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Objectives and Requirements of SWOT for Observing the Oceanic Mesoscale Variability

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

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Objectives and Requirements of SWOT for Observing the Oceanic Mesoscale Variability

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  1. 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

  2. 90% of the ocean’s kinetic energy is carried by eddies which are not well resolved by a nadir-looking altimeter ground tracks of Jason (thick) and T/P (thin) Tandem Mission 10 km scale eddies 100 km scale eddies 100 km

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

  4. D. Chelton (Oregon State U)

  5. Effects of spatial resolution in observing ocean eddies 2 altimeters 4 altimeters 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)

  6. The oceanic submesoscales have not been well observed SSH Wavenumber spectrum from Jason altimeter data

  7. 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)

  8. Feasibility of estimating the vertical velocity of the upper ocean from SSH measurement Simulated W by an OGCM Reconstructed W from SSH Contours are relative vorticity P. Klein (IFREMER)

  9. SWOT SSH error spectral requirement (to resolve signals down to 10 km wavelength) 1000 100 10 km Jason pass 132 (147 cycle average) Signal spectrum k-3 Instrument noise Power density (cm2/cycle/km) k-2 error spectrum 3 cm noise at 1 km2 average 30 km 1cm noise at 1 km2 average 10 km Wavenumber (cycle/km)

  10. Selection of orbit with a desirable subcycle 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. Subcycle (days) 3-day repeat orbit S. Nerem (CU)

  11. Avoid tidal aliasing problem in orbit selection 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. Desirable orbits with 15-, 21-, and 22-day repeat periods for adequate tidal aliasing properties. S. Nerem (CU)

  12. Recommendations • 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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