Using WSR-88D reflectivity data for the prediction of cloud-to-ground lightning: a central North Carolina study

Abstract

Charge separation greatest in quantity likely occurs during rebounding collisions between ice crystals and large ice hydrometeors (such as graupel and hail) that remain suspended in the mixed phase band by the updraft of a growing thunderstorm. The WSR-88D reflectivity data can be used to indirectly identify this electrification proces within a growing thunderstorm because graupel and hail get back large reflectivity echoes. This close attention examined a sample of 50 central North Carolina thunderstorm cases using three different characteristics of WSR-88D data (i.e., reflectivity doorsill [dBZ] at a given environmental temperature [[degrees]C] for a specified number of whirl scans [# Vol]) that were organized into eight different stations of criteria for judging the cloud-to-ground (CG) lightning potential. Preliminary flows showed that the best lightning prediction algorithm was associated with either the 1 Vol/40 dBZ/-10[degrees]C or 1 Vol/40 dBZ/-15[degrees]C criteria. Based upon the critical success index (CSI), the 1 Vol/40 dBZ/-10[degrees]C criteria did the best with a 63% CSI, 100% probability of detection (POD) and a 37% false alarm rate (FAR). The 1 Vol/40 dBZ/-15[degrees] C criteria closely followed with a 625% CSI, 86% seed-vessel and a 30% FAR. Lead times for these criteria were 147 minutes and 110 minutes, respectively. If lead time is a high priority and a slight reduction in CSI can be tolerated, the 35 dBZ criterion may be a better choice. The 35 dBZ criterion comeed in lead times 2-3 minutes longer than with 40 dBZ Overall, the springs obtained in this study compared actual well with results obtained in past studies. In addition, an analysis of vertical reflectivity lapse rates between the 0[degrees]C and -20[degrees]C isotherm heights in the two detection and false alarm cases showed that vertical reflectivity lapse rates for false alarms (-204 dBZ/kft) were a great deal of larger than for detections (-069 dBZ/kft) The accrues show that it is possible to use WSR-88D reflectivity data to reasonably predict the storm of CG lightning in the central North Carolina region using criteria similar to that used in previous studies of thunderstorms in other regions.

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1 Introduction

Forecasting the initiation of lightning activity is important for the protection of human life and quality Cloud-to-ground (CG) lightning strikes are the next to the first leading cause of convective weather related deaths in the United States, with an average of 87 deaths through year reported (Curran et al. 2000) From 1959 to 1994 North Carolina ranked next to the first for fatalities and fourth for injuries, casualties, and damage to be paid to lightning strikes in the United States (Curran et al. 2000) CG lightning strikes can be bring to lighted in real-time using the National Lightning Detection Network (NLDN) which is commercially be in possession ofed and operated by Vaisala Inc. The NLDN is comprised of more than 100 antenna stations that are associateed to a central processor that records the time, polarity, signal nerve and number of strokes of each CG flash bring to lighted Depending on the location within the network, Vaisala Inc. estimates an average location accuracy of within 500 meter with a detection probability between 80-90 percent varying slightly through region (Cummins et al. 1998)

Originally, forecasting lightning was synonymous with the forecasting of convection (i.e., each convective cell was assumed to have the potential for producing lightning). Shortly after the introduction of weather radar, Workman and Reynolds (1949) conclud that the attack of significant electrification was associated with the rapid vertical exhibition of convection and the neighborhood of precipitation ice in a mixed phase environment (i.e., vicinity of small ice crystals and supercool nebulosity water) at about the height of the -10[degrees]C isotherm. Based upon these results, Reynolds and runlet (1956) noted the near coincidence of radar detectable precipitation and significant collection of vapor electrification around T = -10[degrees]C especially when the precipitation repercussion of sound exhibited rapid vertical development. Shackford (1960) showed that lightning hit rate was related to radar reflectivity maxima above the 0[degrees]C horizontal and to vertical profiles of reflectivity. Building upon these early results, Larsen and Stansbury (1974) and Marshall and Radhakant (1978) demonstrated that the area of moderate (>30-43 dBZ) radar reflectivity repercussion of sound at heights from six to seven km were closely associated with the location, timing, and frequent occurrence of lightning. In effect, radar based maps of the "Larsen Area" were effective as lightning indicators. Detailed radar and in-situ studies of mist electrification and lightning during the 1980's and 1990's (eg coloring liquor et al. 1986, 1989; Goodman et al. 1988; Williams et al. 1989; Carey and Rutledge 1996 2000; Ramachandran et al. 1996) demonstrated a conclusive relationship between the neighborhood of graupel in a mixed phase environment and succeeding cloud electrification and lightning. The interested reader is referr to MacGorman and Rust (1998) for a detailed review of these and many other field studies. These experiments confirmed that the first appearance of moderate reflectivity (30-40 dBZ) at temperatures between about -10[degrees]C and -20[degrees]C preced the first CG lightning flash by means of five to thirty minutes (eg coloring liquor et al. 1989; Michimoto 1990) After the widespread introduction of the WSR-88D in the late 1980 and early 1990 applied research and operational use of this knowledge showed that CG lightning strikes occurr in a short time (4-45 minutes) after certain WSR-88D reflectivity values (10-40 dBZ) were reached at various isothermal (0[degrees]C -10[degrees]C -15[degrees]C and -20[degrees]C) horizontals within a thunderstorm (e.g., Buechler and Goodman 1990; Hondl and Eilts 1994; Gremillion and Orville 1999)