Representation of the global emission of oxidized nitrogen by lightning discharges in large-scale models

摘要

In this paper, we describe the global distribution of the lightning source of oxidized nitrogen in a 3-D climatological tracer transport model (MOGUNTIA). The horizontal distribution of the source is based on statistics on cloud top heights from a long-term integration of a General Circulation Model (ECHAM GCM), applying a parameterization of lightning activity. This parameterization makes use of a semi-empirical relationship between convective cloud top heights and lightning flash frequency, with different formulations for continental and marine thunderstorms. The vertical allocation of the source is determined by assuming that the lightning emission of nitrogen oxides (NOx) at each grid point in the vertical is proportional to the local air density. The ratio between the number of cloud-to-cloud and cloud-to-ground flashes occurring per unit time within a grid box is adopted from observations. Because our present understanding of the formation mechanism of NOx in electrical discharges is not sufficient to determine the partitioning between cloud-to-cloud and cloud-to-ground discharges as NOx producers, we distinguish two cases. In the first case, it is assumed that cloud-to-cloud discharges are ten times less effective than cloud-to-ground discharges per discharge in producing NOx. In the second case, we assume that cloud-to-cloud discharges are equally effective as cloud-to-ground discharges per discharge in producing NOx. A global lightning source strength of 5 TgN/yr is adopted in this study. 21 refs, 2 figs, 3 tabs