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Browsing by Author "Rasmussen, Kristen L., advisor"

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    Changes in winter storm characteristics and lake-effect snow in convection-permitting regional climate simulations in the U.S.
    (Colorado State University. Libraries, 2019) Riesenberg, Mark Ryan, author; Rasmussen, Kristen L., advisor; Schumacher, Russ S., committee member; Fassnacht, Steven R., committee member
    Lake-effect snowfall events have extreme regional impacts with some of the largest snowfall totals on record. Previous studies hypothesize that in a future climate, less ice coverage will be present over the Great Lakes in winter, allowing for more latent and sensible heat fluxes released into the atmosphere. An investigation of the changes in winter season precipitation systems, including lake-effect snowstorms, uses two convection-permitting regional climate continuous 13-year simulations driven by: (1) ERA-Interim reanalysis and (2) ERA-Interim reanalysis plus a climate perturbation for the RCP8.5 scenario. These simulations are used to investigate meteorological and land surface changes in a future climate during the winter months across the U.S. Results from this study show that weak precipitation decreases, while moderate to stronger precipitation is enhanced in a future climate with strong signals over the Great Lakes. Therefore, a lake-effect snowstorm event in the Great Lakes region is used to examine the effects of a warming climate on mesoscale lake-effect snowstorm dynamics and their regional impacts. Analysis of these simulations shows that lake-effect snowstorms in a future climate may have enhanced snow accumulations downwind of the lakes due to more frequent ice-free conditions of the Great Lakes. Enhanced latent and sensible heat fluxes, as a result of less ice-coverage, add moisture and energy to the atmosphere to enhance storm development. The increase in surface fluxes are important for meteorological processes within the planetary boundary layer, which interact with the overlaying atmosphere. These interactions may change the mechanisms that are important for lake-effect snowfall events, such as the 850 mb to surface temperature differences, relative humidity, layer instability, and surface pressure. In addition, less ice coverage may enhance mesoscale circulations due to the thermal contrast (i.e., land-lake breeze) and differential surface roughness. This research will improve our understanding of the question, "What will today's weather look like in a future, warmer climate?" to examine possible socioeconomic and public safety implications of changing precipitation patterns in the winter season.
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    Characteristics of hailstorms and enso-induced extreme storm variability in subtropical South America
    (Colorado State University. Libraries, 2019) Bruick, Zachary S., author; Rasmussen, Kristen L., advisor; Schumacher, Russ S., committee member; Chandrasekar, V., committee member
    Convection in subtropical South America is known to be among the strongest anywhere in the world. Severe weather produced from these storms, including hail, strong winds, tornadoes, and flash flooding, causes significant damages to property and agriculture within the region. These insights are only due to the novel observations produced by the Tropical Rainfall Measuring Mission (TRMM) satellite since there are the limited ground-based observations within this region. Convection is unique in subtropical South America because of the synoptic and orographic processes that support the initiation and maintenance of convection here. Warm and moist air is brought into the region by the South American low-level jet from the Amazon. When the low-level jet intersects the Andean foothills and Sierras de Córdoba, this unstable air is lifted along the orography. At the same time, westerly flow subsides in the lee of the Andes, which provides a capping inversion over the region. When the orographic lift is able to erode the subsidence inversion, convective initiation occurs and strong thunderstorms develop. As a result, convection is most frequent near high terrain. Additionally, convection in this region often remains stationary for many hours by back-building over the high terrain, as the low-level jet continues to orographically lift unstable air over the mountains. This thesis expands the TRMM-based findings on convection in this region in two separate studies: (1) Examination of the El Nino-Southern Oscillation (ENSO)-induced convective variability and (2) Characteristics and environmental conditions supporting hailstorms. The first study uses 16 years of TRMM and reanalysis data to identify how El Nino and La Nina affect storm occurrence and characteristics in this region. While the frequency of storms does not vary greatly between ENSO phases, El Nino conditions tend to promote deeper storms with stronger convection, with more robust synoptic environments supporting convective initiation and maintenance. The second study focuses on the characteristics of the powerful hailstorms that frequent subtropical South America. Using TRMM precipitation radar and microwave imager data, hailstorms are investigated based on their probability of containing hail. Results from this study show that hailstorms have an extended diurnal cycle, often occurring in the overnight hours relative to other locations around the world. High-probability hailstorms tend to be taller and larger than storms that contain low probabilities of hail. They also tend to be supported by strong synoptic forcing, including enhanced lower- and upper-level jet streams, an anomalously warm and moist surface, and increased instability. These conditions can be forecast days in advance, which will help promote readiness and preparation for these damaging storms. Overall, these two studies further the knowledge of convection in subtropical South America, providing new information for short- and long-term forecasts of convection and context to the results of the recent RELAMPAGO field campaign.
