Extreme weather events are rare, but disproportionately important for societal and economic risk. Estimating their likelihood is therefore central to applications ranging from infrastructure design to financial risk management. This task is fundamentally constrained by limited data. Observational records are often too short to robustly characterize rare events, while the underlying climate system is changing in ways that can alter the behavior of extremes. At the same time, statistical approaches for estimating extreme event probabilities and physical studies of the processes that generate extremes are often developed separately, limiting our ability to interpret risk in a coherent framework. This thesis investigates how statistical limitations and physical climate processes jointly shape the estimation and interpretation of extreme events under limited data conditions. Using a combination of idealized climate model simulations, observational datasets, and statistical analysis, I examine both the reliability of extreme value estimates and the mechanisms that influence climate extremes, with a focus on extreme heat.
\r\n\r\nI show that short climate records can lead to systematic underestimation of rare event probabilities and large uncertainty in return level estimates. These results quantify how sample size and tail behavior constrain the reliability of statistical inference and provide guidance for identifying when available data are insufficient for robust estimation, including under climate change imposing unknown trends. I also demonstrate that observed changes in the frequency of extreme heat are, to first order, consistent with shifts in the underlying temperature distribution. A novel statistical method of isolating low-frequency variability from noisy climate data yields a simple and generalizable interpretation of observed temperature trends. Finally, I show that in regions with strong land\u2013atmosphere coupling, such as the Central United States, soil moisture deficits and atmospheric circulation patterns can amplify extreme heat beyond what occurs on typical days. These nonlinear processes influence extreme events in a way that may be challenging to anticipate at long timescales in the future. These results highlight both the limits of statistical inference for rare events and the role of physical mechanisms in shaping extreme behavior. This dissertation provides practical and conceptual tools for interpreting and quantifying extremes under limited data in a changing climate.
", "doi": "10.7907/pmnh-6875", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:16527", "collection": "thesis", "collection_id": "16527", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06262024-011209069", "primary_object_url": { "basename": "bonan-db-dissertation.pdf", "content": "final", "filesize": 12539991, "license": "other", "mime_type": "application/pdf", "url": "/16527/1/bonan-db-dissertation.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Explications of a Changing Climate", "author": [ { "family_name": "Bonan", "given_name": "David Brooking", "orcid": "0000-0003-3867-6009", "clpid": "Bonan-David-Brooking" } ], "thesis_advisor": [ { "family_name": "Schneider", "given_name": "Tapio", "orcid": "0000-0001-5687-2287", "clpid": "Schneider-T" }, { "family_name": "Thompson", "given_name": "Andrew F.", "orcid": "0000-0003-0322-4811", "clpid": "Thompson-A-F" } ], "thesis_committee": [ { "family_name": "Adkins", "given_name": "Jess F.", "orcid": "0000-0002-3174-5190", "clpid": "Adkins-J-F" }, { "family_name": "Schneider", "given_name": "Tapio", "orcid": "0000-0001-5687-2287", "clpid": "Schneider-T" }, { "family_name": "Thompson", "given_name": "Andrew F.", "orcid": "0000-0003-0322-4811", "clpid": "Thompson-A-F" }, { "family_name": "Zanna", "given_name": "Laure", "orcid": "0000-0002-8472-4828", "clpid": "Zanna-Laure" }, { "family_name": "Rugenstein", "given_name": "Maria", "orcid": "0000-0002-4541-3277", "clpid": "Rugenstein-Maria" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "Climate models encode our collective knowledge about the climate system and are among the best tools available for estimating past and future climate change. However, in response to greenhouse gas forcing, climate models exhibit a large intermodel spread in various aspects of the climate system, adding considerable uncertainty to future climate projections. This dissertation introduces a series of conceptual models and frameworks to understand the behavior of climate models under greenhouse gas forcing and, consequently, Earth's changing climate.
\r\n\r\nA simple statistical model is used to explain and constrain the intermodel spread in Arctic sea ice projections across climate models. The probability of encountering seasonally ice-free conditions in the twenty-first century is also explored by systematically constraining components of the statistical model with observations.
\r\n\r\nA conceptual framework is introduced to understand controls on the strength and structure of the Atlantic meridional overturning circulation (AMOC) in climate models. This framework is used to explain why climate models suggest the present-day and future AMOC strength are related. This framework, in conjunction with observations, implies modest twenty-first-century AMOC weakening.
\r\n\r\nA simple energy budget framework is used to examine precipitation over a wide range of climates simulated by climate models. It is shown that in extremely hot climates, global-mean precipitation decreases despite increasing surface temperatures because of increased atmospheric shortwave absorption from water vapor, which limits energy available for surface evaporation. These results have large implications for understanding weathering rates in past climates as well as Earth's climate during the Hadean and Archaean eons.
\r\n\r\nFinally, a framework is introduced to reconcile two different approaches for quantifying the effect of climate feedbacks on surface temperature change. The framework is used to examine the influence of clouds on Arctic amplification in a climate model and an energy balance model. This work introduces an important non-local mechanism for Arctic amplification and shows that constraining the mid-latitude cloud feedback will greatly reduce the intermodel spread in Arctic warming.
\r\n\r\nThis dissertation advances our understanding of various aspects of Earth's changing climate and provides a series of conceptual frameworks that can be used to further constrain the behaviour of climate models in response to external forcing.
", "doi": "10.7907/afb5-ns06", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:16724", "collection": "thesis", "collection_id": "16724", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:09132024-172910003", "type": "thesis", "title": "Cloudy with a Chance of Microphysics: Modeling Droplet Collisions for the Climate Scale", "author": [ { "family_name": "de Jong", "given_name": "Emily Katherine", "orcid": "0000-0002-5310-4554", "clpid": "de-Jong-Emily-Katherine" } ], "thesis_advisor": [ { "family_name": "Schneider", "given_name": "Tapio", "orcid": "0000-0001-5687-2287", "clpid": "Schneider-T" } ], "thesis_committee": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Fu", "given_name": "Xiaojing", "orcid": "0000-0001-7120-704X", "clpid": "Fu-Xiaojing" }, { "family_name": "Morrison", "given_name": "Hugh", "orcid": "0000-0001-8480-9787", "clpid": "Morrison-Hugh" }, { "family_name": "Schneider", "given_name": "Tapio", "orcid": "0000-0001-5687-2287", "clpid": "Schneider-T" } ], "local_group": [ { "literal": "3MT Competition (Caltech)" }, { "literal": "div_eng" } ], "abstract": "Feedbacks between a warming atmosphere, emission of aerosols, and clouds and precipitation are some of the most difficult aspects for climate models to accurately capture. While climate models operate at resolutions of tens or hundreds of kilometers, many of the physics that determine how and where clouds form or precipitate function at the micron droplet scale. Due to this disparity in physical scales, most of these cloud physics must be modeled with only a few approximate quantities and physical equations. These simplifications lead to large uncertainties about climate forcings such as the sensitivity of global warming to human-emitted aerosols.
