THE APPLICATION OF RADAR-GAUGE COMPARISONS TO OPERATIONAL PRECIPITATION PROFILE CORRECTIONS

The analyses of data recorded during the past eight years with two Swi ss radars, a network of rain gauges, and river flow measurements have helped to quantify the vertical profile of reflectivity and the influe nces of topography, meteorology, and radar parameters on the precision of radar precipitation estimation. The influence of the topography ar ound the radar, the width of the radar beam, and the vertical echo str ucture produces a complex error distribution in space and time, with e rrors dependent upon storm type, distance from the radar, and the rada r horizon. In spite of excellent agreement between amounts estimated b y the 5-cm radar at close ranges and gauges located below the radar vo lume, underestimation of rainfall increases with range from the radar, The authors' experience dramatically shows how significantly errors a re reduced when precipitation can be estimated close to the ground, a task made easier by choosing a radar site with a good view and by rigo rously eliminating echoes contaminated by ground clutter and anomalous propagation without, however, reducing the detection capability of th e radar for precipitation. Several methods of clutter detection are us ed together to ensure that precipitation estimates are not biased by c lutter. A physical model can correct for a large part of these errors, including brightband effects, or at least tell us something about the validity of the results, if the causes of the long-range underestimat ion are understood. This paper proposes a two-step approach to error c orrection: first, a three-dimensional map of the ''visibility'' from t he radar of each observation point is made, initially assumed constant with time. The vertical profile of precipitation is then estimated (i n real time where possible and from climatological values if not) and used together with a topographical database to estimate the precipitat ion reaching the (usually obscured) ground from a weighted function of all rain-rate estimates made above each point on the surface. The res ults of this analysis, especially appropriate for the Alps but also va luable in ordinary terrain, are being applied to the Swiss Meteorologi cal Institute's new generation of weather radars in order to provide i mproved quantitative precipitation information to support the preparat ion of operational flood warnings in the Swiss Alps. An optimized scan strategy with simultaneous reflectivity and Doppler processing and au tomatic calibration is used to allow corrections in real time and to p roduce products to satisfy a wide variety of user needs.