An Optimal Filter for Geostrophic Mesoscale Currents from Along-Track Satellite Altimetry

Abstract Satellite altimetry has proven to be one of the most useful oceanographic datasets, providing a continuous, near-global record of surface geostrophic currents, among other uses. One limitation of observations from a single satellite is the difficulty of estimating the full geostrophic velocity field. The 3-yr Jason-1–Ocean Topography Experiment (TOPEX)/Poseidon tandem mission, with two satellites flying parallel tracks, promised to overcome this limitation. However, the wide track separation severely limits the tandem mission’s resolution and reduces the observed velocity variance. In this paper, the effective filter imposed by the track separation is discussed and two important consequences for any application of the tandem mission velocities are explained. First, while across-track velocity is simply low-pass filtered, along-track velocity is attenuated also at wavelengths much longer than the track separation. Second, velocity wavenumber spectral slopes are artificially steepened by a factor of k−2 at wavelengths smaller than the track separation. Knowledge of the effective filter has several applications, including reconstruction of the full velocity spectrum from the heavily filtered observations. Here, the hypothesis that the tandem mission flow field is horizontally nondivergent and isotropic is tested. The effective filter is also used to predict the fraction of the eddy kinetic energy (EKE) that is captured for a given track separation. The EKE captured falls off rapidly for track separations greater than about 20 km.

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Geostrophic Currents in the northern Nordic Seas from a Combination of Multi-Mission Satellite Altimetry and Ocean Modeling

10.5194/essd-2019-102 ◽

2019 ◽

Author(s):

Felix L. Müller ◽

Denise Dettmering ◽

Claudia Wekerle ◽

Christian Schwatke ◽

Marcello Passaro ◽

...

Keyword(s):

Sea Ice ◽

Spatial Patterns ◽

Temporal Variability ◽

Satellite Altimetry ◽

Principal Component ◽

Ocean Model ◽

Surface Currents ◽

Geostrophic Velocity ◽

Nordic Seas ◽

Along Track

Abstract. A deeper knowledge about geostrophic ocean surface currents in the northern Nordic Seas supports the understanding of ocean dynamics in an area affected by sea ice and rapidly changing environmental conditions. Monitoring these areas by satellite altimetry results in a fragmented and irregularly distributed data sampling and prevents the computation of homogeneous and highly resolved spatio-temporal datasets. In order to overcome this problem, an ocean model is used to fill in data when altimetry observations are missing. The present study provides a novel dataset based on a combination of along-track satellite altimetry derived dynamic ocean topography (DOT) elevations and simulated differential water heights (DWH) from the Finite Element Sea ice Ocean Model (FESOM). This innovative dataset differs from classical assimilation methods because it substitutes altimetry data with the model output, when altimetry fails or is not available. The combination approach is mainly based on a Principal Component Analysis (PCA) after reducing both quantities by their constant and seasonal signals. In the main step, the most dominant spatial patterns of the modeled differential water heights as provided by the PCA are linked with the temporal variability of the estimated DOT from altimetry by performing a Principal Component Synthesis (PCS). After the combination, the by altimetry obtained annual signal and a constant offset are re-added in order to reference the final data product to the altimetry height level. Surface currents are computed by applying the geostrophic flow equations to the combined topography. The resulting final product is characterized by the spatial resolution of the ocean model around 1 km and the temporal variability of the altimetry along-track derived DOT heights. The combined DOT is compared to an independent DOT product resulting in a positive correlation of about 80 % to provide more detailed information about short periodic and finer spatial structures. The derived geostrophic velocity components are evaluated by in-situ surface drifter observations. Summarizing all drifter observations in equal-sized bins and comparing the velocity components shows good agreement in spatial patterns, magnitude and flow direction. Mean differences of 0.004 m/s in the zonal and 0.02 m/s in the meridional component are observed. A direct pointwise comparison between the drifter trajectories and to the drifter location interpolated combined geostrophic velocity components indicates that about 94 % of all residuals are smaller than 0.15 m/s. The dataset is able to provide surface circulation information within the sea ice area and can be used to support a deeper comprehension of ocean currents in the northern Nordic Seas affected by rapid environmental changes in the 1995–2012 time period. The data is available at https://doi.org/10.1594/PANGAEA.900691 (Müller et al., 2019).

