How is cloud-to-ground lightning density influenced by exposed Canadian Shield bedrock?

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Wednesday, 7 January 2015: 9:15 AM
225AB (Phoenix Convention Center - West and North Buildings)
Daniel Martin Brown, University of Alberta, Edmonton, AB, Canada

This research investigates how the sudden change from soil to exposed bedrock (known as the Canadian Shield) affects the cloud-to-ground lightning density in northern Saskatchewan. The Canadian Shield is a large area of exposed Precambrian bedrock in northern Canada. A very abrupt change from exposed bedrock to soil occurs at the southern boundary of the Canadian Shield, which runs approximately northwest to southeast in western Canada. Patterns in cloud-to-ground lightning density are often caused by physiographic features such as topographic variation or large bodies of water. Understanding the spatial and temporal distribution of cloud-to-ground lightning strikes is important because lightning is a hazard to human life and property, and because it is the ignition source of many wildfires. There has been little discussion of how lightning strike density is affected by soil distribution.

The Canadian Lightning Detection Network has been recording cloud-to-ground lightning strikes since 1999. Data from 1999 through 2011 have been used for this analysis. Lightning density maps were created on a 10 km grid in the vicinity of the Canadian Shield boundary for each year, as well as for the full 13 year period. In addition, the lightning strike density was calculated in 10 km increments from 0 to 200 km north and south of the boundary to highlight changes in lightning strike density in the vicinity of the Canadian Shield boundary.

A very abrupt change between two lightning regimes occurs near the boundary between the Canadian Shield and the soils to the south. Lightning density over the 13 year study period was 7.0 strikes per km2 in the zone 0 to 100 km south of the Canadian Shield boundary, while in the zone from 0 to 100 km north of the boundary the lightning strike density was 4.7 strikes per km2. Data from the 10 km buffers suggested that the lightning density hovered around 7.0 strikes per km2 south of the Canadian Shield, dropped dramatically from 6.5 to 5 strikes per km2 in the vicinity of the Canadian Shield boundary, and then slowly dropped to 3.0 strikes per km2 150 km north of the boundary. Lightning density maps suggest that the transition zone between the two lightning regimes mostly coincides with the irregular Canadian shield boundary throughout Saskatchewan. North of the Canadian Shield boundary there is an average lightning density gradient of 0.015 strikes km-2 km-1, while south of the boundary the gradient is near 0 strikes km-2 km-1. Immediately at the boundary the lightning strike gradient jumps to 0.06 strikes km-2 km-1. The cloud-to-ground lightning density gradient along the Canadian Shield boundary does not occur every year; it takes many years of lightning data for the trends to emerge.

It appears that there is a gradual decrease in lightning density farther north as heat, moisture, and instability are limited, and as lightning detection efficiency decreases. The effect of the Canadian shield appears superimposed on this decrease. The following are some possible explanations for the Canadian Shield phenomenon.

1) The lightning detection efficiency of the lightning detection network is lower in northern Saskatchewan than in southern regions, however this does not help explain why there would be a sharp discontinuity along the Canadian shield boundary. It may be possible that the Canadian Shield somehow affects the lightning detection network's ability to detect lightning.

2) Variations in land-atmosphere interactions may cause fewer thunderstorms north of the Canadian shield boundary. For example, fewer plants on the bare bedrock may transpire less causing lower humidity. Large areas of lakes on the bare bedrock may cause cooling at the surface reducing the instability available for thunderstorms to access.

3) The Canadian Shield bedrock has a much higher soil resistivity than the soils on the south side of the boundary. Variations in soil resistivity may affect the ratio of cloud-to-ground lightning versus in-cloud lightning by making it more difficult to redistribute charge in the earth's surface. Charge separation should still occur within the cloud, but charging the ground by induction would be more difficult, suggesting that a higher ratio of in-cloud lightning would exist. It would be difficult to show whether this is the case because the lightning detection network detects less than 5 percent of in-cloud lightning.

There is still not enough data to support any one of these three possibilities. Further research on this topic is required to reach more definitive conclusions.