TEAMx - Orographic convection

Rainfall over mountains plays a crucial role in the Earth system, with a large proportion of the Earth's population dependent on water resources from mountain catchments for drinking water, agriculture and hydroelectric power. Extreme storms can also contribute to a range of natural hazards including lightning, flooding and landslides. Accurate predictions of rainfall, particularly intense convective rainfall ("thunderstorms"), over mountain areas on both weather and climate timescales are therefore crucial for society.

All weather and climate models rely on parametrisation schemes. A parametrisation scheme is a simplified set of equations which attempt to represent the large-scale impact of an unresolved physical process based on the resolved large-scale flow. Convective clouds are not resolved at the relatively coarse (>10km) horizontal resolutions used for global weather and climate models and so the effects of the convection need to be parametrised through a convection scheme. High resolution (~1km) weather forecast models can now explicitly represent convective clouds however they are computationally expensive and so limited to a particular region and relatively short time periods. These high resolution simulations demonstrate improvements in the timing and organisation of rainfall however they do not capture the smaller scale processes which trigger convection or the details of what happens in the clouds and so these processes still need to be parametrised. Parametrisation schemes are typically developed and tested for relatively idealised situations over flat ground. This often leads to biases in the location, timing and intensity of rainfall over mountain areas. There is therefore a pressing need to improve the representation of convection over mountain areas in weather and climate models across a range of scales through improved model parametrisations.

The project will use the unique observational dataset from the international TEAMx field campaign in the Alps alongside high resolution numerical weather forecasts to improve our understanding of the processes controlling the initiation and development of convection over mountain regions and the role of water vapour transport through valleys in supplying the moisture to drive the convection. We will evaluate how well the current Met Office forecast model (the MetUM) and the next generation model (Momentum Unified Earth Prediction Framework) capture these processes at a range of resolutions. We will use our process understanding to improve the parametrisation of convection over orography through improving the initiation of convection in the new CoMorph convection parametrisation scheme being developed for Momentum and to identify the extent to which other parametrisation schemes such as the turbulence and orographic drag schemes represent the impact of orography on transport of moisture through mountain ranges.

By working to improve the representation of orographic convection in the Met Office model we will ensure wide benefit to end-users of Met Office rainfall forecasts including the general public, emergency response services and flood forecasters leading, allowing mitigation of high impact rainfall events to prevent loss of life and reduce damage and disruption from flooding. Improved seasonal and climate predictions of rainfall will benefit hydrologists, water authorities, farmers by enabling them to better manage and utilise water resources and will allow government and planners to design infrastructure resilient to changes in rainfall in a changing climate.