# Hailstorm Detection by Satellite Microwave Radiometers
## 2 Data Selection  
The database used in this study is based on the AMSU-B/MHS (hereafter MHS) measurements from satellite overpasses of intense hailstorms events over three selected continental areas: Europe (EU), United States (US) and Argentina (AR). The radiances were used as a first quantification of the TB reduction due to hail both in terms of different environmental conditions (global geographic domain) and of the size distribution of ice grains. Additional data based from AMSU-B overpasses co-located with surface hail reports assembled at NOAA's Storm Prediction Center (SPC) and discussed in [29] were used on one side to improve the quantification of hail signature and on the other to validate the algorithm performances. Note that no intersatellite correction between AMSU-B and MHS radiometers was done since the TB differences between the channels of the two radiometers are usually less than 5 K and do not impact dramatically on the performances of the hail detection model [5]. Table 1 lists the events for which the satellite overpasses were very close in time with hailstorms as recorded by the official reports. The events were selected to evaluate the magnitude of the TB depression due to the scattering by hail clouds. Ice aggregates scatter the upwelling radiation by progressively reducing the radiation to the satellite as a function of their size distribution and vertical  
span. Therefore, to get a first evaluation of the net impact of hail on the MHS measurements a selection of thunderstorms on the basis of their geographical location was done. Furthermore, the events were also chosen on the basis of hail size as described by the corresponding official reports. The first step of the analysis was the quantification of the TB reduction due to the scattering from hail aggregates. As expected, the mapping of the satellite signal in the vicinity of the storm revealed that the TBs rapidly drop down when approaching the storm cores. Figure 1 shows the TB of the MHS channels (184, 186, 190 GHz stand for 183.31 \(\pm\) 1, 3, 7, respectively) measured along a transect crossing the hailstorm cores at the time of the events listed in Table 1. Several differences characterize each storm. The first four plates display a lower overall decrease of TBs for the EU events than in the rest of the cases. All MHS frequencies measure TBs \(>\) 200 K with the exception of Bosnia-Herzegovina where the storm signal drops down to 180 K. This is strictly connected to the hail size increasing from 2 to 3 cm for Bulgaria and Germany up to 5-8 cm observed over Bosnia-Herzegovina as reported by the European Severe Storms Laboratory (ESSL) report (not shown). As observed by [PERSON] et al. (2019) and [PERSON] (2019), MHS frequencies are very sensitive to the hail bulk and size distribution showing high skills in distinguishing the different concentration of ice into the clouds. Thus, consistent with theory, the magnitude of TB reduction grows with increasing observation frequencies as a function of effective size distribution of ice into the satellite instantaneous field of view (IFOV). Therefore, a high concentration of very large hail strongly depresses TBs at all MHS frequencies revealing not much difference moving from 90 to 157 and 190 GHz. On the contrary, smaller aggregates enhance TBs at the highest frequencies showing low perturbation in the lowest ones.