Publications

Thunderstorm case studies and earlier observations are described which illuminate the relationship between cloud vertical development and the prevalence of intracloud (IC) and cloud-to-ground (CG) lightning. A consistent temporal evolution starting with peak IC activity changing to predominant CG activity and concluding with strong outflow (microburst) suggests that ice is responsible for both the electrical (i.e., lightning) and dynamical (i.e., microburst) phenomena. The IC activity is attributed to the updraft-driven accumulation of graupel particles in the central dipole region, and the subsequent CG activity to the descent of ice particles beneath the height of the main negative charge. The subsequent descent and melting of ice particles beneath the height of the 0 degree C isotherm are associated with the acceleration of the downdraft and outflow. The IC lightning precursor can provide a valuable short-term (5-10 min) warning for microburst hazard at ground level.

Coordinated Doppler radar and electrical measurements of thunderstorm microbursts were initiated by Lincoln Laboratory and the MIT Weather Radar group in Huntsville, AL in 1987. These measurements were intended to identify electrical precursors to aviation hazards at ground level and to study the relationship between the state of cloud convective development and the prevalent lightning type. The results of the Huntsville Study (Williams and Orville, 1988; Williamd et al., 1988) showed pronounced peaks in intracloud lightning activity and radar reflectivity above the melting level 5-10 minutes prior to maximum outflow velocities at the surface. A similar behavior has been reported by Goodman et al. (1988) for a thunderstorm observed in COHMEX in the same region. These observations support a prominent role for ice, both in promoting the intracloud lightning aloft and in subsequently driving the outflow by virtue of the melting process. All Huntsville cases studied were 'wet' microbursts with maximum low level reflectivity factors greater than 50 dBZ. The parent storms were deep (H>11km) and electrically active (flash rate greater than or equal to 1min^-1). Recent microburst studies in Denver (Hjelmfelt, 1987); Biron and isaminger, 1989) have identified, in addition to a majority of 'wet' microbursts, substantial numbers of dry microburst-producing storms (Z<10^3 mm^6/m^3) with elevated cloud bases and modest radar cloud tops. The present studies were aimed at determining to what extent the electrical manifestations observed in Huntsville were prevalent in Denver. This paper presents some preliminary results for the Denver measurements from the summer of 1988.