Long-term Comparisons of Collocated Radar Reflectivity Observations at X Band in the Mountainous Region of the Southern French Alps

Three X band dual-polarization radars were deployed in the Alps in the south east of France during the RHYTMME project to evaluate the benefit of providing real-time quantitative precipitation estimations (QPE) to the local forecasters in order to minimize the economic and social impact of hazardous weather (Westrelin et al. 2012). Work has now been undertaken at Météo France to incorporate these X band gap-filling radars into the French radar network that already operate S and C band radars in the same area. The complex terrain on one hand that is responsible for the beam blockages and the large differences in altitude between the radar beams, and the problem of the extinction at X band on the other hand are making the direct comparison of the reflectivity between overlapping radars extremely difficult. A first step towards the integration of the individual QPE products of these radars into the French national composite is to analyze the differences in the observed reflectivities in order to assess the strength of the corrections used in the current processing chain (Figueras i Ventura and Tabary 2013).

In this paper, we present the evaluation framework that was developed for comparing long-term time series of co-located reflectivity measurements from two overlapping radar volumes (Ribaud et al. 2014) and produce robust statistics on the temporal and spatial variability of the error differences. 23 precipitation (rain or snow) events from March 2014 to February 2015 are used for the evaluation. Errors related to ground cluster, partial beam blockage, wet radome and bright band were carefully studied and disentangled. The results suggest that the current partial beam blockage correction suffers from poor resolution of digital elevation model used (250m), that the current attenuation correction based on differential phase do not account well for the attenuation due to the bright band, and that the bright band can be less pronounced at X band then at C or S band. These results will be used in a future version of Météo France radar processing chain to produce better QPE estimates at X band.

Figueras i Ventura, J., and P. Tabary, 2013: The New French Operational Polarimetric Radar Rainfall Rate Product. J. Appl. Meteor. Climatol., 52, 1817–1835.

Ribaud, J. F., O. Bousquet, and H. Al-Sakka, 2014: Evaluation of hydrometeor classification algorithms from multi-frequency dual-polarimetric radar observations collected during HyMeX. Proc. European Conf. on Radar in Meteorology and Hydrology, ERAD 2014, Garmisch-Partenkirchen, Germany, 7 pp.

Westrelin, S., P. Mériaux, P. Tabary, and Y. Aubert Y, 2012: Hydrometeorological risks in Mediterranean mountainous areas RHYTMME Project: Risk management based on a radar network. Proc. European Conf. on Radar in Meteorology and Hydrology, ERAD 2012, Toulouse, France, 5 pp.