LIGHTNING-RAINFALL RELATIONSHIPS IN AN'ISOLATED THUNDERSTORM OVER THE MID-ATLANTIC STATES Richard J. Kane National Weather Service Forecast Office Sterling, Virginia Abstract weak and dissipating radar echo. Afterwards, the echo Temporal and spatial relationships between c1oud-to­ promptly began to regenerate and intensify in the same vol­ ground (CG) lightning and precipitation were examined for ume through which the lightning passed, apparently the result an isolated nocturnal thunderstorm over the mid-Atlantic of rapidly growing precipitation particles. states. The lightning flash density field was compared to Kinzer (1974) used sferics (electromagnetic signals from the rainfall pattern. ~dditionally, the volumetric and spatial lightning) to correlate CG strikes with radar reflectivity for distribution of rainfall were related to the concentration of thunderstorms in Oklahoma. His results suggest that the CG lightning strikes. Also, the peak occurrence of CG light­ areas of greater reflectivity were likely regions of greater CG ning strikes within 10 km, 20 km, and 30 km of the National lightning frequency and that, on the average, the lightning Weather Service forecast office at Sterling, Virginia was increased rapidly with an increase in the radial depth of compared to the amount and time of the greatest rainfall reflectivity. Furthermore, there was a disproportionate and rainfall rate. increase in CG strikes as the amount of radar-estimated rain­ The maximum rainfall coincided well with those areas that fall increased. Battan (1965) examined the relationship received the highest concentration of CG lightning strikes. between rainfall and CG lightning frequency for thunder­ The greatest concentration of strikes (57% of the tot.al storm storms in Arizona. On days when it rained heavily (mean CG strikes) produced just over half of the total volumetric rainfall per day per rain gage exceeded 0.1 in.) there was an precipitation over only 16% of the area that received rainfall. average of 56 times more lightning strikes than on those The heaviest rainfall on station began jllst after the 5-min days with light rain. That is, the greater the rainfall from CG lightning peaked within the 10 km, 20 km, and 30 km convection over the area of study, the greater the CG strikes. radii of the station. The greatest rainfall rate was recorded Furthermore, results indicated that the rainfall at the ground in the 5 to 40 min period following the peak in the 5-min CG and the frequency of CG strikes increased as the number, lightning on station. size and duration of the convective clouds increased. Reap and MacGorman (1989) examined a large number of CG lightning strikes over two warm seasons in Oklahoma. 1. Introduction They found a high correlation between the peak CG activity There is an intrinsic relationship between lightning and and echo intensity, especially for negative discharges to precipitation which emanates from charge separation mecha­ ground. Similarly, Rutledge and MacGorman (1988) col­ nisms. When water exists below freezing in multiple phases, lected data for the entire life of a midwestern Mesoscale vertically developing clouds tend to become very thermo­ Convective System (MCS). The peak in negative CG activity electrically active (Stow 1969). In all probability, several was well correlated with the maximum in--convective rainfall, different charge mechanisms act simultaneously to achieve while the peak in positive CG lightning was in agreement cloud electrification (Latham 1981; Pierce 1986). Workman with the maximum in the trailing stratiform rainfall. and Reynolds (1949) found that precipitation was an impor­ Nielsen et al. (1990) observed a substantiallightning-rain­ tant part of the electrification process. They observed that fall correlation during the life (II hrs) of an MCS over the cloud-to-ground (CG) lightning strikes were generated from southern Great Plains. The total (positive and negative) CG a developing cloud a few minutes after precipitation appeared lightning rates closely followed the trends in convective rain from the base of the cloud. Additionally, Reynolds and Brook flux during the entire event. Similarly, Goodman (1990) (1956) observed that precipitation was a necessary, but not observed a high correlation between the rain flux and CG sufficient, condition in thunderstorm electrification. The flash rate for an MCS in the Tennessee Valley. In addition, presence of a detectable radar echo alone did not automati­ Goodman (1990) presented a comparison of the CG lightning cally result in electrification unless there was rapid vertical and rainfall for the life-cycles of 10 Mesoscale Convective development. Likewise, Lhermitte and Williams (1984) Complexes (MCCs; Maddox 1980) . A substantial correlation found that convective development and the subsequent existed between the highest CG lightning rates and rainfall growth of the radar echo were correlated with bursts of approximately 2 