\r\n\r\nThis work presents several promising new techniques for modeling and understanding hydrometeors in the climate system, with a particular focus on processes that involve collisions between droplets. First, I extend a high-complexity high-fidelity Lagrangian microphysics method to represent the process of breakup, in which colliding droplets fragment upon collision. Next, I introduce two new methods which attempt to reduce the assumptions inherent to modeling droplet coalescence, in which colliding droplets combine to form a larger drop. The first method uses a spectral finite element approach, while the second generalizes this technique using a method of moments to create a fully flexible microphysics scheme. Finally, I turn to remote observations of clouds, aerosols, and lightning over busy shipping regions to offer new techniques for quantifying aerosol-cloud interactions from creative data resources. This combination of high-fidelity modeling tools, observational data, and efficient numerical methods offers a path toward improving our understanding of the role of cloud microphysics in our climate system.
", "doi": "10.7907/yv2y-kg55", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:16895", "collection": "thesis", "collection_id": "16895", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:12032024-211328064", "primary_object_url": { "basename": "Caltech_Dissertation_v3_Final.pdf", "content": "final", "filesize": 12342692, "license": "other", "mime_type": "application/pdf", "url": "/16895/1/Caltech_Dissertation_v3_Final.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Towards Hybrid Physics-Machine Learning Parameterizations: Employing Data Assimilation for Online Learning of Turbulence and Convection Closures in a Unified Scheme", "author": [ { "family_name": "Christopoulos", "given_name": "Costa", "orcid": "0000-0002-8552-465X", "clpid": "Christopoulos-Costa" } ], "thesis_advisor": [ { "family_name": "Schneider", "given_name": "Tapio", "orcid": "0000-0001-5687-2287", "clpid": "Schneider-T" } ], "thesis_committee": [ { "family_name": "Frankenberg", "given_name": "Christian", "orcid": "0000-0002-0546-5857", "clpid": "Frankenberg-Christian" }, { "family_name": "Schneider", "given_name": "Tapio", "orcid": "0000-0001-5687-2287", "clpid": "Schneider-T" }, { "family_name": "Ross", "given_name": "Zachary E.", "orcid": "0000-0002-6343-8400", "clpid": "Ross-Z-E" }, { "family_name": "Beucler", "given_name": "Tom", "orcid": "0000-0002-5731-1040", "clpid": "Beucler-Tom" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "Despite advances in climate modeling, the spread in equilibrium climate sensitivity estimates has remained largely unchanged over generations of modeling, mainly due to uncertainties in cloud feedback mechanisms arising from subgrid-scale turbulence, convection, clouds, and the resulting cloud-radiation interactions. Misrepresentations of these processes affect both long-term climate projections and the simulation of short-term atmospheric phenomena, such as the diurnal cycle of precipitation. These limitations are most pronounced in regimes like stratocumulus clouds and their transition to cumulus over ocean basins---areas where climate models have the largest cloud biases in the historical record. This thesis aims to constrain the critical subgrid-scale physics of turbulence and convection by developing and calibrating a hybrid physics\u2013machine learning parameterization using the Eddy-diffusivity Mass-flux (EDMF) framework. By integrating machine learning components into the EDMF and employing data assimilation techniques for online learning, we attempt to directly target some of the processes responsible for uncertainties in cloud feedbacks.\r\n\r\nIn this thesis, we employ ensemble Kalman inversion within a single-column setup to simultaneously perform online calibration of parameters in empirical closures and embedded neural networks, targeting large-eddy simulations as ground truth. The online learning framework ensures stability and physical consistency, as machine learning components are trained within the context of the full model dynamics. By directly targeting poorly constrained processes like lateral entrainment/detrainment and turbulent mixing lengths, we improve the representation of subgrid-scale fluxes and resulting cloud properties across various atmospheric regimes. We uncover limitations of traditional semi-empirical closures, providing insights for future model development. The calibrated hybrid parameterization outperforms existing schemes, particularly in regions where climate models have historically underperformed, and maintains accuracy in out-of-sample forcings from a warmer climate. This work demonstrates that integrating machine learning with physics-based parameterizations through data assimilation offers a systematic and robust approach for reducing biases in climate models and understanding the physics of elusive subgrid-scale closures.", "doi": "10.7907/ptr2-0r80", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:17390", "collection": "thesis", "collection_id": "17390", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06032025-003651371", "primary_object_url": { "basename": "PhD_Thesis__Polina_Khapikova_.pdf", "content": "final", "filesize": 30711121, "license": "other", "mime_type": "application/pdf", "url": "/17390/1/PhD_Thesis__Polina_Khapikova_.pdf", "version": "v7.0.0" }, "type": "thesis", "title": "From Hadley Cells to Heat Extremes: Novel Physical and Statistical Frameworks for Evaluating Climate Models", "author": [ { "family_name": "Khapikova", "given_name": "Polina", "orcid": "0000-0001-8142-5577", "clpid": "Khapikova-Polina" } ], "thesis_advisor": [ { "family_name": "Schneider", "given_name": "Tapio", "orcid": "0000-0001-5687-2287", "clpid": "Schneider-T" } ], "thesis_committee": [ { "family_name": "Thompson", "given_name": "Andrew F.", "orcid": "0000-0003-0322-4811", "clpid": "Thompson-A-F" }, { "family_name": "Schneider", "given_name": "Tapio", "orcid": "0000-0001-5687-2287", "clpid": "Schneider-T" }, { "family_name": "Frankenberg", "given_name": "Christian", "orcid": "0000-0002-0546-5857", "clpid": "Frankenberg-Christian" }, { "family_name": "Baldwin", "given_name": "Jane", "orcid": "0000-0002-4174-2743", "clpid": "Baldwin-Jane" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "As climate models become increasingly central to projecting future climate impacts and guiding adaptation strategies, understanding the sources of intermodel spread is critical. This thesis investigates key drivers of model uncertainty through a process-based lens grounded in fundamental climate physics. It focuses on two major contributors to model spread: atmospheric circulation and land-surface processes, which together shape regional hydrological extremes and human-relevant climate outcomes.
