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GOCE in ocean modelling

Résultats exploitables

The Gocino project actively contributed to reach the pre-operational capability in ocean modelling for GMES utilising data from the ESA satellite mission GOCE that was launched in March 2009. In Gocino an effort was made to further promote and facilitate the transfer of knowledge and the exploitation of GOCE data. The project used specific scenarios for the integration of GOCE data products into four major operational oceanography systems, notably Mercator, MFS, FOAM and TOPAZ, and into the seasonal forecasting system at ECMWF. Based on the outcome from the first Gocino workshop, the project partners prepared a series of reports describing the strategies for implementation of GOCE products in operational ocean models together with the ECMWF, TOPAZ, FOAM, Mercator, and MFS operational centres. A second workshop was held with participation of the invited ocean centres. The outcome of the workshop formed the basis for the finalisation of those strategies. Subsequently, the Gocino conference was held to disseminate and transfer the implementation strategies to the Myocean partners to follow up and coordinate the further implementation of GOCE data into the marine component of GMES. Also, a GOCE workshop was held to facilitate interaction and communication between the GOCE data processing consortium and the oceanographic users. Strategy for combining GOCE geoid and altimetry The practical task of computing a mean dynamic topography (MDT) from a mean sea surface (MSS) and a geoid is conceptually very simple, however there are some issues that must be considered in order to obtain a good MDT product. First, both the MSS and the geoid must be represented relative to the same reference ellipsoid and tidal system. Then, it is important that the altimetry used for the MSS in the MDT calculation has the same corrections applied as the altimetry that is used for the computation of the sea level anomalies. Finally, it is vital that when a MDT is calculated and used to reference time varying sea level anomaly data, the time periods defining the MDT and the sea level anomalies do correspond. Subsequently, the separation of the MDT from the MSS and the geoid require a proper filtering of the differences to eliminate the short scale geoid signal present in the MSS to obtain the MDT. This may be carried out in either the space domain, where the MSS is usually represented, or in the spectral domain where global geoid models are usually represented. Both methods have their advantages and their disadvantages. In both cases, it may be recommended to augment the GOCE spherical harmonic series using other higher degree harmonic expansions of the gravity field to reduce the magnitude of the short scale geoid signal in the MSS. The ECMWF model system Altimeter data assimilation was included for the first time in April 2006 in the operational ECMWF system 3 seasonal forecasting system. An analysis of a model MDT showed that an MDT based on a MSS and a GRACE geoid is higher in the Atlantic that the model MDT and lower in the Pacific. The reason for this discrepancy is partly model errors. More sophisticated schemes are needed to use observation-based MDTs. Including bias correction schemes to correct the model or correct the MDT so it is more compatible with the model. ECMWF may still need to adopt the altimeter bias correction scheme to deal with any large scale discrepancy due to model errors. ECMWF plan to operationally implement a system based on Nemovar. The Met office and ECWMF will both be working on the Nemovar so it is likely that there will be close collaboration on the technical development of bias correction schemes in Nemovar. It will be very important that ECMWF thoroughly test any assimilation scheme using a GOCE based MDT to confirm that forcing the model large scale circulation to match does improve the system and forecast results. The TOPAZ model system The TOPAZ system covering the North Atlantic Ocean, Nordic Seas and the Arctic Ocean is presently running its third version (TOPAZ 3) in real-time. A nearly identical copy is also exploited in real-time by the Norwegian Meteorological Institute. The assimilation system in TOPAZ 3 relies on an ensemble Kalman filter (ENKF) and the HYCOM ocean model. Experiments have showed that the efficiency of the ENKF assimilation in taking up the MDT biases are regionally dependent. This means that the differences in the adjustment of the MDT are linked with the different physical processes predominantly acting in the tropics and mid-latitudes. The results above are produced in a case of severe bias in MDT. Nevertheless the results imply that the impact of GOCE data might be expected to be regionally and temporally dependent. In view of the strategy for assimilating GOCE derived MDT the TOPAZ 3 system will be employed. When it comes to the observation errors and the account for the slow-varying geoid error covariance an ensemble of GOCE based MDTs will be generated, which average is the best MDT estimate. Each of those realisations will be affected to one ensemble member. This will include a MDT error term in the observation errors. The FOAM model system The UK Met office operational system FOAM (Forecasting ocean assimilation model) uses an optimal interpolation type scheme to analyse temperature, salinity, currents and sea ice in the open ocean. The system provides daily analysis and 5 day forecasts using a range of nested models from a ¼ degree global to a 1/12 degree limited area. Recently the observation bias method has been introduced and results are shown below. The full bias code, including model biases, has not yet been implemented operationally, however. If we use a new combined MDT or a direct MDT it will be important to use the altimeter bias correction method to correct for or specify the small scale components of the MDT only. With respect to implementation of a GOCE MDT the work has started at the Met office on testing the 3D-Var Nemovar assimilation scheme within the FOAM system. Within the timescales for producing GOCE based MDT it is likely that we will be testing Nemovar for operational implementation so we may transition both to using a GOCE MDT and Nemovar at the same time. Any new scale selective bias correction scheme will be implemented and tested directly in Nemovar. It is expected that by correcting the model MSS on the large scale using a GOCE based MDT and the MDT on the small scale using bias correction we should obtain maximum benefit from altimeter data assimilation. The Mercator model system In the Mercator system, the observed MDT used to assimilate altimetric sea level anomalies is built from two main steps. The first step consists in estimating a large scale ocean MDT from the subtraction and further filtering of an altimetric mean sea surface and a geoid model. This large scale MDT solution can be compared to 'synthetic' estimates of the mean computed combining ocean in-situ measurements and altimetric sea level anomalies, which provide an insight onto the shortest scales of the mean field. The strategy developed at Mercator for assimilating the future GOCE data consists in combining spatial and in-situ data so as to estimate the ocean MDT at the highest resolution for further assimilation into the operational forecasting system. Data from the GOCE mission will allow resolving the geoid at spatial scales down to 100 km with centimetric accuracy. Areas will be identified where the 100 km resolution permitted by GOCE won't be sufficient to fully resolve the mean circulation (mainly in coastal areas or in strong western boundary currents) so that further combination with synthetic heights is needed. In this combination step, the covariance error information from the GOCE data will be very useful. On the other hand, the assimilation scheme from the Mercator system does not need at the moment the MDT covariance error information. The MFS model system A 3D-VAR assimilation scheme has been recently developed to be integrated into the MFS forecasting system. The estimation of the MDT is particularly difficult in the Mediterranean Sea due to the short characteristic spatial scales of the Mediterranean circulation. A strategy has been developed to take benefit of and improve the existing MDT of the Mediterranean Sea, as well as to prepare for the future use of GOCE data into the MFS forecasting system. The strategy is based on the assumption that the error in the MDT field appears in the assimilation system as a temporally constant and spatially variable observational bias. At the present time, no covariance information is used. When available, it will be used for constructing the MDT. In the future, GOCE observations may be used in a variational scheme to further improve the MDT of the Mediterranean Sea. The Gocino web pages were developed for dissemination of information, knowledge, and experiences, so that the knowledge and expertise build up through the research in the Gocina as well as in the Gocino projects are kept together and fully made accessible for operational GOCE data users in the period up to the release of the first GOCE data.