# Weather Radar in Complex Orography
## 4 Solutions and Results
### Data Processing and Product Generation  
As mentioned in Section 3, two aspects are related to data processing in a mountainous region that require special attention, that is, the cancellation of mountain returns (ground clutter) and the proper treatment of beam shielding, visibility and VPR issues. As a result, the quality of radar measurements varies, to a great extent, in space and time, an aspect that needs to be taken into account in the generation of products for the users. If a clutter signal is superimposed onto the signals of hydrometeors, it is necessary to subtract the former from the latter to obtain a clutter-free sample of the weather. This may be done by applying advanced filter techniques to the frequency domain [148] or following recent developments, preferably to the time domain [149, 150], and this is commonly restricted to those gates that had been identified as potentially being cluttered in a preceding step using any robust clutter identification technique. Returns from mountains are often strong, which limits the usefulness of this method in a mountainous region. An alternative approach is to limit the spatial extent of clutter by using short pulses and to recover the meteorological signal from nearby clutter-free signals, see [3] and Figure 15. A variety of solutions have been proposed for the identification and cancellation of clutter signals, see, for instance, the fuzzy logic approaches presented in [151, 152], which make ample use of the dual-polarisation capability of the French network. A multiparameter decision tree is applied in Switzerland on individual 83-m gates, see Figures 15 and 18. The decision tree algorithm is built on the experience of the third radar generation [3, 153, 154] and has been refined with the deployment of the fourth generation of radars [1]. The approach makes use of polarimetric information (see the tests marked in yellow in Figure 18) but the decision tree was designed so that it would also work with single-polarisation. Cluttered gates will reliably be cancelled by the nonpolarimetric tests, should the polarimetric moments suffer from hardware instabilities, even though the number of erroneously cancelled weather signals will be somewhat higher. The order of the tests plays a crucial role and offers extra control over the performance of clutter cancellation, as each step involves taking a binary \begin{table}
\begin{tabular}{l c c c c c c c} \hline  & **2014** & **2015** & **2016** & **2017** & **2018** & **2019** & **2020** \\ \hline (A) Number of radars & \(3/4\) & \(4\) & \(5\) & \(5\) & \(5\) & \(5\) & \(5\) \\ \hline \multicolumn{8}{c}{**Central radar data processing server, time in \% of year**} \\ \hline (B) Productive time & \(>\)99.99 & \(>\)99.99 & 99.74 a  & \(>\)99.99 & \(>\)99.99 & \(>\)99.99 & \(>\)99.99 & \(>\)99.99 \\ \hline \multicolumn{8}{c}{**Radar network, time in \% of year**} \\ \hline (C) Nationwide & 98.1 & 99.8 & 99.9 & 99.9 & 99.8 & 99.8 & 99.9 \\ (D) Max 1 radar missing & 99.8 & 99.5 & 99.5 & 99.8 & 99.8 & 99.9 \\ \hline \multicolumn{8}{c}{**Single radar, operation time in \% of year**} \\ \hline \multicolumn{8}{c}{Average value of all the radars} \\ \hline (E) Preventive + faults & 98.5 & 97.5 & 97.0 & 97.5 & 97.2 & 98.5 \\ (F) Faults only & 99.5 & 98.5 & & & & \\ \hline \multicolumn{8}{c}{Radar with the poorest performance} \\ \hline (G) Preventive + faults & 97.9 & 96.6 & 95.2 & 94.2 & 91.5 b  & 95.7 \\ \hline \end{tabular}
\end{table} 
Table 2: Availability of the operational Swiss radar network, as automatically retrieved by the monitoring system. See the text. decision in a predefined sequence, which is an important difference between this and a fuzzy logic approach. There are many parameters in the decision tree, the settings of which are based on physical and statistical reasoning and careful fine-tuning over the years. A large amount of literature is available on the correction of partial shielding and VPR effects, see [155; 156; 2; 3; 116; 123] and references therein.