Real-time estimation of the vertical profile of radar reflectivity to improve the measurement of precipitation in an Alpine region

Summary

Over the years much research has been directed towards exploring the potential of radar as an instrument for estimating rain. It is shown that with the present reflectivity measuring radar, good qualitative and in certain conditions even quantitative information may be obtained in an Alpine country. In other words, we would need an extremely dense and costly network of gauges to obtain a spatial resolution easily attainable with radar.

But radar only estimates the precipitation it can “see”. Even the most ingenious procedure using sophisticated equipment will not allow measurements in areas not observed fully or at all, e.g., behind mountains. But likewise in flat country we will find — at longer ranges — increased errors due to losses caused by the earth's curvature and errors caused by the reduced resolution of the radar beam. Therefore, choosing a good radar site is important: A higher one will in general improve the visibility, but increase at the same time the amount of clutter by ground echoes. Doppler radar will help in this respect. To what extent will be shown by the results obtained with the new radars planned to replace Albis and La Dôle north of the Alps and the third, new radar site in the south, probably on Monte Lema in the Tessin, during the years 1993 till 1996. Results are presented describing the accuracy obtained with present radars when just applying all year round a fixed correction for reduced visibility at longer ranges. Planned strategies are discussed for eliminating clutter, for making quantitative measurements, and for reducing the time needed to investigate the entire volume by using an electronically scanned antenna. By decreasing the update period for the full volume information down to the order of one minute, contradicting requirements of various users, such as hydrologists and air traffic controllers, could be fulfilled. However, questions relating to price and reliability will have to be answered in an acceptable way.