Rain measurement in hilly terrain with X-band weather radar systems: Accuracy of path-integrated attenuation estimates derived from mountain returns

The authors recently showed that when attenuating wavelengths are used moun tain returns may allow estimation of path-integrated attenuations (PIAs) be tween a ground-based weather radar and a given mountain, an application of the well-known Surface Reference Technique originally proposed for spacebor ne radar configurations. This information proved to be valuable for the qua ntitative interpretation of X-band weather radar data in terms of rainfall rate for an urban hydrological application in Marseilles, France. In this p aper, a further verification of this concept is presented with the comparis on of mountain-derived PIAs and direct measurements obtained by means of a receiving antenna installed in the Falcons de Belledonne mountain ridge nea r Grenoble, France. Maximum PIAs in the range of 8-16 dB are obtained over the considered 9-km propagation path for Various rain events observed betwe en May and July 1997. A physical model of the mountain return power is deve loped leading to the formulation of two mountain PIA estimators under vario us hypotheses concerning 1) the stability of both the radar equipment and t he electromagnetic properties of the mountain surfaces and 2) the effects o f the rain falling over the mountain. A geometric calculation based on the use of a digital terrain model then allows the authors to estimate both the radar-positioning errors and the rain beamfilling factors of the mountain- cluttered radar bins. The dry-weather mountain return time series are also studied, showing a good stability of the average Value from one rain event to the next with, however, a marked effect of the wetting of the mountain s urfaces. The time variability of these values is also characterized in orde r to assess the minimum detectable mountain PIA, estimated in the present c ase to be about 2.25 dB. Finally, a good agreement is observed between the estimated mountain PIAs and the measured ones (with correlation coefficient s and regression slopes reaching 0.92 and 0.95, respectively). Assuming tha t the receiving antenna measurements are error free, the standard error in the mountain PIA estimation is found to be about +/-2.5 dB in the considere d configuration.