On the assessment of Argo float trajectory assimilation in the Mediterranean Forecasting System

Abstract

The Mediterranean Forecasting System (MFS) has been operational for a decade, and is continuously providing forecasts and analyses for the region. These forecasts comprise local- and basin-scale information of the environmental state of the sea and can be useful for tracking oil spills and supporting search-and-rescue missions. Data assimilation is a widely used method to improve the forecast skill of operational models and, in this study, the three-dimensional variational (OceanVar) scheme has been extended to include Argo float trajectories, with the objective of constraining and ameliorating the numerical output primarily in terms of the intermediate velocity fields at 350 m depth. When adding new datasets, it is furthermore crucial to ensure that the extended OceanVar scheme does not decrease the performance of the assimilation of other observations, e.g., sea-level anomalies, temperature, and salinity. Numerical experiments were undertaken for a 3-year period (2005–2007), and it was concluded that the Argo float trajectory assimilation improves the quality of the forecasted trajectories with ~15%, thus, increasing the realism of the model. Furthermore, the MFS proved to maintain the forecast quality of the sea-surface height and mass fields after the extended assimilation scheme had been introduced. A comparison between the modeled velocity fields and independent surface drifter observations suggested that assimilating trajectories at intermediate depth could yield improved forecasts of the upper ocean currents.

Physical forcing and physical/biochemical variability of the Mediterranean Sea: a review of unresolved issues and directions for future research

“…These observational systems are designed to transmit real-time data to operational centers with the ultimate goal of producing accurate model predictions. Recent developments in numerical modeling that assimilate available in situ observations are a promising step towards more reliable ocean predictions and long-term reanalysis simulations in the Mediterranean within the scope of operational oceanography (Adani et al, 2011;Dobricic et al, 2007Dobricic et al, , 2010Pinardi and Coppini, 2010;Nilsson et al, 2011).…”

Variational assimilation of Lagrangian trajectories in the Mediterranean ocean Forecasting System

“…A2) in the observational operator that maps the model to the observational space, hence facilitating comparisons between the Lagrangian observations and the modeled Eulerian fields. The non-linear particle advection equation was discretized and used for this purpose since its application has proved successful for assimilating Argo-float positions in the Mediterranean Sea; detailed descriptions of the trajectory model implementation and evaluations of the numerical experiments are available in Taillandier et al (2006a,b); Nilsson et al (2011).…”

“…A third and final test was done for a 5000-m observational error, which proved to yield stable OceanVar solutions with slightly reduced RMS misfits and faster computations (in terms of CPU time) compared to the results from the "3000-m experiment". The observational error for the Argo float positions was set to 2000 m, as given from sensitivity tests in Nilsson et al (2011). The Eastern Mediterranean surface circulation is dominated by a highly variable meso-scale eddy field and the meandering Libyo-Egyptian current system, which make accurate velocity-field predictions a most challenging task.…”

“…In order to guarantee stateof-art ocean analyses, a three-dimensional data assimilation scheme denoted OceanVar ) is under continuous development, and recently glider observations (Dobricic et al, 2010) and Argo-float trajectories (Nilsson et al, 2011) have been successfully assimilated.…”

Assimilating en-route Lagrangian observations

“…Since the velocity and height field model we are using is spectral, with only a few Fourier components, our numerical experiments are unable to effectively answer this question and further studies of en-route assimilation with operational ocean models (see, e.g. recent work in Dobricic and Pinardi, 2008;Dobricic et al, 2010;Nilsson et al, 2011Nilsson et al, , 2012 would certainly be needed.…”