2015 EUMETSAT Meteorological Satellite Conference, 21-25 September 2015, Toulouse.- 19 pages.- With contributions from: C. Martín-Puig (NOAA); M. Roca & R. Escol`a (IsardSAT); E. García-Ladona & C. González-Haro (ICM-CSIC)Peer Reviewe

Isern-Fontanet, Jordi

Turiel, Antonio

Digital.CSIC

Two-dimensional surface currents fromthe combination of MSG and Jason observationsJ. Isern-Fontanet and A. TurielInstitut de Cie`ncies del Mar (CSIC), BarcelonaContact: jisern@icm.csic.esWith contributions from:C. Mart´ın-Puig (NOAA)M. Roca & R. Escola` (IsardSAT)E. Garc´ıa-Ladona & C. Gonza´lez-Haro (ICM-CSIC)2015 EUMETSAT Meteorological Satellite Conference, 21-25 September 2015, ToulouseMotivation2 of 18• A key problem in oceanography is the estimation of high resolution ocean currents• However:◦ Sampling geometry and noise of present altimeters constrains the spatial scalesthat can be recovered to scales larger than O(100 km) ⇒ altimetric gap◦ Sampling geometry can also introduce errors in the location of ocean currentsMotivation2 of 18• A key problem in oceanography is the estimation of high resolution ocean currents• However:◦ Sampling geometry and noise of present altimeters constrains the spatial scalesthat can be recovered to scales larger than O(100 km) ⇒ altimetric gap◦ Sampling geometry can also introduce errors in the location of ocean currentsPascualetal.GRL2006Need to exploit the synergy between sensors3 of 18• IR & MW (SST): have wide FOV allowing for a good estimation of current patternsbut it is difficult to recover current intensities.• Radar altimeters (SSH): provide measurement at nadir strongly limiting the observationof current patterns but they provide good estimations of current intensities.The transfer function4 of 18• For a non-divergent flow it is possible to define a stream-function, such thatv(x) = ez ×∇ψs(x) (1)• The reconstruction of surface currents from SST (Ts(x)) can be formulated in termsof a transfer functionψˆs(k) = CFn(k)e−i∆θ(k)Tˆs(k) (2)• The transfer function and phase shift can be theoretically derived from GFD◦ Surface Quasi-Geostrophic (SQG) equations predicts that∆θ(k) = 0 Fn(k) ∝ k−1 (3)◦ Many other solutions can be found imposing different stratifications.◦ They have in common that ∆θ(k) = 0 and Fn(k) depends only on k.(Isern-Fontanet et al. JPO 2014)Empirical determination of the transfer function5 of 18• Altimeters provide a direct measurements of the geostrophic stream functionψs(x) =gf0η(x) (4)• The amplitude of the transfer function can be estimated from SSH and SSTobservationsCFn(k) ≈ gf0〈|ηˆ|〉k〈|Tˆs|〉k(5)(Isern-Fontanet et al. JPO 2014)• Transfer functions have been estimated fromEnvisat data◦ Applied to Envisat data (RA and AATSR)for the period 2002-2010◦ Regional analysis: West Australian coast,Mediterranean SeaThe observed transfer functions6 of 18DiscardedmmmmmmmmmmmmmmFn(k) =[1 +(kkc)2α]−12Isern-Fontanet et al. JPO 20142Fn(k) = βk−αPierrehumbert et al. CS & F 1994ﬀFn(k) = βk−1SQG Held et al. JFM 1994Determination of the best Transfer Function7 of 18• Along-track SSH are used to fix theenergy level• Along-track SSH are used to select thebest transfer functionFn(k) =[1 +(kkc)2α]−12• Along-track SSH are used to set transferfunction parametersα = 2, λc = 200 km• Along-track SSH are used to asses thequality of the velocity reconstructionCorrelaton 0.71RMSE 35.14 cm/sPhase8 of 18• We assume that ∆θ(k) = 0◦ This is not always true(e.g. Gonza´lez-Haro and Isern-Fontanet et al. JGR 2014)◦ Furthermore, usually ∆θ(k) = 0(e.g. Isern-Fontanet et al. JGR 2008, Isern-Fontanet et al. JPO 2014)Phase correction9 of 18• Phase shift has many origins: Mixed Layer dynamics, interior PV, salinity distribution...• We have implemented phase corrections due to salinity distribution for theMediterranean Sea◦ Origin: change of sign of SST anomaly of incoming fresh Atlantic waters.◦ Approach: Wavelet-based image segmentation and correctionSATVELS10 of 18• SATVELS is a software created by Almodis Solutions S.L. to diagnose high resolutioncurrents from SST alone or exploiting the synergy between SST and SSH.◦ Includes some standard image processing algorithms (denoise, segementations,...)◦ Includes most of the theoretical as well as optimized empirical transfer functions◦ Fine processing of SSH observations, including SAR mode altimtery◦ Preliminary phase correction algorithms• A new version is under development◦ Move to FORTRAN08 (current version is IDL)◦ Version 2.0 ready before the launch of Sentinel-3◦ Improve and and add new phase correctionalgorithms (version 2.1)SATVELS: Comparison to Maximum Cross Correlation11 of 18Our approach MCCMCC: I. Barton personal communicationSST: CSIROSATVELS: Comparison to altimetric maps12 of 18Our approach AltimetryMADT: AVISO AltimetrySST: CSIROSATVELS: MSG13 of 18SATVELS: Search and Rescue14 of 18ForecastedffffffqObservedffffffqObservedffffffqImproving high resolution15 of 18Discarded• At short scales the transfer function was extrapolated as Fn(k) ∼ k−α• We want to explore scales below 60 kmHigh resolution altimetry16 of 18• The capability to observe structures along-track is strongly limited by the presence ofnoise.◦ A major contribution to noise are ocean waves◦ Signal-to-noise ratio (SNR) is not homogenous• A classical low-pass filter with a fixed cut-off wavelength may not be adequate toremove noise◦ May remove small scales with large SNR◦ Does not attempt to correct large scales with low SNR• Denoising using waveletsη(x) =∑iwiφi(x) ⇒ η˜(x) =∑iw∗iφi(x) (6)(e.g. Isern-Fontanet and Hascoe¨t JGR 2014)Application to CryoSat: a coastal eddy17 of 18CorrelationsRaw 0.64Filtered 0.91Denoised 0.93• Wavelet coefficients are modified based on the derived SWH and waveform fitting• High correlation with SSH derived from SST observations• Results encourage us to extend this approach to conventional altimetry: Jason-1/2Summary and conclusions18 of 18• The synergy between SSH and SST measurements can be exploited to improve thereconstruction of surface ocean currents◦ SST is used to recover the topology of the flow◦ SSH is used to recover the energy of the flow at different scales◦ The comparison of both allows to asses the quality of the reconstruction• Several configurations have been explored◦ Mono-satellite configuration: one IR radiometer (AATSR) and one altimeter onthe same platform → ERS*, Envisat, Sentinel-3◦ Geostationary configuration: one geostationary IR radiometer (SEVIRI) andsimultaneous altimeters → MSG, Jason*• Our approach has the potential to retrieve coastal structures of the order 5-10 km• We are developing an adaptative approach to process SSH measurements to keep theshortest possible wavelength

Two-dimensional surface currents from the combination of MSG and Jason observations

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