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

Abstract

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 700m‐diameter field surrounded by tree plantations, with both sites hosting an instrumented 50m 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 the vertical turbulence and therefore the mixing of the cool near‐surface air with warmer air aloft. The near‐surface is therefore able to cool rapidly, enabling fog to form more readily. Data from additional sites of 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.This article is protected by copyright. All rights reserved.

Vertical Profiles of Liquid Water Content in fog layers during the SOFOG3D experiment

“…1). The Jachère site (44.41 • N, 0.61 • W) has been selected in a fallow field located in a large open area (Thornton et al, 2023) and was specifically equipped for measurements of aerosols and fog microphysics as well as energy balance with in situ instruments at the surface and on masts. The Charbonnière site, 1.4 km away over a flat terrain (Fig.…”

“…But Dione et al (2023) found much larger values (0.01 < σ 2 w < 0.04 m 2 .s −2 ) during the stable/adiabatic transition of the 4 deepest fog events of SOFOG3D and an increase in TKE of up to 0.4 m 2 .s −2 . These discrepancies may be explained by the contrasting environment between the two measurement areas (Thornton et al, 2023). In the same way, Dhangar 2021), during the WIFEX campaign over New Delhi, considered a TKE threshold of 0.10 m 2 .s −2 .…”