Current Visiting Fellows
Assessing extratropical-tropical interactions in the climate system
The deep tropics (10 ̊S-10 ̊N) account for only 17% of the Earth’s surface; however, 32% of globally integrated annual rainfall falls here due to a narrow band of heavy precipitation called the Intertropical Convergence Zone (ITCZ). In addition, atmospheric teleconnections forced by tropical climate modes like the El Niño-Southern Oscillation (ENSO) can have far reaching impacts on the hydrology of extratropical latitudes. The presence of the ITCZ in conjunction with thermally driven atmospheric teleconnections suggests the deep tropics play an outsized role in modulating much of the planet’s regional rainfall. As a result, many studies have examined tropical climate variability and tropical teleconnections to higher latitudes. However, the climate community has recently identified important pathways by which extratropical variability can directly influence the ITCZ and tropical climate modes such as ENSO. My project will utilize Community Earth System Model (CESM) to develop novel modeling techniques to isolate the physical mechanisms that govern these extratropical-tropical connections in an effort to improve their predictability on seasonal to decadal timescales.
Constraining climate and dynamical forcing efficacies
Tara Banerjee will work with members of the Chemical Sciences Laboratory at NOAA to improve fundamental understanding of the drivers of global climate change and large-scale atmospheric dynamics. In today’s changing world, understanding our planet’s climate sensitivity to increasing atmospheric carbon dioxide concentrations is critical to future climate projections that inform policy makers. However, our atmosphere contains a number of other key forcing agents, such as aerosols and non-CO2 greenhouse gases, whose effectiveness in changing Earth’s global surface temperature remains poorly quantified and understood. In this project, Tara will conduct a series of targeted simulations with NCAR’s Earth System Model in order to quantify effectiveness of these forcing agents in changing Earth’s global climate and atmospheric circulation patterns, relative to CO2. The project will also investigate the extent to which internal variability might hinder our detection of these changes. This work will provide valuable results that quantify the degree and impacts of climate change, reconcile model-observational discrepancies, and inform both long- and short-term climate policy.
State-Specific Emissions and Control of Short-Lived Climate Pollutants
Short-lived climate pollutants (SLCP) contribute as much as 40% to anthropogenic radiative forcing. Bart Croes will collaborate with Steve Montzka to apply California’s SLCP emission inventory methods from 2005 to 2030 for the other 49 States, and identify control paths to meet California’s targets – 40% below 2013 levels by 2030 for hydrofluorocarbons and methane emissions, and 50% below for anthropogenic black carbon. Since the 1970 Clean Air Act, the U.S. improved air quality while the economy grew and vehicle miles traveled increased. Nevertheless, unhealthy PM2.5 and ozone exposures are estimated to cause about 90,000 premature deaths each year, and recent data show a general leveling off of progress since 2013-2015. While several publications have identified climate, global background, and emission control factors as likely contributors, a collaboration with Joost de Gouw will synthesize more recent emission models with satellite data to explore additional hypotheses, and apply environmental justice screening techniques demonstrated in California to national air quality trends.
Entrainment and Mixing in Shallow Convective Clouds
Fabian Hoffmann is working with Graham Feingold, and others at CIRES and NOAA, to broaden our understanding of clouds in the climate system. He focuses on the process of entrainment and mixing, i.e., the engulfment of cloud-free air into clouds. This processes affect the micro- and macrophysical properties of clouds by changing number and size of droplets, and hence a cloud’s ability to reflect sunlight as well as its probability to produce rain. For this purpose, Fabian is extending his Lagrangian cloud model, a novel method to represent cloud microphysics by individually simulated particles, by a detailed representation of entrainment and mixing. This approach not only fosters our process-level understanding of entrainment and mixing, e.g., on how and where mixing takes place inside a cloud, but also enables an assessment of the macrophysical properties of an entire cloud field, i.e., at a scale at which entrainment and mixing are usually crudely parameterized in other models.
