Download Illimani avalanche

Near-surface faceted crystals,
avalanches and climate in
high-elevation, tropical
mountains of the Andes
Douglas Hardy, Mark W. Williams, Carlos Escobar
Cold Regions Science and Technology 33 (2001) 291–302
OBJECTIVES
• Determine the feasibility of measuring
solute chemistry in high-elevation snow and
ice of the Andes
• Low-cost alternative to Lonnie Thompson’s
work
• If present, does DOM provide a marker of
biomass burning in the Amazon?
• Serendipitous avalanche work
ACKNOWLEDGEMENTS
• Mark Williams: Fulbright Research Fellowship,
CU Faculty Fellowship, NSF Hydrology,
International Programs, and NWT LTER
• Eran Hood: NSF GRT
• Doug Hardy: NOAA Global Programs
• Logistical support from Carlos Escobar and
Neuvos Horizontes in La Paz
• Geochemical analysis: Lonnie Thompson and
Byrd Polar Research Center; NWT LTER
Antisana
Global Meteoric
Water Line
Elevated XDS
Similar
Amazon
Source
Illimani: extreme diurnal
recrystallization
Hardy, Williams, and Escobar, CRST 2001
Air temperature on Illimani
• Mean temperature during the winter is only
–12.8°C (1996-99), due to the low air
density at high elevation.
• the average diurnal temperature range of
7.8°C is larger than the annual temperature
range.
Met tower near summit
Highest in the world at that
time
Illimani, Bolivia
Illimani example
On 25 September 1999, two climbers
released one slide at about 5200 m in the Cordillera
Apolobamba on El Presidente, which claimed two lives.
One of the climbers killed was Yoshi Brain,
who had just published a climber’s guide to Bolivia.
Avalanches receive no mention in his book,
consistent with the opinion of a guide-intraining recently that “Avalanches don’t happen in
Bolivia” (Arrington, 1999).
Four days later and 200 km to the southeast,
snow scientists servicing a high-elevation
meteorological site triggered another at 6300 m
near the summit of Illimani.
Both slab avalanches fractured through 25–50 cm
of relatively new snow, with deeper pockets of
wind redistributed snow.
Snowpit analyses on Illimani showed the
avalanche ran on a thick layer of near-surface
faceted crystals overlying the austral
winter dry-season snow surface.
Average crystal size was 5–7 mm,
and individual crystals exceeded 10 mm in diameter.
These are the largest NSFC every reported.
South-facing slopes
• Cold air temps and lots of sun
• Conditions for near-surface faceted crystals
(NSFC)
Colbeck et al., 1990
Diurnal Recrystallization
• Temperature gradient positive during day
• Temperature gradient negative during the night
• vapor flux and heat transfer from the warm area to
the cold
• growth toward vapor source
• Facets usually bi-directional
• Enhanced by low density snow
• Optimum conditions: Clear cold nights warmer
sub-freezing days.
• Persistent atmospheric high pressure ridge
We calculate an average night-time temperature
gradient in the lowest 4 cm during these 13
days of 161°C m-1, well within the 100–300°C m-1
range others have recorded.
In this region of the Andes, our results
suggest that ideal meteorological conditions for
faceted crystal growth persist through the entire
climbing season of June–August, and probably occur
most years in high-elevation areas.
Dust on snow increases the diurnal range in temperatures.
Avalanches may occur when synoptic weather conditions
Result in dry-season snow events and high wind speeds that
produce a slab over this weak layer. La Nin˜a conditions,
such as during 1998–1999, may favor this situation.
Changes in climate may be increasing avalanche danger
in the Bolivian Andes.
Summary
•
•
•
•
NSFC to 10 mm in length
Temperature gradients of
Temperature swings of
Caused by:
–
–
–
–
High solar radiation
Dust layers
Air temps always below zero C.
Cooling through long-wave radiation