Representation Error of Oceanic Observations for Data Assimilation

2008 ◽  
Vol 25 (6) ◽  
pp. 1004-1017 ◽  
Author(s):  
Peter R. Oke ◽  
Pavel Sakov
Keyword(s):  
Data Assimilation ◽  
Sea Level ◽  
Standard Technique ◽  
Original Data ◽  
Sea Level Anomalies ◽  
Ocean Data Assimilation ◽  
Representation Error ◽  
The Difference ◽  
Along Track ◽  
The Given

Abstract A simple approach to the estimation of representation error (RE) of sea level (η), temperature (T), and salinity (S) observations for ocean data assimilation is described. It is assumed that the main source of RE is due to unresolved processes and scales in the model. Therefore, RE is calculated as a function of model resolution. The methods described here exploit the availability of mapped sea level anomalies (mSLAs) and along-track sea level anomalies (atSLAs). The mSLA fields or atSLA observations are regarded as the true ocean state. Here, they are averaged according to the resolution of the model grid, and the averaged field is taken as a representation of the true state on the given grid. The difference between the original data and the averaged field is then regarded as the RE for η. Subsequently, the RE is projected for η over depth using a standard technique, giving an estimate of the RE for T and S. Examples of RE estimates for an intermediate- and high-resolution global grid are presented. It is found that there is significant spatial variability in the RE for η, T, and S, with values that are typically greater than or comparable to measurement error, particularly in regions of strong mesoscale variability.

scholarly journals Data-Driven Interpolation of Sea Level Anomalies Using Analog Data Assimilation

2019 ◽  
Vol 11 (7) ◽  
pp. 858 ◽  
Author(s):  
Redouane Lguensat ◽  
Phi Huynh Viet ◽  
Miao Sun ◽  
Ge Chen ◽  
Tian Fenglin ◽  
...  
Keyword(s):  
Data Assimilation ◽  
Sea Level ◽  
Large Scale ◽  
Empirical Orthogonal Functions ◽  
Data Driven ◽  
Optimal Interpolation ◽  
Observation System ◽  
Orthogonal Functions ◽  
Sea Level Anomalies ◽  
Recent Developments

From the recent developments of data-driven methods as a means to better exploit large-scale observation, simulation and reanalysis datasets for solving inverse problems, this study addresses the improvement of the reconstruction of higher-resolution Sea Level Anomaly (SLA) fields using analog strategies. This reconstruction is stated as an analog data assimilation issue, where the analog models rely on patch-based and Empirical Orthogonal Functions (EOF)-based representations to circumvent the curse of dimensionality. We implement an Observation System Simulation Experiment (OSSE) in the South China Sea. The reported results show the relevance of the proposed framework with a significant gain in terms of Root Mean Square Error (RMSE) for scales below 100 km. We further discuss the usefulness of the proposed analog model as a means to exploit high-resolution model simulations for the processing and analysis of current and future satellite-derived altimetric data with regard to conventional interpolation schemes, especially optimal interpolation.

Satellite Data Assimilation for Improvement of Naval Undersea Capability

2004 ◽  
Vol 38 (1) ◽  
pp. 12-23 ◽  
Author(s):  
Peter C. Chu ◽  
Michael D. Perry ◽  
Eric L. Gottshall ◽  
David S. Cwalina
Keyword(s):  
Data Assimilation ◽  
Satellite Data ◽  
Shallow Depth ◽  
Environmental Model ◽  
Coverage Area ◽  
Ocean Data Assimilation ◽  
The Difference ◽  
Data Assimilation System ◽  
Assimilation System

