This week is the 2018 AGU Fall Meeting.  Here's a selection of AGU presentations by Byrd Center members:

Monday, December 10, 2018

Greenland-wide Glacier Response to Runoff and Retreat: Evaluating Outlet Glacier Changes on Monthly to Multi-year Scales

Michalea King, Ian Howat, and Myoung-Jong Noh

We use our newly derived high-resolution times series of outlet glacier discharge (D) for all large Greenland glaciers, produced by combining a suite of remotely sensed observational ice thickness and velocity data with statistical model estimates, to (1) resolve ~monthly and inter-annual patterns of variability, and (2) quantify the response of D to changes in glacier front position and meltwater runoff obtained from the RACMO2.3v2 surface mass balance model. We find that glacier retreat, which is widespread over the 2000-2017 study period, is closely related to multi-year secular trends in D. The impact of runoff, however, is likely confined to the summer months when early runoff coincides to a near-ubiquitous period of seasonal acceleration in D. We also describe how these relationships to runoff and front position vary spatially, considering environmental and topographic differences across various regions of the ice sheet. This work assesses the relative impacts of runoff and retreat on D over a range of temporal scales, helping to understand how we can use observed patterns of variability to deduce glacier stability and future ice sheet mass balance.  (AGU abstract)

7000 year of aerosol deposition to the Alps from the Ortles ice core

Paolo Gabrielli, Mary Davis, and Ping-Nan Lin

In 2011 four ice cores were extracted from the summit of Alto dell’Ortles (3859 m), the highest glacier of South Tyrol in the Italian Alps. Dating of the Ortles ice cores based on ²¹⁰Pb, ³H, beta activity and ¹⁴C determinations, combined with an empirical model (COPRA), provides evidence of a chronologically ordered ice stratigraphy from the modern glacier surface down to the bottom ice layers with an age of ~7 kyrs back to the Northern Hemisphere Climatic Optimum (NHCO; 6-9 kyrs BP), the warmest interval in the European Alps during the Holocene. At the end of the NHCO temperatures started to decrease and progressively more favorable glacial conditions characterized the Eastern Alps in the following period. Here we present a 7000 year record of aerosol deposition in terms of major ions and dust (from core #2 and crustal trace elements from core #3). We observe a long term trend decrease over time in aerosol species that we explain in terms of changes in atmospheric circulation during the Holocene. (AGU abstract)

Trends and Triggers of Glacier Change: Using Satellite Observations to Understand Monthly to Decadal Variability of Greenland’s Glaciers (Invited)

Michalea King, and Ian Howat

Large volumes of ice are exported from the Greenland Ice Sheet to the surrounding ocean by marine-terminating outlet glaciers. Accurate projections of sea level rise require close monitoring of changes at these glaciers, such as in thickness and speed, which can impact the volume of ice discharged from the ice sheet. We combine remotely sensed observations of glacier speed and thickness from a variety of platforms, using statistical modeling to estimate discharge during periods of missing data, to derive a multi-decadal, high-resolution record of discharge for every (> 200) major glacier in Greenland. We use this record to evaluate how glaciers are changing on ~monthly to multi-year scales, and identify where on the ice sheet these changes are most rapid. We find that most glaciers, independent of size, exhibit a seasonal acceleration, typically discharging the most ice in the summer months. In the northern regions of the ice sheet, however, there is also an underlying increasing secular trend, likely due to widespread, sustained retreat of glaciers in those regions. Discharge from the southeast region, where retreat and large increases in discharge occurred from 2001-2005, has recently re-stabilized to pre-acceleration levels. These data are useful for understanding not only how quickly glaciers respond to various local and climatic forcings, but also what timescales are required to equilibrate to more stable states. (AGU abstract)

