Application of a new scheme of cloud base droplet nucleation in a spectral (bin) microphysics cloud model: sensitivity to aerosol size distribution

A new scheme of droplet nucleation at cloud base is implemented into the Hebrew University Cloud Model (HUCM) with spectral (bin) microphysics. In this scheme, supersaturation maximum Smax near cloud base is calculated using theoretical results according to which Smax ∼ w3∕4Nd−1∕2, where w is the vertical velocity at cloud base and Nd is droplet concentration. Microphysical cloud structure obtained in the simulations of a midlatitude hail storm using the new scheme is compared with that obtained in the standard approach, in which droplet nucleation is calculated using supersaturation calculated in grid points. The simulations were performed with different concentrations of cloud condensational nuclei (CCN) and with different shapes of CCN size spectra. It is shown that the new nucleation scheme substantially improves the vertical profile of droplet concentration shifting the concentration maximum to cloud base. It is shown that the effect of the CCN size distribution shape on cloud microphysics is not less important than the effect of the total CCN concentration. It is shown that the smallest CCN with diameters less than about 0.015 µm have a substantial effect on mixed-phase and ice microphysics of deep convective clouds. Such CCN are not measured by standard CCN probes, which hinders understanding of cold microphysical processes