## 5 Conclusions  
Switzerland is a unique and ideal ground validation site for comparing spaceborne weather radar level 2 precipitation estimates (GPM, Section 2.1) with other different precipitation products, as the mountainous and relatively small (~41,000 km\({}^{2}\)) country is equipped with a network of telemetered, automatic rain gauges (GAUGE, Section 2.3), and with a state-of-the-art dual-polarization network of weather radars (RADAR, Section 2.2). A third and innovative MeteoSwiss precipitation product is available (CombiPrecip, Section 2.4), which is a real-time geostatistical merging of RADAR and GAUGE. To the best of the authors' knowledge, there is no existing published work examining the \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **GPM** & **RADAR** & **CombiPrecip** \\ \hline All pixels in the study area & 0.86 & 0.66 & 0.58 \\ Pixels outside Switzerland & 0.87 & 0.74 & 0.64 \\ Pixels inside Switzerland & 0.76 & 0.56 & 0.51 \\
221 pixels with Gauges (inside Switzerland) & 0.68 & 0.51 & 0.46 \\
135 pixels with Gauges in the mountains & 0.67 & 0.53 & 0.49 \\
86 pixels with Gauges in non complex terrain & 0.56 & 0.46 & 0.41 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Coefficient of Variation (st. deviation divided by the mean) of linear precipitation amounts. GPM-DPR precipitation product and three other different precipitation products in mountainous terrain. On average, GPM underestimates precipitation amounts by \(-3.0\) dB with respect to RADAR, and by \(-3.4\) dB with respect to GAUGE. Ground-based and spaceborne radars provide a complementary view of the complex and highly variable precipitation fields: the former measures precipitations from a lateral direction, while the latter looks at the precipitation fields from the top. The radar backscattering volume increases with the square of the range. Consequently, the changes in resolution of the GPM radar are almost negligible, since its measurements originate from similar distances of 400-420 km over the whole swath. On the contrary, the sampling volume of the ground-based radar changes significantly, since it has to measure precipitation from close to large distances. Hence, the GPM can be used as a reference to evaluate the proper spatial variability of both the RADAR and the CombiPrecip products, since the former is used as an external drift to derive the latter in the geostatistical merging based on GAUGE kriging. This explains why, in this note, not only an evaluation is made of the GPM level 2 estimated surface precipitation product against synchronous estimates from the MeteoSwiss RADAR, GAUGE,and Combiprecip products, but GPM is also used as a reference to evaluate the dispersion of the multiplicative difference between each MeteoSwiss products and an unbiased version of the GPM-derived field. The evaluation of the GPM level 2 precipitation product in the Swiss complex terrain is of interest for those who plan to use GPM in other mountainous areas. The GPM core satellite is, in fact, adding significant new coverage to mountainous areas, especially in poorly instrumented parts of the world and at latitudes not previously covered by TRMM. This study shows that, in terms of average precipitation, a clear underestimation by GPM (with respect to the dense ground-based radar or the gauge networks) can be expected also in other mountainous areas of the world. Regarding the dispersion of the GPM error around the mean, even by integrating 2 years of overpasses, the overall scatter associated with the high-resolution DPR cells (20 km\({}^{2}\)) is of the order of 2.6 dB. This figure represents an overall post facto assessment, and it is simply a quantitative assessment of the scatter of the differences between the DPR and the other three ground-based references, namely, the rain gauge network (2.8 dB), the weather radar network (2.6 dB), and a geostatistical combination (CombiPrecip) of these two ground-based networks (2.4 dB).