Guidance for a Cabled Sea-floor Observation Network in the Cascadia Subduction Zone
M. Jakir Hossen will work with Anne Sheehan on tsunami studies using both historic and modern sea-floor and tsunami observation networks. Several initiatives are underway to install offshore earthquake and tsunami monitoring networks at subduction zones that have high potential for megathrust earthquakes. Hossen proposes to apply an adjoint sensitivity method with Green's function based Time Reverse Imaging to identify the optimal sites for deploying new sea-floor sensors in the Cascadia subduction zone offshore Washington and Oregon. Being installed and operated, seismic data can be used for understanding crustal activities and earthquake rupture process. The pressure sensors can provide real-time tsunami data, which can be used to recover a tsunami source model accurately and efficiently. The source model can be used as an input to forecast a reliable near- and far-eld tsunami to reduce the tsunami impact on the coastal communities across the Pacific ocean, particularly, the U.S. territories.
Automatically quantifying the development of retrogressive thaw slumps in northern Alaska
Lingcao Huang will work with Kevin Schaefer, Kristy Tiampo, and Michael Willis on a project using machine learning to automatically quantify the development of retrogressive thaw slumps (RTS) whose formation is due to the thawing the ice-rich permafrost. Retrogressive thaw slumps are the most dynamic landforms in cold regions and in which thaw of ice-rich permafrost on slopes causes mass-wasting of soil and vegetation. As reported by many local studies, their number and affected areas have increased dramatically in recent decades. However, their spatial distribution as well as development are poorly quantified and understood because they are widespread but localized features. By applying machine learning technology, especially, deep learning, Lingcao will develop a method to delineate and quantify RTS occurrence and dynamics in northern Alaska. The objectives are to provide a tool to monitor RTS dynamics in larger areas and also advance the understanding of permafrost degradation in northern Alaska related to external controlling factors such as climatic variables.
Exploring links between degrading permafrost, hydrology, and tree response
Matthias Leopold will work with Kristy Tiampo applying shallow geophysical methods in high alpine and other periglacial environments to obtain non-destructive information about buried bodies of ice or sporadic permafrost, as well as to study linked hydrological processes. Leopold will collect new geophysical data from his field site on top of Mauna Kea Hawaii, where we have currently studied isolated bodies of permafrost (Schorghofer et al. 2018). New locations will be tested for permafrost and the geophysical field data will be processed at CU Boulder. During his stay at CU Boulder, he will initiate a new project near the Martinelli Snowfield at Niwot Ridge to determine if this area holds permafrost and if so, how much it influences the local hydrology. A complete installation of a time-lapse ERT monitoring including soils and trees at the current tree line will be established. Together with a set of moisture sensors, the research aims to better understand links between degrading permafrost, hydrology, and tree response in summer. He will also collaborate with CIRES’ Mylène Jacquemart, who studies glacial surges in southern Alaska.
Drivers of Pacific Decadal Variability and its changing impacts in a warming world
Nicola Maher will work with Jen Kay and Antonietta Capotondi to investigate what drives Pacific Decadal Variability and how this variability and its impacts are projected to change under varying levels of global warming. The far reaching impacts of Pacific Decadal Variability mean that it is highly relevant under a changing climate to investigate these questions. Previously it was difficult to investigate the decadal variability under a changing climate due to the low sampling of decadal modes in both climate models and observations. By using a new set of large-ensembles, where there is enough data to investigate the decadal variability and by combining new deep learning techniques with other more traditional analyses this research project aims to further the understanding of the mechanisms that drive IPO phase transitions, and how extreme events are modulated by the IPO around the globe under different levels of global warming.
The impact of cloud radiative feedbacks on Southern Ocean variability
Eleanor Middlemas will work with Jen Kay and Graham Feingold to study cloud radiative feedback in today’s changing world. Clouds cover Earth, and they reflect, absorb, and re-emit Earth’s incoming and outgoing radiation, altering the Earth’s energy budget. The clouds’ effect on the energy budget may change the sea surface temperature (SST), which could, in turn, change the clouds and their radiative effects, creating a cloud radiative feedback (CRF). CRF is the largest source of model disagreement in evaluating the response to increasing greenhouse gases (Vial et al. 2013). Meanwhile, the contribution of CRF to internal climate variations, or variations that are not a result of changing greenhouse gases, is largely overlooked. Middlemas will use NCAR’s Community Atmospheric Model, version 5 (CAM5) to determine the precise mechanism through which clouds could impact SST variability with a focus on the Southern Ocean. She hypothesizes that the ways CRF impact the global warming response are the same mechanisms through which CRF impacts climate variability or when the global warming response is not apparent.
