Mountain Scholar :: Browsing by Author "Schumacher, Russ S., committee member"

Browsing by Author "Schumacher, Russ S., committee member"

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Open Access
A social-ecological approach to managing agricultural ammonia emissions and nitrogen deposition in Rocky Mountain National Park
(Colorado State University. Libraries, 2017) Piña, Aaron Joshua, author; Denning, A. Scott, advisor; Ojima, Dennis S., advisor; Schumacher, Russ S., committee member; Baron, Jill S., committee member; Ham, Jay M., committee member
Atmospheric nitrogen (N) deposition is harmful to nutrient-limited mountain ecosystems. Annual wet deposition of total inorganic N in Rocky Mountain National Park (RMNP) is dominated by ammonium, which primarily comes from agricultural sources. The most wet N deposition events between 1980 and 2015 occurred during summer months. The confluence of summertime mountain meteorology and the location of pollution sources are a perfect combination that leads to high values of wet N deposition in RMNP. In Chapter 2, we tested the importance of convection as a N transport mechanism in addition to large-scale east winds, typically associated with the summertime mountain-valley circulation on the eastern plains of Colorado. We characterized the meteorological transport by using the Weather Research and Forecasting model at 4/3-km horizontal resolution. We used passive tracers as a simplified representation of emissions from a single agricultural source in eastern Colorado during three summer precipitation events where wet N deposition values in RMNP were among the highest recorded in all summers between 1980 and 2015. In all three cases, anticyclones in north-central United States and monsoonal flow associated with the North American Monsoon brought together the necessary conditions for deep convection over RMNP. Output from our simulations suggested large-scale winds were responsible for slow and steady transport whereas convection was a rapid and intermittent form of transport. This chapter showed two scales of transport had an additive effect that led to high deposition of N in RMNP during the afternoon/evening hours of three case studies. Chapter 3 discusses the development of a pilot early warning system (PEWS) for agricultural operators to voluntarily and temporarily minimize emissions of NH3 during periods of upslope winds. The PEWS was created using trajectory analyses driven by outputs from an ensemble of numerical weather forecasts together with the climatological expertise of human forecasters. In this study, we discuss the methods for the PEWS and offer a preliminary analyses of 21 months of the PEWS based on deposition data from two sites in RMNP as wells as voluntary responses from agriculture managers and producers after warnings were issued. Results from this study showed that the PEWS accurately predicted 5 of 7 high N deposition weeks at the lower-elevation observation site, but only 3 of 8 high N deposition weeks at the higher-elevation observation site. With the higher-elevation site receiving pollution from sources both west and east of the Continental Divide, sources west of the Continental Divide would need to be included in the PEWS to capture all of the sources leading to deposition at the higher-elevation site. Sixty agricultural producers and managers from 39 of Colorado's agricultural operations volunteered for the PEWS, and a two-way line of communication between the producers and the scientists was formed. An average of 21 voluntary responses (s.d. 4.9) per warning occurred, with over 75% of the PEWS participants altering their practices after an alert. Solving a broad and complex social-ecological problem requires both a technological approach, such as the PEWS, and collaboration and trust from all participants, including agricultural producers, university researchers, and environmental agencies. Chapter 4 applies a systems approach that explores the actors involved in a complex social-ecological problem that deals with the competing interests of an unadulterated environment and the contribution towards feeding the global population. Agricultural operations in northeastern Colorado are among the densest in the world. The demand of a growing global population has put pressure on the agricultural community to provide large quantities of food in a short amount of time. The cost for higher yields means more water, nutrients, and energy, and the result is environmental degradation in the forms of atmospheric and water pollution. The problem becomes more complex when we mix bottom-up and top-down management approaches. That is, agricultural producers are asked to work together with state and federal agencies on reducing emissions from their operations. A pilot early warning system employed in Colorado since 2014 helped bring together the actors to work towards the common goal of reducing nitrogen deposition in Rocky Mountain National Park. Our goal in this chapter was to organize the problem using a conceptual, social-ecological framework. The case studies and pilot early warning system from Chapters 1 and 2 document starting points for how institutional decisions can incorporate agricultural stakeholders in a mix of bottom-up and top-down management approaches under current and future climatic conditions.