Impact of satellite data assimilation on naval undersea capability is investigated using ocean hydrographic products without and with satellite data assimilation. The former is the Navy's Global Digital Environmental Model (GDEM), providing a monthly mean; the latter is the Modular Ocean Data Assimilation System (MODAS) providing synoptic analyses based upon satellite data. The two environmental datasets are taken as the input into the Weapon Acoustic Preset Program to determine the suggested presets for an Mk 48 torpedo. The acoustic coverage area generated by the program will be used as the metric to compare the two sets of outputs. The output presets were created for two different scenarios, an anti-surface warfare (ASUW) and an anti-submarine warfare (ASW); and three different depth bands, shallow, mid, and deep. After analyzing the output, it became clear that there was a great difference in the presets for the shallow depth band, and that as depth increased, the difference between the presets decreased. Therefore, the MODAS product, and in turn the satellite data assimilation, had greatest impact in the shallow depth band. The ASW presets also seemed to be slightly less sensitive to differences than did presets in the ASUW scenario.

scholarly journals Impact of SLA assimilation in the Sicily Channel Regional Model: model skills and mesoscale features

2012 ◽  
Vol 9 (2) ◽  
pp. 421-442
Author(s):  
A. Olita ◽  
S. Dobricic ◽  
A. Ribotti ◽  
L. Fazioli ◽  
A. Cucco ◽  
...  
Keyword(s):  
Sea Level ◽  
Qualitative Evaluation ◽  
Regional Model ◽  
Qualitative Comparison ◽  
Sea Level Anomalies ◽  
Mesoscale Structures ◽  
Sicily Channel ◽  
Altimetric Measurements ◽  
Along Track ◽  
The Impact

Abstract. The impact of the assimilation of MyOcean Sea Level Anomalies along track data on the analyses of the Sicily Channel Regional Model was studied. The numerical model has a resolution of 1/32° degrees and is capable to reproduce mesoscale and sub-mesoscale features. The impact of the SLA assimilation is studied by comparing a simulation (SIM, which does not assimilate data) with two analyses, AN0 and AN1, assimilating different reprocessing versions (V0 and V1) of the same set of Along Track altimetric measurements. The quality of the analyses was evaluated by computing RMSE of the misfits between analyses background and observations (sea level) before assimilation. A qualitative evaluation of the ability of the analyses to reproduce mesoscale structures is accomplished by comparing model results with Ocean Color and SST satellite data, able to detect such features on the ocean surface. CTD profiles allowed to evaluate the impact of the SLA assimilation along the water column. We found a significant improvement for AN1 solution in terms of SLA rmse in respect to SIM (the averaged RMSE of AN1 SLA misfits over 1.5 years is about 0.5 cm smaller than SIM) and a weaker improvement in respect to the assimilation of the V0 dataset (0.1 cm average over the same period). Comparison with CTD data shows a questionable improvement produced by the assimilation process in terms of vertical features: AN1 is better in temperature while for salinity it get worse than SIM at the surface. The qualitative comparison of simulation and analyses with synoptic satellite independent data proves that SLA assimilation of V1 data allows to correctly reproduce some dynamical features (above all the circulation in the Ionian portion of the domain) and mesoscale structures otherwise misplaced or neglected both by SIM and AN0.

scholarly journals Mean Dynamic Topography of the Black Sea, computed from altimetry, drifter measurements and hydrology data

Ocean Science ◽  
2011 ◽  
Vol 7 (6) ◽  
pp. 745-753 ◽  
Author(s):  
A. A. Kubryakov ◽  
S. V. Stanichny
Keyword(s):  
Sea Level ◽  
Black Sea ◽  
The Black Sea ◽  
Climatic Data ◽  
Dynamic Topography ◽  
Mean Dynamic Topography ◽  
Sea Level Anomalies ◽  
Temporal Features ◽  
Geostrophic Velocities ◽  
Along Track

