Evaluation of the Meteo-France operational precipitation radar network capacity for the refractivity measurement

The refractivity can be measured by a weather radar, and it is function of meteorological parameters, as the temperature, the water vapor pressure and the air pressure. The refractivity measurement is obtained by an estimation of the refractive index, which is depending of the change in the two-way travel time Δt of electromagnetic waves between the radar and the targets . This measurement by radar could be useful for convection prediction through the measurement of moisture in the boundary layer, particularly in pre-storm conditions. Until now such measurements have only been performed by coherent radars but European weather radars are mostly equipped with non-coherent magnetron transmitters, for which the phase of the transmitted pulse is random, and the frequency can drift over time, mainly due to temperature variations. Previous studies give an analytical expression of the refractivity measurement by a non-coherent drifting frequency magnetron radar. However, the measurement of radar refractivity remains sensitive to phase ambiguities, which can cause significant measurement errors. These sensibilities to the phase aliasing are directly link to the radar frequency, to the range integration between the radar and the ground target, and to the time sampling of the radar.

The main objective of Meteo-France is to evaluate the capacity of their own radar Operational Network (ARAMIS) to perform a good refractivity measurement. Indeed, the network is heterogeneous in frequency (S-, C- and X-Band), contains both analog and digital radars types, and also mono or dual polarization. Moreover, the sampling cycle of each radar, due to the speed antenna rotation, is different between the oldest and the newest generation recently implement.

In this study, refractivity measure by precipitation radars are compared with in situ measurement, for each radar type present in the network. This aims to identify measurement problems specifically related to the radar type. It is also investigate the possibility of the dual polarization radars to lift the phase ambiguity, and so improve the quality of the radar refractivity measurement.