Normal Polarity Severe Thunderstorms Dominated by Negative Cg Lightning in the Dallas-fort worth Area

BACKGROUND Since the U.S. National Lightning Detection Network (NLDN) became fully operational in 1989, meteorologists have spent considerable time trying to identify cloud-to-ground (CG) lightning characteristics that are uniquely associated with severe weather. Many studies focused on the dominant CG lightning polarity as a potential indicator of severe weather. Knapp (1994) found that 39% of the 556 positive CG dominated (PCGD) thunderstorms (≥ 30% positive CG lightning for an hour) analyzed in their study produced severe weather. This is much larger than the generally accepted value of less than 1% of all thunderstorms producing severe weather. Nevertheless, MacGorman and Burgess (1994) found that many severe thunderstorms are negative CG lightning dominated (NCGD), however the false alarm rate is too large for using it as a severe weather indicator because there are so many NCGD storms that are not severe. The use of 3-dimensional lightning mapping systems, such as the Lightning Mapping Array (LMA), in field programs has shown that PCGD thunderstorms usually have an anomalous electrical structure which is inverted in polarity (i.e., main negative above main positive charge layer) from normal (Rison et al., 1999). Krehbiel et al. (2002) found that severe and supercell thunderstorms were inverted in polarity from normal, non-severe thunderstorms during the STEPS field campaign that took place in northwest Kansas and eastern Colorado in 2000. These inverted storms produced predominantly positive CG lightning flashes. Although there appears to be an important relationship between PCGD storms and severe weather, most of these storms have been studied in the central and northern plains of the United States. A number of other studies have shown that dominant storm polarity has an important regional dependence. Branick and Doswell (1992) found regional variations in dominant ground flash polarity for supercell storms. Lowprecipitation (LP) supercells in northern Oklahoma, Kansas and Nebraska produced predominantly positive CG flashes, while high-precipitation (HP) supercells in central and southern Oklahoma produced predominantly negative CG flashes. Perez et al. (1997) studied thunderstorms producing F4 and F5 tornadoes and found that the PCGD storms were concentrated in the central and northern plains states. Negative CG flashes dominated all the F4 and F5 tornado producing thunderstorms in Texas. Recently, Smith et al. (2000) found that the dominant polarity of CG lightning within storms may be dependent on equivalent potential temperature (EPT) gradients. PCGD storms formed upwind of the EPT maximum and produced severe weather more often than NCGD storms, which occurred downwind of the EPT maximum. Orville et al. (2002) found that less than 20% of all CG flashes on an annual basis are positive in the Dallas-Fort Worth (DFW) area, despite DFW being located in the region of maximum probability of severe weather in the U.S. in April, May and fall (www.nssl.noaa.gov/hazard/index.html). According to Smith et al. (2000), the DFW area should be located downwind of the EPT maximum because most storms are NCGD. If that is the case, then why does it experience such a high percentage of severe weather? On the other hand, if the DFW area is often located upwind of the EPT maximum, where a higher percentage of storms are expected to produce severe weather, then why are most storms NCGD? To date, analysis of a set of severe thunderstorms using the DFW Lightning Detection and Ranging (LDAR II) network (Demetriades et al., 2002) and the NLDN has shown that all of them were of normal polarity and dominated by negative CG lightning despite producing hail up to 7 cm in diameter and strong F2 tornadoes. This paper will cover a couple of case studies that support our claim and show the important role that can be played by normal polarity, negative CG lightning dominated thunderstorms in producing extremely damaging severe weather.