Real time Integration of Foreign radar Quantitative Precipitation Estimations (QPEs) in the French National QPE Mosaic

The French metropolitan weather radar network (named ARAMIS) include 28 weather radars (31 in the future) over a territory of 552 000 km², and the real time quantitative precipitation estimation (QPE) is one of the important public duties of Météo-France. Every 5minutes a local QPE is estimated for each radar, and these local QPEs are fused in a national multi-sensor QPE mosaic at the spatial resolution of 1km², delivered within 2 minutes of the end of the observation time.

Although the ARAMIS network density, the QPE mosaic is degraded in some regions in relation to radar beam blockage and the distance from the radars. These areas clearly appear on the annual score maps produced every year by Météo-France, by comparison of the national QPE mosaic with the ground level validated measurements of between 3500 and 4000 daily rain gauges. Several actions have been taken in order to improve the QPE mosaic quality. One of these actions is the real time integration into this mosaic of data from radars of neighbouring countries, covering areas with degraded ARAMIS network coverage. These foreign radars are different from those used in the ARAMIS network, and also the operating procedures and the data processing. As it was not conceivable to receive in real time raw data directly from each foreign radar, and to develop a specific data processing chain, Météo-France has chosen an approach using directly the QPE produced by the meteorological Offices operating these radars. These QPEs are recovered in real time (every few minutes), and post-treated in order to be easily integrated in the French multi-sensor QPE mosaic according to the process used to merge all the local QPEs of the ARAMIS radars (including real time rain gauge calibration over the French territory, and estimation of QPE quality maps). This communication presents two very different integration cases, fully operational in 2015.

The first case is a relatively easy case, integrating the QPE of the single Jersey radar located in the Jersey Island near the North Brittany coast, over an area of the French territory where the Jersey radar measurements have been proved to be of better quality than the combined measurements of the three ARAMIS radars in the region. The partners meteorological Offices are the UK Met Office and the Meteorological Department of the States of Jersey.

The second case is a more complicated case. It concerns the integration of the Swiss national QPE mosaic (provided by MeteoSwiss and currently merging 3D data from five radars), on the North of the French Alps region covering several mountain massifs with altitudes above 3,000 or 4,000 metres, including the Mont Blanc summit at 4,810 metres. The work realised has shown that in this region, where both the French and the Swiss radar networks have degraded coverage, we can observe a patchwork of reduced areas for which alternatively each source of data is of better quality. The challenge is to optimise the final QPE quality in this difficult region by using all the contribution of existing operational radars, in addition to other actions like the deployment of local X band radars.

This communication explains the methodology used to integrate foreign data in the French national QPE mosaïc, in relation with the current process used to merge ARAMIS data in this mosaïc, and highlights knowledge and lessons from this project. The improvement of the French national multi-sensor QPE mosaic is presented for each case. This project is an example of cooperation between neighbouring countries, and anticipates future operational use of collaborative products like the European QPE composite of the OPERA program*.

(*OPERA = Operational Program for the Exchange of weather Radar information, Weather Radar program of the EUMETNET Economic Interest Group, the Network of the European Meteorological Services).