Prospects for future satellite estimation of small-scale variability of ocean surface velocity and vorticity
Year: 2018
Type: article
Abstract: Recent technological developments have resulted in two techniques for estimating surface velocity with higher resolution than can be achieved from presently available nadir altimeter data: (1) Geostrophically computed estimates from high-resolution sea surface height (SSH) measured interferometrically by the wide-swath altimeter on the Surface Water and Ocean Topography (SWOT) Mission with a planned launch in 2021; and (2) Measurements of ocean surface velocity from a Doppler scatterometer mission that is in the early planning stages, referred to here as a Winds and Currents Mission (WaCM). In this study, we conduct an analysis of the effects of uncorrelated measurement errors and sampling errors on the errors of the measured and derived variables of interest (SSH and geostrophically computed velocity and vorticity for SWOT, and surface velocity and vorticity for WaCM). Our analysis includes derivations of analytical expressions for the variances and wavenumber spectra of the errors of the derived variables, which will be useful to other studies based on simulated SWOT and WaCM estimates of velocity and vorticity. We also discuss limitations of the geostrophic approximation that must be used for SWOT estimates of velocity. The errors of SWOT and WaCM estimates of velocity and vorticity at the full resolutions of the measured variables are too large for the unsmoothed estimates to be scientifically useful. It will be necessary to smooth the data to reduce the noise variance. We assess the resolution capabilities of smoothed estimates of velocity and vorticity from simulated noisy SWOT and WaCM data based on a high-resolution model of the California Current System (CCS). By our suggested minimum threshold signal-to-noise (S/N) variance ratio of 10 (a standard deviation ratio of 3.16), we conclude that the wavelength resolution capabilities of maps of velocity and vorticity constructed from WaCM data with a swath width of 1200 km are, respectively, about 60 km and 90 km in 4-day averages. For context, the radii of resolvable features are about four times smaller than these mesoscale wavelength resolutions. If the swath width can be increased to 1800 km, the wavelength resolution capabilities of 4-day average maps of surface velocity and vorticity would improve to about 45 km and 70 km, respectively. Reducing the standard deviation of the uncorrelated measurement errors from the baseline value of σspd=0.50 m s−1 to a value of 0.25 m s−1 would further improve these resolution capabilities to about 20 km and 45 km. SWOT data will allow mapping of the SSH field with far greater accuracy and space–time resolution than are presently achieved by merging the data from multiple nadir altimeter missions. However, because of its much narrower 120-km measurement swath compared with WaCM and the nature of the space–time evolution of the sampling pattern during each 21-day repeat of the SWOT orbit, maps of geostrophically computed velocity and vorticity averaged over the 14-day period that is required for SWOT to observe the full CCS model domain are contaminated by sampling errors that are too large for the estimates to be useful for any amount of smoothing considered here. Reducing the SSH measurement errors would do little to improve SWOT maps of velocity and vorticity. SWOT estimates of these variables are likely to be useful only within individual measurement swaths or with the help of dynamic interpolation from a data assimilation model. By our criterion, in-swath SWOT estimates of velocity and vorticity have wavelength resolution capabilities of about 30 km and 55 km, respectively. In comparison, in-swath estimates of velocity and vorticity from WaCM data with σspd=0.50 m s−1 have a wavelength resolution capability of about 130 km for both variables. Reducing the WaCM measurement errors to σspd=0.25 m s−1 would improve the resolution capabilities to about 50 km and 75 km for velocity and vorticity, respectively. These resolutions are somewhat coarser than the in-swath estimates from SWOT data, but the swath width is more than an order of magnitude wider for WaCM. Instantaneous maps of velocity and vorticity constructed in-swath from WaCM data will therefore be much less prone to edge effect problems in the spatially smoothed fields. Depending on the precise value of the threshold adopted for the minimum S/N ratio and on the details of the filter used to smooth the SWOT and WaCM data, the resolution capabilities summarized above may be somewhat pessimistic. On the other hand, aspects of measurement errors and sampling errors that have not been accounted for in this study will worsen the resolution capabilities presented here. Another caveat to keep in mind is that the resolution capabilities deduced here from simulations of the CCS region during summertime may differ somewhat at other times of year and in other geographical regions where the signal variances and wavenumber spectra of the variables of interest differ from the CCS model used in this study. Our analysis nonetheless provides useful guidelines for the resolutions that can be expected from SWOT and WaCM.
