May 26 - July 31
An Investigation into the Characteristics of Lightning as it Interacts with Towers
Hayden Webb, Kristin M. Calhoun, and Darrel M. Kingfield
What is already known:
- Areas within 1 km of tall towers show an abnormally increased cloud-to-ground (CG) lightning density compared to their surrounding region.
- Lightning can begin at a tower location and propagate upward towards a storm.
- This upward lightning can be triggered by a nearby CG flash, but may also be self-initiated.
What this study adds:
- Analyzed and classified how lightning interacts with towers across different seasons and storm types using a detailed breakdown of each flash from very-high frequency (VHF) lightning mapping.
- Of the 45 flashes investigated, 60% were CG flashes that terminated at the tower, 20% remained in-cloud flashes, and 20% of flashes were classified as upward lightning, mainly triggered by a nearby preceding CG.
- Lightning triggered upward flashes averaged fewer VHF points than other flash types.
Abstract:
Tall structures can be the subject of lightning interaction and often lead to an increased cloud-to-ground (CG) flash rate in the vicinity. Lightning that interacts with towers can take the form of traditional CG flash, beginning in-cloud and propagating downward to connect with the tower, but flashes may also begin at the tower itself. However, the nature and overall percentage of lightning propagating from tower locations is unknown. This study examines lightning interactions with 10 communication towers from 213 to 610 m tall across Oklahoma using CG data from the National Lightning Detection Network (NLDN) and VHF data from the Oklahoma Lightning Mapping Array (LMA), and radar data from the Weather Service Radar - 1988 Doppler (WSR-88D) network. The NLDN data was used to first determine the specific time (in milliseconds), latitude, and longitude of lightning that potentially interacted with these towers. The LMA data was then used to examine the detailed breakdown of each flash as well as any nearby lightning flashes. 45 individual lightning flashes occurring within 1 km of these tower locations were examined in the winter, summer, and fall of 2010 to determine: (1) the initiation location and height, (2) the size (temporally and spatially) of the lightning flash, (3) any seasonal dependence, and (4) the associated the storm structure and storm type. Using these factors, these lightning flashes were classified to better understand the overall percentage of lightning triggered by the tower and the environmental factors contributing to the occurrence of upward- triggered lightning. Of the 45 flashes, 60% of CG flashes interacted directly with the tower, 20% were classified as in-cloud flashes, and 20% of flashes were classified as upward lightning, mainly triggered by a nearby preceding CG. The initiation distance between these three main flash types (44/45 overall flashes) occurred between 7-9 km from the tower. CG flashes, beginning in-cloud and terminating at the tower, were the dominant flash type across all seasons, independent of the convective mode and the storm structure.