Towards parameterising atmospheric concentrations of ice-nucleating particles active at moderate supercooling

Abstract

A small fraction of freezing cloud droplets probably initiates much of the precipitation above continents. Only a minute fraction of aerosol particles, so-called icenucleating particles (INPs), can trigger initial ice formation at 15 C, at which cloud-Top temperatures are frequently associated with snowfall. At a mountaintop site in the Swiss Alps, we found that concentrations of INPs active at 15 C can be parameterised by different functions of coarse (2 um) aerosol particle concentrations, depending on whether an air mass is (a) precipitating, (b) nonprecipitating, or (c) carrying a substantial fraction of dust particles while non-precipitating. Consequently, we suggest that a parameterisation at moderate supercooling should consider coarse particles in combination with air mass differentiation.