ICE NUCLEATION IN THE UPPER TROPOSPHERE - SENSITIVITY TO AEROSOL NUMBER DENSITY, TEMPERATURE, AND COOLING RATE

We have investigated the processes that control ice crystal nucleation in the upper troposphere using a numerical model. Nucleation of ice r esulting from cooling was simulated for a range of aerosol number dens ities, initial temperatures, and cooling rates. In contrast to observa tions of stratus clouds, we find that the number of ice crystals that nucleate in cirrus is relatively insensitive to the number of aerosols present. The ice crystal size distribution at the end of the nucleati on process is unaffected by the assumed initial aerosol number density . Essentially, nucleation continues until enough ice crystals are pres ent such that their deposition growth rapidly depletes the vapor and s huts off any further nucleation. However, the number of ice crystals n ucleated increases rapidly with decreasing initial temperature and inc reasing cooling rate. This temperature dependence alone could explain the large ice crystal number density observed in very cold tropical ci rrus.