Weather radar observations at high altitude—first experiences with the weather radar located on Valluga mountain (2809 m) in the midst of the Alps

Weather radar precipitation measurements in mountainous areas are of high importance for the monitoring and nowcasting of precipitation, for hydrological applications as well as for risk and crisis management in case of heavy precipitation and for the tourism in the European Alps. These measurements suffer generally from severe beam shielding and blocking, strong ground clutter contamination and difficult operating conditions. Additionally, at larger distances from the radar site, beam overshooting gains importance. On the other hand rain gauge measurements in alpine terrain can suffer from wind induced errors. To make full use of the capability of a weather radar for real time monitoring of precipitation with high spatial resolution over a large area in the Alps, the radar must be located on an isolated mountain top - high enough to provide optimal visibility, but as low as possible to obtain reliable estimates of the surface precipitation.

The fifth Austrian weather radar located on the Valluga Mountain (2809m) has become operational by the end of 2007. To the knowledge of the author it is the highest weather radar site in Europe. The main interest of the public authorities funding the radar is the use of rainfall estimates for an automatic warning system of heavy precipitation events. Therefore high precision of the rainfall estimates is required. Generally, the useful range for the precipitation estimates depends strongly on the season. In the case of shallow wintertime precipitation the useful range is strongly reduced. The conversion of reflectivity to rain rate is based on a fixed Z-R relationship Z=200R^1.6. Remaining ground clutter and the limited use of the VPR (high altitude of the lowest elevation angles even close to the radar site) complicate the radar QPE.

The poster is focused on the validation of the radar rainfall estimates observed at high altitude. The estimates are compared with ground based observations of the dense operational rain gauge network. Additionally the ability of the system to produce reliable rainfall warnings is evaluated. The consequences for gauge adjustment are discussed, followed by the comparison of different radar-gauge merging techniques. Given the experience of nearly two years of operation, the significance of rainfall estimates derived from radar measurements at a high altitude will be critically reviewed.