Preprints
https://doi.org/10.5194/acp-2021-1027
https://doi.org/10.5194/acp-2021-1027
 
17 Jan 2022
17 Jan 2022
Status: a revised version of this preprint is currently under review for the journal ACP.

Experimental study on the evolution of droplets size distribution during the fog life cycle

Marie Mazoyer, Fréderic Burnet, and Cyrielle Denjean Marie Mazoyer et al.
  • CNRM, Université de Toulouse, Météo-France, CNRS, Toulouse, France

Abstract. The evolution of the droplet size distribution (DSD) during fog life cycle remains poorly understood and progress is required to reduce the uncertainty of fog forecasts. To gain insights into the physical processes driving the microphysical properties, intensive field campaigns were conducted during the winters of 2010–2013 at the Instrumented Site for Atmospheric Remote Sensing Research (SIRTA) in a semi-urban environment southwest of Paris city center to monitor the simultaneous variations in droplet microphysical properties and their potential interactions at the different evolutionary stages of the fog events. Liquid water content (LWC), fog droplet number concentration (Nd) and effective diameter (Def f) show large variations among the 42 fog events observed during the campaign and for individual events. Our results indicate that the variability of these parameters results from the interaction between microphysical, dynamical and radiative processes. During the formation and development phases, activation of aerosols into fog droplets and condensational growth were the dominant processes. When vertical development of radiation fogs occurred under the influence of increasing wind speed and subsequent turbulent motion, additional condensational growth of fog droplets was observed. DSDs with one mode (around 11 μm) and two modes (around 11 and 22 μm) were observed during the field campaign. During the development phase of fogs with two droplet size modes, a mass transfer occurred from the smaller droplets into the larger ones through collision-coalescence or Ostwald ripening processes. During the mature phase, evaporation due to surface warming induced by infrared radiation emitted by fog was the dominant process. Additional droplet removal through sedimentation is observed during this phase for fog with two droplet size modes. Because of differences in the physical processes involved, the relationship between LWC and Nd is largely driven by the droplet size distribution. Although a positive relationship is found in most of the events due to continuous activation of aerosol into fog droplets, LWC vary at constant Nd in fog with large Def f  (> 17 μm) due to additional collision-coalescence and Ostwald ripening processes. This work illustrates the need to accurately estimate the supersaturation for simulating the continuous activation of aerosols into droplets during the fog life cycle and to include advanced parameterizations of relevant microphysical processes such as collision-coalescence and Ostwald ripening processes, among others, in numerical models.

Marie Mazoyer 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-2021-1027', Anonymous Referee #1, 09 Feb 2022
    • AC1: 'Reply on RC1', Marie Mazoyer, 14 May 2022
  • RC2: 'Comment on acp-2021-1027', Anonymous Referee #2, 21 Mar 2022
    • AC2: 'Reply on RC2', Marie Mazoyer, 14 May 2022

Marie Mazoyer et al.

Marie Mazoyer et al.

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Short summary
The evolution of the droplet size distribution during fog life cycle remains poorly understood and progress are required to reduce the uncertainty of fog forecasts. To gain insights into the physical processes driving the microphysics, intensive field campaigns were conducted during three winters at the SIRTA site in the south of Paris. This study analyzed the variations in droplet microphysical properties and their potential interactions at the different evolutionary stages of the fog events.
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