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 Division 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.
The formation of surface lakes on Antarctic ice shelves and their impact on ice-shelf stability
Alison Banwell is a glaciologist collaborating with Waleed Abdalati at CIRES and Ted Scambos at NSIDC. Her project uses a combination of modeling, satellite remote sensing, and field-derived data analysis to investigate the formation of surface lakes on Antarctic ice shelves, and the effects of those lakes on ice-shelf stability. This is important because the most likely way for the Antarctic Ice Sheet to contribute to sea level rise over the coming centuries involves the breakup of its ice shelves, which buttress approximately 75% of the ice sheet’s edges and currently prevent the rapid discharge of inland ice into the ocean. A trigger of ice-shelf disintegration is thought to be surface-stress variations associated with surface meltwater ponding and draining, causing ice-shelf weakness, flexure, and potential fracture. For example, the widespread break-up of the Larsen B Ice Shelf in 2002 may have been partially triggered by the drainage of over 2500 surface lakes.
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.
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.
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.
Analyzing the impacts of volcanic eruptions on seasonal and interannual time scales
Alan Robok will work with Jen Kay on his project to analyze the impacts of volcanic eruptions on seasonal and interannual time scales using the Community Earth System Model Large Ensemble and the Decadal Prediction Large Ensemble project. Following a large tropical volcanic eruption, the resulting latitudinal gradient of stratospheric heating, ozone depletion, and surface temperature patterns produce a stronger polar vortex in the Northern Hemisphere, with a positive mode of the Arctic Oscillation in the winter, and winter warming of Northern Hemisphere continents. The exact mechanism by which this winter warming is produced by volcanic eruptions, and whether volcanic eruptions are even involved, is still a matter of research. And because insolation reductions cool land more than the ocean, summer monsoons, which are driven by the land-ocean temperature gradient, are weaker following volcanic eruptions. He will examine the predictability arising from large volcanic eruptions, considering the state of the climate at the time of the eruptions, to compare the forced response with the noise that comes from natural chaotic variability, to see whether it is possible to issue a seasonal forecast with any skill after a large volcanic eruption.
Improving Biosand Water Filters using Insights from Microbial Ecology
Tara Webster is a microbial ecologist and environmental engineer. She is collaborating with Noah Fierer and Balaji Rajagopalan to improve our understanding of the microbial processes in biosand filters. Biosand filters hold great promise to provide clean drinking water in both developed and developing countries. Effective pathogen removal relies on the activity of a complex microbial community within the filter. However, little is known about the factors that shape microbial community structure and function in these systems. Currently, filter start-up and maintenance are based on empirical observations, leading to highly variable performance. To address this variability requires a better understanding of how filter design impacts microbial community development and pathogen removal. This research will determine how biotic and abiotic design decisions can shape microbial community structure to improve biosand filter performance. In partnership with a non-profit in Bangladesh these results will be translated to the field where community-scale filter design and operation can be improved.
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.
Meltwater movement, ponding and refreezing on the Greenland Ice Sheet and Antarctic Ice Shelves
Ian Willis will work with Mike Willis to study water production, ponding, and refreezing in Greenland and Antarctic ice shelves, which have important implications for ice sheet/shelf mass balance, runoff, ice dynamics, and ultimately sea level change. The first aim of his proposal is to amass a large, remotely-sensed data set which will be used to quantify spatial and temporal patterns of surface and subsurface water extents across the firn zone of the GrIS (Greenland Ice Sheet) and across several AISs (Antarctic Ice Shelves). The recent advent of Google Earth Engine will help considerably with this task. The second aim of his proposal is to work towards incorporating the horizontal advection of water and its storage, refreezing, and draining in snow/firn aquifers, on both the GrIS and AISs, into a numerical modelling framework. Such a model will help to better make sense of recent observations made on the GrIS and AISs, increase understanding of important hydrological processes operating on those ice masses, and ultimately enhance the ability to predict their future behavior.
Reconstructing Pliocene precipitation: Constraining El Niño teleconnections in a warm world
Jody Wycech is working with Peter Molnar, Balaji Rajagopalan, and Kris Karnauskas to study the atmospheric teleconnections of El Niño during the Pliocene (5.3-2.6 Ma). The Pliocene is the most recent time interval in Earth history when global climate was warmer than the present, and as such is considered an analog to future climate conditions. Temperature reconstructions argue for a mean El Niño-like state during the Pliocene, but global precipitation anomalies produced by Pliocene El Niño are understudied. To this end, Wycech will reconstruct Pliocene rainfall in the southeastern United States and India, which respectively experience wetter conditions and a weaker summer monsoon during modern El Niño events. The Pliocene precipitation reconstruction will be completed using Ba/Ca ratios in the shells of marine protists (planktic foraminifera) recovered from sediments near river mouths to gauge freshwater runoff. These results will provide novel insights into El Niño conditions in our future warmer world.