1,721,001 research outputs found
NERC Data Grid
No full textThe NERC Data Grid (NDG) provides the infrastructure which allows users to:
- Find data
- Explore what is known about datasets
- Access, manipulate and visualise data!
Like the web, the NDG has no “owner” or “central control”; data remain with data providers - be they managed data centres in the UK or abroad, or semi-managed data archives in large research groups. The location of the data can be transparent to the user, while still allowing data providers to maintain their intellectual investment by controlling access.
The NDG team provide two community services which all NDG participants may avail themselves of:
- The “NDG Discovery Service”: A database of discovery information, harvested from data providers around the world, together with a portal web-site and a set of web-services, for exploiting that database.
- The “NDG Vocabulary Service”: Databases of environmental thesauri and ontology tools to map between terms
A study of currents in the Southern Ocean using satellite altimeter and model data
No full textThe aims of the project are outlined and the satellite altimeter is introduced as a new method for the physical oceanographer to study ocean dynamics. The principles of the satellite altimeter are explained and a description of the sources of error found in the data is given, together with two possible methods for removal of residual errors. The principles of primitive equation ocean circulation models are outlined and the Fine Resolution Antarctic Model (FRAM) is introduced. Methods of hydrographic data collection are discussed and historical data on the circulation of the Southern Ocean are reviewed. The methods for Geosat data extraction, quality control and error correction used in the study are discussed. Data processed by collinear techniques have been used to obtain height profiles along tracks in two regions of the Southern Ocean; South of Africa and in the Central South Pacific. Mesoscale variability fields calculated from the along track altimeter data are presented and explained in terms of historical in situ current data and bottom topography. Geostrophic current speeds calculated from the residual height profiles yield velocity anomalies of the order of 20 cm s"1 for the Antarctic Circumpolar Current (ACC) and up to 1.38 m s1 for the Agulhas Current System, similar to values obtained by in situ measurements. Autocorrelation function calculations show that the characteristic length scales of height anomalies in the ACC are smaller than those of the Agulhas Current System. The FRAM data set is used as a source of artificial sea surface heights to give model altimeter data which are processed and analyzed for the Geosat study regions. The resultant variability fields are compared with the Geosat results. The effects of Geosat's sampling strategy and altimeter processing on the FRAM data are analyzed and are given as an indication of the possible reliability of Geosat variability fields. The results of feature recognition and autocorrelation function analyses are compared to those from Geosat both as a verification for the FRAM mesoscale circulation and as an aid to altimeter data interpretation.</p
Just-in-time altimetry: international collaboration in provision of altimetry datasets
Full text linkEnvironmental research requires access to quality controlled, calibrated data. Satellite altimeter data are used in a range of environmental research, including oceanography, ice and land surface studies. Users who are not altimeter specialists may not be aware of, or have access to, the latest updates and most appropriate corrections to use for their application. We propose a GRID based methodology to give all users access to the best possible altimeter data product at the time of the request, tailor made for their specific application. A data portal system would be based on a "Network of Trust" consisting of the data providers and a certificating authority. Data could be served through a fully interactive web 'front-end' or directly from within analysis programmes. This system would build on the experiences gained in combining two existing Altimeter Data services (GAPSand RADS) to produce a coherent data service with alternative web interfaces and configurable users access
GOCE user toolbox specification: scientific trade off study and algorithm specification
Full text linkAs part of the GOCE User Toolbox Specification (GUTS) project, the GUTS team has carried out a scientific trade-off study, to select the best algorithms to fulfil the user requirements for the toolbox. In some cases the selection is straightforward. However, in other cases, the choice depends on scientific applications as well as the algorithm efficiency and more practical considerations. Studies need to be performed to demonstrate the best possible algorithm. One example is the selection of filtering functions and the choice of interpolation schemes used in calculation of a mean dynamic topography from combined GOCE and satellite altimeter data. The trade-off study has also selected the functionality of the toolbox, given the user requirements and the recommended algorithms. In this paper we present the proposed functionality of the toolbox, and the most important algorithm selections. As part of the scientific trade-off, a pilot application is investigating validation of mean dynamic topography, generated from pseudo observations using proposed toolbox algorithms, against ocean model data. The study includes results from research into methods of calculation of mean dynamic topography and filtering methods presented at this worksho
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Coupled Ocean–Atmosphere Processes and European Climate (COAPEC): Improved Understanding of the Coupled Climate System
Full text linkIntroduction to the special section of the Journal of Climate dedicated to results from the COAPEC (Coupled Ocean Atmosphere Processes and European Climate) programme
EUMETSAT Invitation To Tender 14/209556: JASON-CS SAR Mode Sea State Bias Study. Final report
Full text linkThis document represents the final report of a study funded by EUMETSAT about SAR mode Sea State Bias (SSB) for the Sentinel-6/Jason-CS mission. The study comprises a critical review of SSB estimation methods in conventional (low-resolution mode or LRM) altimetry, theoretical considerations about the effect of swell on SAR altimeter waveforms and empirical investigations with Cryosat-2 SAR mode data to detect swell effects in L1B and Level 2 Sea Surface Height (SSH). The report concludes by summarising the basis for the selection and derivation of the SAR altimeter sea state bias correction algorithm and the methods available to calibrate and validate SAR mode SSB corrections.
