Cloud-top microphysics evolution in the Gamma phase space from a modeling perspective

Abstract. This research employs the recently introduced Gamma phase space to study the evolution of warm cloud microphysics, to evaluate different microphysics parameterizations and to propose an adjustment to bulk schemes for an improved description of cloud droplet size distributions (DSDs). A bin parameterization is employed to describe the main features of observed cloud-top DSD paths in the Gamma phase space. The modeled DSD evolution during the warm cloud life cycle is compared to the results obtained from HALO airplane measurements during the ACRIDICON-CHUVA campaign in the Amazon dry-to-wet season transition. The comparison shows an agreement between the observed and simulated trajectories in the Gamma phase space, providing a suitable qualitative representation of the DSD evolution. The degree of similarity between the trajectories is defined by the conditions of the environment, such as the aerosol number concentration, which modify the DSD evolution through modulation of its driving forces. The modeled DSD properties were also projected in the N_d − D_eff space to obtain further insights into their life cycle. Two different bulk microphysics parameterizations were evaluated regarding the evolution of the DSD and using the bin scheme as a reference. The results show the weakness of bulk schemes in representing trajectories in the Gamma phase space; thus, a new closure is proposed for better comparisons to the reference. The new closure resulted in an improvement in the representation of the DSD evolution, cloud droplet effective diameter and rain mixing ratio.