Preprints
https://doi.org/10.5194/acp-2020-1314
https://doi.org/10.5194/acp-2020-1314

  22 Feb 2021

22 Feb 2021

Review status: a revised version of this preprint was accepted for the journal ACP.

A New Conceptual Model for Adiabatic Fog

Felipe Toledo1, Martial Haeffelin2, Eivind Wærsted3,a, and Jean-Charles Dupont4 Felipe Toledo et al.
  • 1Laboratoire de Météorologie Dynamique, École Polytechnique, Institut Polytechnique de Paris, 91128 Palaiseau, France
  • 2Institut Pierre Simon Laplace, École Polytechnique, CNRS, Institut Polytechnique de Paris, 91128 Palaiseau, France
  • 3Laboratoire de Météorologie Dynamique, École Polytechnique, Institut Polytechnique de Paris, 91128 Palaiseau, France
  • 4Institut Pierre-Simon Laplace, École Polytechnique, UVSQ, Université Paris-Saclay, 91128 Palaiseau, France
  • acurrent affiliation: The Norwegian Meteorological Institute, Henrik Mohns Plass 1, 0313, Oslo, Norway

Abstract. We propose a new paradigm to describe the temporal evolution of continental fog layers. This paradigm defines fog as a layer saturated from the surface to a known upper boundary, and whose liquid water path (LWP) exceeds a critical value, the critical liquid water path (CLWP). When the LWP is less than the CLWP the fog water cannot extend all the way to the surface, leading to a surface horizontal visibility greater than 1 km. On the opposite, when the LWP is larger than the CLWP, the fog water extends all the way to the surface, inducing a horizontal visibility less than 1 km. The excess water with respect to the critical value is then defined as the reservoir liquid water path (RLWP).

The new fog paradigm is formulated as a conceptual model that relates the liquid water path of adiabatic fog with its thickness and surface liquid water content, and allows the critical and reservoir liquid water paths to be computed. Both variables can be tracked in real time using vertical profiling measurements, enabling a real time diagnostic of fog status.

The conceptual model is tested using data from seven years of measurements performed at the SIRTA observatory, combining cloud radar, microwave radiometer, ceilometer, scatterometer and weather station measurements. In this time period we found 80 fog events with reliable measurements, with 56 of these lasting more than three hours.

The paper presents the conceptual model and its capability to derive the LWP from the fog CTH and surface horizontal visibility with an RMS uncertainty of 10.5 g m−2. The impact of fog liquid water path and fog top height variations on fog life cycle (formation to dissipation) is presented based on four case studies, and statistics derived from 56 fog events. Our results show in particular that the reservoir liquid water path is consistently positive during the mature phase of the fog and that it starts to decrease quasi monotonously about one hour before dissipation, reaching a near-zero value at the time of dissipation. The reservoir liquid water path and its time derivative could hence be used as an indicator for life cycle stage and support short range forecasting of fog dissipation.

Felipe Toledo et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2020-1314', Anonymous Referee #1, 29 Mar 2021
    • RC2: 'Reply on RC1', Anonymous Referee #2, 21 Apr 2021
      • AC2: 'Reply on RC2', Felipe Toledo, 17 Jun 2021
    • AC1: 'Reply on RC1', Felipe Toledo, 17 Jun 2021

Felipe Toledo et al.

Felipe Toledo et al.

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Short summary
The article presents a new paradigm to describe the temporal evolution of continental fog layers, based on seven years of fog measurements performed at the SIRTA observatory. This paradigm consists of a conceptual model that relates the liquid water path of fog with its thickness and surface liquid water content. The model provides diagnostic variables that could substantially improve the reliability of fog dissipation nowcasting at local scale, based on real-time profiling observations.
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