Observations of ambient aerosol and warm 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 in describing aerosol-cloud interactions. The Deep Convective Microphysics Experiment (DCMEX) is a project 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 Spectrometry, AMS; Laser Ablation Aerosol Particle Time of Flight mass spectrometry, LAAPToF) and aerosol size distributions, as well as cloud microphysics.

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

A bin-microphysics model was employed to simulate the warm cloud development in this convective system. The simulation results show that both the change of aerosol characteristics and cloud-base conditions affect the warm cloud development, which follow the trends seen in the cloud microphysics observations. Initial cloud base conditions (i.e., initial temperature and relative humidity) mainly affected cloud properties by altering the water mixing ratios while aerosol characteristics mainly affected the initial cloud droplet number concentrations.

Next, we will combine these online aerosol measurements with detailed cloud microphysical measurements and offline INP analysis, to investigate the contributory effect of aerosols on primary ice formation in this deep-convection system and their relationship to secondary ice production processes.