Characterizing Subgrid-Scale Aerosol Variability Near the Southern Great Plains Site during the HI-SCALE Campaign

Abstract:

Complex aerosol distributions evolve in space and time as a function of emissions, turbulent mixing and transport, coagulation, chemical transformation, phase changes, removal processes, and ambient meteorological conditions. The ability of models to represent the multi-scale processes affecting the lifecycle of aerosols depends on their spatial resolution since aerosol properties are assumed to be constant within a grid cell. While decades of work have gone into developing subgrid treatments of clouds in climate models, characterizing and treating subgrid variability of aerosol processes has received far less attention. Subgrid-scale-dependent processes that affect aerosol populations could have a significant impact on the formation of particles, their growth to CCN sizes, aerosol-cloud interactions, deposition and rainout, and hence their burden, lifetimes, and radiative forcing. To address this issue, we have characterized subgrid-scale variability in terms of measured aerosol number, size, and composition by combining ground and aircraft measurements from the HI-SCALE campaign near the ARM Southern Great Plains (SGP) site. Subgrid variability is quantified in terms of percentiles and probability density functions as a function of altitude for two categories: one at grid spacings typically used by cloud-system resolving models (3 km) and another at grid spacings more consistent with climate model simulations (10, 50, 100 km). Even though the SGP site is a rural location, surprisingly large horizontal gradients in aerosol properties were frequently observed. We also characterize the spatial variability associated with new particle formation (NPF) and growth events that occurred frequently during HI-SCALE. Those observations are then compared with predictions made by the Energy Exascale Earth System Model (E3SM), with and without a new treatment of ultrafine particles to help assess the impact of NPF and growth of ultrafine particles on CCN concentrations that would impact cloud-aerosol interactions and cloud radiative forcing.