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Strong winds generated by thunderstorm gust fronts can cause sudden changes in fire behavior and threaten the safety of wildland firefighters. Wildfires in complex terrain are particularly vulnerable as gust fronts can be channeled and enhanced by local topography. Despite this, knowledge of gust front characteristics primarily stems from studies of well-organized thunderstorms in flatter areas such as the Great Plains, where the modification of gust fronts by topography is less likely. Here, we broaden the investigation of gust fronts in complex terrain by statistically comparing characteristics of gust fronts that are pushed uphill and propagate atop the Mogollon Rim in Arizona to those that propagate down into and along the Rio Grande Valley in New Mexico. Using operational WSR-88D data and in situ observations from Automated Surface Observing System (ASOS) stations, 122 gust fronts in these regions are assessed to quantify changes in temperature, wind, relative humidity, and propagation speed as they pass over the weather stations. Gust fronts that propagated down into and along the Rio Grande Valley in New Mexico were generally associated with faster propagation speeds, larger decreases in temperature, and larger increases in wind speeds compared to gust fronts that reached the crest of the Mogollon Rim in Arizona. Gust fronts 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. The potential reasons for these results, and their implications for our understanding of terrain influence on gust front characteristics, are discussed.

Fire safety, aviation, wind energy, and structural-engineering operations are impacted by thunderstorm outflow boundaries or gust fronts (GFs) particularly when they occur in mountainous terrain. For example, during the 2013 Arizona Yarnell Hill Fire, 19 firefighters were killed as a result of sudden changes in fire behavior triggered by a passing GF. Knowledge of GF behavior in complex terrain also determines departure and landing operations at nearby airports, and GFs can induce exceptional structural loads on wind turbines. While most examinations of GF characteristics focus on well-organized convection in areas such as the Great Plains, here the investigation is broadened to explore GF characteristics that evolve near the complex terrain of the Colorado Rocky Mountains. Using in situ observations from meteorological towers, as well as data from wind-profiling lidars and a microwave radiometer, 24 GF events are assessed to quantify changes in wind, temperature, humidity, and turbulence in the lowest 300 m AGL as these GFs passed over the instruments. The changes in magnitude for all variables are on average weaker in the Colorado Front Range than those typically observed from organized, severe storms in flatter regions. Most events from this study experience 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 drops by 0.2°–3°C and turbulent kinetic energy peaks at >4 m 2 s −2 . Vertical profiles reveal that these changes vary little with height in the lowest 300 m.

The sea-breeze (SB) is an important source of summertime precipitation in North Carolina (NC, southeast United States). However, not all SB events produce precipitation. A climatology of wet and dry SB events in NC is used to investigate the conditions that are conducive to precipitation associated with the sea breeze. Radar imagery was used to detect 88 SB events that occurred along the NC coast between May-September of 2009-2012. The majority (85%) of SB events occurred during offshore flow (53%) or during flow that was parallel to the coast (22%). SB events were separated into dry (53%) and wet (47%) events and differences in the dynamic and thermodynamic parameters of the environment in which they formed were analyzed. Significant differences in dynamic and thermodynamic conditions were found. SB dry events occurred under stronger winds (6.00 ± 2.36 ms-1) than SB wet events (4.02 ± 2.16 ms-1). Moreover, during SB wet events larger values of convective available potential energy and lower values of convective inhibition were present, conditions that favor precipitation. Overall, the SB wet events accounted for 20-30% of the May-September precipitation along the NC coastal region. The position of the North Atlantic Subtropical High (NASH) controls both moisture availability and winds along the NC coast, thus providing a synoptic-scale control mechanism for SB precipitation. In particular, it was shown that when the NASH western ridge is located along the southeast coast of the United States, it causes a moist southwesterly flow along the NC coast that may favor the occurrence of SB wet events.

There are more than 2,000 islands across Hawaii and the U.S.-Affiliated Pacific Islands (USAPI), where freshwater resources are heavily dependent upon rainfall. Many of the islands experience dramatic variations in precipitation during the different phases of the El Niño–Southern Oscillation (ENSO). Traditionally, forecasters in the region relied on ENSO climatologies based on spatially limited in situ data to inform their seasonal precipitation outlooks. To address this gap, a unique NOAA/NASA collaborative project updated the ENSO-based rainfall climatology for the Exclusive Economic Zones (EEZs) encompassing Hawaii and the USAPI using NOAA’s PERSIANN Climate Data Record (CDR). The PERSIANN-CDR provides a 30-yr record of global daily precipitation at 0.25° resolution (∼750 km² near the equator). This project took place over a 10- week NASA DEVELOP National Program term and resulted in a 478-page climatic reference atlas. This atlas is based on a 30-yr period from 1 January 1985 through 31 December 2014 and complements station data by offering an enhanced spatial representation of rainfall averages. Regional and EEZ-specific maps throughout the atlas illustrate the percent departure from average for each season based on the Oceanic Niño Index (ONI) for different ENSO phases. To facilitate intercomparisons across locations, this percentage-based climatology was provided to regional climatologists, forecasters, and outreach experts within the region. Anomalous wet and dry maps for each ENSO phase are used by the regional constituents to better understand precipitation patterns across their regions and to produce more accurate forecasts to inform adaptation, conservation, and mitigation options for drought and f looding events.

