Aerosol impacts on marine clouds
File(s)
Author(s)
Tippett, Anna
Type
Thesis
Abstract
Aerosol-cloud interactions (ACI) remain one of the greatest sources of uncertainty in anthropogenic climate change, and complicate assessments of the potential efficacy of marine cloud brightening (MCB), a proposed strategy to intentionally brighten clouds and offset warming. Ship tracks, the linear cloud features caused by ship aerosols, provide a "natural experiment" for quantifying these interactions since they mimic the intended effect of MCB. While aerosols can initially brighten clouds, other cloud properties may subsequently change; adjustments to the liquid water path (LWP) can either enhance or offset the intended cooling effect and remain highly uncertain.
This thesis first identifies a methodological bias in previous ship track studies that led to an overestimation of the LWP response. By correcting for this bias, this work demonstrates a weaker average LWP adjustment that aligns with broader observational evidence, while identifying specific conditions under which both positive and negative adjustments can occur. These improved observations serve as a benchmark for evaluating the representation of LWP adjustments in a km-scale model. Simulations reveal a persistent model bias toward overly positive LWP adjustments. Even with improved physical parameterisations and across diverse cloud regimes, the model fails to reproduce negative LWP adjustments. Through a mechanism-denial experiment, this thesis attributes this deficiency to an inadequate representation of the "enhanced entrainment" pathway, which is currently outweighed by overly sensitive precipitation suppression.
Ultimately, this thesis demonstrates that current model parameterisations may be biased towards an increased cloud lifetime effect because they lack a balanced representation of competing adjustment pathways. The findings provide a robust framework for evaluating MCB strategies and emphasise that accurate ACI simulations require models that can explicitly capture both positive and negative LWP adjustments.
This thesis first identifies a methodological bias in previous ship track studies that led to an overestimation of the LWP response. By correcting for this bias, this work demonstrates a weaker average LWP adjustment that aligns with broader observational evidence, while identifying specific conditions under which both positive and negative adjustments can occur. These improved observations serve as a benchmark for evaluating the representation of LWP adjustments in a km-scale model. Simulations reveal a persistent model bias toward overly positive LWP adjustments. Even with improved physical parameterisations and across diverse cloud regimes, the model fails to reproduce negative LWP adjustments. Through a mechanism-denial experiment, this thesis attributes this deficiency to an inadequate representation of the "enhanced entrainment" pathway, which is currently outweighed by overly sensitive precipitation suppression.
Ultimately, this thesis demonstrates that current model parameterisations may be biased towards an increased cloud lifetime effect because they lack a balanced representation of competing adjustment pathways. The findings provide a robust framework for evaluating MCB strategies and emphasise that accurate ACI simulations require models that can explicitly capture both positive and negative LWP adjustments.
Version
Open Access
Date Issued
2026-04-07
Date Awarded
2026-06-01
Copyright Statement
Attribution-NonCommercial 4.0 International Licence (CC BY-NC)
License URL
Advisor
Gryspeerdt, Edward
Sponsor
Royal Society (Great Britain)
Horizon Europe
Grant Number
URF/R1/191602
101137680
Publisher Department
Department of Physics
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)