Tyler Kranz1, Kenneth Cummins1, Thomas Galarneau1, Ron Holle2
1Department of Hydrology and Atmospheric Sciences, The University of Arizona, Tucson, Arizona
2Vaisala Inc., Tucson, ArizonaThunderstorms are common over the complex terrain of the southwest U.S. during the monsoon season (15 July – 30 September). In northern Arizona, these thunderstorms interact with the complex topographic features that make up the Grand Canyon region (defined here as 35.0 – 37.0° N and 111.0 – 114.0° W). This region is of interest because (i) thunderstorms move over terrain that features an abrupt drop in elevation over a short horizontal distance, and (ii) thunderstorms present a significant hazard to tourists that congregate near the rim of the canyon. Analysis of radar reflectivity imagery for 400 separate thunderstorm events suggests that approximately 90% of thunderstorms weaken as they move over the rim of the canyon and experience an abrupt drop in terrain elevation of 1000 – 1500 m over a horizontal distance of < 500 m. Observations from the National Lightning Detection Network (NLDN) reveal that the occurrence of cloud-to-ground (CG) lightning also decreases as these thunderstorms weaken over the canyon floor. This presentation aims to address the following science question: is lightning preferentially attaching to the canyon rims, or are thunderstorms dissipating over the canyon floor?
To address this question, we will focus on two geographic regions that are subsets of the Grand Canyon. The first domain ranges from 35.7 – 36.2° N and 112.2 – 111.5° W. This region captures the section of the canyon that separates the South Rim from the North Rim – the widest portion of the canyon (~15 km). The second domain spans most the Grand Canyon, ranging from 36.0 – 36.8° N and 112.0 – 113.5° W. Both domains are used to study the morphology of thunderstorms that traverse the Grand Canyon during the monsoon season by establishing a spatial radar climatology and a CG lightning climatology. The CG lightning climatology was created using all NLDN-detected CG stroke counts from the years 2000 – 2015 while the radar climatology used the 2006 - 2015 monsoon seasons.
The radar climatology for both domains shows a clear relationship between thunderstorm frequency and the underlying topography; thunderstorms are relatively more frequent over the higher terrain compared to the lower elevation of the Grand Canyon floor. An even stronger relationship to terrain is evident in the CG lightning climatology. In fact, CG stroke counts along the rims of the canyon are nearly ten times greater than within the canyon at certain locations.