Thermodynamic factors influencing bimodal spectrum formation in cumulus clouds

The mechanism of droplet spectrum formation in cumulus clouds is studied using a microphysical air parcel model with a detailed description of diffusional growth of aerosol particles (AP) and cloud droplets. The model allows simulation of the development of wide bimodal droplet size spectra due to in-cloud nucleation of new droplets on aerosol particles that remain nonactivated at the cloud base and ascend within cloud updrafts along with cloud droplets. New droplets are nucleated at a distance of a few hundred meters above the cloud base, where supersaturation in ascending cloud parcels exceeds the supersaturation maximum near the cloud base. In numerical experiments, the thermodynamic conditions are studied under which the bimodal droplet size distributions can form in convective clouds. It is shown that these conditions really take place in the atmosphere. The effect of thermodynamic conditions on the shape of droplet spectra is illustrated in numerical experiments with different values of relative humidity in the sub-cloud layer and different values of the temperature gradient in the cloud layer. The mechanism of in-cloud nucleation of new droplets helps to explain observations such as the high spatial variability in cloud spectra and the increase in the fraction of samples with bimodal spectra with an increase in cloud layer instability observed by Warner [J. Atmos. Sci. 26 (1969) 1049].