Radiation fogs over two sites at Southern and Northern Europe (Spain and The Netherlands)

Low visibilities due to fog events may cause non desirable effects on air, maritime and terrestrial transport; however, a correct forecasting of fogs is one of the goals not well achieved by the operational meteorological services from their numerical forecasting models. The physical processes involved in the evolution of fogs are not completely well understood, and therefore, not well parameterized in the Numerical Weather Prediction (NWP) models, especially those related to the turbulence in the planetary boundary layer. A comprehensive understanding of these processes would lead to better parameterizations for the NWP models. For this purpose, numerous radiation fog events from several autumn and winter months have been analyzed from two long meteorological datasets: one at the Research Centre for the Lower Atmosphere (CIBA, Spain) and the other at the Cabauw Experimental Site for Atmospheric Research (CESAR, The Netherlands). Both sites are contrasting in terms of geographical situation, typical synoptic patterns, humidity, winds, orography, altitude above sea level, etc. These differences make the occurrence of radiation fogs to be higher in CIBA site, since periods associated with stable situations are more common in the Northern Spanish Plateau (CIBA) than in Northern Europe (CESAR). A large set of radiation fogs at both places has been used to study the threshold values of different meteorological variables during the formation and dissipation of fogs, including turbulent parameters (turbulent kinetic energy, sensible and latent heat fluxes) and stability conditions (Richardson number). This analysis has been completed with a more detailed analysis of two carefully chosen case studies at both places.