Objectives and Requirements of SWOT for Observing the Oceanic Mesoscale Variability

(based on a workshop held at Scripps Institution of Oceanography, April 28-May 1, 2008)

Lee-Lueng Fu

Jet Propulsion Laboratory

C-K. Shum

Ohio State University

NASA SWOT Hydrology Workshop

September 15-17, 2008

The Ohio State University, Columbus, Ohio

90% of the ocean’s kinetic energy is carried by eddies which are not well resolved by a nadir-looking altimeter

100 km

ground tracks of Jason (thick) and T/P (thin) Tandem Mission

100 km scale eddies10 km scale eddies

A snapshot of ocean eddies from merged Jason/Envisat data provided by CLS (the best existing data)

D. Chelton (Oregon State U)

Comparison of altimetry and drifter data of a cyclonic eddy of the Gulf Stream. The white line represents the trajectory followed by a surface float between 14 May 2003 and 28 May 2003. The vectors correspond to the absolute velocity field (geostrophy + Ekman) and the background color field is the SLA + MDT (in cm) on 21 May 2003 from the (left) 2 (T/P/ERS)- and (right) 4 (T/P/Jason/ERS/GFO)- satellite configurations. (from Pascual et al, 2006)

Effects of spatial resolution in observing ocean eddies

2 altimeters 4 altimeters

The oceanic submesoscales have not been well observed

SSH Wavenumber spectrum from Jason altimeter data

The importance of oceanic submesoscales:50% of the vertical motion in the world’s oceans responsible for

heat and CO2 uptake takes place at the submesoscales

R. Ferrari (MIT)

Simulated W by an OGCMSimulated W by an OGCM

Feasibility of estimating the vertical velocity of the upper ocean from SSH measurement

Reconstructed W from SSHReconstructed W from SSH

Contours are relative vorticity

P. Klein (IFREMER)

k-3

k-2

3 cm noise at 1 km2 average

1cm noise at 1 km2 average 30 km

10 km

Jason pass 132 (147 cycle average)

Wavenumber (cycle/km)

Pow

er d

ensi

ty (

cm2 /

cycl

e/km

)

Instrument noise

1000 100 10 km

error spectrum

SWOT SSH error spectral requirement

(to resolve signals down to 10 km wavelength)

Signal spectrum

There was a consensus among the workshop participants for the need of a special experiment of limited duration in which the repeat period is reduced to achieve higher sampling rates for studying the temporal variability of submesoscale processes, especially in coastal zones.

A family of orbits with an inclination of 78 degrees.

A 22-day orbit with a 3-day subcyle can be easily adjusted from an altitude of 970 km to 976 km to get into a 3-day repeat orbit.

Selection of orbit with a desirable subcycle

Su

bcy

cle

(day

s)

3-day repeat orbit

S. Nerem (CU)

We need to avoid aliasing the major tidal components into the low frequencies of interests to the mission (e.g., the annual signal), as well as to avoid aliasing multiple components into the same frequency.

Avoid tidal aliasing problem in orbit selection

Desirable orbits with 15-, 21-, and 22-day repeat periods for adequate tidal aliasing properties.

S. Nerem (CU)

• SSH measurement performance of 1 cm noise at 1 km x 1 km averaging is essential for resolving sub-mesoscale features at wavelengths from 10-30 km, depending on the location. Systematic errors should be at least one order of magnitude less than signals at wavelengths longer than 30 km.

• A candidate orbit emerges with the following characteristics: 78 degree inclination, 22-day repeat period with a 3-day sub-cycle, 970.6 km altitude. This orbit meets the following requirements: cover all of the major rivers of the world; minimize tidal aliasing effects; have a sub-cycle for resolving high-frequency signals with adequate spatial coverage.

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