One-dimensional site-specific forecasting of radiation fog. Part II: Impact of site observations

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 30-40km. A project has been under way to build a Site-Specific Forec ast Model (SSFM) using a one-dimensional version of the Met. Office Unified Model as a basis, with suitable modifications to its surface treatment and , driven by output from three-dimensional NWP models. The SSFM has been app lied to the forecasting of radiation fog as part of the European Commission sponsored project 4MIDaBLE (4Dimensional Met. Information DataBase Linked across Europe). This paper describes the influence of site observations on the model evolution for two real fog cases. This requires a method of forci ng the SSFM via coupling to three-dimensional NWP model output; such a meth od is described. It is shown that when observational data at the site of in terest is incorporated into the model's initial state humidity measurements provide the greatest impact on model evolution. However, their influence i s short-lived (typically less than 6 hours) implying the importance of init ialisation time in the correct forecast of radiation fog.