# Weather Radar in Complex Orography
## 2 Practical Applications, Needs and Requirements
### Operational Applications in the Swiss Alps  
Radar data have been used operationally in the Swiss Alps for quantitative precipitation estimation, which is commonly referred to as QPE [3; 4; 89; 90; 91; 92], for the nowcasting of precipitation [56; 59; 62; 87], for the tracking, severity ranking and nowcasting of thunderstorms [70; 93; 94; 95; 96], for the hail monitoring and hail nowcasting [80; 97; 98], hydrometeor classification [99; 100], wind retrieval and mesocyclone detection [101; 102; 103; 104], data assimilation in numerical weather prediction [63; 105] and climatological studies [52; 53; 55; 79]. The applications are continuously reviewed and expanded with new algorithms, see, for instance, [88; 97; 106; 107; 108]. Radar products and warnings are sent to air traffic control, weather forecasters, the authorities responsible for the intervention in the case of severe weather and flooding, hydrologists [68; 109], and to the private and public sectors. The forecaster on duty provides 24/7 support to users for a correct interpretation of the meteorological data and warnings. The thunderstorm radar tracking algorithm (TRT) [94] automatically tracks thunderstorms and assesses their severity in real time, through a heuristic approach, using volumetric radar data. The thunderstorms are extrapolated forward in time in the direction of their recent movement on the basis of Lagrangian persistence rules, see Figure 5. The uncertainty of the future cell position is estimated from the standard deviation of the motion vectors of the latest three time steps [70]. Warnings have been issued for future thunderstorm positions since 2005. A warning is issued if either the severity of the cell (TRT) or the amount of rainfall (QPE) within the cell exceeds a predefined threshold [62]. These warnings have been provided automatically and operationally to the authorities via dedicated platforms and to the public via the mobile application (app) of MeteoSwiss since 2020 [95]. The complete warning chain from the radar measurement up to the computation of the warnings and their distribution is fully automatic, without any human intervention. In 2021, a total of 906 of these thunderstorm flash warnings were issued throughout the Swiss territory, which has a size of 41,000 km\({}^{2}\). Customised solutions have been developed for particularly demanding applications. The automatic heavy precipitation warning systems developed and configured for a variety of locations and users represent prime examples. These include the operation of a trans-Alpine gas pipeline that crosses a steep Alpine slope that is prone to debris flows, the protection of major construction sites in a river tunnel in the city of Basel [7] and at the railway station in Zurich, the protection of the railway service on a bridge that crosses a catchment with frequent flash floods and debris flows in the Grison Alps (Figure 6) and several other similar applications. Radar measurements are the prime source of information in all these examples of customised nowcasting products. These customised systems often act as prototypes, and the algorithms become part of the standard production for all users in a later step. Figure 7 shows an example of a recently developed dual-polarisation product to identify hydrometors. The two operational radars at Albis and Weissfluhgpifel observe the same hail cell, but from different directions and at different distances. The figure shows the different hydrometeor types along a vertical cross section, as identified by the two radars using polarimetric information [99; 100]. The presence of hail was confirmed through the measurements of a network of automatic hail sensors on the ground deployed in the hail-prone regions in Switzerland. One sensor of a pilot network that was in the centre of the cell detected 425 hail stones in 6 min. The maximum diameter of 3.3 cm that hit the sensor plate matched the maximum expected severe hail size of 4.5 cm estimated by the MESHS radar product for the corresponding 1 km\({}^{2}\) pixel.