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    Influence of terrain on the characteristics and life cycle of convection observed in subtropical South America
    (Colorado State University. Libraries, 2023) Rocque, Marquette N., author; Rasmussen, Kristen L., advisor; Schumacher, Russ S., committee member; Miller, Steven D., committee member; Chandrasekar, V., committee member
    Subtropical South America (SSA) is a hotspot for deep, intense convection that often grows upscale into large mesoscale convective systems (MCSs) overnight. The local terrain, including the Andes and a secondary feature known as the Sierras de Córdoba (SDC) are hypothesized to play a major role in the initiation, development, and evolution of convection in the region. Some satellite studies have investigated this role, but storm-scale and life cycle characteristics of these MCSs have not been studied in depth due to the lack of high-resolution, ground-based instruments in the region. However, in 2018-2019, several research-quality platforms were deployed to Córdoba, Argentina as part of the Remote sensing of Electrification, Lightning, And Mesoscale/microscale Processes with Adaptive Ground Observations (RELAMPAGO) and the Cloud, Aerosol, and Complex Terrain Interactions (CACTI) field campaigns. The data collected during these campaigns is used in the studies presented in this dissertation to investigate how the Andes and SDC contribute to convection initiation and rapid upscale growth under varying synoptic conditions. Determining why convection is so unique in SSA may provide insight into characteristics of other storms around the world. The first two studies in the dissertation evaluate how the Andes and SDC modulate the large-scale environment and storm-scale characteristics under strong vs. weak synoptic forcing. High resolution, convection-permitting simulations in which the terrain is modified are designed to investigate synoptic (Chapter 2) and mesoscale (Chapter 3) processes related to the development of two severe mesoscale convective systems (MCSs) observed during RELAMPAGO-CACTI. Results from the simulations are also compared with radar observations to determine how well the model performs. Under strong synoptic forcing, when the Andes are reduced by 50%, the lee cyclone that develops is weaker, the South American Low-level Jet (SALLJ) is weaker and shallower, and the MCS that develops is weaker and moves quickly off the terrain. When the SDC are removed, there are no substantial changes to the large-scale environment. However, there is no back-building signature of deep convection, likely because cold pools are no longer blocked by the SDC. Under weak synoptic forcing, there are no significant changes to the large-scale environment, even when the Andes are halved. Similar to the strongly forced case though, when the SDC are removed, there are fewer deep convective cores toward the west. In both cases, the model tends to overestimate convection compared to observations. These studies show that the terrain plays varying roles in the evolution of convection in SSA. The third and fourth studies use ground-based lightning observations from RELAMPAGO-CACTI to better understand the electrical and microphysical characteristics of these intense storms. Three-dimensional storm structures are identified in the radar data and lightning flashes are matched with these storm modes to evaluate how lightning varies throughout the convective life cycle (Chapter 4). Results show that lightning flashes associated with deep convective cores are most common along the higher terrain of the SDC and occur in the afternoon hours. They also tend to be the smallest in size. Flashes associated with wide convective cores occur more frequently along the eastern edge of the SDC and are observed around midnight local time. Stratiform flashes are found most frequently in the early morning hours about 50-100 km east of the SDC, and they tend to be the largest in area and occur lower within the cloud. These distributions highlight the life cycle of systems, which initiate along the SDC and grow upscale as they move towards the plains overnight. Flash rates are then related to microphysical properties such as graupel mass and ice water path (Chapter 5). The first lightning flash rate parameterizations are developed for storms in SSA. We find these storms have considerably more graupel associated with them compared to storms in the U.S. These new parameterizations are tested on the simulated strongly forced MCS, and results agree well with observed flash rates. If parameterizations based on U.S. storms had been used instead, the flash rates would have been overestimated by up to a factor of 8. This work, in conjunction with other studies in this dissertation, highlights just how different storms in SSA are compared to the U.S.
