# Weather Radar in Complex Orography
## 2 Practical Applications, Needs and Requirements
### Radar System Requirements  
The requirements for the design of a radar network in a mountainous region are derived from the type of weather, the scales of interest and the applications described above:  
1. Resolution in time: A high temporal update rate, of the order of a few minutes, is required to capture the rapid evolution of thunderstorms and hail cells, which originate flash floods, debris flows and landslides in steep orography. 2. Timeliness: Fast data transmission and processing is critical in the context of air traffic control and the issuing of warnings. The radar products have to be ready for submission to the user within about 60 s after the end of a radar volume scan. 3. Resolution in space: A horizontal mesh size of the order of a kilometre is required to capture rainfall peaks that trigger debris flows, to identify hail pixels inside thunderstorms and to properly describe gradients in thunderstorms and rainfall fields. It is beneficial to have information at scales smaller than 1 km for debris flow warnings and accurate localisation of hail. Figure 6: Inauguration of a new railway bridge in the Grison Alps in eastern Switzerland in 2011. The new bridge is three metres higher than the old one and was built to protect the rails from increasing threats of flash floods and debris flows. A customised radar-based heavy precipitation warning system was put into operation on 7 August 2017. One day later, the nowcasting system issued its first warning, and a few minutes later the rails on the _new_ bridge were flooded by a debris flow. Photograph courtesy of the Rhaetian Railway. Figure 7: Hydrometeor types retrieved from polarimetric radar measurements in the original coordinate system of the polar volume scan [99]. The figure shows a vertical cross section through a hail cell on 27 May 2016. (**a**) The hail cell seen from the Albis radar at an azimuth angle of 57\({}^{\circ}\). (**b**) View from the Weissfluhgipfel radar at an azimuth angle of 316\({}^{\circ}\). The dashed vertical line indicates the intersection. For the positions of the two cross sections, see the thin dotted lines in Figure 1.  
Dynamic range: A network of surveillance radars used for nowcasting has to detect all the different types of precipitation, from weak drizzle to heavy rainfall and hail, for all ranges. This translates into a dynamic range of >100 dB. 5. Sensitivity: Heavy precipitation and thunderstorms result in high reflectivity values. Therefore, their detection does not put the sensitivity of a radar under pressure. However, sensitivity is critical for other reasons. First, surveillance at long ranges, to see the approaching weather for nowcasting purposes, requires high sensitivity. Second, because of the shielding of the radar beam by the terrain, a substantial part of the measurements in orographic regions are made at a high altitude, that is, in snow, where echoes are weak. These measurements from aloft are then extrapolated to the ground using various techniques. Third, in order to use the Sun for monitoring purposes, the minimum detectable signal (MDS) has to be at least a couple of decibels below the level of the Sun's signal [110]. If the Sun is also to be used for the absolute calibration of a dual-polarisation receiver, the MDS has to be about 8 dB below the level of the quiet Sun [111]. Indicatively, a surveillance radar must be able to detect echoes down to 10 dBZ at a distance of 200 km. 6. Stability: Users expect to obtain the same numbers and warning levels in comparable situations. A high stability of the radar hardware is therefore important. One dB is a number that is commonly cited for the stability of reflectivity, and 0.2 dB, which represents an additional challenge, is mentioned for differential reflectivity. 7. Accuracy and resolution in intensity: Accuracy and resolution are obviously important, but it is not possible to provide one number as a generic answer for reflectivity. The same applies for differential reflectivity, radial wind, the specific differential phase and other radar variables.