James While

An operational ocean forecast system incorporating NEMO and SST data assimilation for the tidally driven European North-West shelf

A new operational ocean forecast system, the Atlantic Margin Model implementation of the Forecast Ocean Assimilation Model (FOAM-AMM), has been developed for the European North West Shelf (NWS). An overview of the system is presented including shelf specific developments of the physical model, the Nucleus for European Modelling of the Ocean (NEMO), and the Sea Surface Temperature (SST) data assimilation scheme. Initial validation is presented of the tides and model SST. The SST skill of the system is significantly improved by the data assimilation scheme. Finally, an analysis of the seasonal tidal mixing fronts shows that these, in general, agree well with observation, but data assimilation does not significantly alter their positions.

Spectral analysis of gravity gradiometry profiles

The gravity gradient tensor (whose components are the second derivatives of the gravitational potential) is a symmetric tensor that, ignoring the constraint imposed by Laplaces equation, contains only six independent components. When measured on a horizontal plane, these components generate, in the spectral domain, six power spectral densities (PSDs) and fifteen cross-spectra. The cross-spectra can be split into two groups: a real group and a pure imaginary group. If the source distribution is statistically stationary, 1D spectra can be found from the 2D spectra via the slice theorem. The PSDs form two power-sum rules that link all gradient components. The power-sum rules, in combination with further equalities between the power and cross-spectra, reduce the number of independent spectra to 13, a number reduced to seven if the power spectrum of the potential is assumed isotropic. The power-sum rules, cross-spectral phases, and coherence between components all provide information on the internal consistency of a set of gradiometry measurements. This information can be used to assess the noise, to determine the isotropy, and, for a self-similar source, to calculate the scaling factor and average depth. When applied to a data set collected in the North Sea, the power-sum rules reveal high-frequency noise that is distributed among only three of the gradient components; additionally, the coherences reveal the source to be anisotropic with a nonzero correlation length.

Interpolation of gravity and gravity gradient data by using the generalized sampling expansion: Theory

The generalized sampling expansion is a method that can be used to combine gravity and gravity gradient data so that the line spacing of a survey can be increased without introducing aliasing into the measured signal. This reduces acquisition costs because the costs of shipborne and airborne gravity and gravity gradient surveys are related to the line spacing of the survey. The generalized sampling expansion combines the Fourier spectra of several undersampled gravity and gravity gradient data sets to produce an unaliased result. To achieve that result, input data are assumed to be noise free, infinite in extent, regularly sampled, and band limited. Deviations from this ideal induce errors, but these can be alleviated to a degree. In theory, the generalized sampling expan-sion can reconstruct an unaliased signal sampled at three times conventional line spacing. Unfortunately, edge effects and the presence of noise limit the practical increase in line spacing to twice normal line spacing. Six methods, each...

Evaluating a new NEMO-based Persian/Arabian Gulf tidal operational model

A 3-D baroclinic pre-operational model, including tides of the Persian/Arabian Gulf, has been developed at the Met Office using the NEMO framework. The non-assimilative model is believed to represent a significant improvement over the existing POLCOMS based system, benefiting from: extended domain; improved resolution; more accurate representation of coasts and bathymetry; improved representation of tides; and improved representation of salinity. As expected, with sea surface temperature (SST) data assimilation, the accuracy of SST is significantly improved. However, data assimilation also appears to help reduce thermal biases throughout the water column, within the limited accuracy of a climatology comparison. Operational implementation occurred in late 2012.

The CO5 configuration of the 7 km Atlantic Margin Model: large-scale biases and sensitivity to forcing, physics options and vertical resolution

We describe the physical model component of the standard Coastal Ocean version 5 configuration (CO5) of the European North West Shelf (NWS). CO5 was developed jointly between the Met Office and the National Oceanography Centre. CO5 is designed with the seamless approach in mind, which allows for modeling of multiple timescales for a variety of applications from short-range ocean forecasting through to climate projections. The configuration constitutes the basis of the latest update 5 to the ocean and data assimilation components of the Met Office’s operational Forecast Ocean Assimilation Model (FOAM) for the NWS. A 30.5 year non-assimilating control hindcast of CO5 was integrated from January 1981 to June 2012. Sensitivity simulations were conducted with reference to the control run. The control run is compared against a previous non-assimilating Proudman Oceanographic Laboratory Coastal Ocean Modelling System (POLCOMS) hindcast of the NWS. The CO5 control hindcast is shown to have much reduced biases compared to POLCOMS. Emphasis in the system description is weighted to 10 updates in CO5 over previous versions. Updates include an increase in vertical resolution, a new vertical coordinate stretching function, the replacement of climatological riverine sources with the pan-European hydrological model E-HYPE, a new Baltic boundary condition and switching from directly imposed atmospheric model boundary fluxes to calculating the fluxes within the model using bulk formula. Sensitivity tests of the updates are detailed with a view to attributing observed changes in the new system from the previous system and suggesting future directions of research to further improve the system. 15

The EU-FP7 ERA-CLIM2 Project Contribution to Advancing Science and Production of Earth System Climate Reanalyses

ERA-CLIM2 is a European Union Seventh Framework Project started in January 2014. It aims to produce coupled reanalyses, which are physically consistent data sets describing the evolution of the global atmosphere, ocean, land-surface, cryosphere and the carbon cycle. ERA-CLIM2 has contributed to advancing the capacity for producing state-of-the-art climate reanalyses that extend back to the early 20th century. It has led to the generation of the first ensemble of coupled ocean, sea-ice, land and atmosphere reanalyses of the 20th century. The project has funded work to rescue and prepare observations, and to advance the data51 assimilation systems required to generate operational reanalyses, such as the ones planned by the European Union Copernicus Climate Change Service. This paper summarizes the main goals of the project, discusses some of its main areas of activities, and presents some of its key results.