# Weather Radar in Complex Orography
## 4 Solutions and Results
### Scanning the Atmosphere over Complex Orography  
The quality of radar products in complex orography depends to a great extent on the way the antenna scans the atmosphere, which is commonly referred to as the antenna scan strategy. This is part of the radar configuration and determines:  
* The total number of sweeps in a volume scan, which is important for a thunderstorm severity estimation, hail identification, QPE, hydromteeor classification, mesocyclone detection and wind retrieval;
* The number of sweeps and angular overlappings at low heights close to the terrain, a critical aspect for QPE processing;
* The number of independent measurements per ray, which determines the precision of an individual measurement;
* The maximum height up to which echoes are sampled, which is relevant for the estimation of the echo top, VIL, the wind profile, thunderstorm severity and hail identification;
* The conical volume above the radar, where no measurements are made (the so-called cone of silence);
* The total time required to complete a volume scan, a critical parameter in the context of air traffic control, nowcasting and issuing automatic warnings;* The pulse repetition frequency, PRF, which determines, in opposite directions, the maximum unambiguous range and the Nyquist interval. The latter has an impact on wind retrieval and mesocyclone detection;
* The diversity in the Nyquist velocity between neighbouring sweeps, which helps in de-aliasing;
* The order of the sweeps: for instance, an interleaved approach reduces the effects of temporal undersampling of rapidly evolving storms.  
The scan strategy of the networks in the Austrian and Swiss Alps are both characterised by a comparatively large number of sweeps (16 and 20, respectively), which are sampled in an interleaved scheme, and completed in a short time (5 min), see Figure 14 and [1,8]. As a result of the high elevation angles, the cone of silence above the radar is relatively small. These configurations were driven by the above considerations and the needs and requirements discussed in Section 2. The interleaved scan leads to zigzag patterns in the vertical cross section in rapidly evolving storms, see, for instance, the melting hail signature in Figure 7. Zigzagging is an artefact that is caused by the rapid evolution of a storm during the time the antenna takes to complete half of the volume scan. A zigzag pattern is apparently a disadvantage, but in reality, it conveys more information about the evolving storm than an identical scan without the interleaved scheme. Cartesian radar products in Austria and Switzerland are generated every 2.5 min, combine the two most recent half-volume scans, and hence take advantage of the interleaved approach.  
Technically speaking, it is possible to adapt the antenna scan program in a _dynamic_ manner, depending on the type and location of the precipitating clouds. Dynamic scanning allows an increased resolution to be obtained in space and time in a dynamically selected target area, which has weather of particular interest. This may be an interesting option for gap-filling or any research radar that is not strictly necessary for long-range surveillance, see, for instance, [96], or for a network that was designed for dynamic scanning from the start [141]. It is questionable whether the gain in resolution in some selected areas of a long-range surveillance radar in a mountainous region, where precipitation is frequently present in several different azimuth sectors, is worth the extra complication of the configuration, antenna control, data processing and product generation. Moreover, there is the risk of missing precipitation in some locations outside the area of the targeted scanning.  
Figure 14: The Swiss volume scan consists of 20 elevation sweeps and is completed every 5 min. The antenna scans the atmosphere up to a maximum range of 246 km and maximum height of 18 km. The sequence follows an interleaved scheme with two half-volume scans (blue and yellow), which allows products to be generated every 2.5 min.