The role of CCN in precipitation and hail in a mid-latitude storm as seen in simulations using a spectral (bin) microphysics model in a 2D dynamic frame

A hail storm at Villingen-Schwenningen, southwest Germany, on 28.06.2006 was simulated using the Hebrew University Cloud Model (HUCM) with spectral (bin) microphysics. The model allows the simulation of hail stones with diameters up to 6.8 cm. To investigate whether the amount of hail is sensitive to atmospheric instabilities, the simulations were performed for two different temperature gradients within the boundary layer. The response of precipitation, the hail mass and hail size distribution to aerosol was investigated in the simulations with cloud condensation nuclei (CCN) concentrations ranging between 100 cm− 3and 6000 cm− 3(at the supersaturation of 1%). An increase in the surface temperature by one degree leads to an increase in accumulated rain by ~ 80% and nearly doubles the mass of hail falling to the surface. An increase in CCN concentration from 100 cm− 3 to 3000 cm− 3 leads to a certain increase in accumulated rain and to a dramatic increase in the hail mass, as well as to the increase in the hail diameter from a few mm to 1–4 cm. The mechanisms by means of which aerosols affect precipitation and hail stones size are discussed. It is shown that formation of hail increases the precipitation efficiency of deep convective clouds.