Prediction of Subgrid Fog due to Subgrid Valley Cold Air Pools Diagnosed from Numerical Weather Prediction (NWP) Data

Nocturnal or early morning fog can profoundly impact road safety and aviation (where it leads to disruptive and costly flight cancellations in addition to the obvious hazard factor). In clear, calm conditions, cold air pooling in lowland areas forms a conducive local environment for fog, with lowered near-surface temperatures leading to an increase in relative humidity. Since many valleys are smaller in scale than operational NWP grid box sizes, this variability is poorly captured, and fog may go unforecast. Forecasts of fog made with the Met Office Unified Model (MetUM) use a built-in visibility diagnostic which depends on grid box screen temperature, moisture and aerosol. In previous work, a physically-based algorithm was developed which employs a very high resolution terrain model to downscale NWP temperatures. In this presentation, the use of the MetUM visibility diagnostic formulation in conjunction with this temperature downscaling method is explored, in order to develop an effective method to predict the impact of subgrid temperature variability such as cold pools on fog formation.