Friday, August 19, 2011

August Update

This pair of images provides a glimpse of how the Northern Ice Field has changed over the past ten months. Last week the surface was relatively flat (left), and ~70 cm lower than when penitentes covered the glacier in October of last year (right).
October's penitentes were the result of sublimating snow from the previous wet season (i.e., masika or long rains, March-May 2010). Subsequently, little snow accumulated during either the 2010 short rains or the 2011 long rains (although there was an important event in February, see details). Less snow results in higher net radiation at the glacier surface, leading to ablation.

The right-hand image shows the AWS after repositioning the tower into the ice; note the lowest of the three enclosures, gray in color with the blue strap barely visible. Ten months later the same enclosure is well above the surface, with nearly half the tower base section exposed.

Paradoxically, the glacier surface appears relatively bright in the August image (i.e., high albedo). Two observations help account for this, and indicate why there hasn't been even more than 70 cm ablation. The first is a SPOT satellite image from 17 June (courtesy Nicolas Cullen) showing partial snow cover within the crater. This snowcover - likely to an even greater depth on the glacier - may have been residual from the long rains and/or the result of snowfall only 8 days earlier, a relatively uncommon event in June. The second observation is from the photographer of the left-hand image, Dr. Clavery Tungaraza (Faculty of Science, Sokoine University of Agriculture in Morogoro, Tanzania). In crossing the crater on 8 August, he reported walking through ~15 cm of snow showing no signs of melting, in dry, very cold weather. Again, some of this snow may have been residual, and some may have been associated with a couple small snowfall events during July. The very next day, as Clavery was desending, telemetry from the AWS indicates a multiday snowfall event began!

Nonetheless, dark old ice is just beneath the mantle of snow or superimposed ice creating the bright appearance in last week's image. In another two or three months, it is quite possible that the one-year net loss of ice from the Northern Ice Field surface may amount to a meter or more. This would make 2010-11 among the most negative years since 2000, perhaps not surprising in the context of drought impacting the Horn of Africa just to the north.

[UPDATE 9/4:  The snowfall event mentioned above continued from 9 to 15 August and resulted in net accumulation of ~5 cm. This brightened the glacier surface and temporarily suppressed ablation for a week. Net surface lowering for the month was similar to July at ~8 cm.]

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