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A coastally improved global dataset of wet tropospheric corrections for satellite altimetry

Earth System Science Data ◽

10.5194/essd-12-3205-2020 ◽

2020 ◽

Vol 12 (4) ◽

pp. 3205-3228

Author(s):

Clara Lázaro ◽

Maria Joana Fernandes ◽

Telmo Vieira ◽

Eliana Vieira

Keyword(s):

Sea Level ◽

Coastal Zone ◽

North American ◽

Satellite Altimetry ◽

Microwave Radiometer ◽

Satellite System ◽

Path Delay ◽

Coastal Zones ◽

The North ◽

Along Track

Abstract. The accuracy of satellite radar altimetry (RA) is known to deteriorate towards the coastal regions due to several reasons, amongst which the improper account for the wet path delay (WPD) can be pointed out. The most accurate WPDs for RA are derived from the on-board microwave radiometer (MWR) radiance measurements, acquired simultaneously as the altimeter ranges. In the coastal zone, however, the signal coming from the surrounding land contaminates these measurements and the water vapour retrieval from the MWR fails. As meteorological models do not handle coastal atmospheric variability correctly yet, the altimeter measurements are rejected whenever MWR observations are absent or invalid. The need to solve this RA issue in the coastal zone, simultaneously responding to the growing demand for data in these regions, motivated the development of the GNSS (Global Navigation Satellite System) derived Path Delay (GPD) algorithm. GPD combines WPD from several sources through objective analysis (OA) to estimate the WPD or the corresponding RA correction accounting for this effect, the wet tropospheric correction (WTC), for all along-track altimeter points for which this correction has been set as invalid or is not defined. The current GPD version (GPD Plus, GPD+) uses as data sources WPD from coastal and island GNSS stations, from satellites carrying microwave radiometers, and from valid on-board MWR measurements. GPD+ has been tuned to be applied to all, past and operational, RA missions, with or without an on-board MWR. The long-term stability of the WTC dataset is ensured by its inter-calibration with respect to the Special Sensor Microwave Imager (SSM/I) and SSM/I Sounder (SSMIS). The dataset is available for the TOPEX/Poseidon (T/P); Jason-1 and Jason-2 (NASA and CNES); Jason-3 (NASA and EUMETSAT); ERS-1, ERS-2, Envisat and CryoSat-2 (ESA); SARAL/AltiKa (ISRO and CNES); and GFO (US Navy) RA missions. The GPD+ WTC for Sentinel-3 (ESA and EUMETSAT) shall be released soon. The present paper describes the GPD+ database and its assessment through statistical analyses of sea level anomaly (SLA) datasets, calculated with GPD+, the ECMWF Reanalysis Interim (ERA-Interim) model or MWR-derived WTCs. Global results, as well as results for three regions (the North American and European coasts and the Indonesia region), are presented for ESA's recent Envisat Full Mission Reprocessing (FMR) V3.0. Global results show that the GPD+ WTC leads to a reduction in the SLA variance of 1–2 cm2 in the coastal zones, when used instead of the ERA WTC, which is one of the WTCs available in these products and can be adopted when the MWR-derived WTC is absent or invalid. The improvement of the GPD+ WTC over the ERA WTC is maximal over the tropical oceans, particularly in the Pacific Ocean, showing that the model-derived WTC is not able to capture the full variability in the WPD field yet. The statistical assessment of GPD+ for the North American coast shows a reduction in SLA variance, when compared to the use of the ERA-derived WTC, of 1.2 cm2, on average, for the whole range of distances from the coast considered (0–200 km). Similar results are obtained for the European coasts. For the Indonesia region, the use of the GPD+ WTC instead of that from ERA leads to an improvement, on average, on the order of 2.2 cm2 for distances from the coast of up to 100 km. Similar results have been obtained for the remaining missions, particularly for those from ESA. Additionally, GPD+ recovers the WTC for a significant number of along-track altimeter points with missing or invalid MWR-derived WTCs, due to land, rain and ice contamination and instrument malfunctioning, which otherwise would be rejected. Consequently, the GPD+ database has been chosen as the reference WTC in the Sea Level Climate Change Initiative (CCI) products; GPD+ has also been adopted as the reference in CryoSat-2 Level-2 Geophysical Ocean Products (GOP). Strategies to further improve the methodology, therefore enhancing the quality of the database, are also discussed. The GPD+ dataset is archived on the home page of the Satellite Altimetry Group, University of Porto, publicly available at the repository https://doi.org/10.23831/FCUP_UPORTO_GPDPlus_v1.0 (Fernandes et al., 2019).

