Preprints
https://doi.org/10.5194/egusphere-2024-3397
https://doi.org/10.5194/egusphere-2024-3397
16 Dec 2024
 | 16 Dec 2024
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Adiabatic and radiative cooling are both important causes of aerosol activation in simulated fog events in Europe

Pratapaditya Ghosh, Ian Boutle, Paul Field, Adrian Hill, Marie Mazoyer, Katherine J. Evans, Salil Mahajan, Hyun-Gyu Kang, Min Xu, Wei Zhang, and Hamish Gordon

Abstract. Aerosol-fog interactions affect the visibility in, and life cycle of, fog and are difficult to represent in weather and climate models. Here we explore processes that impact the simulation of fog droplet number concentrations (Nd) at sub-kilometer scale horizontal grid resolutions in the UK Met Office Unified Model. We modify the parameterization of aerosol activation to include droplet activation by radiative cooling in addition to adiabatic cooling, and determine the relative importance of the two cooling mechanisms. We further test the sensitivity of simulated Nd to: (a) interception of droplets by trees and buildings, (b) overestimation of updrafts in temperature inversions, which leads to artificially high Nd values; and (c) potential mechanisms for droplet deactivation due to downward fluctuations in supersaturation.

We evaluate our model against observations from the ParisFog and LANFEX field campaigns, building on evaluation described in the companion paper. Including radiative cooling in the activation mechanism improves how accurately we represent the liquid water path and the vertical structure of the fog in our LANFEX case study. However, with radiative cooling the Nd are overestimated for most of the ParisFog cases, and the LANFEX case. The time-averaged overestimate exceeds a factor of four (the normalized mean bias factor exceeds 3.0) in four out of eleven ParisFog cases. Our sensitivity studies demonstrate how these overestimates can be mitigated. Assuming the overestimate affects both radiative and adiabatic cooling, we find that although radiative cooling is more often the dominant source, both cooling sources can sometimes dominate activation.

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Pratapaditya Ghosh, Ian Boutle, Paul Field, Adrian Hill, Marie Mazoyer, Katherine J. Evans, Salil Mahajan, Hyun-Gyu Kang, Min Xu, Wei Zhang, and Hamish Gordon

Status: open (until 27 Jan 2025)

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Pratapaditya Ghosh, Ian Boutle, Paul Field, Adrian Hill, Marie Mazoyer, Katherine J. Evans, Salil Mahajan, Hyun-Gyu Kang, Min Xu, Wei Zhang, and Hamish Gordon

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Adiabatic and radiative cooling are both important causes of aerosol activation in simulated fog events in Europe Pratapaditya Ghosh, Ian Boutle, Paul Field, Adrian Hill, Marie Mazoyer, Katherine J. Evans, Salil Mahjan, Hyun-Gyu Kang, Min Xu, Wei Zhang, and Hamish Gordon https://doi.org/10.5281/zenodo.14005081

Pratapaditya Ghosh, Ian Boutle, Paul Field, Adrian Hill, Marie Mazoyer, Katherine J. Evans, Salil Mahajan, Hyun-Gyu Kang, Min Xu, Wei Zhang, and Hamish Gordon
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Latest update: 16 Dec 2024
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Short summary
We study the lifecycle of fog events in Europe using a weather and climate model. By incorporating droplet formation and growth driven by radiative cooling, our model better simulates the total liquid water in foggy atmospheric columns. We show that both adiabatic and radiative cooling play significant, often equally important roles in driving droplet formation and growth. We discuss strategies to address droplet number overpredictions, by improving model physics and addressing model artifacts.