## 4 Validation and Results
### Livorno Test Case  
Figures 8 and 9 summarize the QPF values provided by the different modelling experiments for the 12 h time period from 18 UTC on 9 September 2017 to 06 UTC on 10 September 2017. Figure 8 refers to GFS-driven cases, while Figure 9 refers to IFS-driven cases. The MODE analysis results for the three different thresholds are reported in Table 10 (24 mm), Table 10 (48 mm) and Table 10 (72 mm). In the case of the GFS-driven experiments, the best performing run is WIND+SM+ZTD_GNSS3h_list (3 DVAR wind at the analysis time, SM assimilation at the analysis time, 3 DVAR 3h cycling with the 1-minute ZTD observations temporally nearest to the analysis time) with a total of 15 best scores (Tables 10 and 11). Conversely, in the case of the IFS-driven experiments, the best performing one is WIND (3 DVAR at the analysis time, namely 18 UTC on 08 September 2018), with a total of 16 best scores. Looking at the different rain depth threshold values, the behaviour of the assimilation of different variables can be discussed. For example, the assimilation of SST performs well at the lowest threshold, which is in line with the findings that the average SST controls the larger scale precipitation [46; 47; 48]. In fact, these works show that a good representation of the SST is important for a good precipitation forecast, indicating, in particular, that the average SST in the area near the convective event controls the total amount of rain (the warmer the sea, the higher the volume of rain). Other recent works, such as Reference [49], show that the small scales in the SST field may affect the surface wind response and, thus, the formation of convergence lines, that can drive upward motion and precipitation [50]. One of the reason why the SST experiments do not show good performances at the high rain intensity might be the fact that SST is retrieved from Sentinel-3 observations in the optical bands. This means that the presence of clouds limits the extent of the observed area, especially in the vicinity of the convective activity. For the higher thresholds, thus, the assimilation of the SST does not produce positive results, probably because of the lack of fine spatial structure in the SST field near the convective activity. Assimilating the wind field shows very good values of the maximum rainfall values and gets better and better the higher value of the threshold, meaning that the dynamical features are of crucial importance to model the rain peaks. Also the location of the rainfall area gets more realistic, both in terms of mean position and in terms of extent, with respect to the OL when assimilating the wind field. Assimilating the ZTD only seems to improve the orientation of the rainfall area consistently through the cases analyzed. This is probably related to a more correct spatial distribution of the water vapor field with respect to the OL case. Moreover, an improvement in the maximum rainfall values and in the extent of the rainfall area is observed, also at the higher thresholds. Among the runs that assimilate only one variable, SST has the highest number of best scores for the low threshold, WIND has the highest number of best scores for the two higher thresholds with GFS; instead, with IFS, the OL has the highest number of best scores for 24 mm and WIND dominates for 48 mm and 72 mm: this confirms the importance of the wind structure in the simulation of an intense rain event. The rather heterogeneous response to the assimilated Sentinel and GNSS derived variables by the GFS and IFS-driven experiments is most probably due to the difference in their respective initial conditions fields. For example, Figure 10 represents the difference map between the IFS-driven SM experiment and the Sentinel-derived SM (SMIFS-SMS1, on the right) compared to the difference between the GFS-driven SM experiment and the Sentinel-derived SM (SMGFS-SMS1, on the left). Furthermore, the SM mean value over the Area Of Interest (AOI) hit by the event has been calculated for the IFS and GFS driven experiments and the Sentinel observation.