Plume evolution modeling

Uncertainties in how sulfur dioxide (SO₂) mixes in the aircraft wake and how the resulting plume (the cloud of injected material as it disperses) dilutes over time have outsized impacts on the final aerosol sizes, potentially affecting SAI cooling efficiency by factors of two or more. To design a successful aerosol microphysics experiment, researchers must first understand precisely what observations are needed to test and improve models of plume evolution. This requires simulating, in advance, what measurements could be taken at different spatial and temporal scales — from the aircraft wake (where processes evolve over minutes) through larger regional scales (where processes evolve over days and weeks) — and determining how those observations would reduce current uncertainties. The first step is to develop 3-4 viable representations of plume evolution over a one-month period so there are detailed simulations of the plume to test different sampling cadences and instruments.

This activity develops multi-scale simulations of SO₂ plume evolution spanning the first month following aircraft release. At the smallest scale, high-fidelity computational fluid dynamics (CFD) simulations will model how air flows around and behind the aircraft, capturing the three-dimensional structure of the plume as it forms in the aircraft wake — including its size, shape, and how material distributes within it — during the first several minutes after release. These detailed simulations will account for how different aircraft characteristics (weight, speed, wing design) and stratospheric conditions (altitude, temperature, atmospheric density) influence this initial mixing.

Results from these near-field wake simulations will then initialize a reduced-order model that tracks how the wake vortices decay and break apart over the next 10–30 minutes. Once the vortex system has fully dissipated, the activity will incorporate larger-scale dispersion modeling, using wind observations from global weather databases to simulate how atmospheric winds stretch, twist, and fold the plume, and how it gradually mixes with surrounding air over days to weeks.

Throughout all these scales, detailed microphysics models will simulate the chemical and particle-formation processes occurring simultaneously — specifically how SO₂ oxidizes into sulfuric acid, how particles nucleate and grow, and how they collide and coagulate into larger sizes. The output will be 3–4 representations of plume evolution that can simulate the observational capabilities of various instrument suites and help improve microphysics modeling.