C33A-0335 1340h
Sensitivity of Simulated Greenland Ice Sheet Mass Balance Terms to Atmospheric Model Configuration
An ensemble of atmospheric model simulations over the Greenland ice sheet has been conducted as part of a regional atmospheric reanalysis project. The model simulations were performed with the Polar MM5 mesoscale atmospheric model and were conducted in order to determine an appropriate model configuration for use in the regional atmospheric reanalysis. The ensemble consists of members that used multiple model physics options and data assimilation options. Results from this ensemble have been evaluated in terms of ice sheet averaged precipitation and evaporation over Greenland. Differences in the simulated ice sheet average precipitation and evaporation will be presented, and the results will be used to identify the degree of uncertainty in future simulations of ice sheet mass balance terms over the Greenland ice sheet.
C33A-0336 1340h
Data Assimilation Methods for Estimating the Mass Balance of Large Ice Sheets.
We describe progress towards applying data assimilation methods for estimating the mass balance of the large ice-sheets covering Greenland and Antarctica. Data assimilation has the advantage over conventional methods of bringing all relevant measurements to bear on the estimation of changes in the mass of ice sheets. Observations of elevation change, ice flow velocity, ice thickness, and snow accumulation can all be incorporated. An optimal estimation algorithm has been developed, and used successfully to estimate mass balance of the small ice cap of Berkner Island. This paper discusses the additional challenges posed by extending the approach to the much larger scale of Antarctica. To perform successful inversions on this scale, methods for reduced rank approximation of the covariance of the state vector of an isothermal ice flow model have been developed. Numerical simulations are used to identify the data sources and sampling necessary for succesful inversion using this technique.
C33A-0337 1340h
Modelling the coupled evolution of the ice shelf/stream flow system and the oceanic circulation in the ice-shelf cavity.
Recent satellite-based observations highlight the importance of coastal processes on the dynamics of the West Antarctic and Greenland ice sheets. In particular, the ice streams of the Amundsen Sea embayment, West Antarctica, as well as Jakobshavns Isbrae, Greenland, appear to be thinning in response to a recent oceanic trigger. One hypothesis is that enhanced melt from the underside of floating ice shelves and ice plains is causing the ice to thin and reducing the traction exerted by ice rises and other bedrock protrusions. This, in turn, may lead to accelerated flow over the grounding line and thinning of the ice streams. While there is a large amount of observational evidence suggesting that the ice sheets are more tightly coupled to their surrounding oceans than previously thought, the modelling of these interactions is still in its infancy. We investigate these processes by coupling a two-dimensional plume model of sub-shelf oceanic circulation with a three-dimensional, higher-order model of ice flow in the shelf/stream system. Coupling between the two systems arises in three ways. Firstly, through the basal melt rates determined by the plume model which largely control the mass balance of the ice shelf. Second, through the changing basal geometry of the ice shelf that plays an important role in the momentum balance of the plume. Finally, through the subglacial meltwater flux across the grounding line that provides the initial impetus for plume development. We conduct a series of coupled experiments in which the ice shelf/stream and plume are allowed reach equilibrium. We then assess the sensitivity of the predicted melt and freezing rates to the three primary oceanic inputs of shelf water temperature and salinity, as well as turbulent mixing coefficients. Finally, we conduct a series of perturbation experiments in which the effects of incremental changes to these parameters on the coupled system will be assessed.
C33A-0338 1340h
A Snow Accumulation Map For the Dry Snow Region of Greenland Derived from InSAR Correlation Observations
Snow accumulation in remote regions such as Greenland and Antarctica is often estimated from radar brightness or reflectivity. However, incompleteness of the radar scattering models used results in inaccuracies in the derived accumulation maps. Even maps derived from in situ ice core stations are not completely accurate because of the sparseness of the stations in these areas. We have developed an interferometric ice scattering model that relates insar correlation and reflecting to ice grain size and layer spacing, and can be used to invert the radar observations. Accumulation rates derived by our improved method showed superior agreement with in situ core measurements for a small area in the dry snow zone of Greenland, while other models using only reflectivity data match poorly. Our model's improvements follow because larger grain size and thicker annual layers both make a radar image brighter; the first corresponds to lower accumulation and the second corresponds to higher accumulation. However, addition of correlation data can resolve the ambiguity seen using brightness only. Here we apply our method for estimating snow accumulation rates to a larger area of Greenland. Raw RADARSAT-1 data acquired from the Alaska Satellite Facility, collected between September and December of 2000, are currently being processed. Raw data about 400GB in size are required to form interferograms of the entire Greenland area. Accumulation rates for the dry snow zone are derived by applying our new model to radar reflectivity and correlation of these C-band RADARSAT-1 data. The high resolution of the map derived from insar remote sensing data leads to an accurate and detailed map of accumulation. Comparison of our model inversion results and in situ core measurements for this large area will show whether our new method is applicable for the entire Greenland dry snow zone.
