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 p rovides guidance on the likely formation of radiation fog on the spatial sc ale of 30-40 km, A project has been under way to build a Site-Specific Fore cast Model (SSFM) using a one-dimensional version of the Met. Office Unifie d Model as a basis, with suitable modifications to its surface treatment dr iven by output from three-dimensional NWP models. The SSFM has been applied to the forecasting of radiation fog as part of the European Commission spo nsored project 4MIDaBLE (4Dimensional Met. Information DataBase Linked acro ss Europe). The first part of this paper describes the model formulation an d discusses its behaviour simulating radiation fog in idealised cases and i ts sensitivity to geostrophic wind speed and initial humidity. In addition to the expected sensitivity to wind and humidity, it has been found that th e treatment of surface exchange processes can have a small but significant impact on the formation of fog, its formation and clearance times and its d epth.