MACLOUD (MArine CLOUD Brightening)

MACLOUD will utilise state-of-the-art multi-scale modelling to assess the potential of several marine cloud brightening (MCB) strategies and scenarios. MCB strategies frequently propose aerosolising sea-water through powerful sprayers on ocean-going ships. The resulting sea-salt aerosol can act to enhance the brightness of clouds leading to a reflection of an additional amount of sunlight back out to space and an associated cooling of the planet. It has been postulated that MCB could therefore play a role in combatting climate change caused by increased anthropogenic emissions of greenhouse gases. However, the impact of aerosols in cloud brightening is shrouded with considerable uncertainties. The sea-salt aerosol produced by sprayers would need to create particles that are smaller than 1/1000th of a mm. Under high atmospheric humidity, these aerosol can 'activate' into cloud droplets. If MCB sprayers create more aerosols then a greater number of small cloud droplets form. This can reduce the amount of drizzle from clouds which can lead to the clouds lasting for longer. Increases in marine cloud brightness or cloud amount lead to a cooling of the planet, but are particularly effective at cooling the sea-surface directly beneath them. This can lead to local, regional, and at sufficient magnitude, global changes in atmospheric and oceanic circulation patterns. To develop a fundamental understanding of the impacts of MCB therefore requires a range of models. They must be able to capture: the evolution of the aerosol spray via coagulation, evaporation, and deposition between the sprayer and any cloud the detailed sub-micron processes of activation of the aerosol within cloud the representation of the turbulent environment of clouds including entrainment, detrainment, evaporation, rain-out Larger-scale updrafts and down drafts that are important in the self-organisation of cloud-decks Regional responses in temperature Teleconnections via large scale dynamical feedbacks One model cannot do all of this. We propose utilising a comprehensive suite of parcel-models and box-models over effective domains of order metres, through large-eddy-simulations and limited area models with domains of order hundreds or thousands of km, right through to global scale weather and climate models. By confronting and improving the large scale global climate models with cloud-resolving models that have themselves been confronted by parcel-models capable of modelling sub-micron processes we will develop a true traceable multi-scale capability for MCB modelling. We recognise that there are a range of future climate scenarios that are possible depending on the range of mitigation measures that are put in place by society. We choose to utilise so-called shared socio-economic pathways (SSPs) that are widely used by policy-makers as our baseline climate scenarios. We adopt the United Nations COP21 target of maintaining global mean climate well below a warming of 2 degrees Celsius as compared to pre-industrial conditions; this is widely accepted as being 1.5degrees Celsius. We propose modelling MCB across a range of deployment strategies and scenarios in both high resolution cloud-resolving models and global climate models. We will investigate the time of implementation, how and when MCB is phased-out, the altitude of the injection, the scheme representing aerosol activation etc. We will vary the pattern of the MCB deployment which will have a very strong influence in inducing very different teleconnections. Case studies such as how MCB might help or hinder key components of the Earth system such as the Amazon, crops and sea-ice will be investigated.