Sensitivity of raindrop formation in ascending cloud parcels to cloud condensation nuclei and thermodynamic conditions

This paper uses a 2000-bin spectral microphysics cloud parcel model to investigate the effects of aerosol particles on droplet spectrum evolution and warm rain formation in ascending cloud parcels under maritime, intermediate, and continental unstable thermodynamic conditions. Cloud parcels of different cloud depth are simulated for each thermodynamic condition. Concentration and size distribution of cloud condensation nuclei (CCN) were varied within a wide range for each cloud depth. Cloud parcels were divided into three groups with respect to the relationship between cloud depth H_C and the distance of the collision triggering level H_col above cloud base. The sensitivity of precipitation to CCN variations is quite different for the different groups. Parcels of the first group (H_c < H_col) do not precipitate. Precipitation from cloud parcels belonging to the second group, in which collisions start at a small distance below the maximum cloud-top height (H_c ≃ H_col), turns out to be highly sensitive to the variation of CCN concentration and size distribution. For these parcels an increase in the concentration of large CCN can result in an increase in precipitation amount by several factors. Many cloud parcels arising under continental and intermediate conditions, as well as not very deep maritime clouds, can be assigned to this group. The precipitation amount from cloud parcels belonging to the third group (H_c ≫ H_col), represented by deep tropical clouds, turns out to be insensitive to CCN distribution and droplet concentration within a wide range of their variations. Comparable effects of small and large CCN on precipitation formation are analysed for each group of cloud parcels. These results are closely related to the problem of rain enhancement via hygroscopic seeding. It is shown that there is a certain range of cloud depths within which cloud seeding can potentially lead to a significant (several times) increase in rain within ascending parcels. Rain formed in cloud parcels with depths beyond this range is only slightly sensitive to the CCN concentrion.