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| Titel |
The examination of a downslope warming wind event over the Larsen Ice Shelf in Antarctica through modeling and aircraft observations. |
| VerfasserIn |
Daniel Grosvenor, Thomas Choularton, John King, Thomas Lachlan-Cope |
| 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 |
250040847
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| Zusammenfassung |
| During the last 50-60 years temperatures over the Antarctic Peninsula region have increased
more rapidly than anywhere else in the southern hemisphere, at several times the
global average rate. At one station, the near-surface warming between 1951 and
2004 was 2.94 oC compared to a global average of 0.52 oC. However, the seasonal
pattern of this regional warming has varied with location, with the east side having
warmed more than the west in the autumn and summer seasons. This is important
since the process of surface melting on the Larsen ice shelves, which are located on
the east side, predominately occurs in summer. Crevasse propagation due to the
weight of accumulated melt water is currently thought to have been the major factor
in causing the catastrophic near-total disintegration of the Larsen B ice shelf in
2002, representing a loss of ice of area 3200 km2. The larger and more southerly
Larsen C ice shelf could also suffer a similar fate if the warming continues, with
consequences for the ecology and for increased glacier flow, leading to sea level
rise.
The difference in warming between the east and west side in these seasons is thought to have
been driven by circulation changes that have led to increases in the strength of westerly
winds. The high mountains of the Antarctic Peninsula provide a climatic barrier between the
warmer oceanic air of the west and the cold continental air of the east. It has been
suggested that increased westerlies allow warm winds to cross to the east side more
frequently. The warming of westerly flow can also be enhanced by latent heat release
on the upslope side and/or adiabatic descent of air from above, on the downslope
side.
In January 2006 the British Antarctic Survey performed an aircraft flight over the Larsen C
ice shelf on the east side of the Peninsula, which sampled a strong downslope warming wind
event. Surface flux measurements over the ice shelf suggest that the sensible heat provided by
the warm jets would be likely to be negated by latent heat losses from ice ablation. The main
cause of any ice melting was likely to be due to shortwave radiation input. However, the
warming from the jets is still likely to be important by acting as an on/off control for melting
by keeping air temperatures above zero. In addition, the dryness of the winds is
likely to prevent cloud cover and thus maximize exposure of the ice shelf to solar
energy.
This case study has been simulated using the WRF mesoscale model. The model reproduces
the strong downslope winds seen by the aircraft with good matches of wind speed and
temperature profiles through the wind jets. The modeling agrees with the results of the
aircraft study in suggesting that solar radiation input is likely to provide the largest amount of
energy for melting of the ice surface.
The simulation also provides insight into the physics of the downslope winds. They are found
to be driven by descent of air from high above the mountain caused by breaking mountain
waves. This is a mechanism that is different from that often perceived to occur in the region,
whereby air from below the mountain crest rises over the obstacle and descends on the lee
side. The case is also characterized by a large degree of upstream blocking, a situation in
which the previous literature has tended to assume such warming winds would not occur for
this region. In fact, theoretical work suggests that the blocking may play a necessary
role in producing the windstorm in this case. The production of warming on the
east side of the Peninsula in blocked regimes is important as it would mean that
it occurs at lower upstream wind speeds than previously thought. The possible
consequence of this is that the suggested increase in the frequency of warming events over
the past 50 years might not be justified by consideration of the observed westerly
wind speed increases alone, but that it may also be influenced by other factors,
such as wind direction or the stability conditions. This work also shows that a high
resolution is needed to model the small scale wave breaking that drives these events
and thus the effect is likely to be completely absent at climate model resolution. |
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