Abstract
Abstract. Although lightning-generated oxides of nitrogen (LNOx) account for only approximately 10 % of the global NOx source, they have 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, where the oceanic parameterisation is known to greatly underestimate flash rates. We conduct a critical assessment of flash-rate parameterisations that are based on cloud-top height and validate them within the Australian Community Climate and Earth System Simulator – United Kingdom Chemistry and Aerosol (ACCESS-UKCA) global chemistry–climate model using the Lightning Imaging Sensor and Optical Transient Detector (LIS/OTD) satellite data. While the PR92 parameterisation for land yields satisfactory predictions, the oceanic parameterisation, as expected, underestimates the observed flash-rate density severely, yielding a global average over the ocean of 0.33 flashes s−1 compared to the observed 9.16 flashes s−1 and leading to LNOx being underestimated proportionally. We formulate new flash-rate parameterisations following Boccippio's (2002) scaling relationships between thunderstorm electrical generator power and storm geometry coupled with available data. The new parameterisation for land performs very similarly to the corresponding PR92 one, as would be expected, whereas the new oceanic parameterisation simulates the flash-rate observations much more accurately, giving a global average over the ocean of 8.84 flashes s−1. 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−1; the ozone (O3) burden increases by 8.5 %; there is an increase in the mid- to upper-tropospheric NOx by as much as 40 pptv, a 13 % increase in the global hydroxyl radical (OH), a decrease in the methane lifetime by 6.7 %, and a decrease in the lower-tropospheric carbon monoxide (CO) by 3 %–7 %. Compared to observations, the modelled tropospheric NOx and ozone in the Southern Hemisphere and over the ocean are improved by this new flash-rate parameterisation.
Highlights
Oxides of nitrogen (NOx ≡ NO + NO2) play an important role in tropospheric chemistry by acting as a precursor to ozone (O3) and the hydroxyl radical (OH), which are the principal tropospheric oxidants (Labrador et al, 2005)
While prescribing a minimum convective cloud thickness of 5 km for lightning is somewhat arbitrary, having no such threshold value is unrealistic because it would be implicitly assumed that a convective cloud always translates to a thunderstorm, and this would likely lead to unrealistically high flash rates. (We found that increasing or decreasing the minimum cloud thickness value by 1 km from 5 km resulted in a change of −3.2 % and 1.7 %, respectively, in the modelled global flash rate using the PR92 scheme.)
We have critically examined parameterisations of lightning flash rate that are based on the cloud-top-height approach
Summary
Oxides of nitrogen (NOx ≡ NO (nitric oxide) + NO2 (nitrogen dioxide)) play an important role in tropospheric chemistry by acting as a precursor to ozone (O3) and the hydroxyl radical (OH), which are the principal tropospheric oxidants (Labrador et al, 2005). Using a global chemistry transport model, Labrador et al (2005) observed marked sensitivity of NOx, O3, OH, nitric acid (HNO3), and peroxyacetyl nitrate (PAN) to the magnitude and vertical distribution of LNOx. Modelling studies by Grewe (2007) and Dahlmann et al (2011) found that of all the major sources of NOx, LNOx is the dominant source for tropospheric ozone (up to 40 %) in the tropics and Southern Hemisphere. Price and Rind (1992, hereafter PR92) developed simple empirical parameterisations for calculating lightning flash rate in terms of convective cloud-top height over land and ocean. The impacts of the modelled LNOx based on the new flashrate parameterisations on tropospheric composition involving NOx, ozone, the hydroxyl radical, methane lifetime, and carbon monoxide are examined, including comparison with observations where available or appropriate
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