Contrasting the evolution of radiation fog over a heterogeneous region in southwest France during the SOFOG3D campaign

Observations made during the recent SOuth-west FOGs 3D experiment (SOFOG3D) have been used to investigate the formation and evolution of radiation fog over heterogeneous forest plantations. The focus was on comparing measurements made at a relatively open site on arable land with those made in an approximately 700-m diameter field surrounded by tree plantations, with both sites hosting an instrumented 50-m mast. These data showed that at the more sheltered site radiation fog tended to form earlier than at the more open site. This coincided with more rapid decreases and lower minima in both near-surface temperatures and vertical turbulence from the late afternoon. It is proposed here that the surrounding forest creates a sheltering effect, which can cause a reduction in vertical turbulence and therefore the mixing of cool near-surface air with warmer air aloft. The near-surface region is therefore able to cool rapidly, enabling fog to form more readily. Data from additional sites with varying surroundings supported the findings that the more sheltered sites tended to exhibit lower near-surface nocturnal temperatures. However, the onset of fog formation observed at these additional sites suggested that there could be a limit to how sheltered a site may be before fog formation is inhibited rather than enabled by the surroundings. The effects of fog on human life and economic efficiency can be severe, however models still struggle to predict fog accurately due to a general lack of understanding of fog processes. The SOFOG3D campaign in southwest France looked to address fog evolution over heterogeneous forest plantations. We find that reduced vertical turbulence due to a sheltering effect provided by the plantations allowed enhanced cooling at the surface, and consequently earlier formation of shallow radiation fog.image