Observations of Ambient Aerosol and Liquid Cloud Formation in a New Mexico Summer Deep-Convection System

Aerosol particles can affect the formation and properties of clouds by acting as cloud condensation nuclei (CCN) and ice nucleating particles (INP). The accurate representation of aerosol size distribution and composition along with cloud nucleating properties play an important role for describing aerosol-cloud interactions. The Deep Convective Microphysics Experiment (DCMEX) was aimed at improving the representation of microphysical processes in deep convective clouds. The DCMEX campaign (July to Aug 2022) was conducted using the UK FAAM (Facility for Airborne Atmospheric Measurements) BAe-146 Atmospheric Research Aircraft and characterized the aerosol-cloud system over the isolated Magdalena Mountain region in New Mexico. The aircraft was equipped with a range of online instruments to measure aerosol chemical composition (i.e., Aerosol Mass Spectrometer, AMS; Laser Ablation Aerosol Particle Time of Flight mass spectrometer LAAPToF) and aerosol size distribution, as well as cloud microphysics.

A 6-days backward dispersion analysis of this region shows that the experimental region was influenced by changing air masses from Northwest (NW, California coast) to Southeast (SE, Gulf of Mexico) during the campaign period. This air mass source transfer coincided with changes in meteorological conditions, including enhancement of convection available potential energy (CAPE), decreased cloud-base height, and increased boundary layer humidity. The aerosol chemical and physical properties in out-of-cloud runs also show variations under different air mass source conditions. Number size distributions show enhanced Aiken-mode particles but lower accumulation-mode particles before and after the air mass transition. Larger sulphate and lower organic contributions were observed in the sub-micron (<1 μm) aerosol mass fraction in the SE ocean flow when compared to airflow from the NW, with the organic components being more oxidized. The LAAPToF single particle measurements (0.5-2.5μm) indicate a greater proportion of aged sea salt (NaNO₃) particle number within the SE ocean flow. The calculated kappa values suggest more hygroscopic aerosols with the source transfer.

A bin-microphysics model was employed to simulate the liquid cloud development in this deep-convective system, using aerosol and meteorological measurements as model inputs. The SE-flow period (monsoon) generally had higher temperature, pressure and relative humidity at Cloud-base than the NW-flow period (pre-monsoon). Simulation and observation results both suggest that the change of aerosol characteristics and cloud-base conditions lead to enhanced water mixing ratio and cloud droplet size but decreased cloud droplet number concentration in the monsoon period compared to pre-monsoon. The comparison between simulation and observation also highlights the importance of considering entrainment processes from surrounding environments, especially the aerosol mixing, in future studies of simulating convective clouds.

Next, we will test the secondary ice formation mechanisms in the bin-microphysics parcel model by comparing simulations with DCMEX observations. We will also Investigate the contribution of different aerosol types in this deep-convection cloud system using the measurements of cloud residues.