Lightning and thunderstorms, Part I: Observational data and model results

Abstract

Lightning is an important source for nitrogen oxides (NOx) in the atmosphere. To differentiate between the impact of lightning produced NOx and anthropogenic NOx emissions on the global atmospheric chemistry, we present a simulation of flash densities and the corresponding NOx emissions from the global climate chemistry model ECHAM4.L39(DLR)/CHEM (E39/C). The basis for a realistic simulation of flash densities is the realistic representation of deep convection in the model E39/C. This will be validated by observations from the International Satellite Cloud Climatology Project (ISCCP). We will discuss the difficulties that arise, when cloud parameters from these satellite measurements will be compared with the corresponding properties taken from a global model. The flash densities in E39/C are represented by a new lightning parameterization that connects the flash frequency of a thunderstorm with the convective air mass transport within the thunderstorm cell. This property is a reasonable and physically based indicator for lightning within a deep convective cloud. The number of convective events is simulated realistically by E39/C. The simulated heights of convective cloud tops are consistent with the observations. However, in midlatitudes over Europe, the simulated cloud top heights seem to be underestimated. The flash densities show a good agreement with the OTD observations, although the ratio of flash densities over land and over the oceans seems to be underestimated. In midlatitudes over Europe, the simulated flash densities are too low compared to the observations, this can be explained through the shortcomings of the convection parameterization. The simulated NOx emissions show maxima in the tropics and in the midlatitudes, being separated significantly from the aircraft emissions. The maximum values of lightning NOx emissions (in terms of ng(N) s-1 m-3), lying approximately 5 km below the tropopause, are higher than the corresponding emissions from aircrafts by a factor of three. Furthermore, the maximum aircraft emissions tend to be at higher altitude and farther north compared to the lightning NOx production.