The Application of Radar Gauge Comparisons to Operational Precipitation Profile Corrections

Abstract

The analyses of data recorded during the past eight years with two Swiss radars, a network of rain gauges, and river flow measurements have helped to quantify the vertical profile of reflectivity and the influences of topography, meteorology, and radar parameters on the precision of radar precipitation estimation. The influence of the topography around the radar, the width of the radar beam, and the vertical echo structure produces a complex error distribution in space and time, with errors dependent upon storm type, distance from the radar, and the radar horizon. In spite of excellent agreement between amounts estimated by the 5-cm radar at close ranges and gauges located below the radar volume, underestimation of rainfall increases with range from the radar. The authors' experience dramatically shows how significantly errors are 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 rigorously eliminating echoes contaminated by ground clutter and anomalous propagation without, however, reducing the detection capability of the radar for precipitation. Several methods of clutter detection are used together to ensure that precipitation estimates are not biased by clutter. 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 underestimation are understood. This paper proposes a two-step approach to error correction: first, a three-dimensional map of the ?visibility? from the radar of each observation point is made, initially assumed constant with time. The vertical profile of precipitation is then estimated (in real time where possible and from climatological values if not) and used together with a topographical database to estimate the precipitation reaching the (usually obscured) ground from a weighted function of all rain-rate estimates made above each point on the surface. The results of this analysis, especially appropriate for the Alps but also valuable in ordinary terrain, are being applied to the Swiss Meteorological Institute's new generation of weather radars in order to provide improved quantitative precipitation information to support the preparation of operational flood warnings in the Swiss Alps. An optimized sun strategy with simultaneous reflectivity and Doppler processing and automatic calibration is used to allow corrections in real time and to produce products to satisfy a wide variety of user needs.