A New Lightning Parameterization and its Implementation in a Weather Prediction Model

Based on a straightforward physical model, a new lightning parameterization has been developed: A two-plate capacitor represents the basic dipole charge structure of a thunderstorm, which is charged by the generator current and discharged by lightning. In this approach, the generator current as well as the discharge strength are parameterized using the graupel-mass field. If these two quantities are known, and if the charging and discharging are in equilibrium, then the flash rate is uniquely determined. This approach remedies shortcomings of earlier theoretical approaches that relate the flash rate e.g., to generator power. No distinction is made between intracloud and cloud-to-ground discharges in this approach.

In order to test this approach, polarimetric radar data were used, from which the graupel distribution in observed thunderstorms could be inferred. The lightning activity was detected using the LINET network. The comparison between theoretically-predicted and measured flash rates is encouraging: Over a wide range of flash rates, the new approach yields accurate results for isolated thunderstorms. The investigated cases include intense supercellular hailstorms, as well as weak and comparatively shallow storms at the weak end of the spectrum. This approach is compared with an existing approach which is based on the cold cloud depth of the thunderstorm.

These approaches were implemented in the convection-resolving model, COSMO-DE. The output includes the time and the geographic location of each simulated discharge. Real-world scenarios were simulated and results will be shown for a severe hailstorm that affected southern Germany in August 2008.