# Weather Radar in Complex Orography
## 4 Solutions and Results
### Radio Frequency and Polarimetry  
The choice of the radar hardware and configuration is driven by the needs and requirements of the practical applications (Section 2). For a mountainous site of the \"peak\" or \"offside\" type (Section 4.2 and Figures 11 and 13), limited space, difficult terrain for civil work, harsh weather and the budget lead to an upper limit on the size of the building, the radome and thus also of the antenna. For a given antenna size, shorter wavelengths result in a higher antenna gain, a narrower beam, lower sidelobes and a more favourable ratio between weather and mountain returns (clutter) [1; 2]. Gain, beam width, sidelobes and the weather-to-clutter ratio are all critical aspects for operational radars in complex orography. On the other hand, the shorter wavelengths of X-band radars and, to a certain extent, also C-band radars, suffer from signal attenuation as a result of water on the radome and of large rain drops, melting snow and melting hail along the propagation path, whereas attenuation can largely be neglected for S-band radars, except for hail. The choice of the wavelength should be a compromise between the above counteracting arguments. Complex orography countries often opt for C-band radars for volume-scanning surveillance [1; 8; 37] and X-band radars for research and field campaigns [135], gap-filling [9; 126; 136] and urban hydrology [131].  
X-band radars in complex terrain are sometimes promoted because of their low costs and high spatial resolution. However, this is a little misleading. The low costs are mainly the result of the small overall size of the X-band system, which has typical antenna diameters of between 1 and 2.5 m. However, the resulting beam width of these antenna diameters is comparable with or even _broader_ than that of a C-band radar with a 4-m antenna. The advantage in the spatial resolution that can be achieved with X-band wavelengths is lost if a small antenna is used to limit the overall size and costs.  
Many of the radar measurements in a mountainous region are made in solid precipitation and the applications of polarimetry, based on the axis ratio of drops, are limited to the measurements that are made below the melting layer. Furthermore, polarimetric moments are more sensitive to contamination from ground clutter from the main and the side lobes [121] and, in the case of differential reflectivity \(Z_{DR}\), to calibration errors and hardware instabilities in the transmission and reception paths. In spite of this, dual polarisation produces a large benefit in complex orography as it offers a better distinction between weather echoes and ground clutter (Section 4.10), and it can be used to obtain information on the type and mixing of hydrometeors (Figure 7 and [99; 100]), to diagnose hardware anomalies, to correct signal attenuation and to improve quantitative estimates of the precipitation rates (Section 4.10). Last but not least, the vertical channel (V) can be used as a backup for single-polarisation algorithms whenever the horizontal channel (H) suffers from instabilities. The extra costs of dual polarisation are modest and the gain in  
Figure 13: Picture of the radar on Pointe de la Plaine Morte, 2937 m above sea level. The lightning protection consists of two groups of lightning poles, one below the lowest point of the antenna, and the other one on top of the radome. This design results in minimal interference of the lightning poles on the radar beam and polarimetric measurements.  
quality and quantity of information is large, but a robust solution for the calibration and monitoring of both channels is critical [137].