Tuesday, December 11, 2018

Analysis of Present-Day Horizontal Crustal Motions in West Antarctica

David Saddler, and Terry Wilson

The POLENET/ANET project provides a network of continuously operating GPS instruments that span the margin of the East Antarctic craton in the Transantarctic Mountains and across West Antarctic crustal blocks. GPS time series of sufficient length are now available to define the horizontal and vertical velocity fields at mm level. Deformation in Antarctica arises due to superimposed effects that may include rigid plate rotation, internal tectonic displacements, viscoelastic glacial isostatic adjustment (GIA) response to past ice mass change, and elastic displacements due to changes in modern ice mass loads. There is continued debate on the activity of the West Antarctic Rift System that divides East Antarctica from the crustal block assemblage of West Antarctica, which we address using our GPS data set. Our approach removes estimated elastic displacements from each measured GPS velocity. Euler poles are then calculated for individual crustal blocks, as well as combinations of crustal blocks, to test for relative motions between blocks and with respect to stable East Antarctica. Euler poles are calculated using elastic-corrected GPS velocities, and then using velocities adjusted for both elastic displacement and the predicted viscoelastic displacements from a series of GIA models. We compare Euler pole solutions and analyze spatial patterns of horizontal velocities to better understand whether the pattern of horizontal crustal motions across West Antarctica resolves any crustal block displacements across the West Antarctic Rift System, or is best attributed to GIA-induced deformation. (AGU abstract)

New insights on Antarctic ice shelf basal channels from Reference Elevation Model of Antarctica (REMA) digital elevation models

Allison Chartrand, Ian Howat, Myoung-Jong Noh, and Michalea King

Basal channels are distinctive features in several Antarctic ice shelves, but little is known about how they evolve over time and contribute to changes in ice shelf stability. Large basal channels are often associated with surface depressions that deepen as the ice settles toward hydrostatic equilibrium, so the basal channels can be studied using only surface elevation data. We have developed new methods for studying changes in ice shelves and basal channels in particular using 2-8 meter resolution digital elevation models (DEMs) from the Reference Elevation Model of Antarctica (REMA) and NASA Operation IceBridge airborne ice-penetrating radar and laser altimetry data. Our investigation focuses on a 55 km long, 4.5 km wide basal channel under the Getz ice shelf along the Amundsen Sea in West Antarctica. Our data over the period 2010 – 2016 extend the time series of Alley et al. (2016) for this basal channel and show that it has continued to thin the ice shelf at rates of up to 27 ma-1, and that the associated surface depression has continued to deepen along the entire channel and lengthen toward the grounding line. The surrounding ice shelf has also continued to experience a decrease in surface elevation. The ice shelf is near hydrostatic equilibrium in all areas except for directly above the basal channel, indicating a delayed response between basal incision and deepening of the surface depression. These findings suggest that the assumption of hydrostatic equilibrium may not hold if a channel is actively changing. Thus, estimates of basal incision depths derived from surface depression elevations may underestimate the size of basal channels. Furthermore, tracking of the lateral motion of the bottom of the channel surface depression has revealed that the depression does not advect with ice flow, as expected, in several places along the channel. The ability to track the change in surface depressions in three dimensions is a powerful new capability of the REMA DEMs, and the observation that basal channel surface depressions are moving at different rates and directions than ice flow indicate active basal melt along more of the channel than previously thought. Basal channels remain an important component of ice shelves, and the new REMA dataset has proven a powerful tool for investigating basal channel and ice shelf change. (AGU abstract)

UAV-borne remote sensing platforms for glacier-related hazard monitoring in high-mountain environments

Forrest Schoessow, Nischay Soni, Bryan Mark, and Evan Vega

The high-mountain glacier environments of the tropical Andes are prone to geohazards (e.g. seismic activity, mass movements, outburst floods) where changing regional climate patterns are predicted to increase the frequency and magnitude of hazard events. Reliably evaluating glacier-related hazards is complex, requiring not only an understanding of regional land-air-ice interactions in the climate system but also detailed characterization of the complex topography. Digital terrain models are important inputs for risk assessments and numerical hazard models which often rely upon available satellite-derived remote sensed data products of global coverage (e.g. ASTER, SRTM) when analyzing high-mountain environments. However, these datasets are known to be error-prone and of limited spatial resolution in high-mountain environments typified by complex topography where shadows, clouds, and ice reflectivity obscure optical sensors and logistical difficulties impede ground validation efforts. The capabilities of unmanned aerial vehicle-borne remote sensing platforms can greatly improve our capability to assess and respond to natural hazards – particularly in these harsh environments. Designed to operate at extreme altitudes in the Andes of Peru (4500-6800m AGL), our custom-built unmanned aerial system (UAS) is able to overcome the limitations of spaceborne and airborne remote sensing platforms while also reducing the cost and potential risk incurred by researchers. The UAS houses active and passive remote sensing instruments, enabling us to generate high resolution (centimeter-level) surface elevation time-series data using structure-from-motion algorithms and differential lidar techniques. These techniques are capable of revealing minute changes in complex topography and reveal clues about the driving forces of accumulation, surface displacement, slope deformation, and climate control over topography. This presentation will review the platform development as well as discuss instrument techniques and hazard management applications enabling us to observe hazard dynamics at finer spatial-temporal scales, better understand the processes that drive them, and improve our capability for predicting their future evolution in high mountain environments. (AGU abstract)

