L4 derived variables generation.

The L4 derived products consist of the Absolute Dynamic Topography (ADT) (maps and along-track) and maps of geostrophic currents (absolute and anomalies).

The ADT products are obtained by adding a Mean Dynamic Topography (MDT) to the SLA field. The MDT used in the DT2014 reprocessing is described in section Homogenization and cross-calibration.

The anomaly of the geostrophic current disseminated to users is derived from gridded SLA field. It is computed using a 3-point stencil width methodology (Arbic et al, 2012) for latitudes outside the ± 5°N band. In the equatorial band, the Lagerloef methodology (Lagerloef et al, 1999) introducing the β plane approximation is used. The absolute geostrophic current is obtained by adding to this anomaly the mean geostrophic current associated with the MDT field previously used.

The reader can refer to Pujol et al (2016) and Taburet et al (2019) for additional details.

 L3 and L4 Quality control

The production of homogeneous products with high-quality data and within a short time period is the key feature of the DUACS processing system. But some events (failure on payload or on instruments, delay, maintenance on servers), can impact the quality of measurements or the data flows. A strict quality control on each processing step is indispensable to appreciate the overall quality of the system and to provide the best user services.

The Quality Control is the final process used by DUACS before product delivery. In addition to daily automated controls and warnings to the operators, each production delivers a large QC Report composed of detailed logs, figures and statistics of each processing step. An overview of the diagnostics used is given in section Products Quality. Altimetry experts analyse these reports twice a week (only for internal validation, thus those reports are not disseminated).