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In this article, we report the first investigation over time of the atmospheric conditions around terrestrial gamma-ray flash (TGF) occurrences, using GPS sensors in combination with geostationary satellite observations and ERA5 reanalysis data. The goal is to understand which characteristics are favorable to the development of these events and to investigate if any precursor signals can be expected. A total of 9 TGFs, occurring at a distance lower than 45 km from a GPS sensor, were analyzed and two of them are shown here as an example analysis. Moreover, the lightning activity, collected by the World Wide Lightning Location Network (WWLLN), was used in order to identify any links and correlations with TGF occurrence and precipitable water vapor (PWV) trends. The combined use of GPS and the stroke rate trends identified, for all cases, a recurring pattern in which an increase in PWV is observed on a timescale of about two hours before the TGF occurrence that can be placed within the lightning peak. The temporal relation between the PWV trend and TGF occurrence is strictly related to the position of GPS sensors in relation to TGF coordinates. The life cycle of these storms observed by geostationary sensors described TGF-producing clouds as intense with a wide range of extensions and, in all cases, the TGF is located at the edge of the convective cell. Furthermore, the satellite data provide an added value in associating the GPS water vapor trend to the convective cell generating the TGF. The investigation with ERA5 reanalysis data showed that TGFs mainly occur in convective environments with unexceptional values with respect to the monthly average value of parameters measured at the same location. Moreover, the analysis showed the strong potential of the use of GPS data for the troposphere characterization in areas with complex territorial morphologies. This study provides indications on the dynamics of con-vective systems linked to TGFs and will certainly help refine our understanding of their production, as well as highlighting a potential approach through the use of GPS data to explore the lightning activity trend and TGF occurrences. publishedVersion

Continuous estimates of the vertical integrated precipitable water vapor content from the tropospheric delay of the signal received by the antennas of the global positioning system (GPS) are used in this paper, in conjunction with the measurements of the Meteosat Second Generation (MSG) spinning enhanced visible and infrared imager (SEVIRI) radiometer and with the lightning activity, collected here by the ground-based lightning detection network (LINET), in order to identify links and recurrent patterns useful for improving nowcasting applications. The analysis of a couple of events is shown here as an example of more general behavior. Clear signs appear before the peak of lightning activity on a timescale from 2 to 3 h. In particular, the lightning activity is generally preceded by a period in which the difference between SEVIRI brightness temperature (TB) at channel 5 and channel 6 (i.e., ∆TB) presents quite constant values around 0 K. This trend is accompanied by an increase in precipitable water vapor (PWV) values, reaching a maximum in conjunction with the major flash activity. The results shown in this paper evidence good potentials of using radiometer and GPS measurements together for predicting the abrupt intensification of lightning activity in nowcasting systems.

The NASA/JAXA Global Precipitation Measurement (GPM) Core Observatory (CO) carries, for the first time, a dual-frequency Precipitation Radar (DPR). The DPR provides insights into the three-dimensional structure of precipitating clouds and intensity and is currently considered the main calibration instrument for radiometer derived global precipitation estimates. The DPR employs three scanning modes: Matched Scan (MS), Normal Scan (NS) and High Sensitivity scan (HS). The Ka- and Ku-band radar (MS) footprints of the inner swath consist of 25 angle bins with a vertical resolution of 125 m while the Ku-band radar (NS) covers the full swath with 49 angle bins with the same vertical resolution. The HS Ka-band radar footprints are interlaced with matched Ku/Ka-band footprints and consist of 24 angle bins with range sampling at 250 m. The swath widths of Ka- and Ku-band radars are 120 km and 245 km, respectively, while the diameter of both Ka- and Ku-band footprints at nadir is 5 km about. While the precipitation retrieval algorithms of the Ka- and Ku-band radar Matched (MS) footprints provide a double-frequency based output, the Ku-band Normal Scan (NS) and Ka-band High Sensitivity scan (HS) precipitation estimates for same footprints, are single-frequency based. The goal of the present works is to carry out an intercomparison of the single- and double-frequency DPR algorithms by considering the respective outputs over the Mediterranean area during rain events. A number of DPR related variables have been considered (i.e. measured and corrected reflectivity, surface rainfall rate, mean mass diameter, normalized intercept parameter, liquid and ice water content vertical profiles) to perform the analysis. The data have been categorized for surface type (land and sea) and precipitation type (stratiform and convective). The results show the differences in the performance of the single- and double-frequency algorithms as function of the surface and precipitation type. The vertical profiles of the single- and double-frequency based retrieved parameters are compared in order to assess their variability along the precipitating column.

The Italian radar network is currently composed by 20 C-band and 2 X-band systems, managed by 11 administra- tions, able to cover most of the country. The product generation at national level is carried out by the Department of Civil Protection (DPC) that currently manages 7 C-band and 2 X-band systems, all with dual-polarization capa- bility. The national mosaic provides an estimation of the precipitation intensity with a five minutes time resolution over a equi-spaced 1 km grid. A Quality Index (QI) map is associated to the rainfall rate map, taking into account the radar artifacts, the partial beam blockage, the height of measurements with respect to the freezing layer, the beam broadening and the rain path attenuation. The geostationary Meteosat Second Generation (MSG) Spinning Enhanced Visible and Infrared Imager (SEVIRI) provides high-quality measurements across twelve visible (VIS) and infrared spectral (IR) bands. Several tech- niques have been developed exploiting VIS or IR observations (individually or in a combined way) to relate the cloud features to the surface precipitation. One of them is the blending Rapid Update (RU) technique combining IR observations with precipitation estimates obtained from spaceborne passive microwave radiometers, and used operationally within the EUMETSAT H SAF program. Within the collaborative agreement between DPC and the Institute of Atmospheric Sciences and Climate (ISAC) of the National Research Council (CNR) a new blended algorithm has been developed that combines the ground radar precipitation estimates and the IR observations from SEVIRI to provide a precipitation map over the whole Italian territory every 15 minutes, at 1 km spatial resolution. The goal is to integrate the low QI ground radar mea- surements with the precipitation estimates inferred from IR observations. The IR precipitation estimates are based on regression curves built from a training dataset based on coincident IR and high QI radar observations. The present work shows the preliminary results of the blended algorithm. The validation is carried out through a comparative study with the Italian rain gauges network.