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

摘要

Anew scheme of droplet nucleation at cloud base is implemented into theHebrew University Cloud Model (HUCM) with spectral (bin) microphysics. Inthis scheme, supersaturation maximum near cloud base iscalculated using theoretical results according to which  ∼ , where is the vertical velocity at cloud base and is droplet concentration. Microphysical cloud structure obtained inthe simulations of a midlatitude hail storm using the new scheme iscompared with that obtained in the standard approach, in which dropletnucleation is calculated using supersaturation calculated in grid points.The simulations were performed with different concentrations of cloudcondensational nuclei (CCN) and with different shapes of CCN size spectra.It is shown that the new nucleation scheme substantially improves thevertical profile of droplet concentration shifting the concentration maximumto cloud base. It is shown that the effect of the CCN size distributionshape on cloud microphysics is not less important than the effect of thetotal CCN concentration. It is shown that the smallest CCN with diametersless than about 0.015 µm have a substantial effect on mixed-phase andice microphysics of deep convective clouds. Such CCN are not measured bystandard CCN probes, which hinders understanding of cold microphysicalprocesses.