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Global changes in oceanic mesoscale currents over the satellite altimetry record

Nature Climate Change ◽

10.1038/s41558-021-01006-9 ◽

2021 ◽

Author(s):

Josué Martínez-Moreno ◽

Andrew McC. Hogg ◽

Matthew H. England ◽

Navid C. Constantinou ◽

Andrew E. Kiss ◽

...

Keyword(s):

Satellite Altimetry ◽

Global Changes ◽

Mesoscale Currents

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Comparison of gridded multi-mission and along-track mono-mission satellite altimetry wave heights with in situ near-shore buoy data

Ocean Engineering ◽

10.1016/j.oceaneng.2014.03.014 ◽

2014 ◽

Vol 83 ◽

pp. 24-35 ◽

Cited By ~ 17

Author(s):

P.R. Shanas ◽

V. Sanil Kumar ◽

N.K. Hithin

Keyword(s):

Satellite Altimetry ◽

Mission Satellite ◽

Near Shore ◽

Wave Heights ◽

Buoy Data ◽

Along Track

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Geostrophic currents in the northern Nordic Seas from a combination of multi-mission satellite altimetry and ocean modeling

Earth System Science Data ◽

10.5194/essd-11-1765-2019 ◽

2019 ◽

Vol 11 (4) ◽

pp. 1765-1781 ◽

Cited By ~ 1

Author(s):

Felix L. Müller ◽

Denise Dettmering ◽

Claudia Wekerle ◽

Christian Schwatke ◽

Marcello Passaro ◽

...

Keyword(s):

Sea Ice ◽

Spatial Patterns ◽

Temporal Variability ◽

Satellite Altimetry ◽

Principal Component ◽

Ocean Model ◽

Surface Currents ◽

Geostrophic Velocity ◽

Nordic Seas ◽

Along Track

Abstract. A deeper knowledge about geostrophic ocean surface currents in the northern Nordic Seas supports the understanding of ocean dynamics in an area affected by sea ice and rapidly changing environmental conditions. Monitoring these areas by satellite altimetry results in a fragmented and irregularly distributed data sampling and prevents the computation of homogeneous and highly resolved spatio-temporal datasets. In order to overcome this problem, an ocean model is used to fill in data when altimetry observations are missing. The present study provides a novel dataset based on a combination of along-track satellite-altimetry-derived dynamic ocean topography (DOT) elevations and simulated differential water heights (DWHs) from the Finite Element Sea ice Ocean Model (FESOM) version 1.4. This innovative dataset differs from classical assimilation methods because it substitutes altimetry data with the model output when altimetry fails or is not available. The combination approach is mainly based on a principal component analysis (PCA) after reducing both quantities by their constant and seasonal signals. In the main step, the most-dominant spatial patterns of the modeled differential water heights as provided by the PCA are linked with the temporal variability in the estimated DOT from altimetry by performing a principal component synthesis (PCS). After the combination, the annual signal obtained by altimetry and a constant offset are re-added in order to reference the final data product to the altimetry height level. Surface currents are computed by applying the geostrophic flow equations to the combined topography. The resulting final product is characterized by the spatial resolution of the ocean model around 1 km and the temporal variability in the altimetry along-track derived DOT heights. The combined DOT is compared to an independent DOT product, resulting in a positive correlation of about 80 %, to provide more detailed information about short periodic and finer spatial structures. The derived geostrophic velocity components are evaluated by in situ surface drifter observations. Summarizing all drifter observations in equally sized bins and comparing the velocity components shows good agreement in spatial patterns, magnitude and flow direction. Mean differences of 0.004 m s−1 in the zonal and 0.02 m s−1 in the meridional component are observed. A direct pointwise comparison between the combined geostrophic velocity components interpolated onto the drifter locations indicates that about 94 % of all residuals are smaller than 0.15 m s−1. The dataset is able to provide surface circulation information within the sea ice area and can be used to support a deeper comprehension of ocean currents in the northern Nordic Seas affected by rapid environmental changes in the 1995–2012 time period. The data are available at https://doi.org/10.1594/PANGAEA.900691 (Müller et al., 2019).

