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| Titel |
Using upper-air meteorological data to model mass balance and total iceberg production: Columbia Glacier, Alaska, 1948-2007 |
| VerfasserIn |
L. A. Rasmussen, H. Conway, R. M. Krimmel |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250033395
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| Zusammenfassung |
| Daily temperatures at four standard levels in the atmosphere and wind and relative humidity
at one level are used in a simple accumulation-ablation model tuned to mass balance
observations during 1977-1978, 1983-1995, and 1992-1997 at 67 locations on Columbia
Glacier at altitudes ranging from 135 to 2645 meters. Precipitation is taken to be proportional
to the product of the 850-hPa humidity and wind component from 140 -. Precipitation, when
temperature T interpolated in the upper-air data at the altitude of the site is -¤ +2-C, is taken
to be snow. When T > +2Â -C, it is assumed to be rain that immediately runs off without
refreezing in the glacier. Root mean square model error is 1.1 m ice equivalent, with
r2 = 0.88.
The model uses a single ablation coefficient in a positive degree-day (PDD) model, even
though the seasonal duration of snow cover on the glacier surface increases with
altitude. By contrast, most published PDD models use a larger coefficient for ice than
for a snow-covered surface. The model also assumes that precipitation does not
increase with altitude. Although these simplifications would both seem to contribute to
negative model bias at higher altitude — overestimating ablation and underestimating
precipitation there — they are justified by the fact that model error is uncorrelated with
altitude.
The evolving glacier topography is represented as a temporally nonlinear combination of
the 1957 and 2007 area-altitude distributions. When integrated over that topography, the
model yields a 1948-2007 total mass balance of 45 km3 of ice. This represents about a
quarter of the iceberg production over the period, the remainder resulting from loss of glacier
volume due to downwasting and to disintegration of the part of the lower reach between the
pre-retreat terminus and the present terminus. |
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