La brisa marina (BM) es una importante fuente de precipitación de verano en Carolina del Norte (NC en su sigla en inglés), sudeste de Estados Unidos. Sin embargo, no todos los eventos de BM producen precipitación. En este trabajo se utiliza una climatología de eventos de BM lluviosos y secos en NC para investigar las condiciones que conducen a la precipitación. Se utilizaron imágenes de radar para detectar 88 eventos de BM ocurridos a lo largo de la costa NC entre mayo y septiembre de 2009 a 2012. La mayoría (85%) de los eventos de BM ocurrieron durante períodos de viento hacia el mar (53%) o viento paralelo a la costa (22%). Los eventos BM se separaron en eventos secos (53%) y lluviosos (47%) y se analizaron las diferencias en los parámetros dinámicos y termodinámicos del entorno en el que se formaron. Se encontraron diferencias significativas en las condiciones dinámicas y termodinámicas. Eventos de BM secos ocurrieron bajo vientos más fuertes (6,00 ± 2,36 ms-1) que los eventos de BM lluviosos (4,02 ± 2,16 ms-1). Las BM lluviosas ocurrieron bajo valores de energía potencial convectiva disponible más altos y valores del parámetro de inhibición convectiva más bajos, condiciones que favorecen la lluvia. En general, los eventos de BM lluviosos representaron el 20-30% de la precipitación a lo largo de la región costera de NC de mayo a septiembre. La posición de la Alta Subtropical del Atlántico Norte (ASAN) controla la disponibilidad de humedad y los vientos a lo largo de la costa de NC, proporcionando así un mecanismo de control de escala sinóptica para la precipitación de la BM. En particular, cuando la cresta occidental de la ASAN se localiza a lo largo de la costa sureste de los Estados Unidos, se produce un flujo de sudoeste húmedo a lo largo de la costa NC que puede favorecer la ocurrencia de eventos de BM lluviosos.

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 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 (∆wdir = 76.5°) compared to GFs that reached the crest of the Mogollon Rim in Arizona (pspf = 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.

Forecasting SB genesis and evolution is often a difficult task for coastal meteorologists. This is especially the case when attempting to forecast SB-induced precipitation, as not every SB front induces rainfall. The two primary objectives in this thesis were to 1) study why some SB fronts induce precipitation while others do not, and 2) to explore the effects of a future warmer climate on SB evolution. To explore these objectives, a SB climatology for 2009–2012 along the NC coast was constructed using radar and reanalysis datasets. Additionally, current and future climate SB simulations were produced using the Weather Research and Forecasting (WRF) model. Future climate WRF simulations utilized the pseudo-global warming (PGW) approach, which involves rerunning current climate SB simulations using modified thermodynamic initial conditions that represent a warmer, late 21st century climate. The 88 SB events that were detected between 2009–2012 were nearly evenly distributed into SB dry (53%) and SB wet (47%) events. Significant differences in kinematic and thermodynamic conditions were present during SB dry and wet events. On average, SB dry events occurred under stronger synoptic-scale winds (6.00 ± 2.36 m/s), while SB wet events occurred under lighter synoptic-scale winds (4.02 ± 2.16 m/s). Moreover, most (85%) SB events occur during offshore (53%) or parallel (22%) flow. However, as is the case throughout the literature, the maximization of SB fronts during offshore synoptic flow is sensitive to the synoptic wind speed. SB events that occurred under offshore synoptic-scale flow in the 0 to 4–6 m/s range were more likely to be categorized as SB wet events, while SB events that occurred under offshore synoptic-scale flow above 0 to 4–6 m/s were more likely to be categorized as SB dry events, results similar to those seen in the literature. In terms of thermodynamic controls, results from this climatology show that the atmosphere has larger values of CAPE and lower values of CIN and is therefore more conducive to deep convection on SB wet than on SB dry event days. This study suggests that favorable conditions for the formation of precipitation along the SB include enhanced early morning instability, minimal stable air aloft, and synoptic-scale offshore wind flow with speeds between 0 and 4-6 m/s throughout the duration of the event. Seven of the observed SB precipitation events were simulated in WRF under current climate conditions and repeated for future climate conditions under the RCP 4.5 and RCP 8.5 scenarios. Under current climate conditions WRF performed well in simulating the horizontal extent and late day veering/backing of the SB front, as well as the timing of initiation and peak SB-induced precipitation. However, it struggled to simulate the inland penetration distance of the SB front, as well as the spatial distribution and total accumulation of precipitation along the SB front. Under future climate conditions the evolution of WRF simulated SB fronts was altered resulting in some future SB fronts that induced more precipitation, while other future SB fronts induced less precipitation when compared to the current climate WRF simulations. Additionally, under future climate conditions the inland penetration timing and distance was altered for all of these SB cases when compared to the current climate WRF simulations. In both the current and future climate simulations the synoptic-dynamic shifts in the atmospheric flow appear to have more of an influence on SB evolution and associated precipitation than enhanced temperatures, moisture content, and instability. Subtle shifts in the synoptic-scale wind direction and speeds along the coast, associated with a westward migration of the North Atlantic Subtropical High’s western ridge, had considerable influence on the amount and spatial distribution of future climate SB-induced precipitation. From a climatological perspective, the results herein suggest that understanding the effects of climate change on mesoscale precipitation patterns is a very complex task. From a forecasting perspective, the results presented herein suggest that subtle kinematic and thermodynamic shifts in the atmosphere will influence NC SB evolution in both the current and future climate.