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Regional Mean Sea Surface and Mean Dynamic Topography Models Around Malaysian Seas Developed From 27 Years of Along-Track Multi-Mission Satellite Altimetry Data

Frontiers in Earth Science ◽

10.3389/feart.2021.665876 ◽

2021 ◽

Vol 9 ◽

Author(s):

Mohammad Hanif Hamden ◽

Ami Hassan Md Din ◽

Dudy Darmawan Wijaya ◽

Mohd Yunus Mohd Yusoff ◽

Muhammad Faiz Pa’suya

Keyword(s):

Satellite Altimetry ◽

Sea Surface ◽

Dynamic Topography ◽

Tide Gauges ◽

Regional Models ◽

Mean Dynamic Topography ◽

Mission Satellite ◽

Mean Sea Surface ◽

Along Track ◽

Good Agreement

Contemporary Universiti Teknologi Malaysia 2020 Mean Sea Surface (UTM20 MSS) and Mean Dynamic Topography (UTM20 MDT) models around Malaysian seas are introduced in this study. These regional models are computed via scrutinizing along-track sea surface height (SSH) points and specific interpolation methods. A 1.5-min resolution of UTM20 MSS is established by integrating 27 years of along-track multi-mission satellite altimetry covering 1993–2019 and considering the 19-year moving average technique. The Exact Repeat Mission (ERM) collinear analysis, reduction of sea level variability of geodetic mission (GM) data, crossover adjustment, and data gridding are presented as part of the MSS computation. The UTM20 MDT is derived using a pointwise approach from the differences between UTM20 MSS and the local gravimetric geoid. UTM20 MSS and MDT reliability are validated with the latest Technical University of Denmark (DTU) and Collecte Localisation Services (CLS) models along with coastal tide gauges. The findings presented that the UTM20, CLS15, and DTU18 MSS models exhibit good agreement. Besides, UTM20 MDT is also in good agreement with CLS18 and DTU15 MDT models with an accuracy of 5.1 and 5.5 cm, respectively. The results also indicate that UTM20 MDT statistically achieves better accuracy than global models compared to tide gauges. Meanwhile, the UTM20 MSS accuracy is within 7.5 cm. These outcomes prove that UTM20 MSS and MDT models yield significant improvement compared to the previous regional models developed by UTM, denoted as MSS1 and MSS2 in this study.

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Estimating Relative and Absolute Sea Level Rise at Ny-Ålesundwith satellite altimetry and in-situ observations

10.5194/egusphere-egu21-2836 ◽

2021 ◽

Author(s):

Stefano Vignudelli ◽

Francesco De Biasio

Keyword(s):