Open Access
African easterly wave energetics on intraseasonal timescales
(Colorado State University. Libraries, 2014) Alaka, Ghassan J., Jr., author; Maloney, Eric D., advisor; Schubert, Wayne H., committee member; Schumacher, Russ S., committee member; Venayagamoorthy, Subhas K., committee member
African easterly waves (AEWs) are synoptic-scale eddies that dominate North African weather in boreal summer. AEWs propagate westward with a maximum amplitude near 700 hPa and a period of 2.5-6-days. AEWs and associated perturbation kinetic energy (PKE) exhibit significant intraseasonal variability in tropical North Africa during boreal summer, which directly impacts local agriculture and tropical cyclogenesis. This study performs a comprehensive analysis of the 30-90-day variability of AEWs and associated energetics using both reanalysis data and model output. Specifically, the PKE and perturbation available potential energy (PAPE) budgets are used to understand the factors that contribute to PKE maxima in West Africa and the extent to which these surges of AEW activity are modulated by the Madden-Julian oscillation (MJO). The role of the MJO in the intraseasonal variability of AEWs is assessed by comparing PKE sources as a function of an MJO index and a local 30-90-day West African PKE index. Since East Africa is an initiation zone for AEW activity and is modulated by the MJO, the relationship between this region and West Africa is a primary focus in this study. The intraseasonal variability of AEW energetics is first investigated in reanalysis products. While reanalysis data depicts a similar evolution of 30-90-day PKE anomalies in both the MJO and a local PKE index, the MJO index describes only a small (yet still significant) fraction of the local 30-90-day variance. In boreal summers with more significant MJO days, the correlation between the two indices is higher. Baroclinic energy conversions are important for the initiation of 30-90-day West African PKE events east of Lake Chad. In West Africa, both barotropic and baroclinic energy conversions maintain positive PKE anomalies before they propagate into the Atlantic. The primary role of diabatic heating is to destroy PAPE in a negative feedback to baroclinic energy conversions in West Africa. More frequent East Atlantic tropical cyclone generation is associated with positive PKE events than with negative PKE events. Easterly wave activity is then examined in a regional model. The Advanced Research Weather Research and Forecasting (WRF-ARW) simulates West African monsoon climatology more accurately than the WRF Nonhydrostatic Mesoscale Model (WRF-NMM). Although the WRF-NMM produces more realistic boreal summer rainfall than the WRF-ARW, it fails to accurately simulate the AEJ and other key West African monsoon features. Parameterizations within the WRF-ARW are scrutinized as well, with the WRF single-moment 6-class microphysics and the Noah land surface model outperforming Thompson microphysics and the RUC land surface model. Three ten-year WRF-ARW experiments are performed to investigate the role of external forcing on intraseasonal variability in West Africa. In addition to a control simulation, two sensitivity experiments remove 30-90-day variability from the boundary conditions (for all zonal wavenumbers and just for eastward zonal wavenumbers 0-10). Overall, intraseasonal variability of AEWs shows only modest differences after the removal of all 30-90-day input into the model boundary conditions. PKE and PAPE budgets reveal that simulated positive PKE events in West Africa are preceded by extensions of the AEJ into East Africa, which enhance barotropic and baroclinic energy conversions in this region. This jet extension is associated with warm lower-tropospheric temperature anomalies in the eastern Sahara. In West Africa, the amplitude of PKE and PAPE budget terms exhibit a similar evolution (even in the sensitivity experiments) as in the reanalysis products.
Open Access
An assessment of streamflow production mechanisms for dam safety applications in the Colorado Front Range
(Colorado State University. Libraries, 2019) Woolridge, Douglas, author; Niemann, Jeffrey D., advisor; Schumacher, Russ S., committee member; Morrison, Ryan R., committee member
Hydrologic analyses are used for dam safety evaluations to determine the flow a dam must pass without failing. Many current guidelines model flood runoff solely by an infiltrationexcess mechanism. Saturation-excess runoff and subsurface stormflow mechanisms are known to be important for common events in forested regions, but few studies have analyzed their role for extreme events. The objectives of this study are to determine the active streamflow mechanisms for large historical storms and design storms in the Colorado Front Range and to propose methods to model these mechanisms that can be used by consultants. Hydrologic models were developed for five basins to simulate historical events in 1976, 1997, and 2013. The model results show saturation-excess was the dominant mechanism during the 2013 storm, which had a long duration and low rainfall intensities. Infiltration-excess runoff was dominant for the 1976 storm, which had a short duration and high intensities. Surface runoff was not observed during the 1997 storm. Similarly, infiltration-excess dominates for short duration design storms, and saturation-excess dominates for longer design storms.
Open Access
Approaching Arctic-midlatitude dynamics from a two-way feedback perspective
(Colorado State University. Libraries, 2019) McGraw, Marie C., author; Barnes, Elizabeth A., advisor; Randall, David A., committee member; Schumacher, Russ S., committee member; Venayagamoorthy, Karan, committee member
Arctic variability and the variability of the midlatitude circulation are closely intertwined. Although these connections are interrelated and bi-directional, and occur on a variety of timescales, they are not often studied together. Modeling studies generally focus on a single direction of influence--usually, how the midlatitude circulation responds to the Arctic. Studying these relationships in a two-way feedback perspective can offer new insights into these connections, providing information on how they feed back upon each other. This work approaches Arctic-midlatitude dynamics from a two-way feedback perspective, mostly on sub-monthly timescales. Particular emphasis is placed on the influence of midlatitude circulation variability upon the Arctic, as this direction of influence is less-studied than the converse pathway. Reinforcing feedback loops are identified between the North Pacific and North Atlantic jet streams and the Arctic. Variability in both the North Atlantic and North Pacific jet streams drives Arctic variability, which then drives further variability in the jet streams. The circulation variability in many regions, including North America, the east Pacific and Alaska, and Siberia, drives Arctic variability far more than it is driven by Arctic variability. These relationships exhibit substantial regional variability, stressing the important role of an analytical approach that incorporates this spatial heterogeneity. The two-way nature of Arctic-midlatitude connections is also explored in the context of Arctic moisture fluxes. The circulation response to sea ice loss also drives changes in Arctic moisture fluxes, with moisture flux out of the Arctic increasing more than moisture flux into the Arctic. The two-way feedback perspective explored in this research is built around the ideas of causal discovery, particularly Granger causality. Most of these two-way Arctic-midlatitude relationships are considered in the context of added variance explained, or added predictive power. That is, these relationships are characterized by comparing how much an additional predictor improves predictability beyond autocorrelation. Limiting the ability of autocorrelation to color these results emphasizes added variance explained--how much additional variance in the circulation can be explained by Arctic temperature variability, and vice versa? As an example, many recent studies have concluded that warm Arctic temperatures or low sea ice conditions drive a strengthening of high pressures and an increase in cold temperatures over Siberia. However, when memory and autocorrelation are accounted for, it emerges that the circulation variability over Siberia drives a response in the Arctic more than the other way around--results that are in concordance with modeling studies that have also disputed the veracity of the claim of the Arctic driving a strong response in Siberia. Ultimately, this research seeks to offer a different perspective on analyzing climate dynamics, with an emphasis on two-way feedbacks. While targeted climate modeling studies offer great physical insights, and provide substantial opportunities to explore and test physical mechanisms, they are often limited to exploring only one pathway of influence. In reality, these relationships do go in both directions, and a comprehensive understanding of such large-scale interactions between different parts of the atmosphere must ultimately consider the two-way relationships. The causal discovery methods used in much of this research can be used in conjunction with modeling studies to better understand these two-way relationships, and improve interpretation of results. While this research has focused on the relationships between the Arctic and the midlatitude circulation on sub-seasonal timescales, the broad framework and ideas presented within can be more widely applied to many other questions in climate variability studies. Thus, this work has also put a special emphasis on describing and implementing these causality-based methods in a manner that is accessible and interpretable for atmospheric and climate scientists.
