Radiation Fog. Part II: Large-Eddy Simulations in Very Stable Conditions

Despite the long interest in understanding fog processes and improving fog parametrization, numerical modelling of fog remains an important challenge in short-term forecasts, due to the diversity and scales of the mechanisms involved with fog parametrization. In this study, we focus on the key processes that govern performance in fog modelling in very stable conditions. Large-eddy simulations at very high resolution are tested against the observations from Cardington, UK, presented in Part I of this study. The radiation fog forms in statically stable conditions. Five hours after its formation, the fog deepens rapidly and a significant cooling associated with the formation of positive curvature can be seen in the vertical profiles of potential temperature around 50 m. After roughly 8 h of development, a mixed layer has formed at the base of the fog, driven by surface instability. We show that the model captures well the change in static stability, but fails at capturing correctly the mechanisms associated with the deepening of the fog layer. Different possible mechanisms are discussed and tested with the model, such as additional drainage flow and cold air advection, which might result from local heterogeneity. The sensitivity of these results to different microphysical parametrizations is also briefly addressed.