Mountain reference technique: Use of mountain returns to calibrate weatherradars 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 wav elength in a mountainous environment. Two convective rain events observed d uring the Grenoble 97-98 Experiment are selected here for an illustration a nd 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 abou t 15 dB over a 9-km path, while the July 3, 1998, event is quite extraordin ary with (1) a maximum PIA of 50 dB over the same distance and (2) the temp orary presence of hail. An improved scheme is proposed for the MRT paramete r estimation procedure with a more satisfactory treatment of such high-atte nuation 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-y ear period. Although the MRT is based on reflectivity and attenuation measu rements only, validation of the rain rate retrieval algorithms with respect to rain gage data for the June 16, 1997, rain event showed that this techn ique is relevant in terms of rain rate estimation. In particular, the MRT-c alibrated Hitschfeld-Bordan algorithm provides satisfactory results for thi s medium-attenuation event. However, the July 3 case clearly demonstrates t hat this algorithm cannot correct for such high-attenuation effects because of its inherent instability. For both rain events the Marzoug-Amayenc algo rithm, originally proposed for spaceborne configurations, is found to be st able and efficient in terms of rain rate estimation. These interesting feat ures are counterbalanced by the fact that the algorithm implementation is l imited to directions for which PIA measurements are actually available.