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https://doi.org/10.5194/acp-2020-885
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-885
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  02 Oct 2020

02 Oct 2020

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This preprint is currently under review for the journal ACP.

Assessing and improving cloud-height based parameterisations of global lightning flash rate, and their impact on lightning-produced NOx and tropospheric composition

Ashok K. Luhar1, Ian E. Galbally1, Matthew T. Woodhouse1, and Nathan Luke Abraham2,3 Ashok K. Luhar et al.
  • 1CSIRO Oceans and Atmosphere, Aspendale, 3195, Australia
  • 2National Centre for Atmospheric Science, Department of Chemistry, University of Cambridge, Cambridge, UK
  • 3Department of Chemistry, University of Cambridge, Cambridge, UK

Abstract. Although lightning-generated oxides of nitrogen (LNOx) account for only approximately 10 % of the global NOx source, it has a disproportionately large impact on tropospheric photochemistry due to the conducive conditions in the tropical upper troposphere where lightning is mostly discharged. In most global composition models, lightning flash rates used to calculate LNOx are expressed in terms of convective cloud-top height via the Price and Rind (1992) (PR92) parameterisations for land and ocean. We conduct a critical assessment of flash-rate parameterisations that are based on cloud-top height and validate them within the ACCESS-UKCA global chemistry-climate model using the LIS/OTD satellite data. While the PR92 parameterisation for land yields satisfactory predictions, the oceanic parameterisation underestimates the observed flash-rate density severely, yielding a global average of 0.33 flashes/s compared to the observed 9.16 flashes/s over the ocean and leading to LNOx being underestimated proportionally. We formulate new/alternative flash-rate parameterisations following Boccippio’s (2002) scaling relationships between thunderstorm electrical generator power and storm geometry coupled with available data. While the new parameterisation for land performs very similar to the corresponding PR92 one as would be expected, the new oceanic parameterisation simulates the flash-rate observations more accurately, giving a global average of 8.84 flashes/s. The use of the improved flash-rate parameterisations in ACCESS-UKCA changes the modelled tropospheric composition—global LNOx increases from 4.8 to 6.6 Tg N/yr; the ozone (O3) burden increases by 8.5 %; there is an increase in the mid- to upper-tropospheric NOx by as much as 40 ppt; a 13 % increase in the global hydroxyl (OH); a decrease in the methane lifetime by 6.7 %; and a decrease in the lower tropospheric carbon monoxide (CO) by 3–7 %. Overall, the modelled tropospheric NOx and ozone are improved compared to observations, particularly in the Southern Hemisphere and over the ocean.

Ashok K. Luhar et al.

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Ashok K. Luhar et al.

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Short summary
Lightning-generated nitrogen oxides (LNOx) greatly influence tropospheric photochemistry. The most common parameterisation of lightning flash rate used to calculate LNOx in global composition models underestimates measurements over the ocean by a factor of 20–25. We formulate and validate an alternative parameterisation to remedy this problem. The new scheme causes an increase in the ozone burden by 8.5 % and the hydroxyl radical by 13 % and these have implications for climate and air quality.
Lightning-generated nitrogen oxides (LNOx) greatly influence tropospheric photochemistry. The...
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