Simulating shipping impacts on clouds in a high-resolution weather model

Aerosol-cloud interactions remain a significant source of uncertainty in our understanding of climate change. Ship emissions provide a unique natural experiment, artificially brightening clouds through the injection of aerosols that act as cloud condensation nuclei. These "ship tracks" offer a valuable opportunity to evaluate the impacts of aerosols on clouds. Comparing satellite observations of aerosol-cloud interactions with model simulations is often challenging. Direct case studies of natural experiments, such as those involving ship tracks, allow for more precise investigation of the representation of physical processes in models, enabling a clearer assessment of where models succeed or fail. Such simulations are also critical for evaluating the potential of marine cloud brightening as a climate intervention strategy.  

In this study, we simulate ship tracks using the UK Met Office Unified Model in km-scale resolution. By incorporating real ship locations and representing ships as moving aerosol sources within a two-moment cloud microphysics and coupled aerosol scheme, we successfully reproduce the cloud droplet number concentration of observed ship tracks for a specific day in a hindcast (relative to observations). Direct comparisons between these simulated tracks and MODIS satellite observations reveal differences in the model’s ability to represent the magnitude and timescales of cloud and precipitation processes, such as the LWP adjustments to aerosols and the width/depth of the resultant ship tracks. These comparisons shed light on the requirements for accurately simulating ship tracks. 

To further investigate the representation of aerosol-cloud interactions within models, we examine the sensitivity of aerosol impacts to grid resolution. This analysis addresses a critical question for scaling high-resolution simulations to global climate models (GCMs): Can small-scale constraints (such as ship tracks) reliably constrain the parametrisations of large-scale models? We explore whether the scaling of aerosol effects from high resolution models to GCM resolutions are linear or if saturation and concentration effects must be considered. The findings from this study contribute to improving the representation of aerosol-cloud interactions in models and enhancing our understanding of their role in climate systems.