\r\n\r\nFirst, I develop a zonal-mean energetic framework to explain variations in the meridional extent of the Hadley Circulation (HC) across seasonal, interannual, and multidecadal timescales. I show that changes in tropical net energy input and eddy energy export can be used to explain HC migrations in a variety of contexts. For example, this framework can be used to explain the larger migrations of the ascending HC branch relative to the descending branch over the seasonal cycle, and the contrasting HC migration observed during El Nino events and in response to global warming. This approach provides a physically grounded method for interpreting both observed short-term variability and projected future changes in the tropical atmospheric circulation.
\r\n\r\nNext, I address uncertainty in terrestrial water storage (TWS) trends across CMIP6 models. To circumvent the limitations of short observational records, I introduce a novel method that compares the seasonal cycle of TWS using empirical orthogonal functions derived from GRACE satellite data. This approach enables the ranking of models based on their fidelity to observed seasonal variability. Models with stronger agreement with GRACE observations exhibit more consistent and often larger trends in TWS, and share features such as deeper soils and spatially distinct precipitation patterns. These insights can guide model selection and development efforts.
\r\n\r\nFinally, I explore model spread in the temperature-humidity combinations that produce extreme wet-bulb temperatures (WBTs), a key metric of human heat stress. While WBT projections are relatively consistent across models, the partitioning between temperature and humidity\u2014quantified using the \"stickiness\" metric\u2014varies substantially. I show that these differences lead to large uncertainty in the physiological impacts of heat stress at a given WBT. This work underscores the importance of accurately capturing surface energy partitioning to constrain present and future heat stress risks.
\r\n\r\nTogether, the chapters of this thesis offer several innovative approaches to evaluating uncertainty in climate projections. By applying simple theory, observational constraints, and novel metrics, this work demonstrates how physically grounded methods can identify more reliable models and improve the accuracy of future climate projections---particularly for hydrological extremes and human-centered impacts.
", "doi": "10.7907/px1t-j224", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:15158", "collection": "thesis", "collection_id": "15158", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05092023-230615398", "type": "thesis", "title": "The Role of Small-Scale Cloud, Aerosol, and Radiation Processes for Earth's Climate", "author": [ { "family_name": "Singer", "given_name": "Clare Emilie Elmendorf", "orcid": "0000-0002-1708-0997", "clpid": "Singer-Clare-Emilie-Elmendorf" } ], "thesis_advisor": [ { "family_name": "Schneider", "given_name": "Tapio", "orcid": "0000-0001-5687-2287", "clpid": "Schneider-T" } ], "thesis_committee": [ { "family_name": "Wennberg", "given_name": "Paul O.", "orcid": "0000-0002-6126-3854", "clpid": "Wennberg-P-O" }, { "family_name": "Schneider", "given_name": "Tapio", "orcid": "0000-0001-5687-2287", "clpid": "Schneider-T" }, { "family_name": "Callies", "given_name": "Joern", "orcid": "0000-0002-6815-1230", "clpid": "Callies-J" }, { "family_name": "Feingold", "given_name": "Graham", "orcid": "0000-0002-0774-2926", "clpid": "Feingold-Graham" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "What makes clouds ethereal and beautiful also makes them complex and challenging to understand and to model. The important (thermo)dynamical processes of clouds occur at scales from microns (cloud-aerosol interactions), to meters (turbulence), to thousands of kilometers (synoptic weather patterns), and every scale in between. In this thesis, I explore several facets of how clouds interact with, respond to, and shape Earth's climate. I focus on small-scale processes, using high-resolution models and theory, to understand phenomena that can have large-scale impacts.
\r\n \r\nIn the first three chapters of this thesis, I explore the idea of stratocumulus-cumulus transitions. Chapters 1 and 2 develop and demonstrate a conceptual model of a cloud-topped atmospheric boundary layer, which is rooted in mixed-layer theory. This model is able to concisely explain both the spatial stratocumulus-cumulus transition observed in the historical period, as well as a transition that has only been hypothesized by models, which may occur in the future as the direct effect of extreme concentrations of atmospheric CO2, or which may have occurred in the past. I use this conceptual model to show the importance of sea surface temperature variations for driving the climatological transition, and on sea surface warming as a positive feedback for the CO2-induced transition. Chapter 3 extends this work to understand the global response to CO2-induced stratocumulus-cumulus transitions and the role for spatial teleconnections by embedding this conceptual model of the boundary layer into a global climate model (GCM). In the GCM we see both a fast adjustment in low cloud cover to CO2, as well as a slower surface temperature-mediated feedback. Under CO2 quadrupling, the stratocumulus cloud regions shrink in extent as the cloud-top longwave cooling is inhibited by CO2 and surface temperatures also increase.
\r\n\r\nThe final two chapters diverge from the previous theme to present two studies using very high-resolution models to explore how clouds interact with i) aerosols and ii) radiation. In Chapter 4, using a particle-based cloud microphysics model, I find that aerosol hygroscopicity, determined by the chemical composition of the particles, can alter stratocumulus cloud macrophysical properties, like liquid water path by up to 25% (in the regime of small aerosol sizes). I compare these results to a more standard moment-based microphysics model and find that this model is overly sensitive to aerosol hygroscopicity in the regime of small aerosol sizes, but realistically represents the negative sensitivity for large aerosol sizes. Finally, in Chapter 5, I use a Monte Carlo 3D radiative transfer solver to estimate the global albedo bias introduced in models which make the standard assumption that photon fluxes in the horizontal are zero (the so-called Independent Column Approximation). I extrapolate globally from a set of resolved tropical cloud fields, using a learned empirical relation between top-of-atmosphere flux bias and cloud water path. I conclude that in a global model that resolves clouds at small-enough spatial scales, the tropical-mean, annual-mean bias may be on the order of 3 W m-2.
