Improving fog prediction through improved representation of deposition processes
Supervisors: Dr Andrew Ross (Leeds), Prof Alan Blyth (Leeds and NCAS), Dr Jeremy Price (Met Office, MRU Cardington), Dr Adrian Lock (Met Office, Boundary-Layer processes group)
Enquiries: A.N.Ross(at)leeds.ac.uk
Forecasting the onset and duration of fog is an important, and difficult, forecasting problem. Fog causes significant disruption to road traffic and aviation and can be a significant factor in road traffic accidents. Receiving accurate and timely forecasts can help reduce the impact of such weather, minimising disruption, accidents and casualties. The current Met Office Unified Model (MetUM), along with many other models, has a systematic bias to producing fog too often and too quickly compared with observations. This project seeks to investigate the role of one key process – deposition – in controlling the onset of fog to see whether more accurate representation of this could lead to better forecasts. It will combine both field observations and modelling and will involve collaboration with the Met Office, providing broad research training and opportunities for an enthusiastic student.
The work is partly motivated by recent field observations made during the Cold Air Pooling Experiment (COLPEX) (Price et al, 2011) (see figure 1) and at the Met Research Unit at Cardington. The observations show a number of clear, stable nights where cold air pooling occurred and the relative humidity is close to 100% yet fog does not form, or does not form until late in the night. In the MetUM, as soon as the air becomes saturated then fog forms. Clearly there are some physical processes missing or inadequately represented in the model to explain this discrepancy. Significant benefits to visibility forecasts have already been gained from including purely the gravitational settling of fog droplets to the surface which reduces the number of fog droplets in the air to form fog. One possibility is that turbulence or the mean wind could enhance this through deposition on vegetation. This enhanced deposition is not included in the current model parametrisation. Another possibility is that low supersaturation rates due to very weak air ascent rates leads to low aerosol activation and hence few fog droplets.
- Figure 1: Fog forming in the Clun valley during the COLPEX campaign. (Photo: Jeremy Price)
This project will firstly involve analysis of existing measurements from the COLPEX campaign and observations made at Cardington. It will also involved making new observations including direct measurements of deposition using a new instrument being developed by the MRU at Cardington. These measurements will help to assess the likely importance of deposition in controlling fog onset. These observations will be used to test new model developments designed to represent enhanced deposition in the model. Initial development will be done with a single column version of the model, before testing with the full MetUM. An important test will be to see whether inclusion of deposition leads to improved prediction of fog onset in the MetUM for foggy cases from the COLPEX campaign.
There is potential for high impact publications from this project, both in improving our understanding and forecasting of fog events, and in underpinning the development of new parametrisations of the important processes which control fog formation. Collaboration with the Met Office on this project will provide a valuable insight into the operational requirements for fog forecasting and will help ensure that the research leads to real impacts in terms of improving forecast quality.
The School of Earth and Environment is a leading centre for atmospheric research, with both observational and modelling expertise in the dynamics of boundary-layer meteorology and stable boundary layers as well as the microphysical processes in cloud and fog. This studentship would form a part of this active and stimulating research environment. The School has a well-knit group of around 40 atmospheric-science research students. It provides an excellent programme of NERC approved skills training for research students, as well as a number of subject specific activities including the Arran Summer School and Masters-level taught modules.
This project has been approved as a "potential CASE studentship" by the Met Office. Three Met Office CASE studentships will be awarded in the School of Earth and Environment in the 2012 competition, and will be assigned to those students whose applications are ranked highest.
References
Price, J. et al (2011) COLPEX: Field and Numerical Studies Over a Region of Small Hills, Bulletin of the American Meteorological Society.