The effects of environmental water vapor on hailstone size distributions

The aim of this work is to analyze the effects of environmental water vapor on the hailstone size distributions as observed at the ground in the four different times-of-the-day (i.e. local morning, afternoon, evening and night) through the measurements obtained by a network of hailpads posed on the plain of Friuli Venezia Giulia (Italy). The hypothesis tested is that the observed differences in the hailstone size distributions, i.e. larger hailstones during nighttime than during daytime, could be related to differences in the vertical profile of environmental mixing ratio. Data obtained by a sounding carried out every 6 h in the middle of Friulian plain are used to compute the average vertical profiles in the four times-of-the-day during the warm period (i.e. from April to September). The differences in the average vertical profile of environmental mixing ratio are then used in a simple analytical model to compute their effects on the hailstone size distributions. The results of the analytical model show that the differences in the average mixing ratio vertical profile produce effects on the hailstone size distributions that are qualitatively (i.e. stretching of hailstone size distribution) in agreement with the observations and with the right order of magnitude. Because of this qualitative and semi-quantitative agreement between observations and model results, the differences in the environmental water vapor are then considered as one of the main causes of the observed differences in the hailstone size distributions in the four times-of-the-day on the plain of Friuli Venezia Giulia.