Open Access
Assessment of numerical weather prediction model re-forecasts of atmospheric rivers along the west coast of North America
(Colorado State University. Libraries, 2018) Nardi, Kyle M., author; Barnes, Elizabeth A., advisor; Schumacher, Russ S., committee member; Ham, Jay M., committee member
Atmospheric rivers (ARs) - narrow corridors of high atmospheric water vapor transport - occur globally and are associated with flooding and maintenance of the regional water supply. Therefore, it is important to improve forecasts of AR occurrence and characteristics. Although prior work has examined the skill of numerical weather prediction (NWP) models in forecasting ARs, these studies only cover several years of re-forecasts from a handful of models. Here, we expand this previous work and assess the performance of 10-30 years of wintertime (November-February) AR landfall re-forecasts from nine operational weather models, obtained from the International Subseasonal to Seasonal (S2S) Project Database. Model errors along the West Coast of North America at leads of 1-14 days are examined in terms of AR occurrence, intensity, and landfall location. We demonstrate that re-forecast performance varies across models, lead times, and geographical regions. Occurrence-based skill approaches that of climatology at 14 days, while models are, on average, more skillful at shorter leads in California, Oregon, and Washington compared to British Columbia and Alaska. We also find that the average magnitude of landfall Integrated Water Vapor Transport (IVT) error stays fairly constant across lead times, although over-prediction of IVT is more common at later lead times. We then show that northward landfall location errors are favored in California, Oregon, and Washington, although southward errors occur more often than expected from climatology. We next explore the link between the predictability of ARs at 1-14 days and synoptic-scale weather conditions by examining re-forecasts of 500-hPa geopotential height anomaly patterns conducive to landfalling ARs. Finally, the potential for skillful forecasts of IVT and precipitation at subseasonal to seasonal (S2S) leads is explored using an empirical forecast model based on the Madden-Julian oscillation (MJO) and the quasi-biennial oscillation (QBO). Overall, these results highlight the need for model improvements at 1-14 days, while helping to identify factors that cause model errors as well as sources of additional predictability.
Open Access
Axisymmetric and asymmetric processes contributing to tropical cyclone intensification and expansion
(Colorado State University. Libraries, 2020) Martinez, Jonathan, author; Bell, Michael M., advisor; Birner, Thomas, committee member; Schumacher, Russ S., committee member; Davis, Christopher A., committee member; Anderson, Brooke Georgiana, committee member
This dissertation endeavors to advance our understanding of tropical cyclones (TCs) by investigating axisymmetric and asymmetric processes contributing to TC intensification and expansion. Chapter 2 examines the extreme rapid intensification (RI) and subsequent rapid over-water weakening of eastern North Pacific Hurricane Patricia (2015). Spline-based analyses are created from high-resolution observations collected between 22--23 October during the Office of Naval Research Tropical Cyclone Intensity (TCI) experiment and the National Oceanic and Atmospheric Administration Intensity Forecasting Experiment (IFEX). The first full-tropospheric analysis of the dry, axisymmetric Ertel's potential vorticity (PV) in a TC is presented without relying on balance assumptions. Patricia's structural evolution is characterized by the formation of a "hollow tower" PV structure during RI that persists through maximum intensity and subsequently breaks down during rapid over-water weakening. Transforming the axisymmetric PV analyses from radius-height to potential radius-potential temperature coordinates reveals that Patricia's RI occurs "in-place"; eyewall heating remains fixed to the same absolute angular momentum surfaces as they contract in physical space, contributing to rapid PV concentration. Eddy mixing processes are inferred to concentrate PV radially inward of the symmetric heating maximum during RI and hypothesized to be a primary factor underlying the rearrangement of Patricia's PV distribution during rapid over-water weakening, diluting the PV tower while approximately conserving the absolute circulation. Chapter 3 raises the question: do asymmetries facilitate or interfere with TC intensification? An idealized, high-resolution simulation of a rapidly intensifying TC is examined to assess asymmetric contributions to the intensification process. The inner-core asymmetric PV distribution remains on the same order of magnitude as the symmetric PV distribution throughout the intensification period. Scale-dependent contributions to the azimuthal-mean PV tendency are assessed by partitioning asymmetries into low-wavenumber (large-scale) and high-wavenumber (small-scale) categories. Symmetric PV advection and generation are approximately twice larger than asymmetric contributions throughout the intensification period, but the two symmetric contributions largely oppose one another in the eyewall region. Low-wavenumber advection concentrates PV near the axis of rotation during the early and middle stages of RI and low-wavenumber heating concentrates PV in the hollow tower during the middle and late stages of RI. Therefore, asymmetric processes produce non-negligible contributions to TC intensification and may indeed facilitate the intensification process. Chapter 4 investigates the contributions of incipient vortex circulation and environmental moisture to TC expansion with a set of idealized simulations. The incipient vortex circulation places the primary constraint on TC expansion and in part establishes the expansion rate. Increasing the mid-level moisture further promotes expansion but mostly expedites the intensification process. One of the more common findings related to TC expansion in the literature illustrates a proclivity for relatively small TCs to stay small and relatively large TCs to stay large. Findings reported herein suggest that an initially large vortex can expand more quickly than its relatively smaller counterpart; therefore, with all other factors contributing to expansion held constant, the contrast in size between the two vortices will increase with time. Varying the incipient vortex circulation is associated with subsequent variations in the amount and scale of outer-core convection. As the incipient vortex circulation decreases, outer-core convection is relatively scarce and characterized by small-scale, isolated convective elements. On the contrary, as the incipient vortex circulation increases, outer-core convection abounds and is characterized by relatively large rainbands and mesoscale convective systems. A combined increase in the amount and scale of outer-core convection permits an initially large vortex to converge a substantially larger amount of absolute angular momentum compared to its relatively smaller counterpart, resulting in distinct expansion rates.
