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 ( ), 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 example Illimani, Bolivia

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-in- training 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