Measurement of Precipitation in the Alps Using Dual-Polarization C-Band Ground-Based Radars, the GPM Spaceborne Ku-Band Radar, and Rain Gauges

Abstract

Abstract:The complex problem of quantitative precipitation estimation in the Alpine region is tackled from four different points of view: (1) the modern MeteoSwiss network of automatic telemetered rain gauges (GAUGE); (2) the recently upgraded MeteoSwiss dual-polarization Doppler, ground-based weather radar network (RADAR); (3) a real-time merging of GAUGE and RADAR, implemented at MeteoSwiss, in which a technique based on co-kriging with external drift (CombiPrecip) is used; (4) spaceborne observations, acquired by the dual-wavelength precipitation radar on board the Global Precipitation Measuring (GPM) core satellite. There are obviously large differences in these sampling modes, which we have tried to minimize by integrating synchronous observations taken during the first 2 years of the GPM mission. The data comprises 327 "wet" overpasses of Switzerland, taken after the launch of GPM in February 2014. By comparing the GPM radar estimates with the MeteoSwiss products, a similar performance was found in terms of bias. On average (whole country, all days and seasons, both solid and liquid phases), underestimation is as large as −3.0 (−3.4) dB with respect to RADAR (GAUGE). GPM is not suitable for assessing what product is the best in terms of average precipitation over the Alps. GPM can nevertheless be used to evaluate the dispersion of the error around the mean, which is a measure of the geographical distribution of the error inside the country. Using 221 rain-gauge sites, the result is clear both in terms of correlation and in terms of scatter (a robust, weighted measure of the dispersion of the multiplicative error around the mean). The best agreement was observed between GPM and CombiPrecip, and, next, between GPM and RADAR, whereas a larger disagreement was found between GPM and GAUGE. Hence, GPM confirms that, for precipitation mapping in the Alpine region, the best results are obtained by combining ground-based radar with rain-gauge measurements using a geostatistical approach. The GPM mission is adding significant new coverage to mountainous areas, especially in poorly instrumented parts of the world and at latitudes not previously covered by the Tropical Rainfall Measuring Mission (TRMM). According to this study, one could expect an underestimation of the precipitation product by the dual-frequency precipitation radar (DPR) also in other mountainous areas of the world.

Cross-Validation of Observations between the GPM Dual-Frequency Precipitation Radar and Ground Based Dual-Polarization Radars

“…In contrast, the performance was significantly degraded over complex terrain, especially in winters. Similar results were also reported by Gabella et al [17] while evaluating GPM rain-rate products over the Swiss Alps region using ground radar and rain gauge network. Biswas and Chandrasekar [18] performed ground validation of GPM-DPR observations and rainfall rate measurements over the Dallas Fort Worth region in Texas, USA using S-band NEXRAD.…”

Analytical flow duration curves for summer streamflow in Switzerland

“…The mean monthly values computed over the entire observation period for each catchment are shown (see Table 1). parameters vary in space, it was decided here to treat interception losses explicitly with minimal assumptions about this process: different maximum interception depths are attributed to four different land covers: 4 mm for forests, 2 mm for low vegetation, 1 mm for impervious areas and 0 mm for water bodies (Gerrits, 2010). The catchment-scale maximum interception depth is obtained as the land-use-weighted average of these values, but a minimum interception depth of 1 mm is imposed.…”

“…or several meteorological stations as input (Botter et al, 2007c, a;Botter et al, 2013;Ceola et al, 2010;Basso et al, 2015;Schaefli et al, 2013), which is potentially limiting for the model performance since good area-averaged input estimates are critical. Recent progress in spaceborne precipitation observation, and in particular the Global Precipitation Measurement (GPM) mission, potentially offers an interesting new data source for area-averaged precipitation estimates, even in such complex terrain as the Swiss Alps (Gabella et al, 2017), with the drawback of covering only short historical periods. Here, we use the relatively new spatial precipitation data set of MeteoSwiss with a nominal res- This data set can be assumed to give relatively good estimates of area-averaged precipitation (Paschalis et al, 2014;Addor and Fischer, 2015), even in mountainous areas where there are only few meteorological stations.…”

A 10‐year radar‐based analysis of orographic precipitation growth and decay patterns over the Swiss Alpine region

“…The most relevant uncertainties which could affect the analyses in this paper are the blockage of radar beams at low elevations, the underestimation of precipitation at far ranges and the presence of residual ground clutter. A map of the height of the lowest visible beam from the Swiss four-radar composite can be found in Gabella et al (2017). The artefacts due to beam blockage can be reduced by calculating the precipitation statistics over larger areas (section 3.3).…”