Theoretical considerations based on simple SAR waveform modelling indicate that multipeaked waveforms could occur in the presence of swell, but that effects become clearly detectable only when swell height exceeds 4 meters, which is relatively rare. In the case of the Cryosat-2 data examined in this study, only 2% of samples satisfied this condition.
Experimental investigations of Cryosat-2 SAR mode data in different swell conditions produce no consolidated evidence of swell effects. Although anomalous 20Hz waveforms are occasionally observed, no statistically detectable effect of swell is obtained in the overall results for average L1B waveform shapes and L2 1Hz SSH biases and precisions. However, it is stressed that analyses in this study were limited geographically by the availability of Cryosat-2 SAR mode acquisitions over the ocean that could be collocated with Envisat ASAR swell data. It is strongly advised that analyses should be repeated with a broader geographical scope, including data from the central Pacific and the Southern Ocean where high sea state and swell conditions are more prevalent. It is suggested that this could be achieved using Sentinel-3 SRTM and Sentinel-1 L2 swell products, should such data be available.
Empirical SSB estimation methods offer the only viable way forward at present to estimate SAR mode SSB. Parametric, non-parametric and hybrid methods are all relevant, noting that hybrid methods may provide more robust estimates in those high sea state and swell conditions that are less densely populated and where effects will be more significant. The development of SAR mode SSB corrections should include additional dependence on sea state development, which would be consistent with the tendency in LRM towards three-parameters SSB models (e.g. Tran et al., 2010b; Pires et al., 2016).
The challenges of calibrating and validating SAR mode SSB corrections are the same - i.e. no better, no worse - than for conventional altimetry. For SAR mode altimetry however, P-LRM offer a unique way of calibrating and validating SAR mode SSB against conventional altimetry by providing coincident range measurements that have been shown to be unbiased against conventional LRM. In the case of Sentinel-6/Jason-CS, interleaved SAR mode will deliver true LRM data that make it possible to tie the Jason-CS SAR mode mission to the long-term altimetric data record without the issues linked to the loss of precision seen for SAR burstmode P-LRM
ALES: a multi-mission adaptive subwaveform retracker for coastal and open ocean altimetry
Full text linkSatellite altimetry has revolutionised our understanding of ocean dynamics thanks to frequent sampling and global coverage. Nevertheless, coastal data have been flagged as unreliable due to land and calm water interference in the altimeter and radiometer footprint and uncertainty in the modelling of high-frequency tidal and atmospheric forcing.
Our study addresses the first issue, i.e. altimeter footprint contamination, via retracking, presenting ALES, the Adaptive Leading Edge Subwaveform retracker. ALES is potentially applicable to all the pulse-limited altimetry missions and its aim is to retrack both open ocean and coastal data with the same accuracy using just one algorithm.
ALES selects part of each returned echo and models it with a classic “open ocean” Brown functional form, by means of least square estimation whose convergence is found through the Nelder–Mead nonlinear optimisation technique. By avoiding echoes from bright targets along the trailing edge, it is capable of retrieving more coastal waveforms than the standard processing. By adapting the width of the estimation window according to the significant wave height, it aims at maintaining the accuracy of the standard processing in both the open ocean and the coastal strip.
This innovative retracker is validated against tide gauges in the Adriatic Sea and in the Greater Agulhas System for three different missions: Envisat, Jason-1 and Jason-2. Considerations of noise and biases provide a further verification of the strategy. The results show that ALES is able to provide more reliable 20-Hz data for all three missions in areas where even 1-Hz averages are flagged as unreliable in standard products. Application of the ALES retracker led to roughly a half of the analysed tracks showing a marked improvement in correlation with the tide gauge records, with the rms difference being reduced by a factor of 1.5 for Jason-1 and Jason-2 and over 4 for Envisat in the Adriatic Sea (at the closest point to the tide gauge)
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