The importance of uncertainties in ice microphysics in determining the effect of aerosol on mixed-phase continental convective clouds

Deep convective clouds play an important role in the transfer of heat, moisture, trace gases and aerosols from the boundary layer to the upper troposphere. The interaction between atmospheric aerosols and the microphysics of mixed-phase convective clouds are not well understood. In addition, the processes of primary and secondary ice formation which may play an important role in the development of deep convective clouds have large uncertainties associated with them. We have used the University of Leeds Model of Aerosols and Chemistry in Convective Clouds (MAC3) 2-D, axisymmetric, non-hydrostatic, bin-resolved cloud model to examine the impact of aerosol changes on the development of continental mixed-phase convective clouds using a range of ice parameterisations. MAC3 carries four hydrometeors with thiry four mass-doubling size bins per hydrometeor and includes detailed treatment of both warm and cold microphysical processes. The model aerosol distribution is represented by forty three size bins. We have simulated continental convective clouds from four different sites with a wide range of realistic aerosol loadings and modifications to parameterisations of primary ice formation and ice multiplication. We have also investigated whether the responses of deep continental convective clouds to changes in aerosol and ice microphysics are sensitive to the initial thermodynamic conditions. Predictably, we find that an increase in the aerosol loading leads initially to more numerous and smaller cloud drops. Subsequent development of the cloud in the presence of enhanced aerosol is characterised by weaker latent heating as a result of increased evaporation, slower drop freezing and reduced riming rates due to the presence of smaller drops. In general, an increase in aerosol loading also results in a consistent decrease in both the quantity and intensity of precipitation and a reduction in cloud top height, irrespective of the details of the ice parameterisations used.