Open Access
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.
Open Access
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.
Open Access
Damage analysis and mitigation for wood-frame structures subjected to tornado loading
(Colorado State University. Libraries, 2016) Standohar-Alfano, Christine Diane, author; van de Lindt, John W., advisor; Ellingwood, Bruce R., committee member; Heyliger, Paul R., committee member; Schumacher, Russ S., committee member
Tornadoes are one of the most devastating natural hazards that occur in the United States. While there is an average of approximately 1200 tornadoes per year across the country, the annual likelihood of experiencing a tornado at a particular location is quite small due to their relatively small size. However, the high consequence of a tornado strike necessitates the determination of geographic tornado hazard. A methodology to estimate the annualized probabilistic tornado hazard over the contiguous U.S. was developed and used the most recent 38 years of climatological tornado data. Furthermore, with the use of detailed damage surveys after the April 3-4, 1974 and April-May, 2011 tornado outbreaks, an empirical method was developed and applied to account for the gradient of wind speed along a tornado’s path length and path width. From this, a probabilistic tornado hazard index was developed across the United States which quantified the annual probability of experiencing a tornado of any strength on the Enhanced Fujita scale. Tornado hazard curves were developed from the tornado hazard analysis at six illustrative locations which varied as a function of location-specific occurrence rates. Five different residential wood-frame building archetypes were designed at each of the locations based on current residential building code and/or practice. Fragilities for the roof sheathing, truss to wall top-plate, and wall-to-foundation connections were developed for each archetype. At each of the six locations, the fragility curves for the locally adopted residential building code were convolved with the tornado hazard curve at that specific location in order to compute annual failure probabilities for select components along the vertical load path. This was one of the first times unconditional risk of component failure due to tornadoes has been computed since the tornado hazard curve was convolved with the fragility curves. These probabilities quantify failure probabilities of residential wood-frame construction components to tornado winds. In addition, the more wind-resistant Florida residential building code is applied to other locations in the U.S., fragilities are developed and convolved, and failure probabilities for these modified buildings are computed. This resulted in a quantitative measure of risk reduction from tornadoes by using strengthened construction at various locations across the country. The convolved failure probabilities were first developed for individual components. The system level behavior of the entire structure was also assessed and included the correlated dependencies between individual components. Results indicate that stricter building codes may be beneficial in areas with a high annual tornado risk, such as Tornado Alley. The final portion of this work used a simplified property loss model applied to the April 25-28, 2011 tornado outbreak. This was one of the largest tornado outbreaks in U.S. history and resulted in over $5B in property loss. In order to determine property loss over a broad area, census data regarding household income and home market value was utilized. The performance of manufactured homes had to be considered in conjunction with wood-frame residential construction since the tornado outbreak impacted the southern U.S. which has a high number of manufactured homes. Using the system level fragility analysis, property loss was estimated based on both locally adopted residential codes and the stricter guidelines described in the Florida Residential Building Code. Results indicate that using strengthened construction methodologies would reduce property loss up to 40% as compared to current design guidelines.
Open Access
Diabatic and frictional forcing effects on the structure and intensity of tropical cyclones
(Colorado State University. Libraries, 2013) Slocum, Christopher J., author; Schubert, Wayne H., advisor; DeMaria, Mark, advisor; Schumacher, Russ S., committee member; Kirby, Michael J., committee member; Fiorino, Michael, committee member
Tropical cyclone intensity forecasting skill has slowed in improvement for both dynamical and statistical-dynamical forecasting methods in comparison to gains seen in track forecasting skill. Also, forecast skill related to rapid intensification, e.g. a 30 kt or greater increase in intensity within a 24-hour period, still remains poor. In order to make advances and gain a greater understanding, the processes that affect intensity change, especially rapid intensification, need further study. This work evaluates the roles of diabatic and frictional forcing on the structure and intensity of tropical cyclones. To assess the diabatic forcing effects on intensity change in tropical cyclones, this study develops applications of Eliassen's balanced vortex model to obtain one-dimensional solutions to the geopotential tendency and two-dimensional solutions to the transverse circulation. The one-dimensional balanced solutions are found with dynamical model outputs as well as aircraft reconnaissance combined with diabatic heating derived from microwave rainfall rate retrievals. This work uses solutions from both datasets to make short-range intensity predictions. The results show that for the one-dimensional solutions, the tangential tendency does not match the dynamical model or aircraft wind tendencies. To relax the assumptions of the one-dimensional solutions to the geopotential tendency, solutions for idealized vortices are examined by finding two-dimensional solutions to the transverse circulation. The two-dimensional solutions allow for evaluation of the axisymmetric structure of the vortex on the (r, z)-plane without setting the baroclinicity to zero and the static stability to a constant value. While the sensitivity of tangential wind tendency to diabatic forcing and the region of high inertial stability is more realistic in the two-dimensional results, the solutions still neglect the influence of friction from the boundary layer. To understand further the role of frictional forcing in the boundary layer, two analytical slab models developed in this study provide insight into recent work that demonstrates how dry dynamics plays a role in determining eyewall location and size, how potential vorticity rings develop, and how an outer concentric eyewall forms through boundary layer "shock-like" structures. The analytical models show that when horizontal diffusion is neglected, the u(∂u/∂r) term in the radial equation of motion and the u[ƒ + (∂v/∂r) + (v/r)] term in the tangential equation of motion develop discontinuities in the radial and tangential wind, with associated singularities in the boundary layer pumping and the boundary layer vorticity. The analytical models provide insight into the boundary layer processes that are responsible for determining the location of the eyewall and the associated diabatic heating that ultimately impacts the intensity of the tropical cyclone. This work shows that future research linking the roles of frictional forcing in the boundary layer to the diabatic forcing aloft while using a balanced model will be important for gaining insight into forcing effects on tropical cyclone intensity.