Making the qualitative-to-quantitative transition in ice core nanoparticle studies: a case study from Mt. Ortles in the Italian Alps

Aja Ellis, Paolo Gabrielli, and Lonnie G Thompson

Modern atmospheric particles are currently of great interest to the environmental and health science communities, with ongoing efforts to constrain particle characteristics (chemistry, composition, lifetime) and distributions (spatial and temporal). Historical records of aerosols, particularly accumulation mode particles (<1 μm) and nanoparticles, are limited. Particles in these size ranges represent the vast majority of the atmospheric particle number count, despite larger particles dominating total particle mass. They are large contributors to the uncertainty in global climate models, both directly through radiative forcing and indirectly due to cloud and ice formation in the atmosphere. Additionally, atmospheric nanoparticles have significant human health implications as they are often highly reactive and penetrate deeply into the respiratory tract.

Ice cores drilled from glaciers around the world contain records of atmospheric composition over time, which likely includes an unknown population of nanoparticles. Measurements of atmospheric particles in ice cores have been accomplished through particle number counts with limited size range and without composition information or through electron microscopy detailing particle composition but without statistical rigor. Meanwhile, single particle mass spectrometry is developing rapidly within the materials and atmospheric science communities, with instruments now able to resolve the chemical composition of individual aerosol particles in realtime.

Low- and Mid-latitude ice cores are comparatively rare and more representative of the local source regions than their remote polar counterparts, thereby offering a better opportunity to contain a record of nanoparticles with unknown atmospheric transport and lifetimes. Four ice cores were drilled in 2011 at the summit of Alto dell’Ortles (3859 m), Northern Italy. By combining traditional methods of aerosol characterization (electron microscopy) with techniques like single particle mass spectrometry, we detail the nanoparticle record of human activities in the Mediterranean region through the rise of the Roman Empire, European industrialization, and into the modern age. (AGU abstract)

Thursday, December 13, 2018

Utilizing Flood Inundation Observations to Obtain Floodplain Topography in Data-Scarce Regions for use in a Hydrodynamic Model

Apoorva Shastry, and Michael Durand

Flood models predict inundation extents, and can be an important source of information for flood risk studies. Accurate flood models require high resolution and high accuracy digital elevation models (DEM); current global DEMs do not capture the topographic details in floodplains, and this often leads to inaccurate prediction of flood extents by flood models. Flood extents obtained from remotely sensed data provide indirect information about topography. Here, we attempt to utilize this information along with model predictions to produce better floodplain topography.

We use data assimilation-style methods, and utilize the edges of flood extent to constrain water elevation. The method will be implemented for the UK 2007 summer floods of the Severn river. We use the Shuttle Radar Topography Mission (SRTM) DEM as the prior to produce an ensemble of DEMs. Data assimilation style approach will require a large number of ensemble members or particles to capture the complex spatial pattern of topography, making the process computationally inefficient. An ensemble of priors that are more realistic will make the assimilation more efficient. We achieve this by processing the ensemble of priors to constrain the elevations along the observed flood boundary, by reducing the noise in elevations along the edges of inundation.

The approach produced promising results in a synthetic case: the root mean squared error of elevations along the flood boundaries went from 1.1 m to 0.65 m. When the prior and updated DEMs were used to simulate the forward model, true positive rate (sensitivity) of the maximum inundation extent went from 62% to 74%. We expect the posteriors from this ensemble of priors will better represent floodplain elevations, and yield more accurate inundation patterns. Successful application of this approach to produce an updated DEM will mean that this type of analysis can be performed in data-poor floodplains where high resolution DEMs do not exist (AGU abstract)

The significance of local climate conditions on tree ring d¹⁸O cellulose

George Grant, and Joel Barker

Stable oxygen content of cellulose in annual tree rings is commonly used as a temperature proxy for paleoclimate reconstruction. These reconstructions commonly use samples from different climates to generate calibration curves with which temperature is derived. The trees from which the samples are obtained may grow under vastly different climate conditions and factors influencing the cellulose stable oxygen (d¹⁸O) value may vary. Consequently, the application of a general climate reconstruction equation may not be appropriate, even if a single genus is used for calibration. We use d¹⁸O values of extracted cellulose from globally distributed spruce (Picea) with corresponding climate data to determine if regional environmental factors influence d¹⁸O and if so, which are the most influential.

