Droplet dynamics in intermittent, cloud-turbulence: the vital ingredient to rain formation?

Warm cumulus are the fluffy clouds onto which people project shapes of familiar objects. They are aerosols of liquid water droplets contained within a turbulent body of air and sometimes yield rain, although this is an intermittent process with only around one in a million droplets needing to become big enough to trigger the raindrop-formation process. Turbulence is the chaotic motion of the air within the cloud, and is a familiar source of discomfort for airline passengers. Cloud droplets form at small sizes and grow through condensation of water vapour into liquid, however this growth rate is too slow to explain the observation that rainfall can develop within approximately 30 minutes of a cloud's formation. It is increasingly clear that turbulence within the cloud could be a vital factor in accelerating this droplet-growth since it causes collisions between droplets, which then coalesce making them larger. Current estimates suggest that turbulence increases the rate of collisions two- to three-fold. However, the physics behind this turbulence-enhanced collision and coalescence is not well understood meaning that they are not well parameterised for numerical weather and climate modelling. Droplets within turbulence are exposed to aerodynamic and gravitational forces which determine their trajectories, and hence likelihood of colliding/coalescing. An exact equation exists to describe the motion of a very small droplet in turbulence. However, full implementation of this equation in high-fidelity numerical simulations is extremely expensive meaning that typical simulations rely on an abbreviated form of the equation, only considering some of the aerodynamic forces. Unfortunately, cloud turbulence is intermittent, meaning that there are patches of clouds that are significantly more turbulent than average and in these patches the abbreviated form of the equation is insufficient to accurately describe the motion of the droplets, and indeed the full equation is stretched to breaking point. Our research will focus on understanding and then modelling these intermittent turbulent physics on the behaviour of cloud droplets, in particular the rate at which they collide and coalesce. To achieve this we will exploit a combination of experiments in a national wind tunnel and state-of-the-art simulations on supercomputers. We will identify features of intermittent cloud-turbulence that induce a strong contribution from the "new" forces, currently neglected in today's simulations and therefore with an unknown effect on rain-formation. Next, we will test one of the assumptions behind the particle-force equation to destruction: namely that the droplets are sufficiently small that they do not themselves modify the cloud-turbulence. We expect that this assumption will be violated when the droplets pass through particularly "rough"/turbulent patches of a cloud. Once we have learned how these intermittent cloud physics affect the behaviour of droplets within a cloud our final objective is to take these newly-discovered physics and parameterise them into a form that will be useful for numerical weather prediction (NWP). We are partnering with the UK Met Office and Centre for Climate Research Singapore and will implement our parameterised intermittent cloud physics into their NWP models and validate their predictive capabilities against satellite data. As these NWP becomes ever higher-fidelity accounting for the effects of cloud turbulence becomes increasingly important to improving forecasting accuracy, especially as climate change is expected to increase levels of atmospheric turbulence. Cloud/precipitation formation/decay are key uncertainties in global climate modelling and our research will help to reduce these uncertainties.