Sea Surface KInematics Multiscale monitoring (SKIM ...

Sea Surface KInematics Multiscale monitoring (SKIM) mission

Think Current

1. What is SKIM ?

It is a combination of Ka-band radar altimeter,

+

disco ball, and

speed gun …

+

=

- « super-duper altimeter » : best ever flown (Ka-band, 32 Khz PRF, 200 MHz bandwidth, SAR unfocused) → very low noise for sea level, wave height, ice freeboard … - « disco ball » : well, reflector is fixed (1.2 m parabola – in pink) but the « spotlights » are turning (horn feeds on a rotating plate near focal point). Result : radar beams dancing around ground track Preliminary design : 1 nadir beam (classic altimeter) 7 other beams at 6 and 12° incidence SKIM

SKIM in 10 slides. February 2018

2. How it works As the plate rotates on the spacecraft, beams illuminate different footprints. Switching from one beam to another we measure the « radial current » (in color), that is a projection of the current vector (in black). The result is a map of currents with a full vector at least every 20 km in most of the swath. SKIM also measures : - wave spectrum - sea level - wind stress

SKIM


3. Why we need SKIM ?

KInematics… because the ocean moves ! - where is it all going ? Plankton, plastics, heat, salt, sea ice … - How much and where is the wind working on the Ocean ? - What are the wave heights, period and directions & periods ? What is their contribution to extreme sea level at the coast ? - How waves and currents interact ? - How waves and sea ice interact ? Multiscale : - from waves (Stokes drift …) to global scales (garbage patches ...)

SKIM


4. Got current ?

Models gives currents… but they are poorly constrained by - sea level variability from altimeters - gravimetry (GOCE / GRACE) - in situ drifters (used in Mean Dynamic Topography) Especial in tropics, near & under sea ice ...

SKIM


5. Yes we can measure currents With a Doppler radar from space Zonal current component, 3-month average, Envisat data

From Collard et al. (SEASAR 2008) See also Chapron et al. (JGR 2005), Rouault et al. (JGR 2010)

Section at 120°W

SKIM


6. Coverage (if following S1 orbit)

SKIM


7. What we can sample

2021 : SWOT will make a big step towards high resolution, especially using back-scatter (Rascle et al. 2017) 2025 : SKIM can give the right combination & sampling to fully map the mesoscale (dx = 30 km, dt = 4 days) a pathfinder for Doppler vector Oceanography

& SWOT « glitter »

WaCM ? CIMR (microwave SST)?

202x : WaCM can extend the time coverage Ideal combination ? SWOT + SKIM + CIMR

SKIM


8. How do we know it works ? Thanks to AirSWOT... AirSWOT measures SSH and range-resolved Doppler Doppler + ATI & XTI

2016 LASER experiment : mapped drifter velocity

Average Doppler in Ka band follows currents in range : Nouguier et al. (revised)

SKIM


9. Ice - SKIMming Polar applications : The Arctic is becoming a giant marginal ice zone wave & drift data needed by 2025 to observe this regime shift ...large areas of the Arctic Ocean previously covered by pack ice to the wind and surface waves leads to Arctic pack ice cover evolving into the Marginal Ice Zone. The emerging state of the Arctic Ocean features more fragmented thinner sea ice, stronger winds, ocean currents and waves..

(Aksenov et al., Marine Policy 2017) SKIM


9. Ice - SKIMming Polar applications : The Arctic is becoming a giant marginal ice zone (Aksenov et al., Marine Policy 2017): wave & drift data needed by 2025 to observe this regime shift ...large areas of the Arctic Ocean previously covered by pack ice to the wind and surface waves leads to Arctic pack ice cover evolving into the Marginal Ice Zone. The emerging state of the Arctic Ocean features more fragmented thinner sea ice, stronger winds, ocean currents and waves..

→ Excellent revisit : example of 1-day coverage

- drift in marginal ice zone - Excellent nadir beam quality → freeboard - wave spectra near the ice edge SKIM


9. Ice - SKIMming Polar applications : - ice drift in marginal ice zone : Today's sensors fail at ice edge, more so in Southern Ocean. Instead of displacement, SKIM will give snapshot velocity

AMSR2 drift

100 km

SAR drift SKIM

no data In MIZ

Pictures courtesy of A. Korosov SKIM in 10 slides. February 2018

10. Expected SKIM resolution & accuracy So what are we resolving in terms of ocean scales ? Here is for the Gulf Stream case : effective resolved wavelength Le = 65 km

L2b

SKIM


11. Expected SKIM resolution & accuracy Now for 3 regions from equator to Arctic: in blue, effective resolved wavelength Le Regions

Equator

Gulf-Stream

Fram

Input Level-2a error (12& 6° beams)

0.08 & 0.15 m/s

0.16 &0.20m/s

0.10 & 0.16 m/s

Level-2b error (along&across track) and effective resol.

0.03 & 0.05 m/s 89 km

0.09& 0.14 m/s 65km

0.11 & 0.14 m/s 59km

Level-3a error (zonal&meridional) and effective resol.

0.14 & 0.18 m/s 290km

0.23 & 0.24 m/s 71km

0.12 & 0.13 m/s 62km

0.08& 0.12 m/s 51 km

0.10 & 0.13 m/s 53 km

0.22 & 0.24 m/s 71km

0.08 & 0.10 m/s 44km

Level-2b W/O instr. err. Level-3a W/O instr. err.

SKIM

53km 0.14 & 0.17 m/s 277km


8. Conclusions

- SKIM adds new variable for Earth monitoring : total surface velocity focus : strong currents (in particular tropics) & marginal ice zones - leap forward for waves : new applications from nearshore to solid Earth monitoring.

1.5 km

- complementarity with other missions : SWOT, nadir altimeters, SST … Mission status : Phase A & B1 approved, started Nov. 15, 2017. Choice between SKIM and FORUM (infrared Earth Radiation mission) in mid – 2019. Launch for SKIM or FORUM : mid-2025 on VEGA from Kourou. Papers in review :

Ardhuin et al. (2017) : Measuring currents, ice drift, and waves from space: the Sea Surface KInematics Multiscale monitoring (SKIM) concept. doi : 10.5194/os-2017-65 Nouguier et al. :Sea surface kinematics from near-nadir radar measurements Ubelmann et al. : Mapping surface currents from Doppler measurements...

#SKIM4EE9

@FabriceArdhuin

http://tinyurl.com/SKIMonRG SKIM

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Sea Surface KInematics Multiscale monitoring (SKIM) mission

Think Current