We examine reported d¹⁸O values of extracted cellulose from over 700 globally distributed Piceasamples. 16 corrected climate parameters obtained from nearby metrological stations were compared to the collected d¹⁸O values. The statistical relationship between isotopic composition of cellulose and climate parameters were identified through Pearson correlation and georeferenced principle component analysis (PCA).

While tree ring cellulose d¹⁸O have been used for mean annual temperature reconstructions, we found that the most influential climate factors were annual minimum temperature and the number of days per year that the daily maximum temperature is below 0°C. This finding highlights the importance of local climate and isotopic conditions on tree ring cellulose d¹⁸O extracts. This fundamentally impacts the inference of climate conditions through d¹⁸O of cellulose in paleoclimate reconstructions. (AGU abstract)

Utilizing GPS to investigate past ice mass change in the Ross Sea region, Antarctica

Stephanie Konfal, and Terry Wilson

ANET (Antarctic Network) GPS observations from the Polar Earth Observing Network (POLENET) record solid earth deformation in response to ice mass change. In the Transantarctic Mountains (TAM) region, observed horizontal motions are towards modeled regions of uplift in response to West Antarctic centers of ice mass loss, opposite to the radially outward pattern expected. We investigate alternative ice history and earth structure inputs to GIA models in an attempt to reproduce observed motions in the region. The proximity of the Wilkes Subglacial Basin (WSB), to the west of the TAM, prompts us to investigate the GIA response to evidence-based ice unloading from the WSB region, since ice loss there could produce motion toward West Antarctica at GPS sites located along the TAM. We also explore the impact of centennial-scale ice load change in the Siple Coast region – due to stagnation and reactivation of ice streams – on horizontal motions recorded by ANET in the Ross Sea region. Finally, the majority of our modeling employs radially-varying (1D) earth models, but the influence of laterally heterogeneous mantle viscosity is also investigated by coupling loading scenarios with earth models that permit two different viscosity profiles on either side of a longitudinal boundary bisecting East and West Antarctica. Best fitting ice history and earth models are presented, including preferred upper mantle viscosity values. (AGU abstract)

Towards an operational network for continuous measurements of Surface Mass Balance and related firn processes over the interior of Greenland and Antarctica

Santiago de la Peña, and Ian Howat

Covering more than 10% of the total land surface of Earth, the firn layers over Greenland and Antarctica are key components in altimetry-derived mass balance estimates and play an important role in the overall climate system. Short-term departures in surface elevation trends caused by fluctuations in the volume of the firn layer occur over most of the accumulation zone of the Greenland and Antarctic Ice Sheets. Changes in the thickness of the firn column are influenced by variability in Surface Mass Balance, firn compaction, and abrupt seasonal densification near the surface caused by refreezing at depth of variable amounts of surface meltwater in the summer. These processes and dynamic thinning cannot be differentiated from each other by altimetry alone, yet measurements remain sparse in Greenland and Antarctica due in part to the remoteness, cold, and sheer size of the ice sheets.

Until recently, nearly all information on snow density and Surface Mass Balance over the firn layer of glaciers came from ice core and snow pit stratigraphy that provided annual rates with relatively large uncertainties. In an effort to obtain a continuous record of Surface Mass Balance, semi-permanent stations have been installed in Greenland recording snow water-equivalent (s.w.e.) accumulation using sensors that measure the attenuation through snowpack of cosmic ray neutrons. More recently, a similar system designed to withstand extreme-cold temperatures and operate through the Polar Night was assembled over the Antarctic Plateau. The method directly quantifies changes in mass, not in volume, eliminating the reliance on snow density and yielding daily Surface Mass Balance rates with unprecedented precision. The measurements also include firn compaction and relative surface elevation change (from which snow density can be estimated in combination with s.w.e.). These are the first direct, continuous measurements of Surface Mass Balance over the accumulation zone of both Greenland and Antarctica. (AGU abstract)

