# Weather Radar in Complex Orography
## 3 Challenges
### Reflection and Shielding by the Terrain  
Whereas the first two challenges are related to structural engineering and logistics, the third one has to do with the interaction between the radar signal and the mountains. The two most serious problems in this category are (i) the strong reflection (ground clutter) and (ii) the shielding of the radar beam by mountains. Ground clutter is only an issue in a flat region in the vicinity of the radar, up to a distance of about 10 to 20 km. Ground clutter can also occur at longer ranges under certain atmospheric conditions due to abnormal refraction of the beam towards the ground, which is called anomalous propagation, or anaprop. However, ground clutter occurs practically at all times, all ranges and all azimuth angles in a mountainous region, see Figure 8. It is sufficient that a part of the main lobe or any side lobe of the antenna radiation pattern hits the ground somewhere, see, for instance, [113; 114]. The amplitude of the signals reflected by the ground is comparable with that of a thunderstorm, and a patch of ground clutter can easily provoke an erroneous automatic thunderstorm, hail or heavy rainfall warning. A rigorous cancellation of ground clutter is therefore essential. The task is particularly difficult when mountain returns are embedded in echoes from precipitation. Simulation studies have shown that dual-polarisation parameters are more sensitive to sidelobe contamination than single-polarisation reflectivity measurements [115]. Applications that build on dual-polarisation measurements thus put higher demands on clutter cancellation, and automatic data quality control in general. In Figure 8, it is possible to distinguish several different types of cluttered signals. Ground returns are particularly strong and relatively constant in time where the main lobe hits steep mountain slopes and crests, as can be seen from the many yellow-orange-to-red spots within the Alps. The returns from hilly terrain are somewhat weaker and more continuous in space, as pointed out by the violet-blue-to-green areas over France and Italy. Several radial rays can be observed, two of which correspond to signals from the Sun, which was located to the east at the time of this image, while the others refer to signals from external emitters, such as RLAN links. The image also contains ground returns from the side lobes, and possibly a small number of echoes from birds, insects and aeroplanes, which are usually more variable in time but cannot be distinguished with the naked eye in this image. Another severe problem is shielding of the radar beam by mountains. If only a part of the main lobe is shielded, which is referred to as partial shielding, echoes still arrive from the weather behind the mountain as a result of the remaining part of the beam. The loss of power can be corrected if a fairly good estimate of the degree of shielding is available [3,4,116,117]. Some parameters, such as Doppler velocity and the specific differential phase \(\mathrm{K}_{DP}\), a polarimetric moment, are immune to partial shielding (and also to calibration errors and attenuation), see, for instance, [118]. This can be exploited for rainfall estimation purposes [119, 120, 121]. If the beam is completely shielded, the ray behind the mountain will be free of echoes, but the signals sampled by the receiver can be used to study receiver noise and thermal emissions from the terrain, as well as from drops in the atmosphere and water on the radome. The analysis of the signal emitted from water on the radome and in the atmosphere may be used for attenuation correction purposes [122], a concept that needs to be further developed before it can be used routinely in a mountainous  
Figure 8: Nonmeteorological signals (clutter) in a Swiss radar composite map on a clear sky day with the clutter cancellation switched off. The data refer to 13 September 2021, at 05:15 UTC.