Lagrangian trajectories on reanalyses

How effective stratospheric aerosol injection (SAI) is depends partly on how long aerosols remain aloft: aerosols that persist for many months or years reflect substantially more sunlight than those that settle out within a few months. Residence time is governed by stratospheric circulation (the winds and mixing that transport and dilute aerosol plumes over time). Lagrangian trajectory analysis quantifies this by computationally following individual air parcels as they move through the atmosphere, tracing transport and dispersion rather than observing conditions at fixed locations.

The challenge is that current models represent stratospheric circulation inconsistently. Four leading reanalysis products (datasets that combine historical observations with a model to reconstruct past atmospheric conditions) inform this work: ERA5, JRA3Q, ERA-Interim, and MERRA-2. Each constrains models to observational data but produces subtly different, equally plausible pictures of the stratosphere. A Lagrangian analysis using ERA5 alone has been completed (Sun et al., 2023; 2024), but no systematic comparison across all four products exists.

Running the full analysis across all four will quantify how sensitive estimates of aerosol lifetime and dispersion are to the choice of reanalysis, revealing whether current projections of SAI effectiveness could shift substantially depending on which representation of stratospheric circulation is used. Without this comparison, the range of stratospheric transport uncertainty remains uncharacterized — making it impossible to determine whether disagreements between models reflect genuine physical uncertainty or simply undercharacterized circulation differences. This work bounds that uncertainty and provides a more reliable foundation for projections of SAI performance.

This activity runs Lagrangian trajectory analysis across ERA5, JRA3Q, ERA-Interim, and MERRA-2 using the LAGRANTO trajectory tool. Because prior analysis covers ERA5 only, extending to all four products requires (1) designing a consistent set of analyses and scenarios, (2) reformatting each reanalysis dataset to meet LAGRANTO's input requirements, (3) running the full trajectory analysis on each product, and (4) synthesizing results across all four.

A minimum viable study will run at 1° resolution over a 10-year period, focusing on passive tracer and aerosol lifetimes across a few discrete aerosol sizes under two injection scenarios: a hemispherically balanced subtropical injection (30°N/S at approximately 21 km altitude) and a high-latitude, low-altitude injection (approximately 60°N/S at 13–15 km altitude). An extended analysis will increase resolution to 0.5°, run for two years, and add dispersion and diffusivity characterization by latitude, longitude, and season across altitudes from 14 to 20 km, along with additional injection scenarios.