资源环境科技发展态势分析平台

Positive polarity lightning strokes can be useful indicators of thunderstorm behavior. A combination of National Lightning Detection Network and Next Generation Radar retrievals is used to analyze the anomalous positive cloud-to-ground (CG) lightning behavior of a rare, late summer severe weather event that occurred on 26-27 August 2007 in the Northern Great Plains region of the United States and southern Canada. Seven discrete supercells (SC1-SC7) exhibiting frequent and intense lightning were responsible for numerous reports of severe weather (e.g., severe hail and 16 tornadoes) including catastrophic damage to the town of Northwood, North Dakota, caused by SC2. Biomass burning smoke from wildfires in Idaho and Montana was present prior to convective initiation. A positive CG lightning stroke rate of nearly 30 strokes per minute was observed 10min before the EF4 tornado struck Northwood. SC2 was also responsible for all the reports of tornadoes exceeding an EF2 rating. The strongest peak currents (>200kA) were observed in SC1-SC4 with SC2 having a maximum value of 280kA. SC2 dominated the statistics of the line of supercells accounting for 27% of all CG lightning strokes. Positive CG lightning accounted for over 40% of all CG lightning strokes in SC4-SC7 on average, and the maximum exceeded 90% in SC6 and SC7. Increasing positive CG lightning dominance was correlated with an increasing northward gradient of smoke aerosol loading in addition to severe weather being reported before the maximum in positive CG lighting stroke rate (SC5 and SC6). This suggests that a complex combination of synoptic forcing and aerosol perturbation likely led to the observed anomalous positive CG lightning behavior in the supercells.

Plain Language Summary Lightning can be a useful indicator of thunderstorm intensity. Deep convective clouds that form in areas downwind of wildfire smoke have been known to produce significant severe weather (e.g., large hail, tornadoes, and severe wind) as well as frequent and intense lightning. A late summer severe weather event affecting North Dakota, Minnesota, and Canada was responsible for numerous reports of tornadoes and large hail, including the destruction of Northwood, North Dakota, by an EF4 Tornado. The lightning generated by these storms was frequent and intense. Prior to the development of the storms, wildfire smoke from forest fires in Idaho and Montana was present in the area with much of the smoke concentrated north of the international border. On average, negative cloud-to-ground lightning is dominant and accounts for 90% of all lightning strokes globally. The storms in Canada produced an anomalously high percentage of positive cloud-to-ground lightning, while the in the United States, the severe weather events tended to coincide with noticeable increases in positive cloud-to-ground lightning stroke counts.