- ItemOpen AccessThe role of Earth system interactions in large-scale atmospheric circulation and climate(Colorado State University. Libraries, 2023) Yook, Simchan, author; Thompson, David W. J., advisor; Ravishankara, A. R., committee member; Hurrell, James, committee member; Ebert-Uphoff, Imme, committee memberThe complex interactions among different components of the Earth system play a key role in governing the climate variability through various physical processes. For example, an interaction between the fluctuations in one component of the Earth system and associated variations in another component of the Earth system can either amplify or dampen the climate variability depending on the nature of their two-way feedback mechanisms. Thus, understanding the role of various physical interactions among components of the Earth system is critical to understand the changes in climate as well as to reduce the uncertainty in future climate projections. This dissertation focuses on discovering the key processes and interactions among different components of the Earth system on the climate variability using observations and model hierarchies. In Part 1, the interactions between the atmospheric circulation and western North Pacific SST anomalies are explored in two sets of simulations: 1) a simulation run on a coupled atmosphere-ocean general circulation model (GCM), and 2) a simulation forced with prescribed, time-evolving SST anomalies over the western North Pacific. The results support the interpretation of the observed lead/lag relationships between western North Pacific Sea Surface Temperature (SST) anomalies and the atmospheric circulation, and provide numerical evidence that SST variability over the western North Pacific has a demonstrable effect on the large-scale atmospheric circulation throughout the North Pacific sector. In Part 2, the role of moist lapse rate in altering the temperature variability under climate change is explored. To reduce the complexity of the problem, the changes in the temperature variance under global warming are first analyzed in the simplest version of model hierarchy: a single column Rapid Radiative Transfer Model with a simplified convective adjustment. Similar analyses were repeated with varying model hierarchies with additional complexities: a global general circulation model in global Radiative Convective Equilibrium (RCE) setting with fixed SST, and fully coupled Earth system models. The results highlight the role of moist lapse rate as a potential constraint for climate variability in the tropical atmosphere simulated by different model hierarchies. In Part 3, the effects of coupled chemistry-climate interactions on the amplitude and structure of stratospheric temperature variability are quantified in two numerical simulations: A "free running" simulation that includes fully coupled chemistry-climate interactions; and a "specified chemistry" version of the model forced with prescribed chemical composition. The results indicate that the inclusion of coupled chemistry-climate interactions increases the internal variability of temperature by a factor of ~two in the lower tropical stratosphere through dynamically driven ozone-temperature feedbacks. The results highlight the fundamental role of two-way feedbacks between the atmospheric circulation and chemistry in driving climate variability in the lower stratosphere. In Part 4, the effects of coupled chemistry-climate interactions on the large-scale atmospheric circulation are further explored based on two observational case studies of the Antarctic ozone holes of 2020 and 2021. The 2020 and 2021 were marked by two of the largest Antarctic ozone holes on record. It has been demonstrated that the ozone holes of 2020 and 2021 were associated with large changes in the atmospheric circulation consistent with the climate impacts of Antarctic ozone depletion. The ozone holes were also unusual for their associations with aerosol burdens due to two extraordinary events: the Australian wildfires of early 2020 and the eruption of La Soufriere in 2021. The results provide suggestive evidence that injections of both wildfire smoke and volcanic emissions into the stratosphere can lead to hemispheric-scale changes in surface climate. This dissertation provides a detailed look at the complex aspects of the coupled interactions among different components of the Earth system and their roles on climate variability and large-scale dynamics. To clarify the role of the different physical processes contributing to the climate responses, this study performed a comprehensive analysis based on observations as well as a series of numerical experiments run on different configurations of climate model hierarchies. The findings herein improve our understanding of different Earth system interactions and their influences on global climate and large-scale atmospheric dynamics.
- ItemOpen AccessInfluence 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 memberSubtropical 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.
