Greenland Meltpond Formation
The melting edge of the Greenland Ice sheet.
A most profound emerging discovery in ice-sheet glaciology is the realization that melt water derived from the ice-sheet surface can rapidly drain through more than a kilometer of cold ice to reach the subglacial bed where it subsequently can increase the local seaward flow of the ice sheet and, potentially, increase calving rates in outlet glacier fjords.
Summer internship opportunities
We are looking for motivated undergraduates to come spend a summer working with us. This opportunity is through The Leadership Alliance summer research early identification program (SR_EIP). We have a few ideas of summer projects, or, of course, you can come with your own, or create your own once you are here! For more information on the summer internship visit The University of Chicago's Leadership Alliance web page and our web page announcing the opportunity and describing possible summer projects. (glaciology site)
Research Overview
Paramount to understanding the end effect of surface meltwater on the seaward flow of the Greenland Ice Sheet is understanding the processes of supraglacial meltwater production, ponding, and transport (both supraglacial and englacial to the bed). Our research seeks to improve current understandings surface meltwater processes through the creations of numerical models designed to simulate the evolution of meltwater lakes on the Greenland Ice Sheet. Progress on this project is described bellow. We understand that we are not the first to develop models like this, but feel it is important to investigate these processes from multiple intellectual directions, and compare results and conclusions. As part of our efforts to investigate these processes we will be developing a website to encourage model intercomparisons. This project is funded by the National Science Foundation, award number ARC-0907834.
The processes involved in development of surface meltwater lakes on the Greenland Ice Sheet.
Work in progress
In an effort to communicate our work with the glaciology community and anyone else interested, we will be periodically posting updates of our progress on this project.
Radiative transfer model for supraglacial Lakes and other surface features (updated 10/14/09)
We have begun work on a localized surface energy balance model to describe the evolution of surface features. Initial efforts have focused on surface meltwater and surface features effect on the absorption of sun light.
Geometric effects on effective albedos of surface features: Surface roughness at all spatial scales effects the absorption of energy. We extend the pioneering work of Pfeffer and Bretherton (1987) which details the effective albedo of a V-shaped crevasse. Our methodology extends this previous work to arbitrary two-dimensional surface feature geometry. Our numerical model accounts for infinite reflections and shadows, calculating the surface distribution of absorbed energy. Currently, our model can not incorporate surface water, but this is something we are working on including.
Model output showing the effective albedo of an arbitrary two-dimensional surface feature.
Two stream, two-layer radiative transfer model: The presence of water on the surface of an ice sheet significantly effects the absorption of incoming sun light. Accurate incorporation of this effect requires a spectrally dependent two stream radiative transfer model. Bellow we show several plots presented at the 2008 fall AGU meeting in San Fransisco, they illustrate the spectral dependence of the the absorption coefficients and how this spectral dependence effects the energy balance.
Modeled spectral distribution of both incoming light and light reflected off a four meter deep pond.
Melt pond albedo as a function of melt pond water depth for both spectrally dependent absorption coefficients (k(λ)) and averaged absorbtion coefficient .
Two stream, two layer model output for a four-meter melt pond.
Project Motivation
Zwally et al. [2002] observed that ice sheet surface velocities increased during the summer melt season at a position on the flank of the ice sheet in response to increased basal lubrication associated with meltwater drainage into ‘moulins' - vertical conduits that connect the ice sheet surface to the bed through more than a thousand meters of sub-freezing ice. Studies of recently discovered glaciogenic earthquakes [Ekström et al., 2002] emanating from the outlet glacier/fjord systems reveals that earthquake signals correlate with accelerated ice discharge and calving into the fjord systems [Joughin et al., 2008]. These earthquakes occur most frequently during the summer melt season, and their number during each year's melt season has steadily increased, seemingly in step with the observed increase of summer melting on the surface of the ice sheet [Ekström et al., 2006].
These observation-based studies motivate the realization that surface melting may enhance Greenland's contribution to sea level rise through important dynamic and thermodynamic feedbacks that were largely unknown or unappreciated prior to the present decade. This realization is put into the context of the major goal of Greenland Ice-Sheet research-the effort to predict and understand future sea level rise-by the realization that the regional coverage and intensity of surface melting on the Greenland Ice Sheet are increasing [Box and Ski, 2007; Mote, 2007; Fettweis et al., 2007; Hanna et al., 2008; Tedesco, 2008], and that the overall mass balance of Greenland is shifting from a previously balanced state to a state where net mass is lost [Luthcke et al., 2006].
Supraglacial lakes may be the key to getting surface meltwater to the subglacial bed where it can do damage to ice sheet mass balance. The new observations described above have overturned the previously held belief that surface melt water's role in ice-sheet change was primarily embodied by the direct loss of mass through surface runoff. Belief in this previous view was particularly sanguine because glaciology hadn't yet embraced the physics of melt water induced fracture propagation in thick, cold ice as described by [Alley et al., 2005; Van der Veen, 1998; 2007]. As described by simple dynamic analyses of water-filled fractures, when sufficient water is available as a pressure medium, fractures can reach the bed through >1 km of cold glacial ice if sufficient water is available at the surface to support the heat and volume needs associated with the fracture propagation process. Alley et al. [2005] cite the role of supraglacial lakes as being particularly important as initial accumulations of water poised in sufficient volume to create a fracture that eventually leads to the "invasion" of the subglacial bed via drainage through englacial conduits (moulins). The capacity for surface melt water to reach the bed of a thick, cold ice sheet via moulins was recently emphasized by the spectacular surface lake drainage events documented by Das et al. [2008], who observed a Niagara-Falls-like volume flux to the bed in a sudden, abrupt drainage event lasting only hours.
The newly observed indirect effects of surface melting on ice-sheet flow may be more important than direct effect of mass loss through surface runoff alone in governing the ice-sheet's short-term response to surface warming. Given the profound transformation of our understanding of how surface melting affects ice-sheet response to climate change, it is imperative to continue the arduous task of treating the physics of surface melting (and subsequent feedback effects) in ice-sheet models used to predict future sea-level rise, e.g., along the lines begun by Parizek and Alley [2004].
