Abstract
Abstract. Lightning is an important natural source of nitrogen oxide especially in the middle and upper troposphere. Hence, it is essential to represent lightning in chemistry transport and coupled chemistry–climate models. Using ERA-Interim meteorological reanalysis data we compare the lightning flash density distributions produced using several existing lightning parametrisations, as well as a new parametrisation developed on the basis of upward cloud ice flux at 440 hPa. The use of ice flux forms a link to the non-inductive charging mechanism of thunderstorms. Spatial and temporal distributions of lightning flash density are compared to tropical and subtropical observations for 2007–2011 from the Lightning Imaging Sensor (LIS) on the Tropical Rainfall Measuring Mission (TRMM) satellite. The well-used lightning flash parametrisation based on cloud-top height has large biases but the derived annual total flash density has a better spatial correlation with the LIS observations than other existing parametrisations. A comparison of flash density simulated by the different schemes shows that the cloud-top height parametrisation has many more instances of moderate flash densities and fewer low and high extremes compared to the other parametrisations. Other studies in the literature have shown that this feature of the cloud-top height parametrisation is in contrast to lightning observations over certain regions. Our new ice flux parametrisation shows a clear improvement over all the existing parametrisations with lower root mean square errors (RMSEs) and better spatial correlations with the observations for distributions of annual total, and seasonal and interannual variations. The greatest improvement with the new parametrisation is a more realistic representation of the zonal distribution with a better balance between tropical and subtropical lightning flash estimates. The new parametrisation is appropriate for testing in chemistry transport and chemistry–climate models that use a lightning parametrisation.
Highlights
Lightning is always occurring somewhere on Earth with an average of 46 flashes every second (Cecil et al, 2012)
These results show that all parametrisations under-estimate flash density over Central Africa compared to Lightning Imaging Sensor (LIS) satellite measurements, suggesting that either an important component of lightning generation is missing from all parametrisations or the underlying meteorology data are biasing the results of parametrisations
A difference between the two studies is the nature of the upward ice flux variable; an intensive property was used in our case, whilst an extensive property was used in the case of Barthe et al (2010), i.e. mass per area per time was used in our case opposed to only mass per time
Summary
Lightning is always occurring somewhere on Earth with an average of 46 flashes every second (Cecil et al, 2012). Every flash has enormous quantities of energy and can extend over tens of km which allows for the dissociation of nitrogen (N2) and oxygen (O2) molecules in the air. The dissociation products combine to form reactive nitric oxide (NO) which quickly oxidises to NO2, and an equilibrium between NO and NO2 is reached, together they are known as NOx. Air is predominantly detrained in the upper anvil levels of a thunderstorm thereby providing the principal natural source of these ozone precursors to the middle and upper troposphere (Grewe, 2007). Lightning has a large spatial variability as well as a seasonal cycle and interannual variability. As an important but highly variable source of NOx driven by meteorological processes, both chemistry transport models and coupled chemistry–climate models require parametrisations of lightning
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