- ItemOpen AccessLinks between climate feedbacks and the large-scale circulation across idealized and complex climate models(Colorado State University. Libraries, 2023) Davis, Luke L. B., author; Thompson, David W. J., advisor; Maloney, Eric, committee member; Randall, David, committee member; Pinaud, Olivier, committee member; Gerber, Edwin, committee memberThe circulation response to anthropogenic forcing is typically considered in one of two distinct frameworks: One that uses radiative forcings and feedbacks to investigate the thermodynamics of the response, and another that uses circulation feedbacks and thermodynamic constraints to investigate the dynamics of the response. In this thesis, I aim to help bridge the gap between these two frameworks by exploring direct links between climate feedbacks and the atmospheric circulation across ensembles of experiments from idealized and complex general circulation models (GCMs). I first demonstrate that an existing, widely-used type of idealized GCM — the dynamical core model — has climate feedbacks that are explicitly prescribed and determined by a single parameter: The thermal relaxation timescale. The dynamical core model may thus help to fill gaps in the model hierarchies commonly used to study climate forcings and climate feedbacks. I then perform two experiments: One that explores the influence of prescribed feedbacks on the unperturbed, climatological circulation; and a second that explores their influence on the circulation response to a horizontally uniform, global warming-like forcing perturbation. The results indicate that more stabilizing climate feedbacks are associated with 1) a more vigorous climatological circulation with increased thermal diffusivity, and 2) a weaker poleward displacement of the circulation in response to the global warming-like forcing. Importantly, since the most commonly-used relaxation timescale field resembles the real-world clear-sky feedback field, the uniform forcing perturbations produce realistic warming patterns, with amplified warming in the tropical upper troposphere and polar lower troposphere. The warming pattern and circulation response disappear when the relaxation timescale field is instead spatially uniform, demonstrating the critical role of spatially-varying feedback processes on shaping the response to anthropogenic forcing. I next explore circulation-feedback relationships in more complex GCMs using results from the most recent Coupled Model Intercomparison Projects (CMIP5 and CMIP6). Here, I estimate climate feedbacks by regressing top-of-atmosphere radiation against surface temperature for both 1) an unperturbed pre-industrial control experiment and 2) a perturbed global warming experiment forced by an abrupt quadrupling of CO2 concentrations. I find that across both ensembles, the cloud component of the perturbed climate feedback is closely related to the cloud component of the unperturbed climate feedback. Critically, the relationship is much stronger in CMIP6 than CMIP5, contrasting with many previously proposed constraints on the perturbation response. The relationship also explains the slow part of the CO2 response better than the fast, transient response. In general, the strength of the relationship depends on the degree to which the spatial pattern of the response resembles ENSO-dominated internal variability, with "El Niño-like" East Pacific warming and related tropical cloud changes. This is consistent with fluctuation-dissipation theory: Regions with stronger deep ocean heat exchange and weaker net feedbacks must always dominate both 1) internal fluctuations in the global energy budget, and 2) the slow part of the response to forcing perturbations. The stronger CMIP6 inter-model relationships are due to both an amplification of this mechanism and higher inter-model correlations between tropical cloud changes and extratropical cloud changes. Finally, I present emergent constraints on the slow response using a recent observational estimate of the unperturbed cloud feedback. I conclude by discussing some implications of these results. I consider how the relaxation feedback framework might be further developed and reconciled with traditional climate feedbacks to provide future research opportunities with climate model hierarchies.
