1APRIL 2007 K ILINC AND BERINGER 1161 The Spatial and Temporal Distribution of Lightning Strikes and Their Relationship with Vegetation Type, Elevation, and Fire Scars in the Northern Territory MUSA KILINC AND JASON BERINGER School of Geography and Environmental Science, Monash University, Victoria, Australia (Manuscript received 17 October 2005, in final form 3 July 2006) ABSTRACT In this paper the authors explore the spatial and temporal patterns of lightning strikes in northern Australia for the first time. In particular, the possible relationships between lightning strikes and elevation, vegetation type, and fire scars (burned areas) are examined. Lightning data provided by the Bureau of Meteorology were analyzed for a 6-yr period (1998–2003) over the northern, southern, and coastal regions of the Northern Territory (NT) through the use of Geographical Information Systems (GIS) to determine the spatial and temporal characteristics of lightning strikes. It was determined that the highest densities of lightning strikes occurred during the monsoon transitional period (dry to wet) and during the active monsoon periods, when atmospheric moisture is highest. For the period of this study, lightning was far more prevalent over the northern region (1.21 strikes per km2 yrϪ1) than over the southern (0.58 strikes per km2 yrϪ1) and coastal regions (0.71 strikes per km2 yrϪ1). Differences in vegetation cover were suggested to influence the lightning distribution over the northern region of the NT, but no relationship was found in the southern region. Lightning strikes in the southern region showed a positive relationship with elevations above 800 m, but no relationship was found in the northern region, which could be due to the low-lying topography of the area. A comparison of lightning densities between burned and unburned areas showed high variability; however, the authors suggest that, under ideal atmospheric conditions, large-scale fire scars (Ͼ500 m) could produce lightning strikes triggered by either enhanced free convection or mesoscale circulations. 1. Introduction great deal of research on the electrical structure of thunderstorms and the relationships between the po- Information concerning the characteristics of light- larities of the strikes (Latham 1991; Orville 1994; Von- ning strikes in different geographical regions is of in- negut et al. 1995; Orville et al. 2002). However, there is terest and can augment research on the interaction be- very little understanding of the relationship between tween the radiative properties of the surface and the lightning strikes and surface characteristics of eleva- atmosphere. The surface energy balance, albedo, sur- tion, vegetation type, or other surface inhomogeneities, face roughness, and the Bowen ratio are important fac- such as fire scars (burned areas). tors in determining available energy and the partition- Lightning strikes are produced mainly from cumu- ing of the energy fluxes over different surface types lonimbus, which are formed through four mechanisms: (Beringer and Tapper 2002), which are important char- buoyant warm air rising due to intense surface heating, acteristics in determining the microclimate and regional strong heating contrast between surfaces, frontal lifting, climate. The differential heating of two adjacent sur- or by the uplift of air parcels due to orographic lifting faces caused by radiative flux contrasts is likely to cause (Sturman and Tapper 1996). All of these processes may convective activity through uplift and generate meso- trigger convection and, hence, lightning activity. Light- scale circulations (Pielke and Avissar 1990). If the heat- ning originates around 3–4 km above sea level and is ing contrast is large, then enhanced convection may effectively caused by a charge separation that takes occur, leading to the formation of thunderclouds and place within the negatively charged reservoir of the lightning (Dissing and Verbyla 2003). There has been a cloud and the positive electric field of the ground sur- face (Cooray 2003). The resulting discharge is negative, positive, or a cloud-to-cloud stroke. Corresponding author address: Musa Kilinc, School of Geogra- phy and Environmental Science, Monash University, Victoria In the Northern Territory (NT) of Australia (Fig. 1), 3800, Australia. thunderstorms and lightning strikes are most common E-mail: [email protected] during convective periods, when there is a strong influ- DOI: 10.1175/JCLI4039.1 © 2007 American Meteorological Society Unauthenticated | Downloaded 09/25/21 12:47 AM UTC JCLI4039 1162 JOURNAL OF CLIMATE VOLUME 20 FIG. 1. Map of the Northern Territory (Australia) illustrating the three study areas used; northern, southern, and coastal regions. ence from the monsoon. The