Zajac and Weaver (2002), Preprints, Symposium on the Advanced Weather Interactive Processing System (AWIPS), Orlando, FL, Amer. Meteor. Soc. J8.6 Lightning Meteorology I: An Introductory Course on Forecasting with Lightning Data Bard A. Zajac* and John F. Weaver Cooperative Institute for Research in the Atmosphere Colorado State University, Fort Collins, Colorado 1. INTRODUCTION 4. ELECTRIFICATION AND THRESHOLDS The Virtual Institute for Satellite Integration Training Research over the past century has identified the (VISIT) provides forecasters with meteorological training on a gross charge distribution within a typical isolated thunderstorm number of remote sensing topics using distance education and the main mechanism that produces this charge distribution. techniques (Zajac et al. 2002). VISIT has developed a two-part The normal dipole, with positive charge overlying negative course on forecasting with cloud-to-ground (CG) lightning data. charge (Fig. 1c), is a manifestation of the ice-ice collisional The first course, “Lightning Meteorology I: Electrification and charging mechanism. This mechanism describes the exchange Lightning Activity by Storm Scale” describes thunderstorm of charge during collisions between ice particles in the presence electrification and CG lightning activity in most isolated storms of supercooled liquid water (Figs. 2a–b). In the high cloud liquid and mesoscale convective systems (MCSs). The second water (CLW) environment of the convective updraft, the normal course, “Lightning Meteorology II: Anomalous Lightning Activity dipole forms when graupel and ice crystals collide and become and Advanced Electrification” examines the unusual CG oppositely charged, then gravitate to different regions of the lightning activity found in some severe storms and many winter storm due to their large fall speed differential (Saunders 1993; storms. This article provides an overview of Lightning Figs. 2a and 1c). Negatively-charged graupel particles are Meteorology I. suspended at mid-levels or fall out as convective precipitation, while positively-charged ice crystals are lofted to upper-levels. 2. COURSE OBJECTIVES The ice-ice collisional charging mechanism is considered the The broad objective of Lightning Meteorology I is to main charging mechanism in thunderstorms since it is most teach forecasters how to utilize CG lightning data in nowcasting consistent with observations (Saunders 1993). and short-range forecasting. This objective is met by Figure 1 shows how the normal dipole evolves during 1) introducing theoretical concepts on the thunderstorm the thunderstorm lifecycle. Electrification begins with the lifecycle, electrification and CG lightning production and 2) formation of graupel in the towering cumulus stage (Fig. 1b). presenting four AWIPS1 case studies that show consistency The normal dipole is established in the mature cumulonimbus between theory and observation. The theoretical concepts stage as charge is both generated and advected (Fig. 1c). The introduced in Lightning Meteorology I are the simplest concepts normal dipole evolves into a titled dipole as the storm ages and needed to explain the CG lightning activity observed in 80–90% the anvil is advected downshear (Fig. 1d). of the isolated storms and MCSs that occur during the warm The first AWIPS case study examines four storms season. The specific course objectives are: that passed over Fort Collins, Colorado on 28 July 1997. All four • to gain a basic understanding of the ice-ice collisional storms produced heavy rain but only the second two storms, mechanism slightly deeper than the first two, produced CG lightning. An • to identify thresholds in radar reflectivity and satellite analysis revealed the following thresholds for CG lightning. cloud top temperature associated with CG lightning • minimum radar reflectivity at –10°C = 35 to 45 dBZ • to know the charge distributions in thunderstorms and • minimum cloud top temperature = –25 to –30°C their effect on the timing, location and frequency of –CGs The radar threshold indicates that high concentrations of and +CGs. millimeter-sized graupel are needed to support electrification. • to infer storm lifecycle and precipitation type The satellite threshold indicates that a deep cloud is needed for (convective vs. stratiform), location and intensity using oppositely charged graupel and ice crystals to separate. –CGs and +CGs. • to integrate CG lightning data with sounding, satellite 5. ISOLATED THUNDERSTORMS and radar data This section comprises an exercise on induced Lightning Meteorology I is organized into five charge and the second AWIPS case study. The exercise asks sections, which are summarized here in Secs. 3–7. forecasters to consider how the surface of the earth—a