- ItemOpen AccessA potential vorticity diagnosis of tropical cyclone track forecast errors(Colorado State University. Libraries, 2023) Barbero, Tyler Warren, author; Bell, Michael M., advisor; Barnes, Elizabeth A., committee member; Chen, Jan-Huey, committee member; Klotzbach, Philip J., committee member; Zhou, Yongcheng, committee memberA tropical cyclone (TC) can cause significant impacts on coastal and near-coastal communities from storm surge, flooding, intense winds, and heavy rainfall. Accurately predicting TC track is crucial to providing affected populations with time to prepare and evacuate. Over the years, advancements in observational quality and quantity, numerical models, and data assimilation techniques have led to a reduction in average track errors. However, large forecast errors still occur, highlighting the need for ongoing research into the causes of track errors in models. We use the piecewise potential vorticity (PV) inversion diagnosis technique to investigate the sources of errors in track forecasts of four high-resolution numerical weather models during the hyperactive 2017 Atlantic hurricane season. The piecewise PV inversion technique is able to quantify the amount of steering, as well as steering errors, on TC track from individual large-scale pressure systems. Through the systematic use of the diagnostic tool, errors that occur consistently (model biases) could also be identified. TC movement generally follows the atmospheric flow generated by large-scale environmental pressure systems, such that errors in the simulated flow cause errors in the TC track forecast. To understand how the environment steers TCs, we use the Shapiro decomposition to remove the TC PV field from the total PV field, and the environmental (i.e., perturbation) PV field is isolated. The perturbation PV field was partitioned into six systems: the Bermuda High and the Continental High, which compose the negative environmental PV, and quadrants to the northwest, northeast, southeast, and southwest of the TC, which compose the positive environmental PV. Each piecewise PV perturbation system was inverted to retrieve the balanced mass and wind fields. To quantify the steering contribution in individual systems to TC movement, a metric called the deep layer mean steering flow (DLMSF) is defined, and errors in the forecast DLMSF were calculated by comparing the forecast to the analysis steering flow. Lag correlation analyses of DLMSF errors and track errors showed moderate-high correlation at -24 to 0 hrs in time, which indicates that track errors are caused in part by DLMSF errors. Three hurricanes (Harvey, Irma, and Maria) were analyzed in-depth and errors in their track forecasts are attributed to errors in the DLMSF. A basin-scale analysis was also performed on all hurricanes in the 2017 Atlantic hurricane season. The DLMSF mean absolute error (MAE) showed the Bermuda High was the highest contributor to error, the Continental High showed moderate error, while the four quadrants showed lower errors. High error cases were composited to examine potential model biases. On average, the composite showed lower balanced geopotential heights around the climatological position of the Bermuda High associated with the recurving of storms in the North Atlantic basin. The analysis techniques developed in this thesis aids in the identification of model biases which will lead to improved track forecasts in the future.
- ItemOpen AccessInvestigating the impact of forced and internal climate variability on future convective storm environments in subtropical South America: a large ensemble approach(Colorado State University. Libraries, 2023) Chakraborty, Anindita, author; Rasmussen, Kristen, advisor; Hurrell, James, advisor; Anderson, Brooke, committee memberSubtropical South America (SSA) has some of the most intense deep convection in the world. Large hail and frequent lightning are just two of the hazards that profoundly affect people, agriculture, and infrastructure in this region. Therefore, it is important to understand the future convective storm environments over SSA associated with climate change and how these large-scale environmental changes are likely to change high-impact weather events in the future. Previous studies have used convection-permitting regional models and radar data to examine convective storm environments in the current climate across different regions of South America. Here, we use a large ensemble of Earth system model simulations to quantify anthropogenically-driven future changes in large-scale convective environments, as well as how those forced changes might be modified by unforced, internal climate variability. Specifically, we examine changes in different thermodynamic parameters of relevance to severe weather events over SSA in austral spring and summer (September-February). We use daily data from a 50-member ensemble from 1870-2100 performed with version two of the Community Earth System Model (CESM2). Results indicate that no forced changes in convective environments are evident until very late in the 20th century. However, increases in convective available potential energy and atmospheric stability, as well as an increase in lower tropospheric vertical wind shear, became apparent around 1990, and these trends are projected to continue throughout the rest of this century. The implication is that future large-scale environments may be favorable for less frequent, but perhaps more intense and severe convective modes and their associated hazards. Results also demonstrate that anthropogenic changes are likely to be significantly modified, regionally, by internal climate variability.