Evaluating Aerosol-Constrained Bulk- and Bin-Simulated Drop Size Distributions Against Aircraft Observations of Tropical Cumulus Congestus

The evolution of cumulus congestus within tropical oceanic and maritime environments is modulated by the interaction of convective dynamics, liquid- and ice-phase microphysical processes, aerosol loading, and entrainment of ambient environmental air. Characterizing this evolution requires robust observational constraints of aerosol properties and cloud macrophysics and microphysics. The NASA Cloud, Aerosol and Monsoon Processes Philippines Experiment (CAMP²Ex) field campaign in 2019 targeted growing cumulus congestus clouds using airborne in situ and remote sensing platforms. In situ aircraft microphysical measurements and retrievals from the Research Scanning Polarimeter (RSP) both show that cloud droplet number concentrations decrease with increasing cloud top height and droplet effective radii increase, and droplet size distributions (DSDs) broaden with height. These observed components are consistent with an active collision-coalescence process that produces millimeter-sized drops, onsetting warm-rain formation. Here we perform large eddy simulations (LES) at 100-m horizontal grid spacing of a CAMP2Ex case study (RF14, 9/25/2019) using bulk and bin microphysics schemes with periodic boundary conditions. Observed aerosol measurements from the Fast Integrated Mobility Spectrometer (FIMS) are used as input to represent three lognormal modes with fixed geometric mean diameters, standard deviations, and hygroscopicity parameters, and height-varying number concentrations. Application of large-scale vertical motion profiles derived from nested mesoscale model simulations indicate the importance of widespread convergence beneath a persistent subsidence layer in the upper troposphere, as well as surface forcing in this particular case. Both microphysics schemes are able to reproduce realistic cloud base droplet number concentrations as well as the observed systematic decrease in droplet number concentration with cloud top height. However, the quantitative degree of droplet number decrease differs, likely by a magnitude that can be explained by expected observational (case study) and model parameter uncertainties. We quantify the effect of collision-coalescence (versus entrainment) by turning it off in bulk and bin schemes. Lastly, we present initial results of a multi-step optimization process on each scheme, wherein the warm-rain processes are first optimized to match observed DSD spectral broadening to the degree possible within parameterization structural constraints.