# Weather Radar in Complex Orography
## 1 Introduction  
Installing and operating a radar in a region surrounded by high mountains is a particularly difficult technological challenge. Harsh weather conditions, long access routes, exposure to lightning and melting permafrost as well as the limited space on mountain crests make the civil works, power supply, data transmission, building and radar maintenance difficult. However, this is not all. Using radars for meteorological and hydrological applications in a mountainous region poses many other challenges. The radar configurations and algorithms developed for flat regions often fail to work properly over complex orography for several reasons. Moreover, the requirements of the users of radar products are also different. Mountainous regions are sparsely populated and the operators of weather radar networks often focus on the flat parts of the country, where most of the population and industry are located. However, there are exceptions. In Europe, for instance, the Alpine valleys are heavily developed and populated and there are many villages, agricultural mountain farms, winter and summer sports resorts, hydroelectric power plants, regional airports, flight routes, roads and railways. Switzerland started using radars to monitor the weather in the late 1950s [1]. The first generation of weather radars were already sited at the top of mountains and the shielding and reflection of the radar beams by terrain were major issues right from the very beginning. The country had to find suitable solutions [2]and, in recent decades, has gained experience on how to cope with the challenges and to quantitatively use radars over complex orography, see, for instance, [3; 4]. In the years 2011-2016, the Swiss C-band Doppler radar network was upgraded under the umbrella of the Rad4 Alp project to dual polarisation and extended by the addition of two new radars suited on mountain peaks in the central part of the Alpine range (Figure 1). The aims of the extension were to improve coverage of the inner-Alpine regions, to have a substantial overlapping for redundancy purposes and to extend the usage of radar in the Alps [1; 5; 6; 7]. In 2007, Austria took an important step to improve radar coverage in the Alps by installing a dual-polarisation C-band radar with a receiver over elevation design on Valluga, a mountain exposed to harsh weather, at a height of 2.8 km in the central Alps. The other Austrian radars were upgraded to dual polarisation between 2011 and 2013 [8]. Austria has played a pioneering role in the use of radar data for air traffic control in the European Alps. Figure 2 illustrates how radar data are used as guidance to define re-routing and holding loops when take-offs and landing at the Schwechat International Airport in Vienna are suspended, due to the presence of severe thunderstorms. The French weather service made a major effort to fill the gaps in radar coverage in the French Alps as part of the RHYTMME project [9]. While Switzerland opted to use an identical hardware design for all its radars, France took a different approach. To fill the gaps in the Alps in the 2010-2013 period, four X-band radars, which are smaller and cheaper than the C-band and S-band radars that have been in the flatter regions of France for many years [10], were deployed [9; 11]. The installation costs for small X-band radars are lower. On the other hand, the overall development and operating costs may be higher when different technologies are used in parallel, as each type of radar uses different algorithms and requires different spare parts and different maintenance procedures. As a result of the smaller size of an X-band radar, it is easier to find a location in a mountainous environment, and France has long-standing experience in this field [12; 13; 14]. However, signal attenuation and the transmission power limit the use of X-band technology to short ranges of a few tens of kilometres. X-band systems are accompanied by the French C-band and S-band radars for long-range surveillance.