Climate Change ◽

Sea Level Rise ◽

Sea Level ◽

Satellite Altimetry ◽

Adriatic Sea ◽

Tide Gauge ◽

European Space Agency ◽

Data Sets ◽

Space Agency ◽

Along Track

Consistent and long-term satellite-based data-sets to study climate-scale variations of sea level globally and in the coastal zone are available nowadays. Two altimetry data-sets were recently produced: the first one is generated by the European Space Agency&#8217;s (ESA) Sea Level Climate Change Initiative (SL_CCI) over a grid of 0.25 x 0.25 degrees, merging and homogenizing the various available satellite altimetry missions. The second one is a climate-oriented altimeter sea level product that started in the framework of the European Copernicus Climate Change Service (C3S), and is now released as daily-means over a grid of 0.25 x 0.25 degrees, covering the global ocean since 1993 to present. Both reach in the Arctic the latitude of 81.5 N degrees. Therefore, these new altimetry products cover the coastal area surrounding Ny-&#197;lesund (Svalbard Islands, Norway), where a tide gauge station is active since 1976. Near the Svalbard coasts also the along track surface elevations of the CryoSat-2 mission are made available through the European Space Agency&#8217;s Grid Processing on Demand (G-POD) for Earth Observation Applications facility.In this study, we compare sea level measurements from the Ny-&#197;lesund tide gauge with the climate-oriented altimeter sea level gridded products (SL_CCI and C3S) and with the along track data from the only CryoSat-2 mission. This study has three objectives: 1) to assess the performances of the gridded data moving from offshore to near coasts; 2) to explore how the synergy with along track high resolution CryoSat-2 data might help to detail the sea ice impact on the observation of relative and absolute sea level rise around Svalbard; 3) to verify if the differences between satellite altimetry and tide gauges can be used as a proxy of vertical ground movement in the study area by adopting the approaches elaborated in Vignudelli et al. [2018] and De Biasio et al. [2020] that can be validated with ground vertical displacements estimated using Global Positioning System (GPS) data from the stations close to Ny-&#197;lesund.&#160;REFERENCESVignudelli, S., De Biasio, F., Scozzari, A. Zecchetto, S., and Papa, A. (2019): Sea Level Trends and Variability in the Adriatic Sea and Around Venice, Proceedings of the International Review Workshop on Satellite Altimetry Cal/Val Activities and Applications, 23-26 April 2018, Chania, Crete, Greece. DOI:10.1007/1345_2018_51De Biasio, F.; Baldin, G.; Vignudelli, S. Revisiting Vertical Land Motion and Sea Level Trends in the Northeastern Adriatic Sea Using Satellite Altimetry and Tide Gauge Data. J. Mar. Sci. Eng. 2020, 8, 949. DOI:10.3390/jmse8110949

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Multi-Peak Retracking of CryoSat-2 SARIn Waveforms: Potential for Sea Ice Applications

10.5194/egusphere-egu2020-19534 ◽

2020 ◽

Author(s):

Alessandro Di Bella ◽

Ron Kwok ◽

Thomas Armitage ◽

Henriette Skourup ◽

René Forsberg

Keyword(s):

