Wednesday, May 15, 2013

Snowcover: 2010 and 2013

The previous post compared two Quelccaya satellite images discussed on NASA's Earth Observatory website. Among the many interesting features these show is the transient snowline at the time each image was acquired. Landsat images for any particular location (e.g., Quelccaya) can only be acquired when the operative satellite passes overhead, which has typically been limited to an interval of ~16 days. Useful image frequency is further limited by clouds obscuring the scene. In a recent manuscript submitted to The Cryosphere Discussions, authors Maiana Hanshaw and Bodo Bookhagen tabulate many of the best images that include Quelccaya and the Cordillera Vilcanota, beginning with those from Landsat 2 in 1975.

Our measurements on the ice cap provide a context for the 2010 EO image. The graph below shows how surface height at the summit generally decreases through the dry season. Each year of our measurements is shown in black, and for dry season dates common to all years the mean daily height is blue. Note that height decrease is not linear, which reveals important information about the processes involved!

Also shown (in red) is height through the 2010 dry season, with a pink circle indicating the EO image date. Most of the surface lowering (e.g., ablation) took place after the latest available image that year - and the 2010 dry season was the longest of our 8-year record. Before the dry season began, accumulation for the El Niño wet season 2009-10 was the lowest of our record (until this year), with maximum snowdepth reaching 1.79 m on 12 April. So, with lower than normal snowfall and a prolonged dry season - in which albedo steadily decreases - the snowline likely reached a considerably higher elevation than shown in the 2010 image, acquired two months before the dry season ended.

This year, maximum snowdepth was comparable to 2010 (1.78 m), yet reached a month earlier (18 March). A few snowfall events during April and early May have added mass, yet as of 15 May the surface is dropping below the mid-March height. This year and 2010 are the 2 lowest years of accumulation since our measurements began in 2004 (measured as snow depth, without consideration for density). For the first day of May both years, accumulation was >25 cm below the median depth (not shown in figure).

On Quelccaya this July we will measure density profiles and determine the more important measure of accumulation, which is water equivalent. If our team stays strong and our shovels don't all break, we will attempt to reach what remains of 2009-10 accumulation - assuming the 2010 snowline didn't rise above the summit!

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