", "doi": "10.7907/bd4s-w586", "publication_date": "2024", "thesis_type": "phd", "thesis_year": "2024" }, { "id": "thesis:15063", "collection": "thesis", "collection_id": "15063", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:11152022-215747755", "primary_object_url": { "basename": "PhD_thesis__Ignacio_Lopez_Gomez_-revised.pdf", "content": "final", "filesize": 28614789, "license": "other", "mime_type": "application/pdf", "url": "/15063/1/PhD_thesis__Ignacio_Lopez_Gomez_-revised.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "A Unified Data-Informed Model of Turbulence and Convection for Climate Prediction", "author": [ { "family_name": "L\u00f3pez G\u00f3mez", "given_name": "Ignacio", "orcid": "0000-0002-7255-5895", "clpid": "L\u00f3pez-G\u00f3mez-Ignacio" } ], "thesis_advisor": [ { "family_name": "Schneider", "given_name": "Tapio", "orcid": "0000-0001-5687-2287", "clpid": "Schneider-T" } ], "thesis_committee": [ { "family_name": "Callies", "given_name": "Joern", "orcid": "0000-0002-6815-1230", "clpid": "Callies-J" }, { "family_name": "Stuart", "given_name": "Andrew M.", "orcid": "0000-0001-9091-7266", "clpid": "Stuart-A-M" }, { "family_name": "Teixeira", "given_name": "Joao", "clpid": "Teixeira-Joao" }, { "family_name": "Schneider", "given_name": "Tapio", "orcid": "0000-0001-5687-2287", "clpid": "Schneider-T" } ], "local_group": [ { "literal": "Resnick Sustainability Institute" }, { "literal": "div_gps" } ], "abstract": "Resolving atmospheric turbulent and convective processes in global climate simulations is, and will remain for decades, an intractable computational problem. The strong influence of these processes on cloud formation and maintenance makes the task of modeling turbulence and convection one of the grand challenges in climate modeling, due to the outsized effect of clouds on climate. Current operational climate models fail to represent atmospheric turbulence and convection accurately and consistently across dynamical regimes and vertical levels; errors in the representation of these processes explain about half of the spread in climate projections. This dissertation seeks to reduce such representation errors by improving a recently proposed unified framework for modeling turbulence and convection, known as the extended eddy-diffusivity mass-flux scheme, in several ways. First, the framework is rederived by systematically coarse-graining the governing fluid equations, highlighting the assumptions about atmospheric motion that are necessary to yield the scheme. New terms related to turbulent entrainment processes are shown to arise from the derivation. Second, a generalized formulation of turbulent diffusion consistent with the framework is presented. This novel formulation is shown to accurately represent turbulent processes under statically stable and unstable conditions, including regimes with sharp lapse rate inversions such as the stratocumulus-topped boundary layer. Finally, a methodology to calibrate free parameters within the model from indirect data is proposed. The methodology, based on Kalman filtering, is shown to be efficient at calibrating imperfect black-box models from noisy data, and in its regularized unscented version approximately quantifies parametric uncertainty. The resulting unified data-informed model of turbulence and convection is shown to accurately represent a range of low-cloud regimes that are associated with the largest biases in current operational climate models. The response of the model to realistic climate perturbations is also shown to be consistent with the resolved climate response, although structural errors in the amount of condensate are still important at realistic vertical resolutions.", "doi": "10.7907/042m-9686", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:10989", "collection": "thesis", "collection_id": "10989", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05312018-154833107", "primary_object_url": { "basename": "caltech-thesis-zhang-xiyue-final.pdf", "content": "final", "filesize": 5643782, "license": "other", "mime_type": "application/pdf", "url": "/10989/2/caltech-thesis-zhang-xiyue-final.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Dynamics of Resolved Polar Clouds", "author": [ { "family_name": "Zhang", "given_name": "Xiyue", "orcid": "0000-0002-6031-7830", "clpid": "Zhang-Xiyue" } ], "thesis_advisor": [ { "family_name": "Schneider", "given_name": "Tapio", "orcid": "0000-0001-5687-2287", "clpid": "Schneider-T" } ], "thesis_committee": [ { "family_name": "Bordoni", "given_name": "Simona", "orcid": "0000-0003-4771-3350", "clpid": "Bordoni-S" }, { "family_name": "Thompson", "given_name": "Andrew F.", "orcid": "0000-0003-0322-4811", "clpid": "Thompson-A-F" }, { "family_name": "Teixeira", "given_name": "Joao", "clpid": "Teixeira-Joao" }, { "family_name": "Seinfeld", "given_name": "John H.", "orcid": "0000-0003-1344-4068", "clpid": "Seinfeld-J-H" }, { "family_name": "Schneider", "given_name": "Tapio", "orcid": "0000-0001-5687-2287", "clpid": "Schneider-T" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "The polar regions have been experiencing rapid warming and ice loss as greenhouse gas concentrations have risen. The projected warming in the Arctic varies significantly across climate models, part of which is attributed to polar cloud feedbacks. This thesis addresses the question of what drives the changes in polar clouds as the climate warms, using a large eddy simulation (LES) model. LES is a powerful high-resolution model that resolves the most energetic turbulence relevant for clouds. First, we focus on the Arctic boundary layer clouds through three observation based case studies. The cloud and boundary layer characteristics simulated by the LES agree reasonably well with observations and model intercomparisons. We found that during polar night over sea ice, cloud water path increases with temperature and free-tropospheric relative humidity, but it decreases with inversion strength across the cloud top. Most of these changes can be explained by a mixed-layer model. The strength of the estimated positive cloud longwave feedback largely depends on the cloud top inversion strength. Next, we extend the LES domain to cover the entire polar troposphere, and use output from an idealized GCM as forcing to drive the LES. This novel framework allows changes in the large-scale circulation to be parameterized in the LES. The simulated seasonal cycle of liquid clouds resembles observations. In a warmer climate, there is a significant decrease of the low-level liquid clouds during summer and autumn. In spring and winter, liquid clouds increase at all levels. Both the liquid and ice cloud tops rise as the climate warms. Offline radiative transfer calculations estimate a positive cloud feedback that is dominated by longwave feedback.