Mountain reference technique: Use of mountain returns to calibrate weather radars operating at attenuating wavelengths

The Mountain Reference Technique (MRT) was proposed as a means to perform a self-calibration of a weather radar system operating at an attenuating wavelength in a mountainous environment. Two convective rain events observed during the Grenoble 97-98 Experiment are selected here for an illustration and a further verification of the method at X band: the June 16, 1997, event is a medium event with maximum path-integrated attenuations (PIAs) of about 15 dB over a 9-km path, while the July 3, 1998, event is quite extraordinary with (1) a maximum PIA of 50 dB over the same distance and (2) the temporary presence of hail. An improved scheme is proposed for the MRT parameter estimation procedure with a more satisfactory treatment of such high-attenuation effects. It is shown that the optimal calibration factors obtained for the two rain events are almost equal to each other, a comforting result with respect to the radar equipment stability during the corresponding 1-year period. Although the MRT is based on reflectivity and attenuation measurements only, validation of the rain rate retrieval algorithms with respect to rain gage data for the June 16, 1997, rain event showed that this technique is relevant in terms of rain rate estimation. In particular, the MRT-calibrated Hitschfeld-Bordan algorithm provides satisfactory results for this medium-attenuation event. However, the July 3 case clearly demonstrates that this algorithm cannot correct for such high-attenuation effects because of its inherent instability. For both rain events the Marzoug-Amayenc algorithm, originally proposed for spaceborne configurations, is found to be stable and efficient in terms of rain rate estimation. These interesting features are counterbalanced by the fact that the algorithm implementation is limited to directions for which PIA measurements are actually available.