Open Access
Examining chaotic convection with super-parameterization ensembles
(Colorado State University. Libraries, 2017) Jones, Todd R., author; Randall, David A., advisor; Kummerow, Christian D., committee member; Van den Heever, Susan S., committee member; Schumacher, Russ S., committee member; Eykholt, Richard E., committee member
This study investigates a variety of features present in a new configuration of the Community Atmosphere Model (CAM) variant, SP-CAM 2.0. The new configuration (multiple-parameterization-CAM, MP-CAM) changes the manner in which the super-parameterization (SP) concept represents physical tendency feedbacks to the large-scale by using the mean of 10 independent two-dimensional cloud-permitting model (CPM) curtains in each global model column instead of the conventional single CPM curtain. The climates of the SP and MP configurations are examined to investigate any significant differences caused by the application of convective physical tendencies that are more deterministic in nature, paying particular attention to extreme precipitation events and large-scale weather systems, such as the Madden-Julian Oscillation (MJO). A number of small but significant changes in the mean state climate are uncovered, and it is found that the new formulation degrades MJO performance. Despite these deficiencies, the ensemble of possible realizations of convective states in the MP configuration allows for analysis of uncertainty in the small-scale solution, lending to examination of those weather regimes and physical mechanisms associated with strong, chaotic convection. Methods of quantifying precipitation predictability are explored, and use of the most reliable of these leads to the conclusion that poor precipitation predictability is most directly related to the proximity of the global climate model column state to atmospheric critical points. Secondarily, the predictability is tied to the availability of potential convective energy, the presence of mesoscale convective organization on the CPM grid, and the directive power of the large-scale.
Open Access
Exploring the limits of variational passive microwave retrievals
(Colorado State University. Libraries, 2017) Duncan, David Ian, author; Kummerow, Christian D., advisor; Boukabara, Sid-Ahmed, committee member; O'Dell, Christopher W., committee member; Reising, Steven C., committee member; Rutledge, Steven A., committee member; Schumacher, Russ S., committee member
Passive microwave observations from satellite platforms constitute one of the most important data records of the global observing system. Operational since the late 1970s, passive microwave data underpin climate records of precipitation, sea ice extent, water vapor, and more, and contribute significantly to numerical weather prediction via data assimilation. Detailed understanding of the observation errors in these data is key to maximizing their utility for research and operational applications alike. However, the treatment of observation errors in this data record has been lacking and somewhat divergent when considering the retrieval and data assimilation communities. In this study, some limits of passive microwave imager data are considered in light of more holistic treatment of observation errors. A variational retrieval, named the CSU 1DVAR, was developed for microwave imagers and applied to the GMI and AMSR2 sensors for ocean scenes. Via an innovative method to determine forward model error, this retrieval accounts for error covariances across all channels used in the iteration. This improves validation in more complex scenes such as high wind speed and persistently cloudy regimes. In addition, it validates on par with a benchmark dataset without any tuning to in-situ observations. The algorithm yields full posterior error diagnostics and its physical forward model is applicable to other sensors, pending intercalibration. This retrieval is used to explore the viability of retrieving parameters at the limits of the available information content from a typical microwave imager. Retrieval of warm rain, marginal sea ice, and falling snow are explored with the variational retrieval. Warm rain retrieval shows some promise, with greater sensitivity than operational GPM algorithms due to leveraging CloudSat data and accounting for drop size distribution variability. Marginal sea ice is also detected with greater sensitivity than a standard operational retrieval. These studies ultimately show that while a variational algorithm maximizes the effective signal to noise ratio of these observations, hard limitations exist due to the finite information content afforded by a typical microwave imager.
Open Access
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.
