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  • Cambridge University Press  (3)
  • 2020-2023
  • 1995-1999  (3)
  • 1
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    Snow-mass intercomparisons in the boreal forests from general circulation models and remotely sensed data sets (1996)
    Cambridge University Press
    Details
    Publication Date: 1996-07-01
    Description: In much of the boreal forests, snow covers the ground for half of the year. Since these boreal forests comprise approximately 15% of the land normally covered by snow during the winter and upwards of 40% of the land surface normally snow-covered during the spring and autumn, reliable measures of snow cover and snow mass are required for improved energy-balance and water-balance estimates. In this study, results from snow-depth climatological data (SDC), passive microwave satellite data, and output from general circulation models (GCMs) have been intercompared for the boreal forests of both North America and Eurasia. In Eurasia, during the winter months, snowmass estimates from these data sets correspond rather well; however, in North America, the passive microwave estimates are smaller than the estimates from the climatological data and the modeled data. The underestimation results primarily from the effects of vegetation on the microwave signal. The reason why the underestimation is a bigger problem in North America than in Eurasia is likely due to the use of global microwave algorithms that have not accounted for regional differences in the size of snow grains. The GCMs generally produce too much snow in the spring season. This is a result of the models having moisture amounts that are greater and temperatures that are slightly lower than observed, in the late winter and early spring periods. The models compare more favorably with the SDC in the Eurasian boreal forest than in the forests of North America during the winter season. However, in the spring, the model results for the North America boreal forest are in better agreement with the SDC than are the forests of Eurasia.
    Print ISSN: 0032-2474
    Electronic ISSN: 1475-3057
    Topics: Ethnic Sciences , Geography
    Published by Cambridge University Press
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  • 2
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    Ice-sheet mass balance at the Last Glacial Maximum from the GENESIS version 2 global climate model (1997)
    Cambridge University Press
    Details
    Publication Date: 1997-01-01
    Description: At the Last Glacial Maximum (LGM) about 21000 years ago (21 ka BP), the overall mass balance of the Laurentide and Eurasian ice sheets should have been close to zero, since their rate of change of total ice volume was approximately zero at that time. The surface mass balance should have been zero or positive to balance any iceberg/iceshelf discharge and basal melting, but could not have been strongly negative. In principle this can be tested by global climate model (GCM) simulations with prescribed ice-sheet extents and topography.We describe results from a suite of 21 ka BP simulations using a new GCM (GENESIS version 2.0.a), with sea-surface temperatures (SSTs) prescribed from GLIMAP (1981) and predicted by a mixed-layer ocean model, and with ice sheets prescribed from both the ICE-4G (Peltier, 1994) and CLIMAP (1981) reconstructions. This GCM is well suited for ice-sheet mass-balance studies because (i) the surface can be represented at a finer resolution than the atmospheric GCM, (ii) an elevation correction accounts for spectral distortions of the atmospheric GCM topography, (iii) a simple post-processing correction for the refreezing of meltwater is applied, and (iv) the model's precipitation and mass balances for present-day Greenland and Antarctica are realistic. However, for all reasonable combinations of SSTs and ice-sheet configurations, the predicted annual surface mass balances of the LGM Laurentide and Eurasian ice sheets are implausibly negative. Possible reasons for this discrepancy are discussed, including increased ice-age aerosols, higher CLIMAP-like ice-sheet profiles in the few thousand years preceding the LGM, and a surface of the southern Laurentide just before the LGM to produce fleetingly the ICE-4G profile at 21 ka BP.
    Print ISSN: 0260-3055
    Electronic ISSN: 1727-5644
    Topics: Geography , Geosciences
    Published by Cambridge University Press on behalf of International Glaciological Society.
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  • 3
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    Driving a high-resolution dynamic ice-sheet model with GCM climate: ice-sheet initiation at 116 000 BP (1997)
    Cambridge University Press
    Details
    Publication Date: 1997-01-01
    Description: Most dynamic ice-sheet studies currently use either empirically based parameterizations or simple energy-balance climate models for the surface mass-balance forcing. If three-dimensional global climate models (GCMs) could be used instead, they would greatly improve the potential realism of coupled climate ice-sheet simulations. However, there are two serious problems in simulating realistic mass balances on ice sheets from GCM simulations: (i) dynamic ice-sheet models and the underlying bedrock topography need horizontal resolution of 50–100 km or less, but the finest practical resolution of atmospheric GCMs is currently ˜250 km, and (ii) GCM surface physics usually neglects the local refreezing of meltwater on ice sheets.Two techniques are described that address these problems: an elevation correction applied to the atmospheric GCM fields interpolated to the ice-sheet grid, and a refreezing correction involving the annual totals of snowfall, rainfall and local melt at each grid-point. As an example of their use, we have used the GENESIS version 2 GCM at 3.75° × 3.75° resolution to simulate the climate at the end of the last interglaciation at ˜116 000 years ago. The atmospheric climate is then used to drive a standard two-dimensional dynamic ice-sheet model for 10 000 years on a 0.5° × 0.5° grid spanning northern North America. The model successfully predicts ice-sheet initiation over the Baffin Island highlands and the Canadian Archipelago, but at a slower rate than observed. A large ice sheet nucleates and grows rapidly over the northwestern Rockies, in conflict with geologic evidence. Possible reasons for these discrepancies are discussed.
    Print ISSN: 0260-3055
    Electronic ISSN: 1727-5644
    Topics: Geography , Geosciences
    Published by Cambridge University Press on behalf of International Glaciological Society.
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