Abstract. Mean Dynamic Topography (MDT) is a crucial parameter for estimating dynamic topography, and, therefore, geostrophic circulation from satellite altimetry measurements. In this work we use drifting buoy measurements, hydrographic profiles and along-track Sea Level Anomalies (SLA) to reconstruct MDT of the Black Sea by the "synthetic" method. Obtained MDT shows a lot of mesoscale features, which are not present in previous MDT fields of the Black Sea, mostly based on climatic data. Moreover, gradients of sea level in the synthetic MDT are significantly higher compared to other fields, which is evidence of more intense currents in the basin. Validation of determined MDT field with independent dynamic heights and drifter buoy velocities shows good quantitative and qualitative coincidence over all Black Sea basin and improvements compare to previous fields. New Black Sea MDT will improve quality of altimetry-derived geostrophic velocities and lead to better understanding of the spatial and temporal features of the upper layer dynamics.

scholarly journals On retrieving sea ice freeboard from ICESat laser altimeter

2016 ◽  
Author(s):  
Kirill Khvorostovsky ◽  
Pierre Rampal
Keyword(s):  
Sea Ice ◽  
Sea Level ◽  
Arctic Sea Ice ◽  
The Arctic ◽  
Ice Thickness ◽  
First Year ◽  
Laser Altimeter ◽  
Sea Ice Thickness ◽  
The Difference ◽  
Along Track

Abstract. Sea ice freeboard derived from satellite altimetry is the basis for estimation of sea ice thickness using the assumption of hydrostatic equilibrium. High accuracy of altimeter measurements and freeboard retrieval procedure are therefore required. As of today, two approaches for estimation of the freeboard using laser altimeter measurements from Ice, Cloud, and land Elevation Satellite (ICESat), referred to as tie-points (TP) and lowest-level elevation (LLE) methods, have been developed and applied in different studies. We reproduced these methods in order to assess and analyze the sources of differences found in the retrieved freeboard and corresponding thickness estimates of the Arctic sea ice as produced by the Jet Propulsion Laboratory (JPL) and Goddard Space Flight Center (GSFC). For the ICEsat observation periods (2003–2008) it is found that when applying the same along-track averaging scales in the two methods to calculate the local sea level references the LLE method gives significantly lower (by up to 15 cm) sea ice freeboard estimates over thick multi-year ice areas, but significantly larger estimates (by 3–5 cm in average and locally up to about 10 cm) over thin first-year ice areas, as compared to the TP method. However, we show that the difference over first-year ice areas can be reduced to less than 2 cm when using the improved TP method proposed in this paper. About 4 cm of the difference in the JPL and GSFC freeboard estimates can be attributed to the different along-track averaging scales used to calculate the local sea level references. We show that the effect of applying corrections for lead width relative to the ICESat footprint, and for snow depth accumulated in refrozen leads (as it is done for the last release of the JPL product), is very large and increase freeboard estimates by about 7 cm. Thus, the different along-track averaging scales and approaches to calculate sea surface references, from one side, and the freeboard adjustments as applied in the TP method used to produce the JPL dataset, from the other side, are roughly compensating each other with respect to freeboard estimation. Therefore the difference in the mean sea ice thickness found between the JPL and GSFC datasets should be attributed to different parameters used in the freeboard-to-thickness conversion.

scholarly journals Contribution of future wide-swath altimetry missions to ocean analysis and forecasting

Ocean Science ◽  
2018 ◽  
Vol 14 (6) ◽  
pp. 1405-1421 ◽  
Author(s):  
Antonio Bonaduce ◽  
Mounir Benkiran ◽  
Elisabeth Remy ◽  
Pierre Yves Le Traon ◽  
Gilles Garric
Keyword(s):  
Data Assimilation ◽  
Sea Level ◽  
Spatial Scales ◽  
European Space Agency ◽  
Ocean Currents ◽  
Instrumental Error ◽  
Altimetry Data ◽  
Along Track ◽  
The Impact ◽  
Wide Swath

