Development of a nation-wide low-level wind shear mosaic in France

Wind shear is « a change in wind speed and/or direction in space ». Doppler radars can measure the variations of radial wind inside precipitations on a horizontal plan, which is useful for forecasters because these variations are – provided that the geometry of observation is favourable - a good indicator of some dangerous phenomena like gust fronts, convergence lines or mesocyclones.

The French ARAMIS radar network currently consists in 25 Doppler weather radars. All of them are equipped with a triple staggered Pulse Repetition Time (PRT) scheme allowing simultaneous measurement of reflectivity and Doppler velocity up to a distance of 250 km with no ambiguity.

Using the data of this Doppler radar network, which are concentrated in real-time from the 25 radar sites to Météo France national centre in Toulouse, first versions of a nation-wide wind-shear mosaic have been developed. The horizontal wind shear is calculated for each Plan Position Indicator (PPI) of each radar from the local texture of the radial velocity field in Cartesian coordinates (1 km²). The mosaic is then created from the 3D combination of all PPIs of all radars, given a maximum height above ground for the wind-shear measurement (typically 2 to 3 km). That threshold is a compromise between maximising the coverage of the territory and focusing on the lowest levels, where horizontal wind shears are known to be the most intense. At this stage and given the data available, no attempt has been made to distinguish between mesocyclone or gust front for example.

In order to optimise the wind shear signature in the low levels of the atmosphere, different algorithm parameters are currently being tested, such as thresholds on the maximum “realistic” pixel to pixel shear or on the maximum height above ground. Those optimisations are done using a number of significant test cases that are available in the archives and include tornadoes, narrow cold front, squall lines or mesocyclones.

In parallel to those optimisations, a web-based demonstrator has been developed that allows for the mosaic (1536x1536 km²) that is produced in real time every five minutes at a 1 km² horizontal resolution to be visualized and animated. A subjective evaluation of the product by forecasters is currently being prepared and could take place over the Summer of 2011. The aim of this demonstration is to make sure that this new product will help the forecasters to better anticipate and warn for dangerous phenomena.

An objective evaluation of the wind shear estimations will also be conducted using the values of the gusts recorded by the French network surface stations (about 500 stations distributed over the 550,000 km² of the French territory). This comparison will be conducted within the area defined by the track of convective cells. These convective cells are defined by the CONO (COnvection Nowcasting Object) that are mainly based on radar reflectivity but also incorporate lightning data. For this evaluation, these CONO will collect both wind shear values and surface gust measurements along their trajectory. The resulting database – structured by Object - will enable to determine in terms of intensity and frequency the relationships between on the one hand the wind shear observed by the radar network and on the other hand thunderstorms and gusty winds.