conductor—responds to a thunderstorm overhead. Using basic 3. THUNDERSTORM LIFECYCLE physics principles, forecasters determine the location, amount The lifecycle of a typical isolated thunderstorm is and polarity of induced charge as is shown in Figs. 1a–d. reviewed in terms of storm dynamics and microphysics. The The combination of charge in the cloud and induced formation of graupel at mid-levels is emphasized because charge on the earth’s surface produces strong electric fields numerous studies have found correlation between initial and CG lightning. More specifically, charges in the cloud and on electrification and the formation of graupel (e.g., Dye et al. the earth’s surface control the timing, location and frequency of 2 1986). The thunderstorm lifecycle is separated into four stages: –CGs and +CGs . The following characteristics are common to shallow cumulus, towering cumulus, mature cumulonimbus and most warm season isolated thunderstorms: dissipating cumulonimbus (Fig. 1a–d). This four-stage model is an extension of the three-stage model developed by Byers and 1 Braham (1949). A fourth stage is added to capture the growth of The Advanced Weather Interactive Processing System ice crystals into graupel by deposition (i.e., growth from the (AWIPS) is the main tool used by National Weather Service vapor phase) and riming (i.e., the collection of supercooled forecasters. AWIPS merges weather observations and water droplets). numerical model output into a common computer framework. 2 * Corresponding author addresses: Bard Zajac, CIRA, Negative CGs are defined as strikes that neutralize Colorado State University, Fort Collins, Colorado, negative charge within the cloud. Positive CGs neutralize 80523-1375; email: [email protected] positive charge within the cloud. Fig. 1. Four-stage lifecycle of a typical isolated thunderstorm. –40°C [ –10°C 0°C [ Fig. 1a – Shallow Cumulus Fig. 1b – Towering Cumulus Dynamics Dynamics – weak updraft, no downdraft – strong updraft, initial downdraft at mid-levels Microphysics Microphysics – max water supersaturation in updraft – max CLWC in updraft (supercooled above 0°C) – max cloud liquid water content (CLWC) in updraft – small ice crystals ascend; large ice crystals descend – supercooled CLW above freezing level – large ice collects supercooled CLW → graupel – ice nucleation above –10°C Electricity – ice growth by deposition → ice crystals – charge generation at mid-levels Electricity – negative charge on graupel, positive on ice crystals – no graupel, no charging – positive charge induced on surface beneath storm [ [ [ –40°C [ –10°C 0°C [ [ Fig. 1c – Mature Cumulonimbus Fig. 1d – Dissipating Cumulonimbus Dynamics Dynamics – strong updraft, strong downdraft with outflows at sfc – weak updraft, weak downdraft Microphysics Microphysics – max CLWC in updraft (supercooled above 0°C) – weak updraft cannot support water supersaturation – large ice collects supercooled CLW → graupel – no supercooled CLW, no graupel production – melting/evaporation of graupel enhances downdraft – residual graupel falls out of storm Electricity Electricity – charge generation and advection → normal dipole – no charge generation; charge advection continues – normal dipole: positive charge above negative – tilted dipole: positive charge downshear of negative – enhanced positive charge on sfc → –CG strike – enhanced negative charge on sfc → +CG strike 2a ice 2b ice [ crystal crystal aggre- graupel gate [ high CLW low CLW Fig. 2. Conceptual model of the ice-ice collisional charging mechanism as a function of cloud liquid water (CLW). (a) Charging in a high CLW environment representative of convective updrafts. The dotted gray line indicates the ascent of the ice crystal with respect to the stationary graupel particle. (b) As in (a), except for a low CLW environment representative of stratiform updrafts. Less charge is transferred in low CLW collisions than in high CLW collisions. • –CGs are associated with the fallout of convective 8. CONCLUSIONS precipitation Lightning Meteorology I demonstrates the utility of CG • +CGs are associated with upper-levels and often with the lightning data in nowcasting and short-range forecasting, anvil including warning environments. –CGs provide information • –CGs greatly outnumber +CGs about convective precipitation, while +CGs provide information These statements are supported by the second about vertical wind shear and stratiform precipitation. AWIPS case study as well as other studies (e.g., Lopez et al. 1990). Figure 3 shows that the onset of –CGs was roughly Acknowledgements coincident and collocated with the onset of heavy precipitation The authors thank the following individuals for at the surface. As the storm evolved, –CGs continued to be reviewing the session: