Thunderstorm and Terrain Interactions over the Grand Canyon Region

Thunderstorms 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 particular interest since thunderstorms (i) move over terrain that features an abrupt drop in elevation over a short horizontal distance, and (ii) present a significant hazard to tourists that congregate near the rim of the canyon.  Preliminary analysis of composite radar reflectivity imagery for select thunderstorm events suggests that 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 questions: do thunderstorms typically weaken as they traverse the Grand Canyon? What are the physical mechanisms linked to storm weakening?  How does storm intensity evolve within the average in-cloud charging layer as these storms move over the canyon floor?

            To address these questions, 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, which is the widest portion of the canyon (~15 km).  The second domain spans the majority of the Grand Canyon, ranging from 36.0 – 36.8° N and 112.0 – 113.5° W.  Both of these 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 radar climatology for domains one and two were derived from the Next-Generation Radar (NEXRAD) sites located at Flagstaff, AZ (KFSX) and Cedar City, UT (KICX), respectively. Both domains used radar data from the 2006 – 2015 monsoon seasons, specifically when CG lightning (≥ 10 flashes within 1 hr for domain 1, ≥ 30 flashes within 1 hr for domain 2) was also being reported by the NLDN somewhere within the domain.  In addition to this stipulation, only radar data from the second beam tilt was used since this beam height corresponded closest to the average height of the in-cloud charging layer within the thunderstorms.  The CG lightning climatology was created using all NLDN-detected CG stroke counts from the years 2000 – 2015.

            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.  Both the CG lightning and radar climatology suggest there is a direct relationship between the amount of moisture in the mixed-phase region and the underlying terrain.  This presentation will also cover possible hypotheses as to why thunderstorms may or may not weaken as they move over the canyon floor.