6.2 Lightning Characteristics Relative to Radar, Altitude and Temperature for a Multicell, MCS and Supercell over Northern Alabama

Wednesday, 25 January 2017: 10:45 AM
Conference Center: Tahoma 1 (Washington State Convention Center )
Retha Matthee Mecikalski, University of Alabama, Huntsville, AL; and L. D. Carey

Cloud electrification leads to the production of nitrogen oxides (NOx), which has an effect on ozone concentrations. Currently large uncertainty exists regarding the contribution of lightning to the global and local NOx budget, even on a per flash basis. Most of the lightning NOx (LNOx) models distribute the LNOx at reflectivities (Z) ≥ 20 dBZ in the horizontal, while vertically, a Gaussian distribution function with a peak at -15 °C is used for cloud-to-ground (CG) flashes and a bimodal distribution function with peaks at -15 °C and -45 °C is used for inter- and intra-cloud (IC) flashes. This research aims to improve our basic understanding of lightning location relative to radar Z as a function of storm and flash type. Using data from the North Alabama Lightning Mapping Array (NALMA) and the Multi-Radar Multi-Sensor data suite, the results from analyzing an ordinary multicell storm, a mesoscale convective system (MCS) and a supercell complex showed that 29.7 %, 15.9 % and 6.9 % of all flashes initiated in regions where Z < 20 dBZ, respectively; these percentages are higher than was initially thought. The bimodal lightning initiation distribution for IC flashes was also not observed for any of the three storms. What’s more, the -45 °C upper peak for ICs used in certain LNOx models are only applicable to the supercell storm whereas for the multicell and MCS the peak of the distribution of IC flash initiation points occur between roughly -14 °C and -22 °C. Likewise, the peak of the distribution of the CG flashes occurs at warmer temperatures (around 20 °C warmer for the multicell storm and 10 °C warmer for the MCS and supercell storms) than is currently used in some LNOx models.

Previous research have also shown that NOx occurs at temperatures > 3,000 K, while stepped leaders have characteristic channel temperatures of ~10,000 K. Because LMAs record stepped leaders, and these stepped leaders could lead to the production of LNOx, it is imperative that current LNOx models start incorporating the region where the flash propagates by including the location of all LMA radiation sources. Our research shows that when flash propagation is included, the percentage of NALMA lightning sources located in regions where Z < 20 dBZ increases. In fact, for the multicell storm, the percentage of sources located at Z < 20 dBZ increases to ~47 %.

Finally, we introduce a third flash type, Hybrid flashes, to the currently used IC and CG flash types that are distinguished in LNOx models. If flash size is important in LNOx modelling, then it is crucial that this third flash type be included, as our results show that Hybrid flashes have consistently larger sizes than IC and CG flashes; while IC and Hybrid flashes tend to have more NALMA sources located at Z < 20 dBZ than CG flashes.

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