Meteorological and Electrical Conditions Associated with Positive Cloud-to-Ground Lightning

Abstract

Meteorological and electrical conditions associated with the occurrence of positive cloud-to-ground (CG) lightning (i.e., lightning that lowers positive charge to ground) are examined. Results from case studies in winter and summer storms reveal common features and lend support to the tilted dipole hypothesis. Lightning bipoles, whose lengths range from the convective scale to the mesoscale, are aligned with the vertical wind shear, with a predominance of negative locations in proximity to the deepest convection and a mixture of positive and negative locations displaced downshear from the deepest convection. Comparisons with radar data show that all lightning events am located within a distance of 10?20 km of precipitation extending from the surface to several kilometers above the O°C isotherm. Electrostatic field measurements beneath precipitation removed from the deepest convection indicate a positive dipole structure and a tilting deformation by vertical wind shear. These observations suggest that the principal contributor to positive lightning downshear of the deepest convection is mesoscale charge separation by differential particle motions rather than mesoscale advection over distances of 100 km or more.