Open Access
Investigation of the dynamics of tropical cyclone precipitation structure using radar observations and numerical modeling
(Colorado State University. Libraries, 2023) Cha, Ting-Yu, author; Bell, Michael M., advisor; Rasmussen, Kristen L., committee member; Schumacher, Russ S., committee member; Lee, Wen-Chau, committee member; Reising, Steven C., committee member
Precipitation from tropical cyclones (TCs) produces significant damage and causes fatalities worldwide. Forecast skill of the structure of precipitation in a TC remains challenging, due in part to limited fundamental understanding of the underlying complex dynamics and limitations in our observational capability. This dissertation seeks to improve our understanding of the underlying dynamics of TC precipitation structure by using and improving radar retrieval techniques and numerical modeling. In Part 1, the vortex dynamics of TC polygonal eyewall structure during rapid intensification (RI) of Hurricane Michael (2018) are examined from ground-based single‐Doppler radar analysis. Although the organization of polygonal precipitation asymmetries has been theorized to be related to vortex Rossby wave (VRW) dynamics, prior studies have had observational limitations that prevent a detailed description of the phenomena. Here, we present the first observational evidence of the evolving wind field of a polygonal eyewall during RI to Category 5 intensity by deducing the axisymmetric and asymmetric winds at 5‐min intervals. Novel single Doppler radar retrievals show that both tangential wind and reflectivity asymmetries rotate at speeds that are consistent with linear VRW theory. Dual‐Doppler winds from airborne radar provide further evidence of the vortex structure that supports growth of asymmetries during RI. In Part 2, the relationship between VRWs and the polygonal precipitation structure is further explored through a simple modeling framework. A two-layer model consisting of a shallow water fluid on top of a slab boundary layer is used to understand the dynamical relationship. The model maintains an approximate gradient wind balance in the free atmospheric layer and parameterizes the diabatic heating produced by convection from the vertical motion out of the boundary layer. The two-layer model provides insight into the essential dynamics of Hurricane Michael's intensification and precipitation structure observed by radar in Part 1. The results show that the convective maxima located at the vertices of an elliptical vortex are due to boundary layer processes and not the free atmospheric convergence. The simulations further show that continuous intensification of the vortex can happen in the presence of elliptical asymmetries and even after the potential vorticity ring breakdown when the diabatic heating is continuously maintained by boundary layer processes. When TCs approach land they can produce voluminous rainfall totals, especially when interacting with complex terrain. Doppler radar can provide the capability to monitor extreme rainfall events over land, but our understanding of airflow modulated by orographic interactions remains limited. In Part 3, a new Doppler radar technique is developed to retrieve three-dimensional wind fields in precipitation over complex terrain. New boundary conditions are implemented in a variational multi-Doppler radar technique to represent the topographic forcing and surface impermeability. A series of observing simulation sensitivity experiments using a full-physics model and radar emulator simulating rainfall from Typhoon Chanthu (2021) over Taiwan are conducted to evaluate the retrieval accuracy and parameter settings. Analysis from real radar observations from Chanthu demonstrates that the improved retrieval technique can advance scientific analyses for the underlying dynamics of orographic precipitation using radar observations.
Open Access
Latent heating and aerosol-precipitation interactions within mesoscale convective systems
(Colorado State University. Libraries, 2016) Marinescu, Peter James, author; van den Heever, Susan C., advisor; Kreidenweis, Sonia M., advisor; Eykholt, Richard, committee member; Schumacher, Russ S., committee member
Two studies are presented in this thesis that focus on understanding cloud processes within simulations of two mesoscale convective system (MCS) events that occurred during the Midlatitude Continental Convective Clouds Experiment (MC3E). Simulations are conducted with the Regional Atmospheric Modeling System (RAMS) and are compared with a suite of observations obtained during MC3E. It is concluded that the simulations reasonably reproduce the two MCS events of interest. Both studies provide information that can assist in the advancement of cloud process parameterizations in atmospheric models. The first study details the microphysical process contributions to latent heating profiles within MCS convective and stratiform regions and the evolution of these profiles throughout the MCS lifetime. Properly representing the distinctions between the latent heating profiles of MCS convective and stratiform regions has significant implications for the atmospheric responses to latent heating on various scales. The simulations show that throughout the MCSs, condensation and deposition are the primary contributors to latent warming, as compared to riming and nucleation processes. In terms of latent cooling, sublimation, melting, and evaporation all play significant roles. Furthermore, it is evident that throughout the MCS lifecycle, convective regions demonstrate an approximately linear decrease in the magnitudes of latent heating rates, while the evolution of latent heating within stratiform regions is associated with transitions between MCS flow regimes. The second study addresses the relative roles of middle-tropospheric and lower-tropospheric aerosol particles on MCS precipitation during the mature stage. A suite of sensitivity simulations for each MCS event is conducted, where the simulations are initialized with different aerosol profiles that vary in the vertical location of the peak aerosol particle number concentrations. Importantly, the total integrated aerosol mass remains constant between the different initialization aerosol profiles, and therefore, differences between the simulated MCS precipitation characteristics can be more directly attributed to the varied vertical location of the aerosol particles. The simulations from both MCS events demonstrate that during the mature stage, the concentrations of lower-tropospheric aerosol particles are the primary factor in determining the intensity of precipitation near the cold pool leading edge, while middle-tropospheric aerosol particles were entrained within convective updrafts, thus altering the cloud droplet properties. However, the aerosol effects on total surface precipitation is not consistent between the two simulated MCS events, suggesting that the MCS structure and environmental conditions play important roles in regulating the impacts of middle-tropospheric and lower-tropospheric aerosol particles on MCS precipitation. Lastly, changes in precipitation processes can result in dynamical feedbacks that further modify, and hence complicate, the net effect of aerosol particles on MCS precipitation. One such feedback process involving the MCS cold pool intensity and updraft tilt is discussed.
Open Access
Parameter estimation methods for models of major flood events in ungauged mountain basins of Colorado
(Colorado State University. Libraries, 2021) Irvin, Ben Christopher, IV, author; Niemann, Jeffrey D., advisor; Schumacher, Russ S., committee member; Bailey, Ryan, committee member
Accurate hydrologic modeling of ungauged mountain basins plays an important role in ensuring the safety of Colorado's dams. Recent research has shown that infiltration-excess runoff, saturation-excess runoff, and subsurface stormflow can all contribute to streamflow from major storms in Colorado's mountains, and the soil moisture accounting (SMA) method in HEC-HMS has been suggested as an appropriate approach to model these mechanisms. However, SMA requires estimation of parameters that have not been previously considered in dam safety analyses. The objectives of this work are to (1) evaluate simplifications to the modeling process that would reduce the number of required parameters and (2) develop methods to estimate the remaining parameters in ungauged mountain basins of Colorado where calibration to observed discharges is not possible. The proposed simplifications and parameter estimation methods are tested for five basins in the Front Range and three basins in the San Juan Range that have streamflow data available for major flood events. For these historical events, the proposed uncalibrated models produce the same streamflow generation processes as calibrated models and the predicted peak discharges from the uncalibrated models usually have 25% errors or smaller. For design storms, the peak discharges from the proposed uncalibrated models can differ substantially from the peak discharges from calibrated models but are conservative relative to the envelope of observed peak discharges.