hrs prior to MCC maturation. On the aver­ electrical activity. age, MCCs produce their heaviest rainfall just prior to MCC Moore et al. (1962) examined the enigmatic relationship maturation (Kane et al. 1987). that exists when rain gushes (rapid discharges of rain) were Grosh (1978) examined the relationship between the aver­ observed following a lightning flash. Their findings implied age radar echo volume, echo height, rain flux, rainfall rate, that lightning may possibly cause the rain gush by greatly and lightning flash rate. For a single storm, he found that enhancing the coalescence of cloud droplets. Additionally, there was a strong relationship between the peak rain flux, Moore et al. (1964) observed that after a CG lightning strike echo growth, and the lightning frequency. However, the from an area in a weak radar echo, the area sometimes inten­ maximum rainfall rate lagged the peak in flash rate by several sified rapidly. This was suddenly followed by a rapid dis­ minutes. Likewise, Goodman et al. (1988) found a lag of a charge (gush) of rain or hail. Szymanski et al. (1980) observed few minutes between the peak flash rate (total IC and CG an intra-cloud (IC) discharge that passed through a relatively lightning) and the maximum rain flux and rain rate in a micro- 2 Volume 18 Number 3 December, 1993 3 burst-producing storm over the southeastern United States. east just south of the surface trough and intersected it across However, the peak flash rate was well correlated with the the Delmarva peninsula (Fig. I). Also at 850 mb, the mixing l maximum storm mass, vertically integrated liquid (VIL), ratio was at a maximum (10 g kg· ) across northern Virginia echo volume, and cloud height. Ellison (1992) found the lag and central Maryland (Fig. 2). Similarly, an axis of 850 mb time between the peak CG lightning rate and the maximum moisture convergence extended across the Mid-Atlantic with rainfall rate for a convective system in New Mexico to be a maximum over southwest Virginia and southern West Vir­ as much as 45 min. Buechler et al. (1990) investigated 21 ginia (Fig. 2). At 500 mb (Fig. 3), the mean ridge was over the small thunderstorms over the southeastern United States Mississippi Valley and extended from Minnesota southward and found a high correlation between the peak 10-min CG into Arkansas. Meanwhile, a short wave trough was propa­ lightning rate and the maximum rain flux. However, in one gating across the northeastern United States with 30-m height storm there was a lag in the maximum rain flux by 10 min. falls extending from Albany, New York south to Atlantic This temporal lag in precipitation is similar to findings of City, New Jersey. At 300 mb, (Fig. 4) a 26 m S·I to 36 m s' Piepgrass et al. (1982) in which the peak rainfall followed I (50 to 70 kt) jet stretched from the Great Lakes through the peak in total flashes (IC and CG) by 5 to 10 min. Similarly, northern Virginia and into New England. Assuming little Shackford (1960) suggested a temporal lag of about 3 to 4 change in the upper-level wind field from 0000 UTC to 0300 min between heavy ra\n arrival at a particular station and the UTC, the convection developed just on the south edge of first lightning strikes within a one-mile radius of the station. the strong height gradient (jet) and propagated southward in On 3 June 1991, precipitation echoes first appeared (esti­ a diffluent upper-level flow. The 850-500 mb thickness pattern mated from radar reports) about 0245 UTC over north-central (not shown) extended from northwest to southeast and was Maryland. The convective system then intensified as it prop­ also diffluent over Virginia. agated southward into Virginia passing over the National Surface dewpoints ranged from 18.3°C (65°F) to 21.10C Weather Service Forecast Office (NWSFO) at Sterling, Vir­ (70°F) across Virginia and Maryland. Atmospheric stability ginia (WBC) and Dulles Airport (lAD). This investigation parameters I based on the upper-air sounding at lAD (not examines the lightning-rainfall relationships of this isolated shown) were characteristic of relatively slow moving single slow-moving nocturnal thunderstorm. It was characterized or multi-cell thunderstorms. Convective Available Potential l by a slow southward propagation which resulted in a local­ Energy (CAPE) was 1218 J kg· . This amount of buoyant ized precipitation maximum of 11 cm (over 4 in.). The tempo­ energy wasjust below the lower threshold for moderate insta­ 1 l ral and spatial characteristics of the attendant CG lightning bility (1500 J kg· to 2500 J kg· ) defined by Weisman and were examined in relation to the precipitation field and the Klemp (1986). The atmosphere was conditionally unstable amount of rainfall received on station. Unlike previous stud­ with a Lifted Index (U) of -4, K-Index 31, and Total Totals ies, the CG lightning flash density field was compared to the of 45. The precipitable water was 4.17 cm (1.64 in.), indica­ precipitation field.