monsoonal influence dur- tween land and ocean have shown that there is high ing the wet season has a significant role in distributing variability between the distribution of lightning strikes moisture throughout the northern region; however the over the ocean and land (Boccippio et al. 2000; Wil- southern regions are very dry and are influenced by liams and Stanfill 2002; Williams et al. 2002). subtropical cold fronts (Beringer and Tapper 2000). Forced convection, or orographic lifting, triggers con- Thunderstorms develop in homogeneous air masses vection by transporting sensible and latent heat verti- that are associated with convergence zones and insta- cally into the atmosphere via uplift. The convection bility caused by monsoon onset and the active monsoon may then intensify the instability of an area by stronger period (wet season) (Sturman and Tapper 1996). The updrafts and cause lightning discharges. For example, contrast between the heating properties of the ocean the valley/mountain winds produced from differential and land is a trigger for strong sea-breeze development, heating between two sloping surfaces, combined with which can stretch several hundred kilometers inland orographic lifting, can be an instigator of lightning (Simpson 1994). Sea breezes are a form of mesoscale strikes and therefore produces a relationship between circulation and aid in the development of convective elevation and lightning strike density (López and Holle activity via frontal uplift and cause instability, and 1986; Lericos et al. 2002; Orville et al. 2002; Dissing and hence thunderstorms and lightning. Previous studies in- Verbyla 2003). Dissing and Verbyla (2003) found that a vestigating the contrast of lightning strike density be- positive correlation between lightning strike density Unauthenticated | Downloaded 09/25/21 12:47 AM UTC 1APRIL 2007 K ILINC AND BERINGER 1163 and elevation existed up to a maximum elevation of lift over a fire scar. An increased heat source over 1100–1200 m. Mesoscale circulations can aid in the de- burned areas may also lead to buoyancy and result in velopment of convective activity via uplift and cause local and regional instability. In a model used by convergence and instability over the affected area, Knowles (1993), a halving of the albedo over burned which may then trigger thunderclouds and hence light- vegetation areas increased convection and resulted in ning. The relationship between lightning strike distri- the formation of a mesoscale circulation system. bution and vegetation was also studied by Dissing and In this paper we explore the spatial and temporal Verbyla (2003), who found that mesoscale circulations patterns of lightning strikes in northern Australia, for triggered by the differential heating between two con- the first time, and the possible relationships between trasting vegetation types were likely to produce light- lightning strikes and elevation, vegetation type, and fire ning strikes. Surface inhomogeneities provide a heating scars. Through the use of Geographical Information contrast between two adjacent surfaces, which could Systems (GIS), we analyzed lightning data provided by produce mesoscale circulation patterns similar to a sea the Bureau of Meteorology for a 6-yr period (1998– breeze (Segal et al. 1988), though other examples have 2003) over the northern, southern, and coastal regions also been previously studied: snow breeze (Segal et al. of the NT. 1991), salt lake breeze (Tapper 1991), and lake breeze (Laird et al. 2003). 2. Study area Heating contrasts between surfaces arise due to dif- ferences in albedo, surface roughness, and the way in The Northern Territory (Australia) was selected as which energy is partitioned into sensible, latent, and the study area for this project since Christian et al. ground heat flux (Beringer and Tapper 2002). For ex- (2003) estimated that, on average, 44 Ϯ 5 lightning ample, Pielke and Vidale (1995) found that a larger flashes occur around the globe every second, which sensible heat flux over particular vegetation types in- strike between the geographic regions of 30°N and 30°S creased the air temperature and, as a result, triggered and account for approximately 75% of the global light- convection. In the Maritime Continent Thunderstorm ning count (Torancita et al. 2002). In addition, the NT Experiment (MCTEX), Beringer and Tapper (2002) has a variety of vegetation types from grassland to rain- showed that the sensible heat flux among various veg- forest and moderate terrain variability ranging from etation types (savannah, grasslands, forest, and shallow ϳ400 m in the northern region to ϳ1500 m in the south- tidal strait) was an important factor in the production of ern region. A high degree of burning, especially