SWOT Science Team meeting, March 2010

Towards an end‐to‐end of simula2on of SWOT performance for mesoscale and sub‐mesoscale 

circula2on monitoring  

P.Y. Le Traon, P. Klein, B. Chapron (Ifremer) 

M.I. Pujol, G. Dibarboure (CLS)  

SWOT Science Team meeting, March 2010

Ifremer/CLSstudyforSWOTprepara6on

Analysis of sampling capabili2es of SWOT using very high resolu2on basin scale model (1 ‐2 km) (SSH, velocity and ver2cal velocity).   

Analysis of SWOT measurement errors (white/correlated noise, roll errors) and their impact on mission objec2ves.   

Use tools developed and tested for conven2onal mul2ple al2meter and WSOA studies (Le Traon et al. 2001; Le Traon and Dibarboure, 2002).  

A major  issue  is  to  have  a  precise  error  budget  for  SWOT  and  to analyze  the  impact  of  post‐processing  of  SWOT  data  on  ocean signals (e.g. roll error reduc2on)   

Study is on going (see hereaVer).   

SWOT Science Team meeting, March 2010

Highresolu6onal6metry(SWOT)

mesoscale/submesoscaleinves6ga6ons

•  Shouldallowacharacteriza6onofsubmesoscalesignals.Scales(wavelengths/2)between10and100km.

•  Noiseisamajorissue(agoalof1cm/1kmforanuncorrelatednoiseisquitechallenging!).

•  Timesamplingisanissue(exceptforthe3dayrepeatcycles)

Ducet et al., 2000

SWOT noise level ? (1 cm / 1 km)

10-20 km

Conventional altimeter noise level (2.5 cm – 7 km)

SWOT Science Team meeting, March 2010

SWOT OSSE 

Reference data set: 50 days of SLA maps from two models : EarthSimulator (ES) and POP. ES has (much) more energy at short wavelengths (< 100 km) and high frequencies (< 10 days)

ES RMS(SLA)

0 20 cm

38

SLA

-40 40 cm

POP

38 SLA

-40 40

38 RMS(SLA)

0 40 cm

Model resolution: ES : 0.017°x0.032°; 12 h POP: 0.1°x0.1° and 3 days

SWOT Science Team meeting, March 2010

SWOT J2+EN+ GFO+J1N

J2+EN+GFO J2+EN

  4 constellations studied :

  SLA reconstruction with Objective Analysis method:

  final grid resolution :1/8° - every 3 days

  correlation scales: 100 km / 10 days

OSSE study 

SWOT Science Team meeting, March 2010

Sampling effect on SLA 

 SWOT alone gives same results (ES), even better (POP), than for the 4-satellite constellation  ES larger errors are due to small scales signals not present in POP simulation

50 % 2

J2+EN J2+EN+GFO J2+EN+GFO+J1N SWOT

Mapping error (in % of signal variance)

SLA error over high-variability areas (in % of signal variance)

-18% -32%

-28% -29%

-1% -22%

ES

POP

No noise considered in this part. “Perfect” measurements.

SWOT Science Team meeting, March 2010

 With ES, the main part of the errors observed are induced by small scale structures ES : high-frequency↔ short and long wave signals POP : high-frequency↔ long wave signals (> 100 km)

From ES : entire signal

Frm ES : Long-wave (> 100 km) and low frequency (> 10 days)

From ES : Low-frequency part (> 10 days)

From ES : Long-wave signal (> 100 km)

From POP : entire signal

Sampling effect on SLA long/short‐wave and low/high‐frequency component 

SLA error over high-variability areas, deduced from different reference signals (in % of signal variance)

Reference signals used

No noise considered in this part

SWOT Science Team meeting, March 2010

Sampling effect on SLA  Sensibility to the la2tude with SWOT 

In inter-track areas, SWOT SLA error budget is multiplied by 2. SWOT should be combined with others satellites

Formal mapping error (in % of signal variance)

50 % 2

SWOT 38°N

SWOT 45°N

-18% -28%

-28% -23%

-1% +153%

SLA error over high-variability areas (in % of signal variance)

ES (38°N)

ES (45°N)

No noise considered in this part

SWOT Science Team meeting, March 2010

The white noise: 3 cm rms for nadir measurement (corresponds to the real observation case) 0.45 cm rms for SWOT wide swath (corresponds to the optimistic case of white noise reduction)

Roll-error (SWOT): max 3.5 cm rms on swath extremity (pessimistic case [2 arcsec/120s] reduced to [0.1 arcsec/120s] after correction)

Residual orbit error: 1 cm rms / orbit and ½ orbit length

  3 different noises are taken into account :

Impact of measurement noise (on going) 

SWOT Science Team meeting, March 2010

Conclusions / Perspec2ves 

 SWOT alone gives equivalent results as with a conventional 4-altimeter constellation (mean mapping error) (even when taking into account noise)

 These preliminary studies need to be completed:

 Better parameterization of measurement noise and objective analysis

o  higher white noise on SWOT wide swath and variable in space

o  case of nadir absence on SWOT to be considered

o  better parameterization of the noise for objective analysis

o  adjustment of correlation scales and final grid resolution

  Computational method is now extended to velocity (done) and 3D vertical velocity (using SQG approximation).

  Analysis of SWOT results along the swaths only (snapshots)

  Analysis of the 3-day repeat cycle

  Analysis of the SWOT results combined with other satellites

SWOT Science Team meeting, March 2010

Recommenda6ons

Tools are in place to simulate the expected contribu2ons of SWOT.   We plan to report on these studies at the next OST and Lisbon workshop.  

Need a be\er/consolidated error budget for SWOT.    Need also from the project to iden2fy the main scenarii that could be tested.  

Need to simulate the effect of SWOT data (post) processing as part of this study.  This post processing will  likley remove ocean signals and this must be quan2fied (e.g.  removing an along track slope over 1500 km on al2meter SLA does impact the mesoscale signals – see Le Traon, Boissier, Gaspar, JAOT, 1991).  

We will also simulate the impact of the fast repeat cycle (3 days).   Only this mode will allow (over  limited areas) a  space/2me monitoring of mesocale/submesoscale circula2on.   The 10  day  sampling  will  «  only  »  provide  a  series  of  high  resolu2on  snapshots  at  10  day interval.