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.