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On the Formation and Development of Radiation Fog: An Observational Study

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Abstract

Observations from the Local and Non-local Fog EXperiment field campaign are used to investigate the formation and initial development of radiation fog. Data from six locations have been selected: five sites in Shropshire, and one in Bedfordshire, UK. These data show that during night-time clear-sky conditions, thermally-stable radiation fog only forms when the vertical velocity variance near the ground (at 2 m) decreases to 0.002–0.005 m2 s−2 or less. The variation in this threshold appears to be a function of the vertical relative humidity gradient near the ground. When the vertical velocity variance exceeds these values, saturation may still occur near the ground, but fog formation is inhibited due to the more efficient transport of moisture to the ground in the form of dew. Furthermore, once stable radiation fog forms, any increase in the vertical velocity variance to values greater than 0.002–0.005 m2 s−2 may cause the fog to dissipate. The subsequent development of shallow stable fog into deeper, weakly-unstable fog is also examined. Possible mechanisms for this are discussed, including in situ processes and non-local larger scale processes. Clear evidence of the latter is presented, but the relative importance of in situ and non-local processes for fog deepening requires further clarification.

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Notes

  1. Other than deposition by hygroscopic absorption (Price and Clark 2014).

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Acknowledgements

I am very grateful to all of the people who made the LANFEX campaign possible, especially those staff at the Met Office Research Unit, Cardington. The comments of three anonymous reviewers are appreciated.

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Correspondence to J. D. Price.

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Electronic Supplementary Material

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10546_2019_444_MOESM1_ESM.wmv

Online Resource 1. This contains a thermal infra-red time-lapse video showing the evolution of fog during IOP 18. (WMV 4169 kb)

Online Resource 2. This is a text file containing an explanation of the video in Online Resource 1. (PDF 104 kb)

Appendix 1

Appendix 1

See Table 4.

Table 4 Showing the main instrument sensors used for the LANFEX field trial. Reproduced from Price et al. (2018)

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Price, J.D. On the Formation and Development of Radiation Fog: An Observational Study. Boundary-Layer Meteorol 172, 167–197 (2019). https://doi.org/10.1007/s10546-019-00444-5

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  • DOI: https://doi.org/10.1007/s10546-019-00444-5

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