Interactions between tectonics, erosion, climate during Late Cenozoic global cooling and Northern Hemisphere glaciation
Yani Najman will work with Peter Molnar to understand interactions between tectonics, erosion and climate during the onset of Late Cenozoic global cooling and Northern Hemisphere glaciation. The degree of interaction between these entities is debated, in part because the records are incomplete and difficult to disentangle. For example, is the Late Cenozoic proposed increase in global erosion rates the result of a change to a cooler, less equable, perhaps stormier climate, or the result of mountain ranges rising, or simply the result of an incomplete record of sediment accumulation? Moreover, the feedbacks between these variables are multifaceted: erosion affects climate both by increasing silicate weathering which causes drawdown of atmospheric CO2, and because higher sedimentation rates result in faster burial of organic carbon; growth of mountain belts impacts climate by interfering with atmospheric circulation patterns; and the influence of isostasy moderates these variables. Understanding the complex interlinkages and feedbacks requires a knowledge of the precise timing of events. This project will undertake a new approach to interrogating these records, utilising a compilation of low temperature thermochronology data to seek to determine the level of temporal correlation between such events.
Influence of the Ocean Energy Transport on the Annual Cycle of the Intertropical Convergence Zone
Ho-Hsuan Wei will work with Kris Karnauskas and others at CIRES to better understand the dynamics of variability within the Intertropical Convergence Zone (ITCZ). In particular, Ho-Hsuan will focus on the annual cycle of the ITCZ and its relation to the ocean energy transport.
The ITCZ is a zonally-elongated band of deep convective clouds in the tropics, which migrates seasonally with a more northward (southward) position in boreal (austral) summer. Understanding these tropical rainbands, which deliver water to regions strongly dependent on agriculture, is a major challenge for the climate sciences. While recent theoretical advances based on energetic constraints have provide important insight on understanding the response of the ITCZ position, it is shown that the relation between the ITCZ position and energetic quantities is more complicated under subseasonal timescales due to the changes in the Hadley circulation vertical structure. Also, the partition between the ocean and atmospheric energy transports can lead to different responses of the ITCZ position under energetic framework. By prescribing both fixed and seasonally varying ocean energy transport data and analyzing with a coupled atmosphere-ocean model, Ho-Hsuan aims to extend understanding of the annual cycle of the Hadley cells and the ITCZ.
Leveraging multi-mission satellites to assess the impacts of recent storage changes in global lakes and reservoirs on sea level change and land surface modeling
Fangfang Yao is working with Ben Livneh and Balaji Rajagopalan to improve the understanding of recent global changes in surface water storage. Surface water provides easily accessible water resources for human beings. Spatial and temporal variations of surface water storage not only affect local water supply, but also have implications for the hydrological cycle. Despite its importance, changes in open surface water (especially lakes and reservoirs) are not simulated by existing land surface models due to the challenges of modeling human impacts including damming and water withdrawal. Partially owing to this, land surface models have large uncertainties in estimating both seasonality and long-term trends of terrestrial water storage (TWS; the summation of surface water, soil moisture, groundwater, snow and ice). In this context, Fangfang proposes to investigate global surface water storage dynamics and the impacts on the hydrological cycle using a combination of satellite observations and land surface models. He aims to answer two science questions: i) how global open-surface water has contributed to the recent sea level change, and ii) how an improved observation of open-surface water changes can further benefit the modeling of other terrestrial water components (such as soil moisture and groundwater). These questions are essential to the understanding of terrestrial water cycle and the closure of global sea level budget.