Fog prediction using a very high resolution numerical weather prediction model forced with a single profile

Abstract

Fog and low level cloud forecasting is crucial for aviation, land transport and shipping, Current numerical weather prediction (NWP) models can give a broad indication of fog formation and dissipation on scales of a few tens of kilometres, but can provide little detail especially in areas of complex terrain. One-dimensional models for specific locations taking into account the impact of the surrounding orography on the flow have been developed as a cheap solution, but they are unable to treat complex three dimensional flows. A full NWP model running at a horizontal resolution of around 1 km (or better) may be required to address the three-dimensional fog forecasting problem in full. Enhancement of the model resolution to such a scale is restrained by available computing power and is not presently feasible for general coverage. However, in the weakly forced situations leading to fog formation, the use of a small, local area (e.g. 50 km × 50 km), high resolution NWP model may be a viable alternative. In this study, the local area NWP model approach is explored, initializing and forcing the model with a time series of single forcing profiles from either a low resolution NWP model, or from a local radiosonde profile. This method of homogeneous single-profile forcing (SPF) allows the model to be run locally with a small volume of input data. The UK Met Office Unified Model (UM) is set-up for a 50 km by 50 km horizontal domain, with real orography and a grid spacing of 1 km and 76 vertical levels. The model is initialized and forced with temperature, humidity and wind profiles from a 12 km coarse resolution model. Some typical results from a single-profile forcing (SPF) model run will be shown and compared with the output from a full NWP model with 1 km resolution and with observational data. © Crown Copyright 2008.