\r\n", "doi": "10.7907/69E7-0Q10", "publication_date": "2018", "thesis_type": "phd", "thesis_year": "2018" }, { "id": "thesis:9981", "collection": "thesis", "collection_id": "9981", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:11122016-111133209", "type": "thesis", "title": "Dynamics of the Intertropical Convergence Zone", "author": [ { "family_name": "Bischoff", "given_name": "Tobias", "orcid": "0000-0003-3930-2762", "clpid": "Bischoff-Tobias" } ], "thesis_advisor": [ { "family_name": "Schneider", "given_name": "Tapio", "clpid": "Schneider-T" } ], "thesis_committee": [ { "family_name": "Thompson", "given_name": "Frederick B.", "clpid": "Thompson-F-B" }, { "family_name": "Schneider", "given_name": "Tapio", "clpid": "Schneider-T" }, { "family_name": "Bordoni", "given_name": "Simona", "clpid": "Bordoni-S" }, { "family_name": "Adkins", "given_name": "Jess F.", "clpid": "Adkins-J-F" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "Previous studies have shown that the latitude of the Intertropical Convergence Zone (ITCZ) is negatively correlated with cross-equatorial atmospheric energy transport and that the ITCZ shifts southward as the northern hemisphere cools and the northward cross-equatorial energy transport strengthens. However, it has remained unclear what controls the sensitivity of the ITCZ position to cross-equatorial energy transport, and what other factors may lead to shifts of the ITCZ position. In this thesis, it is shown how an energetic perspective using the vertically-integrated moist static energy balance of the atmosphere can be used to address this question. Climate states with a double-ITCZ around the equator also occur, for example, seasonally over the eastern Pacific, and frequently in climate models. Here it is shown how the ITCZ position is connected to the energy balance near the equator under a wide range of circumstances, including states with single and double ITCZs and using a Taylor expansion of the meridional energy transport around the equator quantitative estimates for the ITCZ location are derived. Simulations with an idealized aquaplanet general circulation model (GCM) confirm the quantitative adequacy of these relations. Using these ideas, an idealized precipitation model for the tropics is presented that is able to capture variations of paleoclimatological precipitation records on orbital time scales. The results provide a framework for assessing and understanding causes of common climate model biases and for interpreting tropical precipitation changes, such as those evident in records of climates of the past.", "doi": "10.7907/Z97M05XR", "publication_date": "2017", "thesis_type": "phd", "thesis_year": "2017" }, { "id": "thesis:9107", "collection": "thesis", "collection_id": "9107", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08192015-043550928", "type": "thesis", "title": "Simulations and Mechanisms of Subtropical Low-cloud Response to Climate Change", "author": [ { "family_name": "Tan", "given_name": "Zhihong", "orcid": "0000-0002-7422-3317", "clpid": "Tan-Zhihong" } ], "thesis_advisor": [ { "family_name": "Schneider", "given_name": "Tapio", "orcid": "0000-0001-5687-2287", "clpid": "Schneider-T" }, { "family_name": "Teixeira", "given_name": "Joao", "clpid": "Teixeira-J" } ], "thesis_committee": [ { "family_name": "Seinfeld", "given_name": "John H.", "orcid": "0000-0003-1344-4068", "clpid": "Seinfeld-J-H" }, { "family_name": "Schneider", "given_name": "Tapio", "orcid": "0000-0001-5687-2287", "clpid": "Schneider-T" }, { "family_name": "Teixeira", "given_name": "Joao", "clpid": "Teixeira-Joao" }, { "family_name": "Bordoni", "given_name": "Simona", "orcid": "0000-0003-4771-3350", "clpid": "Bordoni-S" }, { "family_name": "Wennberg", "given_name": "Paul O.", "orcid": "0000-0002-6126-3854", "clpid": "Wennberg-P-O" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "This thesis focuses on improving the simulation skills and the theoretical understanding of the subtropical low cloud response to climate change.
\r\n\r\nFirst, an energetically consistent forcing framework is designed and implemented for the large eddy simulation (LES) of the low-cloud response to climate change. The three representative current-day subtropical low cloud regimes of cumulus (Cu), cumulus-over-stratocumulus, and stratocumulus (Sc) are all well simulated with this framework, and results are comparable to the conventional fixed-SST approach. However, the cumulus response to climate warming subject to energetic constraints differs significantly from the conventional approach with fixed SST. Under the energetic constraint, the subtropics warm less than the tropics, since longwave (LW) cooling is more efficient with the drier subtropical free troposphere. The surface latent heat flux (LHF) also increases only weakly subject to the surface energetic constraint. Both factors contribute to an increased estimated inversion strength (EIS), and decreased inversion height. The decreased Cu-depth contributes to a decrease of liquid water path (LWP) and weak positive cloud feedback. The conventional fixed-SST approach instead simulates a strong increase in LHF and deepening of the Cu layer, leading to a weakly negative cloud feedback. This illustrates the importance of energetic constraints to the simulation and understanding of the sign and magnitude of low-cloud feedback.
\r\n\r\nSecond, an extended eddy-diffusivity mass-flux (EDMF) closure for the unified representation of sub-grid scale (SGS) turbulence and convection processes in general circulation models (GCM) is presented. The inclusion of prognostic terms and the elimination of the infinitesimal updraft fraction assumption makes it more flexible for implementation in models across different scales. This framework can be consistently extended to formulate multiple updrafts and downdrafts, as well as variances and covariances. It has been verified with LES in different boundary layer regimes in the current climate, and further development and implementation of this closure may help to improve our simulation skills and understanding of low-cloud feedback through GCMs.
", "doi": "10.7907/Z9JS9NC9", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:9536", "collection": "thesis", "collection_id": "9536", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:01202016-022601840", "primary_object_url": { "basename": "wills_robert_thesis_2016.pdf", "content": "final", "filesize": 44989292, "license": "other", "mime_type": "application/pdf", "url": "/9536/1/wills_robert_thesis_2016.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Stationary Eddies and Zonal Variations of the Global Hydrological Cycle in a Changing Climate", "author": [ { "family_name": "Wills", "given_name": "Robert Christopher", "clpid": "Wills-Robert-Christopher" } ], "thesis_advisor": [ { "family_name": "Schneider", "given_name": "Tapio", "clpid": "Schneider-T" } ], "thesis_committee": [ { "family_name": "Thompson", "given_name": "Andrew F.", "clpid": "Thompson-A-F" }, { "family_name": "Bordoni", "given_name": "Simona", "clpid": "Bordoni-S" }, { "family_name": "Adkins", "given_name": "Jess F.", "clpid": "Adkins-J-F" }, { "family_name": "Lamb", "given_name": "Michael P.", "clpid": "Lamb-M-P" }, { "family_name": "Schneider", "given_name": "Tapio", "clpid": "Schneider-T" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "This thesis advances our physical understanding of the sensitivity of the hydrological cycle to global warming. Specifically, it focuses on changes in the longitudinal (zonal) variation of precipitation minus evaporation (P - E), which is predominantly controlled by planetary-scale stationary eddies. By studying idealized general circulation model (GCM) experiments with zonally varying boundary conditions, this thesis examines the mechanisms controlling the strength of stationary-eddy circulations and their role in the hydrological cycle. The overarching goal of this research is to understand the cause of changes in regional P - E with global warming. An understanding of such changes can be useful for impact studies focusing on water availability, ecosystem management, and flood risk.
\r\n\r\nBased on a moisture-budget analysis of ERA-Interim data, we establish an approximation for zonally anomalous P - E in terms of surface moisture content and stationary-eddy vertical motion in the lower troposphere. Part of the success of this approximation comes from our finding that transient-eddy moisture fluxes partially cancel the effect of stationary-eddy moisture advection, allowing divergent circulations to dominate the moisture budget. The lower-tropospheric vertical motion is related to horizontal motion in stationary eddies by Sverdrup and Ekman balance. These moisture- and vorticity-budget balances also hold in idealized and comprehensive GCM simulations across a range of climates.
\r\n\r\nBy examining climate changes in the idealized and comprehensive GCM simulations, we are able to show the utility of the vertical motion P - E approximation for splitting changes in zonally anomalous P - E into thermodynamic and dynamic components. Shifts in divergent stationary-eddy circulations dominate changes in zonally anomalous P - E. This limits the local utility of the \"wet gets wetter, dry gets drier\u201d idea, where existing P - E patterns are amplified with warming by the increase in atmospheric moisture content, with atmospheric circulations held fixed. The increase in atmospheric moisture content manifests instead in an increase in the amplitude of the zonally anomalous hydrological cycle as measured by the zonal variance of P - E. However, dynamic changes, particularly the slowdown of divergent stationary-eddy circulations, limit the strengthening of the zonally anomalous hydrological cycle. In certain idealized cases, dynamic changes are even strong enough to reverse the tendency towards \"wet gets wetter, dry gets drier\u201d with warming.
\r\n\r\nMotivated by the importance of stationary-eddy vertical velocities in the moisture budget analysis, we examine controls on the amplitude of stationary eddies across a wide range of climates in an idealized GCM with simple topographic and ocean-heating zonal asymmetries. An analysis of the thermodynamic equation in the vicinity of topographic forcing reveals the importance of on-slope surface winds, the midlatitude isentropic slope, and latent heating in setting the amplitude of stationary waves. The response of stationary eddies to climate change is determined primarily by the strength of zonal surface winds hitting the mountain. The sensitivity of stationary-eddies to this surface forcing increases with climate change as the slope of midlatitude isentropes decreases. However, latent heating also plays an important role in damping the stationary-eddy response, and this damping becomes stronger with warming as the atmospheric moisture content increases. We find that the response of tropical overturning circulations forced by ocean heat-flux convergence is described by changes in the vertical structure of moist static energy and deep convection. This is used to derive simple scalings for the Walker circulation strength that capture the monotonic decrease with warming found in our idealized simulations.
\r\n\r\nThrough the work of this thesis, the advances made in understanding the amplitude of stationary-waves in a changing climate can be directly applied to better understand and predict changes in the zonally anomalous hydrological cycle.
", "doi": "10.7907/Z91V5BX8", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:8854", "collection": "thesis", "collection_id": "8854", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05112015-075223217", "primary_object_url": { "basename": "mbengue_cheikh_2015_thesis.pdf", "content": "final", "filesize": 3849313, "license": "other", "mime_type": "application/pdf", "url": "/8854/1/mbengue_cheikh_2015_thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Storm Track Response to Perturbations in Climate", "author": [ { "family_name": "Mbengue", "given_name": "Cheikh Oumar", "clpid": "Mbengue-Cheikh-Oumar" } ], "thesis_advisor": [ { "family_name": "Schneider", "given_name": "Tapio", "orcid": "0000-0001-5687-2287", "clpid": "Schneider-T" } ], "thesis_committee": [ { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Teixeira", "given_name": "Joao", "clpid": "Teixeira-Joao" }, { "family_name": "Schneider", "given_name": "Tapio", "orcid": "0000-0001-5687-2287", "clpid": "Schneider-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "This thesis advances our understanding of midlatitude storm tracks and how they respond to perturbations in the climate system. The midlatitude storm tracks are regions of maximal turbulent kinetic energy in the atmosphere. Through them, the bulk of the atmospheric transport of energy, water vapor, and angular momentum occurs in midlatitudes. Therefore, they are important regulators of climate, controlling basic features such as the distribution of surface temperatures, precipitation, and winds in midlatitudes. Storm tracks are robustly projected to shift poleward in global-warming simulations with current climate models. Yet the reasons for this shift have remained unclear. Here we show that this shift occurs even in extremely idealized (but still three-dimensional) simulations of dry atmospheres. We use these simulations to develop an understanding of the processes responsible for the shift and develop a conceptual model that accounts for it.
\r\n\r\nWe demonstrate that changes in the convective static stability in the deep tropics alone can drive remote shifts in the midlatitude storm tracks. Through simulations with a dry idealized general circulation model (GCM), midlatitude storm tracks are shown to be located where the mean available potential energy (MAPE, a measure of the potential energy available to be converted into kinetic energy) is maximal. As the climate varies, even if only driven by tropical static stability changes, the MAPE maximum shifts primarily because of shifts of the maximum of near-surface meridional temperature gradients. The temperature gradients shift in response to changes in the width of the tropical Hadley circulation, whose width is affected by the tropical static stability. Storm tracks generally shift in tandem with shifts of the subtropical terminus of the Hadley circulation.
