Evaluating Thunderstorm Outflow Boundaries in Complex Terrain

Luchetti, Nicholas Thomas

Graduate Thesis Or Dissertation

Evaluating Thunderstorm Outflow Boundaries in Complex Terrain Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/cz30pv032

Abstract

Rapid changes in wind and turbulence generated by thunderstorm gust fronts (GFs) can threaten the safety of wildland firefighters. Firefighters are particularly vulnerable to GFs in areas of complex terrain, where local terrain influence on GF winds can add complexity and tactical challenges faced by emergency management teams. Despite this, our traditional understanding of GF structure and behavior has focused primarily on GFs that develop from organized thunderstorms in flatter regions. In this dissertation, the investigation of GFs is broadened to those that occur in areas of complex terrain. First, Chapter 2 focuses on quantifying changes in vertical profiles of atmospheric variables during 24 GFs that occurred in the Colorado Front Range using in-situ and remote-sensing instruments. Second, in Chapter 3, GFs that were pushed uphill atop the crest of the Mogollon Rim in Arizona are statistically compared to those that propagated down into or along the Rio Grande Valley in New Mexico using in situ and radar observations. Lastly, in Chapter 4, idealized microburst and outflow boundaries are simulated using the Weather Research and Forecasting (WRF) model to quantify canyon-enhancement of winds and turbulence generated by microbursts.

In Chapter 2, it was found that the magnitude change in GF characteristics were on average weaker in the Colorado Front Range GFs compared to what is typically observed along GFs from organized, severe storms in flatter regions. Most events from this study experienced an increase in wind speed from 1 to 8 m s-1, relative humidity from 1 to 8 %, and weak vertical motion from 0.3 to 3.6 m s-1 during GF passage, while temperature dropped by 0.2 to 3° C and ii turbulent kinetic energy (TKE) peaked at > 4 m² s^-2. Vertical profiles reveal that these changes vary little with height in the lowest 300 m.

In Chapter 3, GFs that propagated down into and along the Rio Grande Valley in NM were associated with faster propagation speeds (pspd= 8.6 m s^-1), slightly larger decreases in temperature (∆T = -2.2°C), larger increases in horizontal wind speeds (∆wsp= 7.5 m s^-1) and changes in wind direction (∆wdr = 76.5°) compared to GFs that reached the crest of the Mogollon Rim in Arizona (pspd = 5.2 m s^-1; ∆T = -1.5°C; ∆wsp= 3.5 m s^-1; ∆wdir = 53.1°). GFs atop the Mogollon Rim in Arizona behaved less in accordance with density current theory compared to those in the Rio Grande Valley in New Mexico.

In Chapter 4, short-distance microbursts produced stronger canyon-induced enhancements in horizontal wind, upward vertical velocity, and TKE in the canyons and along the canyon walls compared to long-distance microbursts, which were located ~2 km farther away. For canyons located closer to the microburst, the increase in horizontal winds, vertical velocity, and TKE was generally stronger in the canyon and along the walls of the steeper 30° sloped canyons compared to the 10° sloped canyons. Lastly, the maximum increase in horizontal wind is mostly observed near the canyon floors and towards the exit region of the canyons regardless of the proximity to the microburst. Conversely, the maximum increase in upward vertical velocity and TKE is mostly observed at higher elevations on the walls and along the canyon crests in both short- and long-distance canyons.

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