High Resolution Simulations of Cold Air Pooling in the UK Landscape: Results from the COLPEX Campaign

Cold air pool formation under stable boundary conditions presents an interesting challenge for numerical weather prediction. The challenge is increased when the valleys which dominate cold pool formation are on scales unresolved by numerical weather prediction (NWP) models. This is the case in the UK and can lead to substantial local departures from the forecasted temperature. In the current study simulations are presented using the MetUM model in a nested configuration to downscale to 100m horizontal resolution. These high resolution simulations are then used to investigate cold pool formation as part of the COLPEX field campaign using both short-term IOP case studies and a longer climatological simulation.

These simulations are also useful for addressing more general issues associated with running NWP models at resolutions typically associated with Large Eddy Models (LEMs). The performance of the model over a larger number of cases identifies biases in the high resolution model relative to the more standard lower resolution setup. Specifically, the use of a critical relative humidity threshold for cloud formation as a means of representing unresolved variability is shown to have a significant impact on the downscaling behaviour. Finally, the impact of small scale cold pool processes at larger scales is discussed with the aim of improving parametrisation of their effects in NWP / climate models.