C33A-0339 1340h
Glacio-isostatic Uplift Rates in Antarctica: do Field and Modeling Agree ?
This paper investigates the use of new relative sea level (RSL) data from the Antarctic Peninsula to provide constraints for geophysical models of deglaciation. The paper also looks more widely across Antarctica at the (mis)match between rates of post-glacial rebound derived from RSL measurements, GPS measurements and geophysical model predictions. Better-constrained models of ice history are also discussed as possible inputs to future geophysical modeling.
C33A-0340 1340h
A 700-Year Regional History of Greenland Ice Sheet Accumulation
Greenland Ice Sheet mass balance is a key indicator of climate variability, past and present. However, a great deal of uncertainty still exists regarding changes in mass balance at regional scales due to observational data and modeling constraints. The decade-to-century scale variability in regional accumulation was explored using a set of 21 individual site and six regional records developed for the purpose of exploring spatial differences in annual accumulation time histories across the ice sheet. The six regional records: (1) Northwest, (2) Crete area, (3) West central, (4) Southwest, (5) Central, and (6) Southeast, have been developed through statistical and time series analysis based on similarities in individual site accumulation series. Common period of record for five of the regional records is AD 1244-1984, while the remaining record spans AD 1717-1997. From the six regional records (Gaussian filtered and unfiltered), it is apparent that although there are extended periods in which the regional groups show similar accumulation trends, non-stationary regional co-variability is also quite evident. For instance, filtered records from groups 1 and 3 show similar interannual behavior throughout the record but they decouple in the late 1600's and mid-late 1700's. For 1810-1840, negative anomalies extend across the ice sheet except at groups 4-6 where positive anomalies are observed for the decade beginning about 1825. Anomalies become increasingly positive across the ice sheet again by the late 1880's. During the 1950's and early 1960's, groups 2 and 5 indicate positive anomalies while groups 1, 3, and 4 are predominantly negative. Cumulative changes in regional accumulation show a +12-m deviation from the 1245-1984 regional mean for groups 3 and 6 and an opposite 23-m deviation in group 1.
http://faculty.ucmerced.edu/rbales/Greenland
C33A-0341 1340h
Greenland melt, surface mass balance and equilibrium line altitude from microwave radiometry
Surface mass balance of the Greenland ice sheet is thought to respond directly to climate changes. However, knowledge of whether the overall change is positive or negative remains unclear. Several studies have addressed the role Greenland plays in predictions of future sea level under various climate change scenarios; however, there is uncertainty as to whether the ice sheet as a whole is increasing or decreasing in mass. The question of how the ice sheet responds to variations in climate, and potential climate change, can be at least partially addressed by examining the variations in snowpack melt. This project presents an update to the time series of Greenland melt extent to 2003 using SMMR and SSM/I data. Significant positive trends exist in the annual time series, as well as months of June, July and August. The interannual variability is largely driven by western and northern Greenland. Estimates of surface mass balance (SMB) using SSM/I-derived melt frequency are the most positive in 1992 and the negative SMB in 1995. This project presents a comparison of the SSM/I-derived ablation area, SMB and equilibrium line altitude estimates and based on ECMWF reanalysis data. The main goals of this project were to examine interannual variability in melt extent, ablation area, surface mass balance and the location of the equilibrium line of the Greenland ice sheet and to assess how the different data sources agree or disagree over this period.
C33A-0342 1340h
Melt Anomalies on the Greenland Ice Sheet and Changing Atmospheric Circulation Patterns in the North Atlantic
This study analyzes the relationship between interannual variability in melt extent on the Greenland ice sheet and changes in atmospheric circulation and cyclone patterns in the northern high latitudes from 1979 to the present. Melt anomalies are computed annually for the ice sheet by calculating the probability of each passive microwave pixel melting as observed given the average melt behavior over the last 25 years (1979-2003) of satellite data. The anomalies are then analyzed using empirical orthogonal function (EOF) analysis to extract the dominant structures of melt variability as well as cluster analysis to identify specific anomalous melt events. The derived patterns of melt anomalies are then compared with NCEP/NCAR reanalysis data to explore the linkages between observed trends in melt extent and changes in synoptic scale circulation patterns in the North Atlantic. Results indicate significant positive trends and increased variability in melt extent prior to the peak melt expectation indicative of a shift toward larger melt extents earlier in the melt season.