# Weather Radar in Complex Orography
## 3 Challenges
### Reflection and Shielding by the Terrain
The dashed rectangle corresponds to the domain shown in Figure 1. There are strong ground returns, \(>\)50 dBZ, in practically all the regions in Switzerland, which demonstrates the importance of rigorous and reliable clutter cancellation in the automatic warning context. region. Figure 9 shows, for illustration purposes, how the noise floor increases when a heavy thunderstorm cell moves over the Albis radar site. If a beam is completely shielded, it is necessary to rely on the next higher beam to obtain information about any precipitating clouds behind the obstacle. The lower beams in mountainous regions are often shielded, and the radar can only \"see\" the precipitating cloud starting at heights of some kilometres. Figure 10 shows the present situation in Switzerland with its five operational radars. The intensity of radar echoes decreases rapidly with height, due to phase changes and other microphysical processes, which is commonly referred to as the vertical profile of reflectivity, or VPR in short. The further a measurement is from the ground, the more it becomes difficult to derive useful meteorological parameters, such as the rainfall rates on the ground, and the higher the risk of the radar completely missing a precipitating cloud, which is referred to as overshooting. Overshooting is particularly a problem in stratiform precipitation and generally in winter, when the concentration of  
Figure 9: Registration of the noise level in the Albis radar receiver on 28 June 2021. The emissions from the wetted radome and the water in the atmosphere cause an increase in the noise level of more than 1.5 dB at the moment a severe hail cell approaching from the south-west passes over the radar. (**a**) Noise power as a function of time sampled at an elevation of 35\({}^{\circ}\) (red dots) and 40\({}^{\circ}\) (salmon dots). The measurement of a simple rain detector with three intensity classes installed on the tower of the radar is also indicated (blue). (**b**) The same noise power as in (**a**), here shown as a function of the azimuth, between 13:30 and 16:30 UTC, when the cell approaches the radar (salmon and red dots), and at 16:35 UTC (black), at the time the cell hits the radar. The grey ellipse indicates the mean and standard deviation of the wind at the radar site. The antenna occasionally points in the direction of the Sun and the Sun’s signal is superimposed onto the noise level (star), as automatically detected by the online Sun monitoring system. The exact value of the Sun’s signal depends on the relative position between the Sun and the centre of the beam, an aspect that is taken into account in the automatic monitoring of the receiver. All the red, salmon and black dots and stars are averages over 10\({}^{\circ}\) in the azimuth. scattering hydrometeors decreases rapidly with height. If clutter is adequately eliminated, shielding, combined with VPR effects, are likely the dominant sources of uncertainty in quantitative estimates of precipitation (QPE) in a mountainous region [2; 48; 116; 123]. The influence of clutter and VPR effects is less pronounced in a flat region, and other sources of error, such as the space-time variability of the drop size distribution, move into the focus of QPE processing, see, for instance, [124]. The impact of shielding is less critical for nowcasting and the issuing of automatic warnings of thunderstorms because convective storms can arrive well above the height of the lowest visible beam, even when data from one radar are missing (see Figure 10b).