Abstract. The impact of forthcoming wide-swath altimetry missions on the ocean analysis and forecasting system was investigated by means of OSSEs (observing system simulation experiments). These experiments were performed with a regional data assimilation system, implemented in the Iberian–Biscay–Ireland (IBI) region, at 1∕12∘ resolution using simulated observations derived from a fully eddy-resolving free simulation at 1∕36∘ resolution over the same region. The objective of the experiments was to assess the ability of different satellite constellations to constrain the ocean analyses and forecasts, considering both along-track altimeters and future wide-swath missions; consequently, the capability of the data assimilation techniques used in the Mercator Ocean operational system to effectively combine the different kinds of measurements was also investigated. These assessments were carried out as part of a European Space Agency (ESA) study on the potential role of wide-swath altimetry in future versions of the European Union Copernicus programme. The impact of future wide-swath altimetry data is evident for investigating the reliability of sea level values in OSSEs. The most significant results were obtained when looking at the sensitivity of the system to wide-swath instrumental error: considering a constellation of three nadir and two “accurate” (small instrumental error) wide-swath altimeters, the error in ocean analysis was reduced by up to 50 % compared to conventional altimeters. Investigating the impact of the repetitivity of the future measurements, the results showed that two wide-swath missions had a major impact on sea-level forecasting – increasing the accuracy over the entire time window of the 5-day forecasts – compared with a single wide-swath instrument. A spectral analysis underlined that the contributions of wide-swath altimetry data observed in ocean analyses and forecast statistics were mainly due to the more accurate resolution, compared with along-track data, of ocean variability at spatial scales smaller than 100 km. Considering the ocean currents, the results confirmed that the information provided by wide-swath measurements at the surface is propagated down the water column and has a considerable impact (30 %) on ocean currents (up to a depth of 300 m), compared with the present constellation of altimeters. The ocean analysis and forecasting systems used here are those currently used by the Copernicus Marine Environment and Monitoring Service (CMEMS) to provide operational services and ocean reanalysis. The results obtained in the OSSEs considering along-track altimeters were consistent with those derived from real data (observing system experiments, OSEs). OSSEs can also be used to assess the potential of new observing systems, and in this study the results showed that future constellations of altimeters will have a major impact on constraining the CMEMS ocean analysis and forecasting systems and their applications.

scholarly journals Contribution of future wide swath altimetry missions to ocean analysis and forecasting

2018 ◽  
Author(s):  
Antonio Bonaduce ◽  
Mounir Benkiran ◽  
Elisabeth Remy ◽  
Pierre Yves Le Traon ◽  
Gilles Garric
Keyword(s):  
Data Assimilation ◽  
Sea Level ◽  
Spatial Scales ◽  
European Space Agency ◽  
Ocean Currents ◽  
Instrumental Error ◽  
Altimetry Data ◽  
Along Track ◽  
The Impact ◽  
Wide Swath

Abstract. The impact of forthcoming wide-swath altimetry missions on the ocean analysis and forecasting system was investigated by means of OSSEs (Observing System Simulation Experiments) performed with a regional data assimilation system, implemented in the Iberian-Biscay-Ireland (IBI) region, at 1/12° resolution using simulated observations derived from a fully eddy-resolving free simulations at 1/36° resolution over the same region. The objective was to asses the contribution of different satellite constellations to constrain the ocean analyses and forecasts, considering both along-track altimeters and future wide-swath missions, and as consequence the capability of the data assimilation techniques used in Mercator Ocean operational system to effectively combine the different kind of measurements. This was carried out as part of a European Space Agency (ESA) study on the potential role of wide-swath altimetry for the evolution of the European Union Copernicus programme. The impact of future wide-swath altimetry data is clearly evident investigating the reliability of sea-level in the OSSEs. The most significant results were obtained looking at the sensitivity of the system to wide-swath instrumental error: considering a constellation of three nadir and two accurate (small instrumental error) wide-swath altimeters, the error in the ocean analysis was reduced up to the 50 %, with respect to conventional altimeters. Investigating the impact of the repetitivity of the future measurements, the results showed that two wide-swath missions had a major impact on the the sea-level forecasting increasing the accuracy over the entire time-window of the 5-day forecasts, with respect to a single wide-swath instrument. A spectral analysis underlined that the contributions of wide-swath altimetry data observed in the ocean analyses and forecasts statistics were mainly due to resolve more accurately (up to > 25 %), with respect to along-track data, the ocean variability at spatial scales smaller than 100 km. Considering the ocean currents, the results confirmed that the information provided by wide-swath measurements at the surface is propagated also in the vertical and has a considerable impact (30 %) on the ocean currents (up to 300 metres), with respect to the present constellation of altimeters. The ocean analysis and forecasting systems used here are currently adopted by Copernicus Marine Environment and Monitoring Service (CMEMS) to provide operational services and ocean re-analysis. The results obtained in the OSSEs considering along-track altimeters were consistent with those derived with real data (observing system experiments, OSEs). OSSEs also allow to evaluate the potential of new observing systems and in this study the results showed that future constellations of altimeters will have a major impact to constrain the CMEMS ocean analysis and forecasting systems and their applications.

