Preprints
https://doi.org/10.5194/acp-2022-409
https://doi.org/10.5194/acp-2022-409
 
10 Jun 2022
10 Jun 2022
Status: this preprint is currently under review for the journal ACP.

Below-cloud scavenging of aerosol by rain: A review of numerical modelling approaches and sensitivity simulations with mineral dust

Anthony Crawford Jones1, Adrian Hill1, John Hemmings1, Pascal Lemaitre2, Arnaud Quérel3, Claire Ryder4, and Stephanie Woodward1 Anthony Crawford Jones et al.
  • 1Met Office, Fitzroy Road, Exeter, EX1 3PB, UK
  • 2Institute for Radiation Protection and Nuclear Safety (IRSN), PSN-RES, SCA, LPMA, Fontenay-aux-Roses, 92260, France
  • 3Institute for Radiation Protection and Nuclear Safety (IRSN), PSE-SANTE, SESUC, BMCA, Fontenay-aux-Roses, 92260, France
  • 4Department of Meteorology, University of Reading, RG6 6BB, Reading UK

Abstract. Theoretical models of the below-cloud scavenging (BCS) of aerosol by rain yield scavenging rates that are 1–2 orders of magnitude smaller than observations and associated empirical schemes for submicron-sized aerosol. Even when augmented with processes which may explain this disparity, such as phoresis and rear-capture in the raindrop wake, the theoretical BCS rates remain an order of magnitude less than observations. Despite this disparity, both theoretical and empirical BCS schemes remain in wide use within numerical aerosol models. BCS is an important sink for atmospheric aerosol, in particular for insoluble aerosol such as mineral dust which is less likely to be scavenged by in-cloud processes than purely soluble aerosol. In this paper, various widely used theoretical and empirical BCS models are detailed and then applied to mineral dust in climate simulations with the Met Office’s Unified Model in order the gauge the sensitivity of aerosol removal to the choice of BCS scheme. We show that the simulated accumulation mode dust lifetime ranges from 5.4 days in using an empirical BCS scheme based on observations to 43.8 days using a theoretical scheme while the coarse mode dust lifetime ranges from 0.9 to 4 days, which highlights the high sensitivity of dust concentrations to BCS scheme. We also show that neglecting the processes of rear-capture and phoresis may overestimate submicron-sized dust burdens by 83 %, while accounting for modal widths and mode-merging in modal aerosol models alongside BCS is important for accurately reproducing observed aerosol size distributions and burdens. This study provides a new parameterisation for the rear-capture of aerosol by rain and is the first to explicitly incorporate the rear-capture mechanism in climate model simulations. Additionally, we answer many outstanding questions pertaining to the numerical modelling of BCS of aerosol by rain and provide a computationally inexpensive BCS algorithm that can be readily incorporated in other aerosol models.

Anthony Crawford Jones et al.

Status: open (until 22 Jul 2022)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Anthony Crawford Jones et al.

Data sets

Data to support Below-cloud scavenging of aerosol by rain: A review of numerical modelling approaches and sensitivity simulations with mineral dust Jones, A. C.; Hill, A.; Hemmings, J.; Lemaitre, P.; Querel, A.; Ryder, C.; Woodward, S. http://dx.doi.org/10.5285/2e36fe8eb7ee4bd0a0833d3e1edd795a

Model code and software

Python and fortran scripts to support "Below-cloud scavenging of aerosol by rain: A review of numerical modelling approaches and sensitivity simulations with mineral dust" by Anthony C. Jones, Adrian Hill, John Hemmings, Pascal Lemaitre, Arnaud Querel, Claire L. Ryder, and Stephanie Woodward, Submitted to Atmospheric Chemistry and Physics, May 2022 Jones, A. C. https://doi.org/10.5281/zenodo.6617052

Anthony Crawford Jones et al.

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Short summary
As raindrops fall to the ground, they capture aerosol (Below Cloud Scavenging, BCS). Many different BCS schemes are available, and it is unclear what the impact of selecting one scheme over another is. Here, various BCS models are outlined and then applied to dust in climate model simulations. We find that aerosol concentrations are highly sensitive to the BCS scheme, with dust atmospheric lifetimes ranging from 5 to 44 days.
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