Influence of the Vertical Profile of Reflectivity on Radar-Estimated Rain Rates at Short Time Steps

Abstract

The present study aims to demonstrate the major influence of the vertical heterogeneity of rainfall on radar?rain gauge assessment. For this purpose, an experimental setup was deployed during the HYDROMET Integrated Radar Experiment (HIRE-98) based on a conventional S-band weather radar operating at long range (90 km), an X-band vertically pointing radar, and a network of 25 tipping-bucket rain gauges. After calibration and attenuation corrections, the X-band radar data enables the estimation of the vertical profile of reflectivity (VPR) time series. Screening and VPR correction factors are derived for the distant S-band radar measurements. The raw and corrected S-band radar estimates are compared to rain gauge measurements for various integration time steps (6?30 min). Considering about 12 h of intense Mediterranean precipitation, the VPR influence at the X-band radar site is clear for all the time steps considered. For instance, a continuous increase in the Nash efficiency for the corrected radar data compared to the rain gauge data (0.85 for the 6-min time step, up to 0.93 for the 30-min time step) is observed while this criterion remains less than 0.15 for the raw radar data, regardless of the time step. The effect of the low-level reflectivity enhancement on the radar?rain gauge assessment was also found to be very important in the considered configuration. The establishment of reliable VPR climatologies is therefore a challenge in order to better account for such effects that are not observable at long range from the radars. The spatial validity of the VPR correction derived from a point sensor like the vertically pointing radar is also investigated. As a result of the high space?time variability of rainfall, such a punctual VPR correction has an efficiency limited to areas of about 20 km2 (200 km2) for the 6-min (30 min) integration time step.