Evaluating Droplet Size Distribution Evolution in Aerosol-constrained Simulations of Tropical Cumulus Congestus against Airborne Polarimetry Observations

Abstract

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 and effective radius increases with increasing cloud top height, with droplet size distributions (DSDs) that broaden with height. These observed components are responsive to an active collision-coalescence process that produce millimeter-sized drops, onsetting warm-rain formation. Here, we present an analysis of CAMP2Ex RSP data showing the evolution of droplet number concentrations and DSDs with height and its variation with RSP-retrieved aerosol number concentrations. For one case study (RF14, 9/25/2019), we perform large eddy simulations (LES) at 100-m horizontal grid spacing using bin and bulk microphysics schemes. Detailed multi-modal, vertically resolved aerosol measurements from the Fast Integrated Mobility Spectrometer (FIMS) are used as input. The relative ability of the bulk and bin schemes to produce the observed DSD evolution, from activation to warm-rain production, is evaluated. Sensitivity experiments are performed to assess the roles of height-varying aerosol concentrations, rain-forming collision-coalescence, and entrainment in realizing observed droplet number concentration and effective radius profiles. Additionally, we share the prospect of detailed aerosol properties, droplet number concentrations, and DSDs, similar to as acquired by RSP, becoming available from polarimeters on NASA’s PACE satellite mission, launched in early 2024.