Mesobeta Profiles to Extrapolate Radar Precipitation Measurements above the Alps to the Ground Level

Abstract

In the Alps, the volume visible by a radar is reduced because of ground clutter, elevated horizon, and earth curvature. This often inhibits a direct view on precipitation close to the ground. When using radar measurements from aloft to estimate precipitation rates at ground level, the measurements must be corrected for the vertical change of the radar echo (the profile) caused by the growth and transformation of precipitation. In this paper a robust profile-correction scheme for operational use in complex orography is presented. The aim is to correct for the large errors related to the profile in an Alpine environment: frequent underestimation caused by the vertical decrease of the radar echo, and occasional overestimation in the bright band. The profile is determined from volumetric radar data integrated over a few hours within a 70-km range of the radar (mesobeta scale). The correction scheme is verified by comparing radar estimates to gauge measurements of 247 h of summer and winter precipitation in a highly mountainous area. During the selected period, 10 gauges collected a total of 3966 mm of water. Four concepts to estimate ground-level precipitation are compared: the vertical maximum echo, the lowest visible echo, estimates corrected with the average-event profile, and estimates corrected using the mesobeta profile. Comparisons with the ground truth show that in summer profile correction considerably reduces the bias and scatter. The root-mean-square error diminishes by a factor of 2. Thus, corrected radar images give a much better overall view of the precipitation field than uncorrected ones. In winter, the improvement is found in a very significant reduction of the bias. The algorithm is currently being implemented in the operational radar network of MeteoSwiss. Long-term verification is needed after a few years of operation.