Friday, December 14, 2018

A first estimate of the expected distribution of SWOT river discharge accuracy

Michael Durand, Renato Frasson, and Stephen Tuozzolo

Surface Water and Ocean Topography (SWOT) data will be used to estimate river discharge for global rivers greater than 100 m in width (perhaps as small as 50 m) using three SWOT river measurements: height, width, and slope. SWOT measurements provide only part of the information needed to estimate discharge; time-invariant parameters (e.g. bathymetry) are also needed. Mass-conserved flow law inversion (McFLI) algorithms estimate unknown parameters and discharge from SWOT measurements and globally available a priori information. McFLI algorithms improve over globally-available estimates of a priori mean annual flow (MAF), but MAF accuracy plays a major role in final McFLI discharge accuracy.

We characterize the expected cumulative distribution function (cdf) of SWOT discharge accuracy across global rivers using a combination of available global river datasets, recent McFLI studies, and recent work to integrate information across river networks. Note though that current river datasets used will limit our analysis to rivers at latitudes lower than 60°. We restrict our analysis to single-channel rivers, as McFLI accuracy on braided rivers is not well constrained. First, we use recent McFLI results to characterize McFLI retrieval accuracy, as a function a priori discharge error. Second, we classify global sets of mass-conserved reaches based on available a priori data thus: real-time or historical discharge discharge data in the reach, real-time or historical data in the basin, global hydrological model. Third, we assign the expected discharge accuracy associated with each type of a priori data to each river based on its class. Accuracies for real-time vs. historical are based onthe expected mismatch between the period where inversion is performed and the MAF. Accuracies for prior flow for global hydrological models are based on basin size. SWOT accuracy via McFLI retrieval will be based on a reduction in the original prior flow error after applying a simple way of forcing mass conservation across a river network.

The final result is a cdf of expected SWOT accuracy for global reaches, based on prior information. We analyze the cdf of this and compare it to hypothesized accuracy in global models; we also look at how accuracy varies as a function of river size, as expressed through drainage area. (AGU abstract)

How Well Will the Surface Water and Ocean Topography Mission Measure Water Surface Heights and Slopes in Complex Terrain? (Highlighted)

Michael Durand, and Renato Frasson

The Surface Water and Ocean Topography (SWOT) mission will measure water surface heights, widths, and slopes of rivers wider than 100 m and possibly as narrow as 50 m. Unlike traditional altimeters, SWOT’s main payload uses near nadir radar interferometry to observe water bodies inside its measurement swath, allowing more spatially continuous observations. However, due to its measurement principle and viewing geometry, observations may be affected by terrain layover, a condition that happens when the radar returns from water and land targets reach the satellite simultaneously, causing them to become indistinguishable, which may lead to stage-dependent and spatially correlated biases.

We applied a simplified height and slope error model capable of estimating systematic and random errors to all rivers in the Global River Widths from Landsat database with the intent of estimating SWOT height and slope uncertainties and evaluate the effect of terrain layover. The error model considers river width, orientation and location with respect to the satellite ground track, and the topography roughness to estimate height uncertainty at the node scale, i.e. equally spaced points located every 200 m over the river centerline, which are later aggregated into 10 km reaches.

Our results showed that overall, the impact of terrain layover on reach height and slope uncertainties is relatively small, leading to an increase of the 68th height error percentile from 9.4 cm to 10.4 cm and an increase of the 68th slope error percentile from 1 cm/km to 1.7 cm/km. Nevertheless, the impact of layover is not equally distributed over the globe, depending on width, topographic roughness, and location, being more influential for narrower rivers flowing through complex terrain. When height and slope uncertainties are propagated through Manning’s equation, we see that the impact of layover induced errors will lead to a marginal increase of 68th percentile of discharge uncertainty (due to height and slope uncertainties alone) from 12% to 13%. Finally, our analysis shows the unexpected result that the extent of layover is rather large, affecting even rivers that are not surrounded by complex topography, nevertheless, the magnitude of the introduced bias is rather small. (AGU abstract)

Fluid Earth Viewer: An Engaging and Interactive Educational Web Tool for Exploring Earth’s Global and Polar Atmospheric and Oceanographic Phenomena

Jason Cervenec, Aaron Wilson, and Julien P. Nicolas

Using open-source code and publicly available atmospheric and ocean model data sets, a team at The Ohio State University created the Fluid Earth Viewer (FEVer), an interactive, intuitive, and visually appealing web tool that allows users to explore current and past conditions of our planet’s atmosphere and oceans. Building on extensive user-testing with audiences that included undergraduate students, middle school students and the general public, FEVer serves as a vehicle of modern Earth science communication, making complex data accessible and engaging in both informal and formal education settings.

