63 research outputs found

    A new phase in the production of quality-controlled sea level data

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    Sea level is an essential climate variable (ECV) that has a direct effect on many people through inundations of coastal areas, and it is also a clear indicator of climate changes due to external forcing factors and internal climate variability. Regional patterns of sea level change inform us on ocean circulation variations in response to natural climate modes such as El Niño and the Pacific Decadal Oscillation, and anthropogenic forcing. Comparing numerical climate models to a consistent set of observations enables us to assess the performance of these models and help us to understand and predict these phenomena, and thereby alleviate some of the environmental conditions associated with them. All such studies rely on the existence of long-term consistent high-accuracy datasets of sea level. The Climate Change Initiative (CCI) of the European Space Agency was established in 2010 to provide improved time series of some ECVs, including sea level, with the purpose of providing such data openly to all to enable the widest possible utilisation of such data. Now in its second phase, the Sea Level CCI project (SL_cci) merges data from nine different altimeter missions in a clear, consistent and well-documented manner, selecting the most appropriate satellite orbits and geophysical corrections in order to further reduce the error budget. This paper summarises the corrections required, the provenance of corrections and the evaluation of options that have been adopted for the recently released v2.0 dataset (https://doi.org/10.5270/esa-sea_level_cci-1993_2015-v_2.0-201612). This information enables scientists and other users to clearly understand which corrections have been applied and their effects on the sea level dataset. The overall result of these changes is that the rate of rise of global mean sea level (GMSL) still equates to ∼ 3.2 mm yr−1 during 1992–2015, but there is now greater confidence in this result as the errors associated with several of the corrections have been reduced. Compared with v1.1 of the SL_cci dataset, the new rate of change is 0.2 mm yr−1 less during 1993 to 2001 and 0.2 mm yr−1 higher during 2002 to 2014. Application of new correction models brought a reduction of altimeter crossover variances for most corrections

    Comparison of sea-ice freeboard distributions from aircraft data and cryosat-2

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    The only remote sensing technique capable of obtain- ing sea-ice thickness on basin-scale are satellite altime- ter missions, such as the 2010 launched CryoSat-2. It is equipped with a Ku-Band radar altimeter, which mea- sures the height of the ice surface above the sea level. This method requires highly accurate range measure- ments. During the CryoSat Validation Experiment (Cry- oVEx) 2011 in the Lincoln Sea, Cryosat-2 underpasses were accomplished with two aircraft, which carried an airborne laser-scanner, a radar altimeter and an electro- magnetic induction device for direct sea-ice thickness re- trieval. Both aircraft flew in close formation at the same time of a CryoSat-2 overpass. This is a study about the comparison of the sea-ice freeboard and thickness dis- tribution of airborne validation and CryoSat-2 measure- ments within the multi-year sea-ice region of the Lincoln Sea in spring, with respect to the penetration of the Ku- Band signal into the snow

    An Improved and Homogeneous Altimeter Sea Level Record from the ESA Climate Change Initiative

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    Sea Level is a very sensitive index of climate change since it integrates the impacts of ocean warming and ice mass loss from glaciers and the ice sheets. Sea Level has been listed as an Essential Climate Variable (ECV) by the Global Climate Observing System (GCOS). During the past 25 years, the sea level ECV has been measured from space by different altimetry missions that have provided global and regional observations of sea level variations. As part of the Climate Change Initiative (CCI) program of the European Space Agency (ESA) (established in 2010), the Sea Level project (SL_cci) aimed at providing an accurate and homogeneous long-term satellite-based sea level record. At the end of the first phase of the project (2010-2013), an initial version (v1.1) of the sea level ECV has been made available to users (Ablain et al., 2015). During the second phase (2014-2017), improved altimeter standards have been selected to produce new sea level products (called SL_cci v2.0) based on 9 altimeter missions for the period 1993-2015 (https://doi.org/10.5270/esa-sea_level_cci-1993_2015-v_2.0-201612). Corresponding orbit solutions, geophysical corrections and altimeter standards used in this v2.0 dataset are described in details in Quartly et al. (2017). The present paper focuses on the description of the SL_cci v2.0 ECV and associated uncertainty and discusses how it has been validated. Various approaches have been used for the quality assessment such as internal validation, comparisons with sea level records from other groups and with in-situ measurements, sea level budget closure analyses and comparisons with model outputs. Compared to the previous version of the sea level ECV, we show that use of improved geophysical corrections, careful bias reduction between missions and inclusion of new altimeter missions lead to improved sea level products with reduced uncertainties at different spatial and temporal scales. However, there is still room for improvement since the uncertainties remain larger than the GCOS requirements. Perspectives for subsequent evolutions are also discussed

