Analysis of a Small, Vigorous Mesoscale Convective System in a Low-Shear Environment. Part I: Formation, Radar Echo Structure, and Lightning Behavior

Abstract

The evolution of a small, vigorous mesoscale convective system (MCS) over northern Alabama is described using Doppler radar, GOES satellite, surface mesonet, lightning, and sounding data. The MCS formed near noon in a relatively unstable environment having weak synoptic forcing and weak shear. The initiation of separate lines and clusters of deep convection occurred in regions exhibiting cumulus cloud streets, horizontal variations in stratocumulus cloud cover, and variations in inferred soil moisture. MCS growth via merger of storms within clusters and lines, and among the clusters, was accomplished largely through intersection of storm-scale and mesoscale outflow boundaries. The MCS maximum anvil area (∼60000 km2 at 220 K) and lifetime (8 h) were about 50% that of the typical Great Plains mesoscale convective complex (MCC). Despite its smaller size, this MCS displayed many aspects that typify the mostly nocturnal Great Plains MCS. The precipitation output was highly variable due to the transient nature of the intense convective elements, many of which produced microbursts. The radar measurements documented the formation of a stratiform region along the trailing side of an intense convective line. This stratiform region formed as decaying convective cores coalesced, rather than through advection of precipitation particles directly from the convective region. Combined GOES IR imagery and radar reflectivity analyses within the stratiform region show a sinking anvil cloud top in the presence of increases in the vertical radar reflectivity gradient within the cloud during the maturation of the stratiform region. During its intense developing stages, the MCS generated a peak cloud-to-ground (CG) flash rate of 2400 h−1, comparable to rates produced by larger MCCs. Early on, positive CG flashes were most prevalent around intense convective core regions exhibiting strong divergence at anvil level. During the latter stages, the emergence of positive CG was coincident with the formation of a prominent radar bright band within the stratiform region. Thus, a bipole was established, but its length was quite short at approximately 50 km, 25%–50% of the distance documented in other MCSs.