In particular, FEVer provides hands-on visualizations of the important climatic role of the polar regions, their connections to lower latitudes, and the changes they are undergoing. A companion website, FEVer-Ed, provides background information, educational support, and opportunities for additional learning through a gallery of historically interesting atmospheric and oceanic events. Beyond showcasing how FEVer is readily expandable to include additional data sets, the presenter is interested in brainstorming novel applications of the tool for learners of all ages. FEVer is accessible online at fever.bpcrc.osu.edu. (AGU abstract)

Terra Cognita: The Silicon Age of Earth Exploration (Invited)

Ian Howat

The serendipitous convergence of three trends over the past few years has enabled repeat, high resolution terrain mapping at the continental scale; these are the opening of access to large quantities of sub-meter stereoscopic satellite imagery, the creation of efficient, open source image processing software, and the availability of petascale high performance computing (HPC). Within a matter of months, vast areas at high latitudes and other remote regions have gone from having the poorest topographic data, to among the best on Earth; and these data are freely open to everyone. Here l review progress to date in the pursuit of global, time-dependent terrain mapping and the new science it is enabling. I include some lessons learned from the first forays into continental scale mapping with these data: the ArcticDEM and Reference Elevation Model of Antarctica (REMA) projects. I will highlight some of the novel initial approaches taken to ingest and analyze the large body of information these data provide. Based on this early progress, I will conclude with a few recommendations for enhanced utilization and continued improvement of this transformative new resource for understanding the Earth and how it is changing. (AGU abstract)

Climatic and Environmental Linkages Between the Third Pole and the Tropical Andes of South America over the Last 10,000 Years (Highlighted)

Lonnie G Thompson, Tandong Yao, Mary Davis, Ellen Mosley-Thompson, and Stacy Porter

Large-scale teleconnections across the Pacific Basin, linked through the El Niño-Southern Oscillation (ENSO) and movement of the Intertropical Convergence Zone (ITCZ), are elucidated by comparison of ice core records from the tropical Andes and the Third Pole (TP). These connections are demonstrated by integrating proxy records with reanalysis data and ocean/atmosphere indices. The ice core records from the TP and Andes cover multiple millennia and the oldest core from Guliya in the Western Kunlun extends back at least 130 ka. At these trans-Pacific ice core sites the early Holocene was warmer and/or drier than today. In the mid-Holocene (~7 to 4 ka) the TP cores record cooler conditions while the Andean cores suggest generally stable climate. Since 4 ka the TP has experienced warming, while Andean temperatures have remained steady except for a pronounced Little Ice Age cooling. We examine the millennial to decadal-scale drivers (e.g., orbital forcing of the position and migration of the ITCZ, changes in greenhouse gases, etc.) across these regions. Over the last millennium the TP and Andean ice cores reveal large-scale, regional differences between the Indian Monsoon-dominated Himalaya in the south and the arid continental TP in the north and between the arid southern Andes and the wetter South American Monsoon-dominated north. Within the last century the climate proxy records across these widely separate regions show meridional differences in precipitation trends and a consistent, dominant warming. Observations of glacier retreat during the 20^th and 21^stCenturies suggest that the current warming is unusual when viewed relative to the 160-year record of direct temperature measurements and the millennial-scale perspective provided by ice core records. The ongoing, widespread melting of mountain glaciers provides strong evidence that a large-scale, pervasive, and relatively rapid change in Earth’s climate system is underway. Ice core analyses, glacier monitoring by satellite, and in situ techniques provide critical perceptions of modern climate change which have potential economic and societal impacts (such as dwindling water resources) on glacier-dependent nations and geo-hazards triggered by collapsing glaciers, avalanches, and glacial lake outbursts. (AGU abstract)