Open Access
Strong and weak cold pool collisions
(Colorado State University. Libraries, 2022) Falk, Nicholas Michael, author; van den Heever, Susan C., advisor; Schumacher, Russ S., committee member; Venayagamoorthy, Subhas K., committee member
Collisions between convective cold pools commonly initiate new convective storms. This occurs through enhancements to the vertical velocity through mechanical forcing, and increased water vapor content via thermodynamic forcing. The goal of this study is to investigate the impact of the following four parameters on the mechanical and thermodynamic forcing associated with cold pool collisions: (1) the initial temperature perturbation of cold pools, (2) the initial distance between cold pools, (3) the environment in which cold pools exist, and (4) the strength of atmospheric diffusion. To achieve this goal, the dynamical and thermodynamical processes of colliding pairs of cold pools is investigated using a two-dimensional, high- resolution non-hydrostatic anelastic model. The four parameters of interest were varied across a wide range of values in a model suite comprised of 11,200 large eddy simulations in total. To facilitate our analysis, a classification of cold pool collisions into categories of "mechanically strong" and "mechanically weak" is proposed. "Mechanically strong" cold pool collisions occur when the updraft velocities resulting from the collisions are greater than those produced by the flow of air forced up the leading edges of individual cold pools. In "mechanically weak" collisions, the updraft velocities produced by individual cold pools are greater than those from cold pool collisions. An analogous classification of "thermodynamically strong/weak" collisions is also proposed. The results of this analysis show that the initial temperature perturbation of the cold pools has the largest impact on mechanical and thermodynamic forcing from cold pool collisions. Colder cold pools have greater horizontal wind velocities at their heads, leading to greater near- surface horizontal convergence when they collide. This in turn leads to greater updraft velocities which are also more effective at advecting water vapor upwards. The second largest impact on mechanical and thermodynamic cold pool forcing is from the environment in which the cold pools exist. Due to a decreased vertical gradient of potential temperature, weaker low-level static stability increases mechanical forcing as the air lofted by the collisions is decelerated less by negative buoyancy. Environments with larger low-level vertical moisture gradients are associated with increased thermodynamic forcing through enhanced vertical moisture advection. The initial edge-to-edge distance between the cold pools has the third largest impact on the proxies for convective initiation. Mechanical forcing is found to peak at an optimal initial distance between cold pools of ~2.5 km due to a balance between the creation and dissipation of kinetic energy. Thermodynamic forcing, on the other hand, peaks for much greater initial cold pool distances than those associated with the mechanical forcing. This is likely a result of the faster updraft winds generated during collisions for closely spaced initial cold pools also being more effective at advecting moisture away during the collision, thereby decreasing the thermodynamic forcing. The smallest impact on the proxies for convective initiation comes from the atmospheric diffusion rate which impacts cold pool strength through mixing. Thus, this work finds that convective initiation becomes increasingly likely from a cold pool collision when the cold pools are colder, the environment is less stable and has a greater vertical water vapor gradient, the cold pools start close to some optimal separation distance, and the atmospheric diffusion rate is low.
Open Access
The climatology of lightning producing large impulse charge moment changes with an emphasis on mesoscale convective systems
(Colorado State University. Libraries, 2013) Beavis, Nicholas, author; Rutledge, Steven A., advisor; Schumacher, Russ S., committee member; Lyons, Walter A., committee member; Lang, Timothy J., committee member; Eykholt, Richard E., committee member
The use of both total charge moment change (CMC) and impulse charge moment change (iCMC) magnitudes to assess the potential of a cloud-to-ground (CG) lightning stroke to induce a mesospheric sprite has been well described in literature. However, this work has primarily been carried out on a case study basis. To complement these previous case studies, climatologies of regional, seasonal, and diurnal observations of large-iCMC discharges are presented. In this study, large-iCMC discharges for thresholds > 100 and > 300 C km in both positive and negative polarities are analyzed on a seasonal basis using density maps of 2° by 2° resolution across the conterminous U.S. using data from the Charge Moment Change Network (CMCN). Also produced were local solar time diurnal distributions in eight different regions covering the lower 48 states as well as the Atlantic Ocean, including the Gulf Stream. In addition, National Lightning Detection Network (NLDN) cloud-to-ground (CG) flash diurnal distributions were included. The seasonal maps show the predisposition of large positive iCMCs to dominate across the Northern Great Plains, with large negative iCMCs favored in the Southeastern U.S. year-round. During summer, the highest frequency of large positive iCMCs across the Upper Midwest aligns closely with the preferred tracks of nocturnal mesoscale convective systems (MCSs). As iCMC values increase above 300 C km, the maximum shifts eastward of the 100 C km maximum in the Central Plains. The Southwestern U.S. also experiences significant numbers of large-iCMC discharges in summer, presumably due to convection associated with the North American Monsoon (NAM). The Gulf Stream is active year round, with a bias towards more large positive iCMCs in winter. Diurnal distributions in the eight regions support these conclusions, with a nocturnal peak in large-iCMC discharges in the Northern Great Plains and Great Lakes, an early- to mid-afternoon peak in the Intermountain West and the Southeastern US, and a morning peak in large-iCMC discharge activity over the Atlantic Ocean. Large negative iCMCs peak earlier in time than large positive iCMCs, attributed to the maturation of large stratiform charge reservoirs after initial convective development. Results of eight case studies of Northern Great Plains MCSs using the NMQ National Radar Mosaic dataset are also presented. Thresholds described above were used to disseminate iCMC discharges within the MCSs. The radar analysis algorithm on a 5-minute radar volume basis included convective-stratiform partitioning, association of iCMCs and CGs to their respective storms, and statistical analysis on large (100-300 C km) and sprite-class (>300 C km) iCMC-producing storms. Results from these case studies indicated a strong preference of sprite-class iCMCs to be positive and located in stratiform-identified regions. A 2-3 hour delay in the maximum activity of sprite-class iCMCs after the maximum large iCMC activity was noted, and was strongly correlated with the maximum areal coverage of stratiform area. A loose correlation between more frequent sprite-class iCMC production and larger stratiform areas was noted, suggesting that larger stratiform areas are simply more capable, not more likely, to produce high sprite-class iCMC rates. Enhanced maximum convective echo heights corresponded to enhanced sprite-class iCMC activity in stratiform areas, attributed in part to enhanced charge advection from the convective line. In situ charging was also presumed to have a significant role in charge generation leading to sprite-class iCMC discharges in stratiform regions.