Source: Progress In Oceanography
Institutions: Oregon State University, Woods Hole Oceanographic Institution, University of California, Los Angeles, Centre National de la Recherche Scientifique, Ifremer, Université de Bretagne Occidentale, Laboratory for Ocean Physics and Satellite Remote Sensing, Institut Universitaire Européen de la Mer, Institut de Recherche pour le Développement, Université de Bretagne Occidentale
Language: English
Subfield: Oceanography
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: hybrid
Estimated APC paid: $3,720
Awards: PROJECT SUMMARY MANIFESTATIONS OF OCEANIC MESOSCALE, SUBMESOSCALE, AND INTERNAL-WAVE VARIABILITY IN SWOT DATA THIS IS A PROPOSAL TO PARTICIPATE IN SW, AN APPROXIMATE DOUBLING OF THE RESOLUTION AND THE 18-YEAR RECORD LENGTH AFFORDED BY THE SSH FIELDS CONSTRUCTED BY CLS/AVISO FROM PAIRS OF SIMULTANEOU, QUIKSCAT OBSERVATIONS IN COMBINATION WITH OTHER SATELLITE DATA HAVE RECENTLY REVEALED THAT THE STRONG COUPLING BETWEEN SST AND SURFACE WINDS STUDIED, PROPOSED WORK WILL INCLUDE: (1) SIMULATIONS, USING FULLY COUPLED OCEAN-ATMOSPHERE NUMERICAL MODELS OF THE CCS EASTERN BOUNDARY CURRENT REGIME, SUPPLE, THIS IS A COLLABORATIVE PROPOSAL BY FOUR PIS FROM THREE INSTITUTIONS TO PARTICIPATE IN SWOT SCIENCE TEAM ST ACTIVITIES. OUR INTERESTS ARE IN ASSESSIN, OUR PRESENT KNOWLEDGE OF OCEAN DYNAMICS HAS BENEFITTED TREMENDOUSLY FROM 2+ DECADES OF SATELLITE ALTIMETRY AND THE ARCHIVAGE VALIDATION ET INTERPRETATION DES DONNEES DES SATELLITES OCEANOGRAPHIQUES (AVISO) DATA SETS THAT HAVE MERGED THE IRREGULARLY SPACED MEASUREMENTS OF MULTIPLE ALTIMETERS INTO GRIDDED PRODUCTS THAT ARE USED BY VIRTUALLY THE ENTIRE INTERNATIONAL OCEANOGRAPHIC RESEARCH COMMUNITY. KNOWLEDGE OF THE DOMINANT OCEANIC DYNAMICS HAS ADVANCED TO THE POINT WHERE WE NOW CAN CONSIDER REFINEMENTS TO THE COVARIANCE FUNCTIONS AND OPTIMAL INTERPOLATION ALGORITHMS THAT MIGHT REDUCE MAPPING ERRORS AND ADMIT ADDITIONAL DYNAMICAL FEATURES THAT ARE ADEQUATELY SAMPLED BY SATELLITE ALTIMETERS BUT POORLY REPRESENTED IN THE CURRENT AVISO PRODUCTS. THREE DISTINCT DYNAMICAL PROCESSES MOTIVATE AND WILL SERVE AS CASE STUDIES TO GUIDE OUR WORK ON THIS. IN OUR CURRENT OSTST PROJECT WE HAVE BEEN RECONCILING OBSERVATIONS FROM SATELLITE ALTIMETRY WITH NUMERICAL PROCESS MODEL OUTPUT AND SOME SIGNIFICANT CHALLENGES HAVE ARISEN THAT MOTIVATE THE PRESENT PROPOSAL. STRONG EVIDENCE FOR 30-35 DAY BAROTROPIC ROSSBY WAVES RADIATING NORTHWARD FROM TROPICAL INSTABILITY WAVES (TIWS) WAS PREVIOUSLY PUBLISHED BY THE PI. OUR MORE DETAILED RECENT