Impact of the Altitudinal Gradients of Precipitation on the Radar QPE Bias in the French Alps

Dominique Faure,Nicolas Gaussiat,Guy Delrieu

doi:10.3390/atmos10060306

Abstract

In the French Alps the quality of the radar Quantitative Precipitation Estimation (QPE) is limited by the topography and the vertical structure of precipitation. A previous study realized in all the French Alps, has shown a general bias between values of the national radar QPE composite and the rain gauge measurements: a radar QPE over-estimation at low altitude (+20% at 200 m a.s.l.), and an increasing underestimation at high altitudes (until −40% at 2100 m a.s.l.). This trend has been linked to altitudinal gradients of precipitation observed at ground level. This paper analyzes relative altitudinal gradients of precipitation estimated with rain gauges measurements in 2016 for three massifs around Grenoble, and for different temporal accumulations (yearly, seasonal, monthly, daily). Comparisons of radar and rain gauge accumulations confirm the bias previously observed. The parts of the current radar data processing affecting the bias value are pointed out. The analysis shows a coherency between the relative gradient values estimated at the different temporal accumulations. Vertical profiles of precipitation detected by a research radar installed at the bottom of the valley also show how the wide horizontal variability of precipitation inside the valley can affect the gradient estimation.

Highlights

In the French Alps, the quality of radar Quantitative Precipitation Estimation (QPE) provided by the French weather radar network is limited by difficulties of measurement, which increase the impact of well-known sources of error in radar data processing
This may be explained by the radar data processing used for each single radar, which does not take into account the relief or any altitudinal gradient of precipitation
If the rain gauge data underestimated the true precipitation amounts at high altitude, the gradient values were underestimated, as well as the radar QPE. Bias, these QPE clearly underestimating the precipitation in altitude. These results suggest some responses to the questions at the origin of this study, and first how the altitudinal gradients of precipitation could explain the radar QPE bias observed

Summary

Introduction

In the French Alps, the quality of radar Quantitative Precipitation Estimation (QPE) provided by the French weather radar network is limited by difficulties of measurement, which increase the impact of well-known sources of error in radar data processing. The distance to the radars and the altitude of measurement may be important, which expose to non-uniform beam filling effects, the proximity to 0 ◦ C isotherm altitude and the impact of the melting layer (solid and liquid phases in the radar sampling volume affecting the precipitation estimation, bright band, signal attenuation). One can observe that the distant C band radars of the network and the X band radars installed within the mountain chain between 1740 m and 2580 m of altitude, scan the atmosphere above the mountain range and cannot detect precipitation into the valleys This indicates that the radar data processing has no information on the evolution of precipitation in the valleys

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Discussion

Conclusion