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    Synoptic through mesoscale environments of South American thunderstorms
    (Colorado State University. Libraries, 2020) Piersante, Jeremiah Otero, author; Rasmussen, Kristen L., advisor; Schumacher, Russ S., advisor; Nelson, Peter A., committee member
    Subtropical South America east of the Andes Mountains is a global hotspot for deep convection owing to frequent mesoscale convective systems (MCSs) that contribute to over 90% of the region's rainfall and produce severe weather including large hail, flash flooding, high winds, and tornadoes. Investigations of these high impact systems through the Tropical Rainfall Measuring Mission (TRMM) satellite's precipitation radar (PR) determined that unique orographic, synoptic, and mesoscale processes initiate and maintain larger and longer-lasting MCSs in subtropical South America relative to the United States. Prior to the initiation of convection, the South American low-level jet guided by the Andes Mountains advects warm and moist air southward from the Amazon basin into the subtropics. A deep mid-level trough simultaneously approaches the mountains from the west in most cases, inducing dry mid- to upper-level subsidence flow and creating a strong capping inversion over the moist air mass. This cap is overcome via terrain-induced lift by the Andes foothills and the Sierras de Córdoba, a secondary mountain range in northern Argentina, resulting in explosive convection. These unique topographic features often act as a platform for "back-building" in which convection remains tied to the western terrain as storms propagate eastward and grow upscale; the quasi-stationary nature of MCSs inflicts considerable damage to property and agriculture in Argentina. To improve the predictability and understanding of the physical mechanisms leading to dangerous MCSs in subtropical South America relative to those in the U.S., two studies within this thesis employ data from the recently conducted Remote sensing of Electrification, Lightning, And Mesoscale/microscale Processes with Adaptive Ground Observations (RELAMPAGO) field campaign focusing on 1) the comparison of biases in warm-season Weather Research and Forecasting model (WRF) forecasts in North and South America and 2) a synoptic through mesoscale analysis of the driving factors behind upscale growth in subtropical South America. The first study uses WRF output over North and South America verified against Stage IV analyses and radiosonde observations to contrast magnitudes and sources of forecast error between continents. It is found that the cumulus parameterization, which is most active during the warm-season, governs biases in North American precipitation forecasts. While both continents featured a mid-level dry bias, the South American bias is greater. The second study uses TRMM PR, ERA5 reanalysis, high-resolution soundings and GOES-16 infrared brightness temperature data to identify synoptic and mesoscale phenomena that induce upscale growth varying in size and season. Synoptic forcing decreased from large to small systems and from spring to summer, suggesting that terrain-induced lift is more important in the summer. Furthermore, a case study of an MCS that exhibited rapid upscale growth during RELAMPAGO highlights the role of southerly return flow associated with the western edge of the 850-hPa lee trough on low-level convergence, vertical wind shear, and thus convection initiation. These two studies are of importance to the atmospheric science community as they enhance the understanding of some of the world's most violent thunderstorms in a region that has been notably understudied. Knowledge gained can also be applied to similar regions whose convection is also modulated by orography and provide a greater understanding of convective processes on a global scale.
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    Topographic and diurnal influences on storms associated with heavy rainfall in northern Colorado
    (Colorado State University. Libraries, 2024) Douglas, Zoe A., author; Rasmussen, Kristen L., advisor; Bell, Michael M., committee member; Kampf, Stephanie K., committee member
    Despite its profound impacts on agricultural and socioeconomical conditions globally, heavy rainfall is a high-impact weather phenomenon of which we have limited quantitative understanding and forecast skill. The Prediction of Rainfall Extremes Campaign in the Pacific (PRECIP) planned to observe the spectrum of heavy rainfall events in the moisture-rich environment of Taiwan and Japan during 2020, but was delayed until 2022 due to the global COVID-19 pandemic. As a result of this unanticipated delay, the PRECIP science team conducted the Preparatory Rockies Experiment for the Campaign in the Pacific ("PRE"-CIP), which observed precipitation over northern Colorado from May to August 2021 using Colorado State University's ground-based research radars and radiosondes. Extreme precipitation features are identified in the radar data and organized into storm modes based on prior research on the Tropical Rainfall Measuring Mission satellite's Precipitation Radar. An "ingredients-based" approach provides a theoretical framework to separate the storm modes into a spectrum of storm intensity and duration during the entire "PRE"-CIP field project, allowing us to connect storm modes to the topography, diurnal cycle, and overall rainfall characteristics in northern Colorado. While precipitation occurred from the mountains to the plains, the highest concentration of storm tracks calculated from all ground-based radar observations occurred over the Rocky Mountains, regardless of storm duration. The majority of storm tracks are of low intensity and short duration, with over 80% of tracked storms having lifetimes of 1 h or less, suggesting that the general population of warm-season precipitation in northern Colorado is short-lived and of weak intensity. When considering heavy rainfall-producing storms, deep convection is the most dominant storm mode in northern Colorado by up to three orders of magnitude over broader convective and stratiform systems. Deep convection most frequently occurred over the Rocky Mountains in the afternoon, while broader convective and stratiform systems most frequently occurred over the foothills and plains in the evening to nighttime hours. Therefore, diurnal forcing and orographic lift play important roles in the morphology of warm-season precipitation in northern Colorado, as has been seen in mountainous regions across the world. The frequent occurrence of deep convective storms directly over the Rocky Mountains, however, differs from the deep convective hotspots seen in the lowlands downstream of similarly large mountain barriers like the Andes and Himalayas. Ultimately, these radar-based analyses are important for the eventual comparison of heavy rainfall in a semi-arid midlatitude region (Colorado) and a moisture-rich tropical environment (Taiwan and Japan), thus providing an enhanced global understanding of the commonalities of heavy rainfall processes.