On the diurnal cycle of deep moist convection in the southern side of the Alps analysed through cloud-to-ground lightning activity

In this work, eight years of cloud-to-ground lightning data are used as a proxy observable to describe the hourly frequency of deep moist convection occurrence over an area characterized by complex geography in the southern side of the Alps (Friuli Venezia Giulia). The study area is divided in eight sub-zones, defined according to the climatic differences of the southern side of the Alps, in particular those related to the precipitation regime. In the eight sub-zones, the hourly frequency of cloud-to-ground lightning shows two different behaviours: bimodal and single-mode. Single-mode hourly frequencies, in turn, can be divided into two more classes: afternoon maximum and evening maximum. A conceptual model for the explanation of the observed features is proposed. This conceptual model takes into account the atmospheric instability, which has a maximum during afternoon, and the convection forcing represented by the switch in the breezes regime, which is stronger in the late morning and evening. The proposed conceptual model is coherent with the complementary meteorological parameters taken into account (hourly rain, temperature and wind distribution and speed) and it is capable to describe, at least qualitatively, the observed behaviours in cloud-to-ground lightning hourly distribution.