The relationship between lightning type and convective state of thunderclouds

Abstract

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 0øC 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. INTRODUCTIONConvective storms are well recognized to produce two common types of lightning: intracloud (IC) and cloud to ground (CG). Some convective storms are recognized to produce strong downdrafts, often referred to as microbursts, which have been demonstrated to pose a severe hazard to commercial aviation [Fujita, 1985]. This study was initially concerned with a search for a practical short-term precursor to the microburst hazard in the cloud electrical development. In the course of this investigation, consistent relationships between the stage of the convective activity and the lightning type became apparent, and tied in closely with earlier electrical observations of thunderstorms and with contemporary observations of microburst precursors in Doppler radar observations [Campbell, 1988]. This paper is concerned with a description of these relationships and with an interpretation based on ice-phase microphysics. CASE STUDIESThe microburst-producing clouds in this study were almost invariably air mass thunderstorms in weakly sheared environments and exhibited peak lightning rates greater than or equal to three flashes/min. The IC lightning dominated over CG lightning in the early stages of vertical development. The observed temporal relationship between the peak lightning rate and the peak outflow velocity was very systematic in the storms studied. The evolution of total lightning rate, CG lightning rates (all events located within 20 km of the Doppler radar), and maximum differential velocity at the ground are recorded for several case studies, which are discussed briefly in chronological order. The velocity differential across all outflows reported here exceeded 20 m/s. A headwind-tailwind change of 10 m/s is of concern to commercial aviation, while the largest microburst velocity differentials observed by Doppler radars have been roughly 40-50 m/s. All lightning rates were tabulated in 1-min time intervals. All microbursts studied were classified as "wet" 13,213

Characteristics of the Water-Vapor Field over the Kanto District Associated with Summer Thunderstorm Activities

“…However, some studies showed that IC preceded CG in thunderstorm activity (Williams et al 1989). Thus, time lag between maximum PWV and total lightning activity including IC and CG might be shorter than that between maximum PWV and CG analyzed in this study.…”

Lightning in the eastern Alps 1993-1999, part I: Thunderstorm tracks

“…It deliberately excludes cloud-to-cloud flashes since they are irrelevant for the main purpose of the network: detection of strikes leading to damage and the evaluation of lightning risk. This is disadvantageous for meteorological sciences since the main share of all flashes are cloud-to-cloud flashes and the flash activity of a thunderstorm cell often starts with cloud-to-cloud flashes (Williams et al, 1989).…”

Recent progress in the operational forecasting of summer severe weather

“…Another major user of lightning data is the hydro utilities, where lightning strikes can cause havoc with the distribution of electrical power. Recent case studies have shown that lightning is related to other severe weather hazards -downbursts, tornadoes, heavy rains (Williams et al, 1989;Kane, 1991;Keighton et al, 1991). The lightning flash density for slow-moving high-rainfall storms has been related to heavy rain events (Clodman and Chisholm, 1994a).…”

“…Since there are few Doppler radars across the country at this time, this study attempted to verify a three-dimensional reflectivity algorithm for the occurrence of downbursts (Stewart, 1991) with Doppler observations. d Lightning Lightning is a complex electrical phenomenon and is very difficult to forecast, partly because it is not fully understood (Uman, 1987;Williams et al, 1989). Forecasts for the Canadian forestry program have focussed on the expected fire damage in relation to ground lightning counts.…”