Saturday, June 28, 2014

Lightning and Topography

Given that this is lightning week, it seems quite appropriate that a paper examining the influence of topography on lightning strikes came out this week in Monthly Weather Review (Vogt and Hodanish 2014; paywalled, but I'll hit the highlights).

Specifically, they used data from the North American Lightning Detection Network (NLDN) operated by Vaisala to document the characteristics of 12.5 million cloud-to-ground lightning strikes over Colorado from 2003–12.  The topographic map below highlights some of the key topographic features that influence lightning strikes within the state.
Source: Vogt and Hodanish (2014)
The map below shows the stroke density during the warm season (1 April to 31 October).  One can clearly see the influence of elevation, but also some important geographic effects.  Stroke densities are highest in the high terrain encompassing Pikes Peak, the southern Front Range, and along the Palmer Divide east of east of Pikes Peak.  High densities are also found in the high terrain near the southern Sangre de Cristos and along Raton Mesa.  Low densities are found in the San Luis Valley, as well as lower elevations regions along the Gunnison and Uncompahgre Rivers near and southeast of Grand Junction (not shown in the above map), far northwest Colorado, and along the I-25 corridor in northern Colorado. One can clearly see the influence of local terrain throughout the western half of the state, but with this color scale it is especially apparent in the San Juan Mountains.

The influence of elevation and topography is summarized nicely in the graph below, which shows the stroke density as a function of elevation. There is a peak at 5500–6000 ft that reflects the high stroke density of the Palmer Divide and Raton Mesa (point D), a minimum at 7500–8000 ft that reflects the low stroke density of the San Luis Valley (point D), and then a very clear increase above 10,500 feet reflecting the increasing stroke density in the high mountains of the state.
Source: Vogt and Hodanish (2014)
Now is a good time to mention that low stroke density doesn't mean no stroke density. Climatological stroke densities may be lower along the northern I-25 corridor than in the Front Range to the west, but you wouldn't catch me golfing or mowing the lawn there if there was a storm nearby.  Ditto in the San Luis Valley and other broad lowland regions.  If you are in the mountains, you can minimize risk by moving to lower elevations, but if you can, it's still best to seek shelter in an enclosed building or hard-topped vehicle.  If you are car camping in a valley and you hear thunder, best to be in the car than your tent.

It would be great to see a product like this produced for Utah.  For you atmospheric sciences undergraduates out there, producing such a map would make a great senior capstone project.

1 comment:

  1. I think Utah would be a really interesting state for this... including the diurnal variations of lightning activity. Most of the mountainous western U.S. has a strong afternoon maximum, while portions of Utah may be an exception to this rule.