    Marine geoid modeling from multi-mission satellite altimetry data using least squares stokes modification approach with additive corrections

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    Marine geoid is crucial for orthometric height determination. The airborne and shipborne surveys have been used for geoid and gravity surveys in marine areas, but they could only cover a limited coverage area due to the high cost and time constraints. Over the last 30 years, satellite altimeter has become an important tool for global geoid and gravity field recovery, with nearly 60% of the Earth’s surface in relation to the height of the ocean could be covered. This enables researchers to replace the conventional marine geoid models, and surveys can be conducted faster with a larger coverage area at a reduced cost. This study presents an attempt to model marine geoid from multi-mission satellite altimetry data using Least Squares Stokes Modification Approach with Additive Corrections. Six altimetry data were used to derive the mean sea surface which was processed in the Radar Altimeter Database System. The gravity anomaly was computed using Gravity Software, and planar Fast Fourier Transformation method was applied. The evaluation, selection, blunder detection, combination and re-gridding of the altimetry-derived gravity anomalies and Global Geopotential Model data were demonstrated. The cross validation approach was employed in the cleaning and quality control of the data with the combination of the Kriging interpolation method. Marine geoid was computed based on the Least Squares Stokes Modification Approach with Additive Corrections. The optimal condition modification parameters of 4° spherical cap, 0.4 mGal terrestrial gravity data error and 0.1° correlation length were applied. Then, the additive corrections based on Downward Continuation, Atmospheric Effects and Ellipsoidal Corrections were combined with the estimated geoid to provide a precise marine geoid over the Malaysian seas. Three selected levelling observations at tide gauge stations at Geting, Cendering and Pelabuhan Klang were used to verify the accuracy of the computed marine geoid model. The derived mean sea surface represents -0.4945m mean error and 2.2592m root mean square error values after being evaluated with the mean sea surface of Denmark Technical University 13. The gravity anomaly data from tapering window width with block 300 from hhawtimr4 assessments denotes the optimum gravity anomaly results with root mean square error value, 17.8329mGal. The accuracy of marine geoid model corresponds to the standard deviation, 0.098m and the root mean squared error value, 0.177m. The findings suggest that the marine geoid model can be utilized for the orthometric height determination in marine areas. The by-product of this research, the Malaysian Marine Geoid Calculator (MyMG) could assist users in extracting marine geoid in Malaysian seas

    Improved global sea surface height and current maps from remote sensing and in situ observations

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    We present a new gridded sea surface height and current dataset produced by combining observations from nadir altimeters and drifting buoys. This product is based on a multiscale and multivariate mapping approach that offers the possibility to improve the physical content of gridded products by combining the data from various platforms and resolving a broader spectrum of ocean surface dynamic than in the current operational mapping system. The dataset covers the entire global ocean and spans from 1 July 2016 to 30 June 2020. The multiscale approach decomposes the observed signal into different physical contributions. In the present study, we simultaneously estimate the mesoscale ocean circulations as well as part of the equatorial wave dynamics (e.g. tropical instability and Poincaré waves). The multivariate approach is able to exploit the geostrophic signature resulting from the synergy of altimetry and drifter observations. Sea-level observations in Arctic leads are also used in the merging to improve the surface circulation in this poorly mapped region. A quality assessment of this new product is proposed with regard to an operational product distributed in the Copernicus Marine Service. We show that the multiscale and multivariate mapping approach offers promising perspectives for reconstructing the ocean surface circulation: observations of leads contribute to improvement of the coverage in delivering gap-free maps in the Arctic and observations of drifters help to refine the mapping in regions of intense dynamics where the temporal sampling must be accurate enough to properly map the rapid mesoscale dynamics. Overall, the geostrophic circulation is better mapped in the new product, with mapping errors significantly reduced in regions of high variability and in the equatorial band. The resolved scales of this new product are therefore between 5 % and 10 % finer than the Copernicus product (https://doi.org/10.48670/moi-00148, Pujol et al., 2022b).</p

    Observability of fine-scale ocean dynamics in the northwestern Mediterranean Sea

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