# Weather Radar in Complex Orography
## 1 Introduction
Figure 1: Map of the operational Swiss radar network which consists of 5 radars: La Dôle and Albis (since 1959, 1 st generation), Monte Lema (since 1993, 3 rd generation), Pointe de la Plaine Morte and Weissfulbugfel (since 2014/2016, 4 th generation). The heights of the radar sites and selected mountain peaks are in metres above sea level. A number of radars in Italy are also located in complex orography, for instance [15], and several studies have dealt with the challenges of using radars for precipitation measurements over mountainous regions [16; 17; 18; 19; 20; 21; 22]. In 1954, the US Air Force installed the first operational radar designed for meteorological applications in the US, the CPS-9 X-band system, shortly followed by the WSR-57 S-band system of the national weather service, which was first deployed in 1959. Today's operational WSR-88D network was installed in the nineties [23; 24] and upgraded to dual polarisation in 2013 [25]. The driving force was protection against severe weather, such as supercells, hail and tornadoes, which typically occur over the mostly flat regions of the US. Radar applications in the mountainous regions were not the focus of the initial design [26; 27], but have been receiving increasing attention lately, see, for instance, [28; 29; 30]. The optimal choice of a radar site is intricate on the Pacific coast. It is necessary to place a radar at low heights close to the coast to see the weather over the ocean and support traffic at sea. However, this results in limited radar coverage over and behind the mountain ranges along the coast due to blockage of the beam. A similar situation can be found in South Korea and in Taiwan, which have steep mountain ranges close to the coast, and therefore require solutions to deal with severe beam shielding [31] and ground clutter [32; 33]. Several studies have demonstrated the value of polarimetric Doppler radar data for the monitoring of rainfall rates and wind fields over the complex orography of Taiwan and South Korea [34; 35; 36; 31]. As part of its new-generation weather radar network, the Chinese weather service opted to use S-band radar technology in eastern China, which is mainly flat, and C-band radars in the less populated and more mountainous western part of the country [37]. The number of radars decreases as the altitude of the terrain increases, thus resulting in lower radar coverage over the mountainous regions [37], an issue of ongoing research [38; 39; 40]. Figure 2: Re-routing and holding loops of air traffic during thunderstorms over the eastern Austrian Alps on 17 July 2010, 20:15. The aircraft trajectories correspond to a 10-min period. Only reflectivity values above 35 dBZ are shown; yellow corresponds to \(>\)40, red to \(>\)50 dBZ. Image courtesy of [PERSON]. Other countries, such as Australia, Canada, Japan and New Zealand [41], have dealt with the problems of deploying and using radars in mountainous regions. Some countries have just started recently, such as Nepal, which installed its first weather radar in 2019 [42]. Three inexpensive X-band radars have been installed in the high Andes of southern Ecuador, to support local authorities in hazard prevention [43]. Many solutions have been proposed in the literature concerning the use of radars in complex orography, but only a small percentage of them have actually been implemented and used in an operational manner. The fact that the mountainous regions in many countries are sparsely populated, and not the target area of highest priority for radar operators, discourages the development of solutions for operational radar usage over mountains. Complex orography is a challenge for radar applications, but it is also an opportunity. When analysing radar measurements over the orography, patterns are found which, depending on the atmospheric conditions, repeat themselves. This is due to the influence of the terrain on precipitation mechanisms [44; 45; 46].