Influence of ice-nucleating particles on tropical convection

Convective clouds are key components of the global hydrological cycle with important roles in atmospheric energy and heat transfer. Within the mixed-phase region of convective clouds, ice crystals can form when an aerosol particle, termed an ice-nucleating particle (INP), catalyses the freezing of supercooled liquid droplets, or when existing ice hydrometeors facilitate the formation of new ice crystals via secondary ice production mechanisms (SIP). A large number of parameterisations of INP number concentrations are used in atmospheric models but the effect on convective cloud properties of INP parameterisation choice is not known. Firstly, I test the effect of INP parameterisation choice on a tropical convective cloud field in a regional model with advanced microphysics. The daytime domain outgoing radiation is sensitive to INP parameterisation choice and the differences between parameterisations can be as large as the effect of removing INP altogether. In particular, the temperature dependence of the INP parameterisation is important and determines cloud microphysical properties even in the presence of SIP via the Hallett-Mossop process. Next, I examine the effect of INP and the Hallett-Mossop process on the properties of an idealised deep convective cloud using a Latin hypercube sampling method and statistical emulation. At high INP number concentrations, the anvil ice crystal number concentration decreases sharply. At weak INP temperature dependencies, which increase INP number concentrations at warm mixed-phase temperatures, significant increases in anvil extent and anvil ice crystal size occur as a result of enhanced Hallett-Mossop ice production and more extensive cloud glaciation. Finally, INP transport across the tropical Atlantic, the region of interest for this thesis, is found to be overestimated in a global aerosol model. Overall, the results further our understanding of the effects of INP in convective clouds and indicate the importance of quantifying INP number concentrations at all mixed-phase temperatures and improving the representation of cloud glaciation in climate models.