Sea Ice ◽

Satellite Altimetry ◽

Arctic Sea Ice ◽

The Arctic ◽

Ice Thickness ◽

Phase Information ◽

Sea Ice Thickness ◽

Processing Scheme ◽

Thickness Estimation ◽

Along Track

For the last 25+ years, satellite altimetry has proven to be a powerful tool to estimate sea ice thickness from space, by measuring directly the sea ice freeboard. Nevertheless, available thickness estimates from satellite altimetry are affected by a relatively high uncertainty, with the largest contributions originating from the poor knowledge of both the Arctic snow cover and the sea surface height (SSH) in ice-covered regions. The ESA&#8217;s CryoSat-2 (CS2) radar altimetry mission is the first mission carrying on board an altimeter instrument able to operate in Synthetic Aperture Radar Interferometric (SARIn) mode. Previous studies showed how the phase information available in the SARIn mode can be used to reduce the random uncertainty of the SSH in ice-covered regions [1] and, consequently, the average uncertainty of along-track freeboard retrievals [2].This work shows that it is possible to extract even more information from level 1b SARIn data. In fact, while it is not possible to perform full swath processing [3] over sea ice, the contribution from sea ice reflections originating close to the satellite nadir is successfully separated from the specular returns from off-nadir leads for some SARIn waveforms. We find that retracking multiple peaks, in combination with the respective phase information, enables to obtain more than one valid height estimate from single SARIn waveforms over sea ice. The resulting larger amount of freeboard estimates, together with the more precise SSH, is found to contribute to an average reduction of the gridded random and total sea ice thickness uncertainties of ~40% and ~25%, respectively, compared to a regular SAR processing scheme. This study also investigates how the CS2 SARIn phase information can aid thickness estimation in coastal areas, using ESA Sentinel-1 SAR images and airborne data from NASA Operation IceBridge campaigns as a mean of validation.The more precise and, potentially, more accurate freeboard retrievals, as well as the potential for coastal freeboard and thickness estimation shown in this work, support the design of future satellite altimetry missions, e.g. Sentinel-9, operating in SARIn mode over the entire Arctic Ocean.&#160;References[1] Armitage, T. W. K., & Davidson, M. W. J. (2014). Using the interferometric capabilities of the ESA CryoSat-2 mission to improve the accuracy of sea ice freeboard retrievals. IEEE Transactions on Geoscience and Remote Sensing, 52(1), 529&#8211;536. http://doi.org/10.1109/TGRS.2013.2242082[2] Di Bella, A., Skourup, H., Bouffard, J., & Parrinello, T. (2018). Uncertainty reduction of Arctic sea ice freeboard from CryoSat-2 interferometric mode. Advances in Space Research, 62(6), 1251&#8211;1264. http://doi.org/10.1016/j.asr.2018.03.018[3] Gray, L., Burgess, D., Copland, L., Cullen, R., Galin, N., Hawley, R., & Helm, V. (2013). Interferometric swath processing of Cryosat data for glacial ice topography. Cryosphere, 7 (6), 1857&#8211;1867.

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Dynamic Ocean Topography of the Greenland Sea: A comparison between satellite altimetry and ocean modeling

10.5194/tc-2018-184 ◽

2018 ◽

Author(s):

Felix L. Müller ◽

Claudia Wekerle ◽

Denise Dettmering ◽

Marcello Passaro ◽

Wolfgang Bosch ◽

...

Keyword(s):

Sea Ice ◽

Satellite Altimetry ◽

Irregular Sampling ◽

The Arctic ◽

Greenland Sea ◽

Comparison Results ◽

Good Accordance ◽

Ocean Topography ◽

Along Track ◽

Annual Signal

Abstract. The dynamic ocean topography (DOT) in the polar seas can be described by satellite altimetry sea surface height observations combined with geoid information and by ocean models. The altimetry observations are characterized by an irregular sampling and seasonal sea-ice coverage complicating reliable DOT estimations. Models display various spatio-temporal resolutions, but are limited to their computational and mathematical context and introduced forcing models. In the present paper, ALES+ retracked altimetry ranges and derived along-track DOT heights of ESA's Envisat and water heights of the Finite Element Sea-ice Ocean Model (FESOM) are compared to investigate similarities and discrepancies. The study period covers the years 2003–2009. An assessment analysis regarding seasonal DOT variabilities shows good accordance and confirms the most dominant impact of the annual signal in both datasets. A comparison based on estimated regional annual signal components shows 2–3 times stronger amplitudes of the observations but good agreement of the phase. Reducing both datasets by constant offsets and the annual signal reveals small regional residuals and highly correlated DOT time series (correlation coefficient at least 0.67). The highest correlations can be found in areas that are ice-free and affected by ocean currents. However, differences are visible in sea-ice covered shelf regions. Furthermore, remaining constant artificial elevations in the observational data can be addressed to an insufficient representation of the used geoid. In general, the comparison results in good accordance between simulated and altimetry based description of the DOT in the Greenland Sea. Furthermore, the investigation shows that combining both datasets and exploiting the advantages of along-track altimetry observations and those of homogeneous modeled DOT representations leads to a deeper comprehension of the Arctic Ocean's DOT.

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