## 5 Results and Discussion
### Short-Term Analysis  
In this section we discuss the tropospheric delay parameters and PWV retrieved from the analysis of GNSS data with different techniques. We employed GPS-derived PWV data to track and investigate the characteristics of some thunderstorm occurrences and their spatial and temporal evolution, using a selection of stations belonging to NeVoCGPS network. In detail, we report on the tropospheric delay parameters and PWV on the occasions of three heavy rain episodes: 23 and 26 September 2019 [Day of the Year (DoY): 266, 269) and 16 November 2020 [DoY: 322] (Table 2). All these thunderstorms had a common peculiar evolution for the region, where most of the storms followed a nearly west-to-east trend, entering the Campania region through its Tyrrhenian side. For the first two events listed in Table 2, we retrieved the PWV analyzing an 8-day-long GPS data sample (red and yellow squares in Figure 1), spanning 19-26 September 2019. For the third thunderstorm, the GNSS data analyses covered an interval of 5 days, from 14 to 18 November 2020. The analyses were performed with GAMIT/GLOBK and the aid of the sh_metutil script [33], as described in the previous paragraphs. For the last event we compare the results coming from different techniques of GNSS data analysis, namely the GAMIT/GLOBK [33] and GIPSY/OASIS-II software, in a PPP strategy according to the solutions provided by NGL [34] and CSRS-PPP [32]. For one site of the network (TAI1), equipped with a GNSS station, a comparison of PWV as retrieved for GPS, GLONASS and Galileo satellite constellations is provided too. The GAMIT/GLOBK processing was done by including meteorological data (pressure, temperature, humidity), acquired at a meteorological station collocated with MAFE, with a sampling step, every 10 min. For all the sites for which no meteorological observations were available, values from GPT2 and VMF1 models were used [26; 27]. To avoid correlation of the delay parameters in the small regional network [37], the remote ISS station (MATE-Matera) and related meteorological data acquired at a GPS-located station were also included in the tropospheric delay retrieval. The humidity above MAFE (Figure 7), as deduced from the PWV, increases approximately 24-36 h before precipitation events, probably when a low-pressure system enters the area above the station. Then, for about 10-12 h for the 23 September (DoY 266) event and for about 6-8 h for the 26 September (DoY 269) event, the PWV fluctuated about a value. Finally, a clear and steep increase in PWV values was observed a few hours before the two storms (Figure 7) followed by a sharp decrease. Clear anomalies can be seen, for different satellites, in the temporal variation of the phase residuals iono-free LC component at TAI1 and MAFE stations (see Supplemental Material Figure S1). A time interval when the amplitude of the residuals is much higher than before and after the thunderstorm can be clearly seen. It indicates that the increase of PWV had significantly disturbed the propagation of GPS signal. Figure 7: PWV at MAFE GPS station retrieved from GAMIT/GLOBK analysis (blue curve) and rainfall amount on the occasions of two thunderstorms (DoY 266, 269 = 23, 26 September 2019). This temporal evolution of PWV has already been observed by several authors (e.g., [12; 46]). That is, thunderstorms occur after an accumulation of PWV and, during the hours immediately following the event, a rapid decrease in PWV is observed. During the night between 16-17 November 2020, about 30 mm of rain fell on the city of Naples in less than one hour and more than 70 mm in about four hours (Table 2), according to the observations collected at the meteorological station collocated with MAFE (Figures 8 and 9). The main thunderstorm event was preceded by few minor raining episodes that occurred in the morning (7:00 \(\div\) 12:00 UT). The evolution of the main thunderstorm over the Napoli urban area began around 23:00 UT on 16 November and continued through the early hours of 17 November (Figure 8).