## 5 Results and Discussion
### The Simulated Precipitation

Table 4: Rio Croso morphometric data and morphometry of the alluvial fan including the estimation of the debris volume after the 12 August 2019 event. The presented information include original data and data retrieved from IFFI and Arpa Piemonte-SIFraP-Sistema Informativo Frane in Piemonte. Interferometric data (ERS for the time interval 1993–2001; RADARSAT for the time interval 2003–2009; ENVISAT for the time interval 2003–2010) for the Croso stream (from Arpa Piemonte-SIFraP Sistema Informativo Frane in Piemonte). Line Of Sight (LOS); Permanent Scatter (PS); Distributed Scatterer (DS). This precipitation quantity, that activated the Croso basin debris-flow, was predicted by WRF only with 15 min of delay with the value of 25.0 mm and hourly rain rate of 12.5 mm. In this case the cumulated rain was twice as much of the quantity predicted by WRF, and, as aforementioned, the convection scheme (acting at the first domain) and the microphysics scheme had underestimated the lee-side convection and convergence with the South-Alpine airmass. The adoption of a double-moment scheme for the microphysics could partially enhance the performance, even if in other cases that did not change the amount of rain considerably. The planetary boundary layer (PBL) scheme, instead, could be decisive, improving the turbulence, the fluxes, and the PBL convection. Figure 8: The 12 August 2019 precipitation event simulated with WRF, precipitation totals in 30 min from (**a**) 0530 UTC to (**f**) 0800 UTC at 30 min step. Black dots with letters are the locations of weather stations (see Table 1), while the red dot indicates the position of the bridge on the Croso stream. After the peak of 12 August morning, the rainfall stopped, and the totals ranged between 96.2 mm at Alpe Devero and 27.4 mm at Alpe Veglia (first panel in Figure 9). The maximum rainfall predicted by WRF was located easterly, near Locarno (Switzerland), some peaks (greater than 100 mm) at Valle Anzasca and Antrona (South of the study area), and some other peaks (between 80 and 100 mm) along the main Ossola Valley. From Figure 9 emerges that the WRF was able to catch the timing in total precipitation and also in its minor steps. This is a very important aspect that can allow a realistic forecast of the meteorological conditions triggering mass wasting processes affecting slopes, where high connectivity conditions are verified and lithotypes characterised by a high Landslide Susceptibility Index outcrop. The August 2019 event was preceded by a relatively long period of slope stability and without prolonged or intense rainfalls, while the slope instability event on the same basin occurred in October 2000. This latter affected the whole North-Western Italy also inducing river floods. The precipitations  
Figure 9: Comparison of simulated (blue lines) and observed (red dots) precipitations at station locations. The top-left panel represents the total simulated precipitation during the period 10 to 15 August 2019. Letters of panels and on the cumulated precipitation plot refer to Table 1. The white dot on the upper left panel indicates the position of Croso stream bridge. measured at the same locations, except Alpe Veglia that was installed on 2 August 2002, reveal the continuous nature of this event that rained from the 11 to the 16 October. Analysing the station observations, the maximum rain rate over ten-minute intervals, was 9.2 mm at Domodossola (at 1650 UTC on 13 October 2000), while at the other stations it ranged between 3.0 mm (at Formazza) to 8.8 mm (at San Domenico). The mean rain rate over 10 min intervals of the entire event (six days) ranged between 0.3 mm at Formazza to 0.6 at San Domenico. The cumulated precipitations over the entire event (six days) ranged from 354.8 mm at Formazza to 677.8 mm at San Domenico (Figure 10), so the average precipitation was from 60 to 120 mm per day. Figure 10: Comparison of simulated (blue lines) and observed (red dots) precipitations at station locations.