Simulated SWOT ocean observations

Need for the nadir altimeter Wet tropospheric correction

Oceanographic performance requirement

High resolution wide-swath altimetry anticipated from the SWOT mission

The Surface Water and Ocean Topography (SWOT) mission will carry a Ka-band radar interferometer to perform wide-swath altimetry over the global oceans. •  Joint mission of NASA and CNES with

contribution from the Canadian Space Agency

•  Launch planned for 2020

The future SWOT mission

Lee-Lueng Fu(1), Clement Ubelmann(1) and Rosemary Morrow(2)

(1) Caltech, Jet Propulsion Laboratory, Pasadena, CA (2) LEGOS/CNES, Toulouse, France

Sampling characteristics:

•  SAR resolution from 10 -70 m, smoothed to 1kmx1km over the ocean to reduce noise to cm accuracy

•  2x60km wide swath.

•  Non-sun-synch orbit with inclination of 78°, 21-day repeat period

A snapshot of sea surface height (SSH) from Jason-1/2

•  Drop in energy of 2-D gridded SSH spectrum (blue) vs alongtrack spectrum (red) at 200 km Indicates the mapping resolution (AVISO product) from nadir altimeter constellations.

•  The challenge of the SWOT

Mission is to observe SSH at wavelengths shorter than 200 km

The limit of resolution of Jason-1/2

mean

68%

95%

10!100!103!105!

104!

103!

102!

101!

100!10-3! 10-2! 10-1!

Wavenumber (cycle/km)!

Pow

er S

pect

ral D

ensi

ty (c

m2 /c

ycle

/km

)!

baseline threshold

Wavelength (km/cycle)!

Reference wet tropo correction

1-beam residual error

2-beam residual error

•  Conventional nadir-looking one-beam radiometer is not sufficient for correcting the cross-track variability of the range errors caused by the tropospheric water vapor.

•  The two-dimensional swath measurement of SWOT needs a two-beam radiometer for wet-tropospheric corrections.

•  The long-wavelength accuracy of the SSH measurement is expected higher for the nadir altimeter than KaRIn.

•  By combining the two measurements simultaneously, a consistent measurement from short (KaRIn) to long wavelengths (altimeter) can be obtained.

•  Strong heritage of error analysis from nadir altimetry (T/P, Jason1, Jason2, Saral/AltiKa, …)

•  Calibration and validation of SWOT in setting the standard for the next generation altimetry missions to continue the climate data record of sea level and improve its resolution and coverage.

à  For oceanography, requirements are specified in the spectral domain

à  In order to observe fine scales (~20km wavelength) in most of the regions, the level of noise has to be almost 2 orders of magnitude below conventional altimetry

Left: Global mean power spectrum of SSH estimated from Jason series satellites (thick black curve). For wavelength <100km, a linear fit has been applied following (Xu and Fu, 2012). Lower limits of the 68% and 95% percentiles are indicated. Red and blue spectra: baseline and threshold requirement for the SSH accuracy. The baseline (threshold) requirement leads to 15 km (20 km) resolution over 68 % of the ocean.

Spatial distribution of the resolved wavelength (baseline)

à  < 35km everywhere

à  The small values in the tropical band are caused by shallow spectral slopes that may not reflect ocean eddy signals (e.g., internal waves)

References •  Durand et al. et al., 2010: The Surface Water and

Ocean Topography mission: Observing terrestrial surface water and oceanic submesoscale eddies. Proc. IEEE, 98 (5), 766-779.

•  Fu and Ubelmann, 2014: On the Transition from Profile Altimeter to Swath Altimeter for Observing Global Ocean Surface Topography, JTECH (in press)

•  Xu and Fu, 2012:The effects of altimeter instrument noise on the estimation of the wavenumber spectrum of sea surface height, JPO, 42, 2229-2233.

•  Ubelmann et al., 2014: The effect of atmospheric water vapor content on the performance of future wide-swath ocean altimetry measurement, JTECH (accepted)

•  With Jason-1 + Jason-2, only large scales >200km are captured. Eddies are severly attenuated.

•  The finescale features of the reference truth field are captured by the simulated SWOT observations.

•  Drastic Improvement in retrieving the geostrophic velocity (derivative of SSH).

~ 15 km

km

cm

km