Kinematic, microphysical, and electrical aspects of an asymmetric bow-echo mesoscale convective system observed during STEPS 2000

摘要

Five hours of detailed observations of the asymmetric bow-echo mesoscale convective system on 11–12 June 2000 during the STEPS project are presented. Data sources included polarimetric and Doppler radars, a VHF lightning mapper, and the National Lightning Detection Network. Nearly every (99%) VHF lightning source occurred within 10 km of the convective line. Charge identification within the much of the convective line revealed evolution during the first 3 h from a normal polarity dipole (upper positive charge near 9 km MSL/?30°C, over mid-level negative charge near 7 km/?16°C) to a normal polarity tripole with the addition of lower positive charge near 4 km (1°C). During the final 2 h, the charge structure resembled an inverted dipole, with negative charge overlaying positive. When the bow echo occurred during hour 2, part of the cells associated with severe winds developed an inverted tripolar charge structure and produced predominantly positive cloud-to-ground lightning. Stratiform lightning was infrequent, normally initiating in the convective line and propagating rearward along two pathways: a downward-sloping upper pathway near 9 km, and a more common constant-altitude lower pathway near 6 km (?10°C). Some flashes initiated within the stratiform region during this time, but only after stratiform reflectivities increased significantly in the mixed-phase region. These flashes initiated near the bright band (4 km), and tapped negative charge above and positive charge below. Possible mechanisms to explain the observations are discussed.