scholarly journals Quality Assessment of a 1985–2007 Mediterranean Sea Reanalysis

2011 ◽  
Vol 28 (4) ◽  
pp. 569-589 ◽  
Author(s):  
Mario Adani ◽  
Srdjan Dobricic ◽  
Nadia Pinardi
Keyword(s):  
Mediterranean Sea ◽  
Sea Level ◽  
Root Mean Square ◽  
Three Dimensional ◽  
Optimal Interpolation ◽  
Mean Square ◽  
Sea Level Anomalies ◽  
Mean Fields ◽  
Along Track ◽  
Mean Square Errors

Abstract A simulation and two reanalyses from 1985 to 2007 have been produced for the Mediterranean Sea using different assimilation schemes: a reduced-order optimal interpolation [System for Ocean Forecast and Analysis (SOFA)] and a three-dimensional variational scheme (OceanVar). The observational dataset consists of vertical temperature and salinity in situ profiles and along-track satellite sea level anomalies; daily mean fields of satellite sea surface temperature are used for correcting the air–sea fluxes. This paper assesses the quality of the reanalyses with respect to observations and the simulation. Both the SOFA and OceanVar schemes give very similar root-mean-square errors and biases for temperature and salinity fields compared with the assimilated observations. The largest errors are at the thermocline level and in regions of large eddy field variability. However, OceanVar gives 20% better results for sea level anomaly root-mean-square error.

scholarly journals Predictability of non-phase-locked baroclinic tides in the Caribbean Sea

Ocean Science ◽  
2019 ◽  
Vol 15 (5) ◽  
pp. 1287-1305
Author(s):  
Edward D. Zaron
Keyword(s):  
Sea Level ◽  
Forecast Error ◽  
Caribbean Sea ◽  
Forecast Model ◽  
Sea Surface ◽  
Spatial And Temporal Variability ◽  
Tide Gauges ◽  
Sea Level Anomalies ◽  
The Caribbean ◽  
The Difference

Abstract. The predictability of the sea surface height expression of baroclinic tides is examined with 96 h forecasts produced by the AMSEAS operational forecast model during 2013–2014. The phase-locked tide, both barotropic and baroclinic, is identified by harmonic analysis of the 2-year record and found to agree well with observations from tide gauges and satellite altimetry within the Caribbean Sea. The non-phase-locked baroclinic tide, which is created by time-variable mesoscale stratification and currents, may be identified from residual sea level anomalies (SLAs) near the tidal frequencies. The predictability of the non-phase-locked tide is assessed by measuring the difference between a forecast – centered at T+36, T+60, or T+84 h – and the model's later verifying analysis for the same time. Within the Caribbean Sea, where a baroclinic tidal sea level range of ±5 cm is typical, the forecast error for the non-phase-locked tidal SLA is correlated with the forecast error for the subtidal (mesoscale) SLA. Root mean square values of the former range from 0.5 to 2 cm, while the latter ranges from 1 to 6 cm, for a typical 84 h forecast. The spatial and temporal variability of the forecast error is related to the dynamical origins of the non-phase-locked tide and is briefly surveyed within the model.

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