One-dimensional site-specific forecasting of radiation fog. Part 1: Model formulation and idealised sensitivity studies

The Met. Office currently operates a high resolution mesoscale model that provides guidance on the likely formation of radiation fog on the spatial scale of 30-40 km, A project has been under way to build a Site-Specific Forecast Model (SSFM) using a one-dimensional version of the Met. Office Unified Model as a basis, with suitable modifications to its surface treatment driven by output from three-dimensional NWP models. The SSFM has been applied to the forecasting of radiation fog as part of the European Commission sponsored project 4MIDaBLE (4Dimensional Met. Information DataBase Linked across Europe). The first part of this paper describes the model formulation and discusses its behaviour simulating radiation fog in idealised cases and its sensitivity to geostrophic wind speed and initial humidity. In addition to the expected sensitivity to wind and humidity, it has been found that the treatment of surface exchange processes can have a small but significant impact on the formation of fog, its formation and clearance times and its depth.