Estimating Twomey forcing sensitivity to aerosol plume spreading rates
- 1University of Washington (Seattle, United States of America)
Abstract
The amount of sunlight that reaches Earth's surface can be reduced by increasing cloud droplet number and decreasing droplet size (i.e., Twomey forcing), a central idea underpinning the Marine Cloud Brightening strategy. Cloud albedo depends nonlinearly on cloud droplet concentration, meaning the spatial extent of aerosol plumes could be an important constraint on the brightening potential. In this study, horizontal aerosol spreading is simulated using a Langevin particle model driven by a library of realistic, 2 d Large Eddy Simulations (LES) spanning the meteorological and aerosol phase space of northeast Pacific stratocumulus. The 2D reflectance fields from the LES are superimposed onto the 2D perturbed aerosol concentrations to calculate the 2D Twomey forcing response. The Day 1 and Day 2 LES regimes have distinct meteorological, aerosol, and turbulence characteristics associated with equatorward movement of the LES domain. Our results indicate that the Day 2 regime has substantially faster plume spreading than Day 1, with ensemble median differences exceeding 2 km h ⁻¹ . Despite these differences, Twomey forcing is insensitive to the natural variability in spreading rate. This study suggests that Twomey forcing is resilient to variations in meteorology, aerosols, and turbulence, with its efficacy governed primarily by aerosol lifetime and assumptions surrounding cloud adjustments. Although the natural variation in spreading rate does not materially affect Twomey forcing, idealized plume spreading simulations suggest that current GCM assumptions of an infinitely fast spreading rate could lead to a 10 %–200 % overestimation of cooling.
Versions
Related
- Forcing closure suggests stronger aerosol forcing and higher climate sensitivity than the consensus
- Anthropogenic aerosol forcing of European windstorms in CMIP6 climate models
- Major volcanic eruptions episodically disrupted the continuity of polar warming through aerosol forcing
- Stratospheric aerosol forcing for CMIP7 – Part 2: Volcanic sulfur dioxide emissions
- Sensitivity of simulated storm surge to tropical cyclone wind forcing around islands: insights from Hurricane Irma
BibTeX
@article{mcmichael2026estimating,
title = {{Estimating Twomey forcing sensitivity to aerosol plume spreading rates}},
author = {Lucas A. McMichael and Ehsan Erfani and Robert Wood and Knut von Salzen},
journal = {Atmospheric Chemistry and Physics},
year = {2026},
month = oct,
doi = {10.5194/acp-26-14111-2026},
url = {https://acp.copernicus.org/articles/26/14111/2026/},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Estimating Twomey forcing sensitivity to aerosol plume spreading rates
AU - Lucas A. McMichael
AU - Ehsan Erfani
AU - Robert Wood
AU - Knut von Salzen
PY - 2026
DA - 2026/10/09
JO - Atmospheric Chemistry and Physics
DO - 10.5194/acp-26-14111-2026
UR - https://acp.copernicus.org/articles/26/14111/2026/
AB - The amount of sunlight that reaches Earth's surface can be reduced by increasing cloud droplet number and decreasing droplet size (i.e., Twomey forcing), a central idea underpinning the Marine Cloud Brightening strategy. Cloud albedo depends nonlinearly on cloud droplet concentration, meaning the spatial extent of aerosol plumes could be an important constraint on the brightening potential. In this study, horizontal aerosol spreading is simulated using a Langevin particle model driven by a library of realistic, 2 d Large Eddy Simulations (LES) spanning the meteorological and aerosol phase space of northeast Pacific stratocumulus. The 2D reflectance fields from the LES are superimposed onto the 2D perturbed aerosol concentrations to calculate the 2D Twomey forcing response. The Day 1 and Day 2 LES regimes have distinct meteorological, aerosol, and turbulence characteristics associated with equatorward movement of the LES domain. Our results indicate that the Day 2 regime has substantially faster plume spreading than Day 1, with ensemble median differences exceeding 2 km h ⁻¹ . Despite these differences, Twomey forcing is insensitive to the natural variability in spreading rate. This study suggests that Twomey forcing is resilient to variations in meteorology, aerosols, and turbulence, with its efficacy governed primarily by aerosol lifetime and assumptions surrounding cloud adjustments. Although the natural variation in spreading rate does not materially affect Twomey forcing, idealized plume spreading simulations suggest that current GCM assumptions of an infinitely fast spreading rate could lead to a 10 %–200 % overestimation of cooling.
SN - 1680-7324
ER -