ANALYSIS OF THE GRIDDED AVISO PRODUCT SHOWS THAT SSH VARIABILITY COHERENT WITH THE TIWS CAN BE FOUND THROUGHOUT MOST OF THE NORTH PACIFIC; AS FAR POLEWARD AS THE GULF OF ALASKA AND THE ALEUTIAN ISLANDS. THE IMPLICATION THAT THIS WIDE-SPREAD COHERENCE IS DUE TO THE PREVIOUSLY IDENTIFIED BAROTROPIC ROSSBY WAVES IS SUPPORTED BY THE COHERENCE PATTERNS WITH ONE PUZZLING EXCEPTION. THE OBSERVED SSH VARIANCE IN THIS PERIOD BAND DECREASES BY A FACTOR OF ABOUT 100 IN THE 20-40 LATITUDE BAND AND IT INCREASES AGAIN AFTER THE WAVES PROPAGATE THROUGH THE REGION AND REACH HIGHER LATITUDES. WE FOLLOWED SEVERAL CONJECTURES FOR EXPLAINING THIS BEHAVIOR BUT WE WERE UNABLE TO REPRODUCE THIS LOCALIZED MINIMUM WITH PROCESS MODELS OR COME UP WITH A PLAUSIBLE WAY OF RATIONALIZING ITS EXISTENCE. WE ONLY RECENTLY DISCOVERED THAT THE MINIMUM IS PRODUCED BY THE SPATIAL DEPENDENCE OF THE TIME SCALES INCORPORATED IN AVISO S OBJECTIVE MAPPING ALGORITHM. AN INDEPENDENT MAPPING OF THE ALONG-TRACK DATA WITH A SPATIALLY UNIFORM MAPPING TIMESCALE PRODUCES MUCH BETTER AGREEMENT WITH MODEL PREDICTIONS AND PHYSICAL REASONING. THE ABOVE EXAMPLE HIGHLIGHTS A PARTICULAR LIMITATION RESULTING FROM THE ASSUMPTIONS THAT GO INTO THE AVISO GRIDDED ALTIMETRY PRODUCT. ANOTHER POTENTIAL WEAKNESS CONCERNS ASSUMPTIONS ABOUT SSH PROPAGATION SPEEDS. AT EACH LOCATION THE AVISO MAPPING SCHEME ASSUMES A PROPAGATION SPEED BASED ON THEORY AND OBSERVATIONS. WHILE THIS MAY REPRESENT THE DOMINANT PROPAGATION CHARACTERISTICS AT MOST LATITUDES THERE ARE IMPORTANT REGIONS OF THE WORLD OCEANS (THE SOUTHERN OCEAN AND THE KUROSHIO EXTENSION FOR EXAMPLE) WHERE SIMULTANEOUS EASTWARD AND WESTWARD PROPAGATION CAN BE BOTH PREDICTED AND OBSERVED BUT IS NOT HANDLED CONSISTENTLY IN THE A PRIORI AUTOCOVARIANCE FUNCTION USED FOR THE AVISO MAPPING. WE HAVE DISCUSSED THESE ISSUES WITH THE PERSONNEL AT CLS RESPONSIBLE FOR THE AVISO/DUACS PROCESSING AND HAVE DEVELOPED A COLLABORATIVE PLAN TO WORK TOWARD IMPROVING THE AVISO MAPPING ALGORITHM. OUR PART OF THE EFFORT WILL ENTAIL DYNAMICAL MODELING AND INDEPENDENT ANALYSIS OF AVAILABLE ALONG-TRACK ALTIMETRY DATA TO QUANTITATIVELY DESCRIBE AND UNDERSTAND OCEAN DYNAMICS PRESENTLY NOT REPRESENTED IN THE AVISO MAPPING AUTOCOVARIANCE FUNCTIONS AND SUBSEQUENT WORK TO CONSIDER DIFFERENT WAYS OF HANDLING SUCH SIGNALS IN AN OBJECTIVE MAPPING TO PRODUCE AN IMPROVED SSH PRODUCT., THE LONG-TERM GOAL OF THIS PROJECT IS TO OBSERVE AND IMPROVE UNDERSTANDING OF WIND FORCING AND SEA-SURFACE HEIGHT VARIABILITY AT HIGH FREQUENCIES (PE
OpenAlex ID: W2896257549