Open Access
The role of inner-core and boundary layer dynamics on tropical cyclone structure and intensification
(Colorado State University. Libraries, 2018) Slocum, Christopher J., author; Schubert, Wayne H., advisor; DeMaria, Mark, advisor; Schumacher, Russ S., committee member; Randall, David A., committee member; Kirby, Michael, committee member; Fiorino, Michael, committee member
Inner-core and boundary layer dynamics play a vital role in the tropical cyclone life cycle. This study makes use of analytical solutions and numerical models to gain insight into the role of dynamical processes involved with the incipient, rapidly intensifying, and eyewall replacement stages. A simplified, axisymmetric, one-layer, analytical model of tropical cyclone intensification is developed. Rather than formulating the model with the gradient balance approximation, the model uses the wave-vortex approximation, an assumption to the kinetic energy of the system, which limits its use to flows with small Froude numbers. Through filtering the inertia-gravity waves and adding a mass sink so that potential vorticity is not conserved in the system, the model is solved and provides analytical, time-evolving solutions that provide insight into long incubation periods prior to rapid intensification, potential vorticity tower development without frictional effects, and storm evolution in time through the maximum tangential velocity, total energy phase space. To understand the applicability of the forced, balance model for tropical cyclone intensification, the model is compared to a model using gradient balance. The comparison shows that the model based on the wave-vortex approximation is appropriate for fluids with flow speeds indicative of the external vertical normal mode in which case the deviation to the fluid depth is small. To understand another aspect of the inner-core dynamics that influence the radial location of the mass sink associated with the eyewall convection in the tropical cyclone, boundary-layer dynamics are considered. Motivated by abrupt jumps in the horizontal wind fields observed in flight-level aircraft reconnaissance data collected in Hurricanes Allen (1980) and Hugo (1989), an axisymmetric, f-plane slab boundary layer numerical model with a prescribed pressure forcing is developed. From this model, two simple analytic models are formulated in addition to two local, steady-state models. These models allow for the role of shock dynamics in the tropical cyclone boundary layer to be assessed. Two local models are also developed to evaluate the role of the nonlinear terms in the full numerical slab model. The local models adequately describe the boundary layer winds outside of the eyewall region. If a storm is weak or broad, the local models can explain a portion of the structure that develops in the eyewall region. This result shows that, to capture the hyperbolic nature of the eyewall region, the nonlinear terms are needed. The nonlinear response allows for the boundary-layer Ekman pumping to shift radially inward into the region of high inertial stability. The results from the local models and full numerical model also show that as the vortex wind field broadens, the convergence associated with the primary eyewall decays and that a secondary maximum displaced radially outward forms, a feature indicative of the formation of a secondary eyewall.
Open Access
Transport of pollutants from eastern Colorado into the Rocky Mountains via upslope winds
(Colorado State University. Libraries, 2013) Piña, Aaron J., author; Denning, A. Scott, advisor; Schumacher, Russ S., committee member; Ham, Jay, committee member
The confluence of mountain meteorology and major pollution sources come together to transport pollutants across the Front Range, especially nitrogen species (NH3, NH4+, orgN, NO3-, and HNO3) from agricultural and urban regions, into the Rocky Mountains. The focus of this study was to examine the meteorological conditions in which atmospheric wet deposition of inorganic nitrogen in the Rocky Mountains was anomalously high. We analyzed 19 years (1994-2013) of precipitation and concentrations of wet inorganic nitrogen data from three National Atmospheric Deposition Program (NAPD) sites in the Rocky Mountains: Beaver Meadows (CO19), Loch Vale (CO98), and Niwot Ridge (CO02). Beaver Meadows (2477 m), Loch Vale (3159 m), and Niwot Ridge (3520 m) are all within 40 km but differ in elevation, resulting in different seasonal precipitation composition and totals. The North American Regional Reanalysis (NARR) was used to observe synoptic conditions that influenced two high wet deposition events from August 18-20, 2006 and July 6-8, 2012. Interestingly, anti-cyclones in southern Canada and high precipitable water values associated with monsoonal flow played significant roles in initiating convection that caused high values of wet deposition of inorganic nitrogen in the Rocky Mountains. The Advanced Research WRF model was then used to simulate the meteorology at a high spatial and temporal resolution for the two time periods to examine the contribution of cloud-scale convection to wet nitrogen deposition in the Rocky Mountains. A mesoscale mountain circulation caused by differential heating between mountains slopes and the plains was the main driver of the slow westward transport towards the mountains while cloud-scale convection contributed greatly to the transport of nitrogen along the Colorado Front Range.