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DOI	10.5194/acp-21-7271-2021
	Is a more physical representation of aerosol activation needed for simulations of fog?
	Poku C.; Ross A.N.; Hill A.A.; Blyth A.M.; Shipway B.
发表日期	2021
ISSN	1680-7316
起始页码	7271
结束页码	7292
卷号	21 期号:9
英文摘要	Aerosols play a crucial role in the fog life cycle, as they determine the droplet number concentration and hence droplet size, which in turn controls both the fog's optical thickness and lifespan. Detailed aerosol-microphysics schemes which accurately represent droplet formation and growth are unsuitable for weather forecasting and climate models, as the computational power required to calculate droplet formation would dominate the treatment of the rest of the physics in the model. A simple method to account for droplet formation is the use of an aerosol activation scheme, which parameterises the droplet number concentration based on a change in supersaturation at a given time. Traditionally, aerosol activation parameterisation schemes were designed for convective clouds and assume that supersaturation is reached through adiabatic lifting, with many imposing a minimum vertical velocity (e.g. 0.1 m s-1) to account for the unresolved subgrid ascent. In radiation fog, the measured updraughts during initial formation are often insignificant, with radiative cooling being the dominant process leading to saturation. As a result, there is a risk that many aerosol activation schemes will overpredict the initial fog droplet number concentration, which in turn may result in the fog transitioning to an optically thick layer too rapidly. This paper presents a more physically based aerosol activation scheme that can account for a change in saturation due to non-adiabatic processes. Using an offline model, our results show that the equivalent cooling rate associated with the minimum updraught velocity threshold assumption can overpredict the droplet number by up to 70 % in comparison to a typical cooling rate found in fog formation. The new scheme has been implemented in the Met Office Natural Environment Research Council (NERC) Cloud (MONC) large eddy simulation (LES) model and tested using observations of a radiation fog case study based in Cardington, UK. The results in this work show that using a more physically based method of aerosol activation leads to the calculation of a more appropriate cloud droplet number. As a result, there is a slower transition to an optically thick (well-mixed) fog that is more in line with observations. The results shown in this paper demonstrate the importance of aerosol activation representation in fog modelling and the impact that the cloud droplet number has on processes linked to the formation and development of radiation fog. Unlike the previous parameterisation for aerosol activation, the revised scheme is suitable to simulate aerosol activation in both fog and convective cloud regimes. © 2021 Author(s).
语种	英语
scopus关键词	aerosol composition; aerosol formation; atmospheric convection; cloud droplet; concentration (composition); droplet; fog; saturation; size distribution
来源期刊	ATMOSPHERIC CHEMISTRY AND PHYSICS
文献类型	期刊论文
条目标识符	http://gcip.llas.ac.cn/handle/2XKMVOVA/246892
作者单位	School of Earth and Environment, University of Leeds, Leeds, United Kingdom; Met Office, Exeter, United Kingdom; National Centre of Atmospheric Sciences, University of Leeds, Leeds, United Kingdom; Wolfson Atmospheric Chemistry Laboratories, University of York, York, United Kingdom
推荐引用方式 GB/T 7714	Poku C.,Ross A.N.,Hill A.A.,et al. Is a more physical representation of aerosol activation needed for simulations of fog?[J],2021,21(9).
APA	Poku C.,Ross A.N.,Hill A.A.,Blyth A.M.,&Shipway B..(2021).Is a more physical representation of aerosol activation needed for simulations of fog?.ATMOSPHERIC CHEMISTRY AND PHYSICS,21(9).
MLA	Poku C.,et al."Is a more physical representation of aerosol activation needed for simulations of fog?".ATMOSPHERIC CHEMISTRY AND PHYSICS 21.9(2021).