Aerosol-Fog interactions

Project Summary
Fog is a high-impact weather event. It can cause severe disruption. The formation of fog involves a number of different physical processes and that makes it very difficult to model. Aerosols are an essential component in the development of fog and cloud droplets. The way that aerosols interact with fog and influence fog evolution, i.e. onset, development and dissipation of fog, and the means by which fog influences aerosol are very uncertain. The aim of this PhD project is to investigate and understand the role of aerosol and aerosol-cloud interactions in the evolution fog. The student will be able to use two different state-the-art high resolution numerical models. One is the newly upgraded large-eddy model (Met Office/NERC Cloud model, MONC) and the other the bulk Cloud AeroSol Interactions Microphysics (CASIM) scheme in the Met Office Unified Model. In addition to the strong numerical modelling element to this project, there will be a focus on the comparison with observation and development of theoretical understanding to answer important science questions, e.g. what is the role of turbulence in the activation of aerosol in stable boundary layers?

Background
Fog can cause severe disruption to transport, sometimes with loss of life. Major health problems can also occur due to air-pollution events that are caused by the same meteorological conditions, and that can cause enhancements in fog intensity (e.g. the Beijing smogs). Rapid urbanisation combined with changing climate may have a big impact on fog/air quality. Recent disruption at Heathrow Airport due to fog has created huge costs to the aviation industry. Furthermore fog creates severe problems for the transport industry in general, with risk to life. Improved fog forecasting is therefore a high priority. Fog formation and development is controlled by subtle interactions of dynamical and microphysical processes at fine scales which makes this a challenging problem. The short-term forecasting of fog in numerical weather prediction (NWP) models is therefore vitally important. Despite significant advances in NWP, the numerical modelling of fog and short-term forecasting remains a significant challenge because of the diversity of processes involved in fog parametrisation.

Alongside detailed observations of fog, a method that is often employed to understand and develop fog parametrisation is high resolution (1 to 10 m) large eddy simulation, which permits a detailed analysis of the processes involved in fog formation and dissipation. Previous studies that employ such models have demonstrated that the simulation of fog is very sensitive to the representation of cloud microphysics (e.g. Porson et al. 2011). While aerosol are a vital component in the development of fog and clouds, the role of aerosol, the way that aerosol interacts with fog and the means by which fog influences aerosol are all very uncertain.

Scientific Aims of the Project:
The aim of the project is to study the role of aerosol in fog formation and dissipation. This will involve:

•Participation in a major field campaign and analysis of data; and
•Using the newly developed Met Office-NERC cloud model (MONC) and the bulk Cloud AeroSol Interactions Microphysics (CASIM) scheme

A field campaign will take place in the South Shropshire Hills for a 6-month period during the winter of 2015/16. Comprehensive measurements of the thermodynamics, radiation, dynamics, aerosol and microphysics of the fog and its environment will be measured using several towers, instruments on a tethered balloon and ground-based instruments at several different sites.
The new model, MONC is new, highly scalable idealised atmospheric process model, which has been developed for new massively parallel (MPP) architectures. The underlying science of MONC is based on the Met Office Large Eddy Model (LEM), while the code structure and parallelism has been completely re-written and re-engineered. The use of MONC permits the efficient simulation of very high resolution, big domain fog cases that can be compared with available observations. Initially, this project will use MONC to replicate the simulations of Porson et al (2011) and investigate the role of dynamics and radiation in fog evolution, with a simple microphysics representation. This project will then use MONC alongside a detailed cloud microphysics scheme and the bulk Cloud AeroSol Interactions Microphysics (CASIM) scheme to investigate and understand the role of aerosol and aerosol processing in the evolution of fog. In addition to the strong numerical modelling element to this project, there will be a focus on the comparison with observation and development of theoretical understanding to answer important science questions, e.g. what is the role of turbulence in the activation of aerosol in stable boundary layers?