Aerosol-cloud interactions in mixed-phase convective clouds. Part 1: Aerosol perturbations
Authors: Miltenberger Annette K. , Field Paul R. , Hill Adrian A. , Rosenberg Phil , Shipway Ben J. , Wilkinson Jonathan M. , Scovell Robert , Blyth Alan M. 
Published: September 2017

Abstract. Changes induced by perturbed aerosol conditions in moderately deep (cloud top at about 5 km) mixed-phase convective clouds developing along sea-breeze convergence lines are investigated with high-resolution numerical model simulations (grid spacing of 250 m). The simulations utilise the newly developed Cloud-AeroSol Interacting Microphysics module (CASIM) for the Unified Model, which allows for the representation of the two-way interaction between cloud and aerosol fields. Simulations are evaluated against observations collected during the COPE field campaign over the southwestern peninsula of the UK in 2013. The simulations compare favourably with observed thermodynamic profiles, cloud-base cloud droplet number concentrations (CDNC), cloud depth, and radar reflectivity statistics. Including the modification of aerosol fields by cloud microphysical processes in the simulations improves the match to observed cloud-base CDNC, increases the CDNC variability and leads to a larger decrease of CDNC with height above cloud base. However, it also reduces the average cloud size and cloud top height, which is less compatible with observations. Before clouds become organised along the sea-breeze convergence lines, precipitation is suppressed by increasing aerosol due to less efficient precipitation production by warm-phase microphysics. The precipitation suppression is less evident if aerosol processing is taken into account. After the sea breeze convergence zone is established, accumulated precipitation from the on average deeper and wider clouds increases with aerosol concentrations as long as cloud top heights are not limited by an upper level stable layer. The precipitation enhancement is controlled by changes in condensate production and precipitation efficiency. Enhanced condensate production in high aerosol scenarios is related to higher vertical velocities in the convective cores, i.e., convective invigoration, and stronger latent heating below the 0 °C level, while changes in latent heating in the mixed-phase region are negligible. Perturbed aerosol concentrations alter the cloud field structure with fewer larger cells developing in high aerosol environments, but inducing only small changes in cloud fraction. It is hypothesised that the stronger latent heating from convection is related to the changes in the cloud field structure reducing the mixing of environmental air into the convective core. For very high aerosol concentrations, the translation of convective invigoration into deeper clouds and enhanced precipitation is limited by thermodynamic constraints. The aerosol-induced changes in shallow warm-phase clouds prior to the development of a strong convergence zone is consistent with ideas based on parcel models. However, the precipitation response of the deeper mixed-phase clouds along well-established convergence lines suggest that when clouds begin to interact with the pre-existing thermodynamic environment and modifications to the cloud field structure occur, i.e., processes other than microphysics effect the cloud evolution, and the precipitation behaviour can be opposite to predictions from parcel models.