Dynamical Controls on Radiation Fog in the Teme River Valley

Fog is a significant natural hazard that can lead to financial and life losses. Accurate prediction of fog has been, and remains, a long-standing challenge for numerical weather prediction models. Improved fog forecasting is therefore a high priority. Despite steady progress in understanding and modelling of the life cycle of fog over the years, as illustrated by the development of the state of the art between reviews of the body of knowledge on fog controls by Willett (1928), Mason (1982) and Gultepe et al. (2007), much still remains to be understood, particularly for radiation fog that are mostly associated with anticyclonic conditions. Given synoptic weather conditions and aerosol loadings suitable for radiation fog formation at a certain location, the local surface conditions and the orographic features of the area often determine the location and timing of fog formation (e.g. Pilié et al., 1975a,b; Duynkerke, 1991; Golding, 1993).

Several observational studies of radiation fog have been reported in the literature, mostly presenting observations of atmospheric stability within fog layers, and a discussion of the physics involved (e.g. Roach et al., 1976; Choularton et al., 1981; Meyer et al., 1986; Duynkerke, 1991; Fuzzi et al., 1992; Guédalia and Bergot, 1994; Fuzzi et al., 1998; Duynkerke, 1999; Haeffelin et al., 2010; Porson et al., 2011). However, detailed process-level observations characterizing or even documenting the effects of surface and orographic variability on dynamical controls on fog during the complete life cycle of radiation fog are largely non-existent, most probably owing to the challenges in making extensive observations of fog (in complex terrain).

The Local And Non local Fog EXperiment (LANFEX) was designed to examine how local is the develop- ment and evolution of (primarily) radition fogs in complex terrain (Lane et al., 2015).The experimental phase of LANFEX ran between mid-November 2014 to end of March 2016. Networks of instrumentation were deployed at selected sites in the shallow valley of the River Great Ouse around Bedford, UK, and deeper valleys in south-west Shropshire, UK. The present study uses a subset of the campaign data, collected around Skyborry in the section of the Teme River Valley between Knighton and Dutlas in Shropshire during the winter season 2015–2016.

The sites and main instruments used in this work, namely a Doppler LiDAR (DL), temperature and relative humidity data loggers, referred to as HOBOs, and Optical Particle Counters (OPCs) will be presented. The data were collected from 18 December to 26 March 2016, as part of the LANFEX field campaign. We also use radiosonde/rawinsonde (RS) and OPC upper-air data collected during the intensive observation periods (IOPs) of the field campaign. Initial results will be presented, focusing on relating observed changes in droplet size distribution throughout fog to dynamical controls on fog, following the ideas pioneered by Pilié et al. (1975a,b).

Acknowledgements. We wish to thank the land owners for providing access to their fields. The authors would like to acknowledge the National Centre for Atmospheric Science (NCAS) Atmospheric Measurement Facility (AMF) for the use of the Doppler LiDAR and HOBOs during this project. The contribution by GA to this work was supported by a grant from LabEx Osug@2020 (Investissements d'avenir – ANR10LABX56).