GNGTS 2013 - Atti del 32° Convegno Nazionale

Results and conclusions. Al- though the WV and PW content can- not be directly linked to the precip- itation amounts, nevertheless some interesting correlations between the time changes of PW and rainfall can be envisaged (Fig. 1d). We fo- cused on the rainfall event occurred on October 12 th 2012 (Julian day: 276), when a cold front coming from Tyrrhenian Sea and moving towards SE has struck the South of Italy. A double thunderstorm cluster enucle- ates since 07:00 UTC breaking out a heavy event with large rain amount concentrated in a very short time span; about 40 mm of rain in about 50 minutes were detected at MAFE station. Infrared channel of the Me- teo satellites (MSG) in the Euro-At- lantic area allowed clearly following the front evolution and migration. Aimed at detecting any traces in the GPS signals, we analysed the phase residuals for each visible GPS satel- lite at MAFE station above the eleva- tion cut-off angle. In Fig. 2 the time series of the phase residuals around 7:00 UTC are plotted; a “normal” phase plot showing the typical in- crease of the phase scattering of the radio signal at low elevation is even drawn as a reference (see PRN1 in Fig. 2a). The large phase residuals when the satellites transit at high elevation angles (Fig 2b, …) clearly indicate that the observed delays are likely due to tropospheric effects. To better follow the migration of the cold front and the related WV fluxes, the PW time series were even computed for MATE and CAGL IGS stations (Fig. 3). Unfortunately no rain data were available for MATE station. An increase of about 13 mm peak to peak was observed at MAFE in the PW accumulating during the 24 hours preceding the pouring rain event. Co- herently to the evolution shown by the sequence of the satellite radio images, at CAGL station no rainfall was recorded and no significant variation in the PW can be envisaged. On the con- trary, in Matera (SE of Napoli) an increase of the PW is observed but it appears to be delayed of about 4 hours in respect to MAFE. The GPS observations in the vicinity of the most intense rainfall area are all consistent with the WV depicted in the satellite meteorological radar obser- vations. Moreover we found that the maximum amplitude of the wet gradients is not correlated with the rain itself but with rapid changes in both the orientation and the amplitude of the wet gradients. Our study confirms that, thanks to the nowadays dense covering of national networks, the integration of ground based and satellite meteorological data with GPS technique is a useful tool for monitoring atmospheric parameters and for capturing their temporal variability. Ac- tually we acknowledge that raining phenomena are a very complex physical process, hence is naive claiming that PW alone could be enough for rainfall forecast. Nevertheless in a mul- Fig. 3 – Precipitable water (PW) assessed at MAFE and two remote CGPS stations on the occasion of the extreme rainfall event occurred on 2012 October 12 th . 204 GNGTS 2013 S essione 3.3