\r\n\r\nWe develop a one-dimensional diffusive energy-balance model that links changes in the Hadley circulation to midlatitude temperature gradients and so to the storm tracks. It is the first conceptual model to incorporate a dynamical coupling between the tropical Hadley circulation and midlatitude turbulent energy transport. Numerical and analytical solutions of the model elucidate the circumstances of when and how the storm tracks shift in tandem with the terminus of the Hadley circulation. They illustrate how an increase of only the convective static stability in the deep tropics can lead to an expansion of the Hadley circulation and a poleward shift of storm tracks.
\r\n\r\nThe simulations with the idealized GCM and the conceptual energy-balance model demonstrate a clear link between Hadley circulation dynamics and midlatitude storm track position. With the help of the hierarchy of models presented in this thesis, we obtain a closed theory of storm track shifts in dry climates. The relevance of this theory for more realistic moist climates is discussed.
", "doi": "10.7907/Z9FT8J05", "publication_date": "2015", "thesis_type": "phd", "thesis_year": "2015" }, { "id": "thesis:7309", "collection": "thesis", "collection_id": "7309", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:12062012-130125923", "primary_object_url": { "basename": "levine_xavier_2013_thesis.pdf", "content": "final", "filesize": 2544446, "license": "other", "mime_type": "application/pdf", "url": "/7309/1/levine_xavier_2013_thesis.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Dynamics of Earth's Hadley Circulation", "author": [ { "family_name": "Levine", "given_name": "Xavier Josselin", "clpid": "Levine-Xavier-Josselin" } ], "thesis_advisor": [ { "family_name": "Schneider", "given_name": "Tapio", "clpid": "Schneider-T" } ], "thesis_committee": [ { "family_name": "Bordoni", "given_name": "Simona", "clpid": "Bordoni-S" }, { "family_name": "Ingersoll", "given_name": "Andrew P.", "clpid": "Ingersoll-A-P" }, { "family_name": "Schneider", "given_name": "Tapio", "clpid": "Schneider-T" }, { "family_name": "Wennberg", "given_name": "Paul O.", "clpid": "Wennberg-P-O" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "This thesis advances our understanding of the mechanisms controlling the Hadley circulation, and its interaction with eddies on planetary scales in particular. On Earth, and more generally in a rapidly rotating and differentially heated planet, planetary scale eddies in the extratropics interact with the mean flow in the tropics, contributing to the driving of the Hadley circulation. A hierarchy of numerical models is used to simulate and understand the relative importance of eddies in the driving of the Hadley circulation. In a global warming experiment, the Hadley circulation is found to strengthen in colder climates and weaken in warmer climates, with a maximum strength in a climate close to present-day Earth\u2019s. This nonmonotonicity is shown to be consistent with variations in the eddy activity in the midlatitudes. The cells are also found to widen over the entire range of this climate change. A criterion quantifying the importance of baroclinic waves in setting the depth of the troposphere, which is modified to account for the effect of convective adjustment on planetary Rossby waves activity, is used to explain the shifts in the terminus of the Hadley circulation for a wide range of climate scenarios. Additionally, by comparing simulations with and without ocean heat transport, it is shown that accounting for low-latitude ocean heat transport and its coupling to wind stress is essential to obtain Hadley circulations in a dynamical regime resembling Earth\u2019s. These changes in the strength and extent are found to be captured in a simple one-dimensional model that relies on standard assumptions about the thermodynamic properties of the atmosphere in the low-latitude regions and with a simple representation of eddy fluxes. Further work with this model, which may be amenable to analytical progress, could provide a quantitative understanding for the sensitivity of the Hadley circulation in comprehensive GCM simulations of 21st century global warming scenarios.
", "doi": "10.7907/Z25V-TA20", "publication_date": "2013", "thesis_type": "phd", "thesis_year": "2013" }, { "id": "thesis:6778", "collection": "thesis", "collection_id": "6778", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:01252012-143955675", "primary_object_url": { "basename": "soto_thesis.pdf", "content": "final", "filesize": 13198117, "license": "cc_by_nc_nd", "mime_type": "application/pdf", "url": "/6778/1/soto_thesis.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Dynamical Paleoclimatology of Mars", "author": [ { "family_name": "Soto", "given_name": "Alejandro", "clpid": "Soto-Alejandro" } ], "thesis_advisor": [ { "family_name": "Schneider", "given_name": "Tapio", "clpid": "Schneider-T" } ], "thesis_committee": [ { "family_name": "Ingersoll", "given_name": "Andrew P.", "clpid": "Ingersoll-A-P" }, { "family_name": "Schneider", "given_name": "Tapio", "clpid": "Schneider-T" }, { "family_name": "Ehlmann", "given_name": "Bethany L.", "clpid": "Ehlmann-B-L" }, { "family_name": "Bordoni", "given_name": "Simona", "clpid": "Bordoni-S" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "We investigated the dynamical paleoclimatology of Mars with a focus on three areas: large scale dynamics, atmospheric collapse, and controls on precipitation and aridity of a warm, wet Mars. We explored the changes, and lack of changes, in the large scale circulation over a range of atmospheric masses. We present the results here, with an emphasis on the response of the winds and the meridional transport.
\r\n\r\nThe conditions for continuous condensation of the CO2 atmospheres in the polar regions, often called 'atmospheric collapse', were explored by simulating the Martian atmosphere over a wide range of obliquities for a wide range of atmospheric thicknesses. As expected, atmospheric collapse occurs at low obliquities, but surprisingly, collapse occurs for high obliquities (up to 40\u25e6) for moderate atmospheric thicknesses (100's of millibars up to 1000 millibars). Using the MarsWRF model, we show that a competition between atmospheric heating feedbacks, including the greenhouse feedback and the heat transport feedback, and the condensation temperature feedback determines whether atmosphere collapse occurs.
\r\n\r\nFinally, we explored the precipitation and aridity of a warm, wet Mars with an active hydrological system. Even an extremely wet climate with a northern hemisphere ocean produces an extremely dry, desert climate in the southern hemisphere, with an equatorial band of rain and run off. Cross-equatorial flows deliver moist air from the northern ocean into the southern region, but topography and the distribution of land versus ocean limit the extent of the rainfall.
