)

• India: study of the East India Coastal Current (EICC) (Durand et al., 2009) • West Florida Shelf: performance assessment over a wide shelf (Liu et al., 2011)

• Still about the improvement of along-track data:

On-going developments in coastal altimetry

• waveform analysis (PISTACH, COASTALT)

• wet tropospheric correction (PISTACH, COASTALT): - better account for land in the altimeter footprint (Desportes et al., 2007)- combine altimeter radiometer measurements, local measurements and

improved atmospheric models (Fernandes et al., 2010)

Zenith wet delay (ZWD, m) from the GPD method

(black), the inboard radiometer (red) and

ECMWF model (blue). Green area are

polar/coastal zones. Adapted from Fernandes et

al., 2011.

On-going developments in coastal altimetry

• tidal correction: - today: global tidal model in many zones; nested high-resolution DA

regional model (G. Egbert, 2011 Coastal Altimetry Workshop recommendation)- along-track harmonic analysis

• dynamic atmospheric correction (DAC): - signal due to wind and pressure removed with barotopic model (Carrère

and Lyard, 2003)- today: global model real-time products; delayed-time?; regional model

soon?

Short-term perspectives

• all these upgrades have to be evaluated

• the evaluation phase has to take into account local conditions (bathymetry, currents etc.)

Configuration of in-situ moorings array under a Jason track. From

Beal et al., 2009.

• mean sea surface (several datasets)

Example of a possible test bed: the Florida Keys area

• Complex topography

- broad SW Florida shelf and Florida Bay- narrow Atlantic Florida Keys shelf- deep Straits of Florida

• Complex dynamics

- wind-driven shelf flows- buoyancy-driven shelf flows (river runoffs)- intense coastal to offshore interactions: Florida Current front and eddiesAdapted from Lee et al., 2002.

Example of a possible test bed: the Florida Keys area

• Jason track 102 is perfectly oriented: perpendicular to the bathymetry and the main current orientation

SSH (cm) from the HYCOM-FKeys simulation on Aug 28, 2009. Isobaths

(black) are 200, 500 and 2000m.

HYCOM-FKeys SSH w/ surface currents.

Apr 25 - salinity

Example of a possible test bed: the Florida Keys area

HYCOM-FKeys SSH w/ along-track 5 Hz surface currents.• observe the Florida Current in details

• able to observe meso-scale eddies along the Keys (right timing)

Apr 22 Apr 23 Apr 24

Example of a possible test bed: the Florida Keys area

• standard altimetry misses the eddy

=> The detection of small frontal eddies is possible thanks to:- data availability closer to the coast- high rate data

5 Hz coastal data Standard 1 Hz data

HYCOM-FKeys SSH w/ along-track 5 Hz surface currents (left) and 1 Hz currents (right)

Example of a possible test bed: the Florida Keys area

• Validation opportunities:- in-situ mooring (Looe Key, 30 km upstream)- high resolution ocean color and/or SST maps (GHRSST)

Location of in-situ moorings along the Florida Keys.

sanDT

A1B1

LKsmk

SW FL Shelf

CR

SRB2C2

Provided by C. Hu (USF/IMARS)

• ideal strategy…

Example of a possible test bed: the Florida Keys area• What can we expect from such dataset?- medium to long-term trends (long time series)- isolated event (when simultaneous with obs)- limitation: one track every 10 days (Topex-Jason orbit)

-> lucky to observe short-term event (eddy passage, wind burst current)

… but with multi-satellite: more opportunities for detection

Location of Jason 2 (red), Jason 1 (magenta), Envisat (yellow) and GFO (green)

tracks.

Example of a possible test bed: the Florida Keys area

• Connection to models

- first: compare the physical content of the model and the observations (need complementary data)

- long-term: assimilate the data and see how they affect the simulation

• existing issues with data assimilation in coastal areas:

- coastal areas (shelf and near-shelf areas): superposition of multiple scales adapt the assimilation strategy: observation data treatment and covariance scales (Yi Chao); 5th coastal altimetry workshop recommendation need multivariate DA to constrain the model (A. Kurapov; 5th CAW)

- tides: how do we assimilate SLA in the presence of tides?- large signal on shelf areas; largest tidal model errors 1st idea: remove the tidal signal from the model before assimilating de-tided SLA (Xie et al., 2011). How can we optimize such filtering? Do we lose some information?- the tidal signal is included in the altimetric data themselves: can we use that information directly? idea: correct the tidal boundary forcing (Barth et al., 2011, HF radar); possible to correct bathymetry (Mourre et al., 2006)?

open questions, crucial when it comes to assimilating altimetry in shelf areas

Conclusions:

• Coastal altimetry is recent: need for users to know the content of the products (corrective terms)

• Achievements so far:• 2D: extend SLA to the coast, improve the resolution• along-track: validation, derive geostrophic current in poorly or uncovered areas still a big effort for the assessment; need collaboration between users and data providers; benefit for SWOT

• Florida Keys area: • multiple scales (Western boundary current, eddies, shelf dynamics)• perfect orientation of Jason 2 track• expected improvements by coastal altimetry (presence of the Keys islands, adapted corrections)

• Long-term objective: data assimilation• adapted multi-scale strategy• treatment of the tidal signal is a major issue

 

Thanks!