", "doi": "10.7907/3D9Y-DC44", "publication_date": "2012", "thesis_type": "phd", "thesis_year": "2012" }, { "id": "thesis:6533", "collection": "thesis", "collection_id": "6533", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:07012011-191902511", "type": "thesis", "title": "The General Circulation of the Tropical Atmosphere and Climate Changes", "author": [ { "family_name": "Merlis", "given_name": "Timothy Moore", "clpid": "Merlis-Timothy-Moore" } ], "thesis_advisor": [ { "family_name": "Schneider", "given_name": "Tapio", "clpid": "Schneider-T" } ], "thesis_committee": [ { "family_name": "Ingersoll", "given_name": "Andrew P.", "clpid": "Ingersoll-A-P" }, { "family_name": "Adkins", "given_name": "Jess F.", "clpid": "Adkins-J-F" }, { "family_name": "Bordoni", "given_name": "Simona", "clpid": "Bordoni-S" }, { "family_name": "Schneider", "given_name": "Tapio", "clpid": "Schneider-T" }, { "family_name": "Thompson", "given_name": "Andrew F.", "clpid": "Thompson-A-F" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "I examine the general circulation of the tropical atmosphere and climate changes. First, the response of the zonal surface temperature gradients and zonally asymmetric tropical overturning circulations (Walker circulations) to substantial changes in the longwave optical depth of the atmosphere in an idealized general circulation model (GCM) is compared with scaling theories. Second, the response of the hydrological cycle and monsoonal Hadley circulations to changes in top-of-atmosphere insolation associated with orbital precession is examined in an idealized GCM.
\r\n\r\nZonal surface temperature gradients and Walker circulations are examined over a wide range of climates simulated by varying the optical thickness in an idealized atmospheric GCM with a climate-invariant zonally asymmetric ocean energy flux. The tropical zonal surface temperature gradient and Walker circulation generally decrease as the climate warms in the GCM simulations. A scaling relationship based on a two-term balance in the surface energy budget accounts for the changes in the zonally asymmetric component of the GCM-simulated surface temperature gradients. A scaling estimate for the Walker circulation based on differential changes (precipitation rates and saturation specific humidity) in the hydrological cycle accounts for the GCM simulations provided locally averaged quantities are used in the estimate.
\r\n\r\nThe results of atmospheric GCM simulations with varied top-of-atmosphere insolation are analyzed to constrain orbitally-forced changes in the tropical atmospheric circulations and precipitation. When the perihelion is varied between solstices, there is more annual-mean precipitation in the hemisphere in which perihelion occurs during the summer solstice. In aquaplanet simulations, this is primarily associated with thermodynamic changes: there is a correlation between the seasonal cycle of the perturbed water vapor and the seasonal cycle of the Hadley circulation convergence. The monsoonal Hadley circulation does not respond to insolation gradients in a simple manner, as the atmosphere\u2019s energy stratification changes. An idealized continent that has a simple treatment of land surface hydrology and inhomogeneous heat capacity allows an assessment of how land-sea contrasts can mediate the response to orbital precession. In these simulations, the response of precipitation to orbital precession depends on changes in the atmospheric circulation, which strengthens when perihelion occurs in the summer of the hemisphere with the land region. The changes in atmospheric circulation are related to changes in both the top-of-atmosphere energy balance and the thermodynamic properties of the surface.
", "doi": "10.7907/DH11-0926", "publication_date": "2012", "thesis_type": "phd", "thesis_year": "2012" }, { "id": "thesis:2105", "collection": "thesis", "collection_id": "2105", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05262006-111949", "primary_object_url": { "basename": "thesis_nyk.pdf", "content": "final", "filesize": 1927391, "license": "other", "mime_type": "application/pdf", "url": "/2105/1/thesis_nyk.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Characterizing Carbon-Dioxide Fluxes from Oceans and Terrestrial Ecosystems", "author": [ { "family_name": "Krakauer", "given_name": "Nir Yitzhak", "orcid": "0000-0002-4926-5427", "clpid": "Krakauer-Nir-Yitzhak" } ], "thesis_advisor": [ { "family_name": "Schneider", "given_name": "Tapio", "clpid": "Schneider-T" } ], "thesis_committee": [ { "family_name": "Schneider", "given_name": "Tapio", "clpid": "Schneider-T" }, { "family_name": "Adkins", "given_name": "Jess F.", "clpid": "Adkins-J-F" }, { "family_name": "Randerson", "given_name": "James T.", "clpid": "Randerson-J-T" }, { "family_name": "Wennberg", "given_name": "Paul O.", "clpid": "Wennberg-P-O" }, { "family_name": "Yung", "given_name": "Yuk L.", "clpid": "Yung-Y-L" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "Understanding the processes that change the amount of carbon stored in the ocean and in the land biota, with their implications for future climate and ecology, is a fundamental goal of earth-system science. I have developed, refined, and applied several approaches that combine data analysis and modeling to better understand processes affecting carbon fluxes.
\r\n\r\n(1) Using a database of tree-ring widths from some 40,000 trees, I looked at the impact of large volcanic eruptions in the past millennium on tree growth globally. I found a decline in growth north of 45\u00b0 N lasting for several years after eruptions, presumably due to eruption-associated cooling, and no significant impact at lower latitudes. This argues against the hypothesis that the increased diffuse-light fraction due to volcanic aerosols greatly increased plant carbon uptake after the 1991 Pinatubo eruption, suggesting that other explanations are needed for the slow increase in atmospheric CO2 levels in the early 1990s.
\r\n\r\n(2) I applied generalized cross-validation (GCV) to the problem of estimating a regional CO2 source/sink pattern consistent with observed geographic variation in atmosphere CO2 levels. I showed that GCV works for selecting data and regional-flux uncertainty levels to assume for this inverse problem; these have usually been estimated rather arbitrarily, though they can have a large impact on the solution.
\r\n\r\n(3) The air-sea gas transfer velocity determines how fast the surface ocean adjusts to a change in atmospheric composition, and hence is important for understanding ocean CO2 uptake. By modeling the ocean\u2019s adjustment to fluctuations in atmospheric carbon isotope composition and analyzing a variety of atmosphere and ocean bomb-14C and 13C measurements, I estimated regional and global mean gas transfer velocities, concluding that there may be less latitudinal variation in the gas transfer velocity than usually thought \u2013 implying, for example, relatively low CO2 uptake in the Southern Ocean.
", "doi": "10.7907/8B12-1G19", "publication_date": "2006", "thesis_type": "phd", "thesis_year": "2006" } ]