Tuesday, August 15, 2017

dry-season weather


We visited Quelccaya for fieldwork during July this year and experienced a long interval of sunny, dry weather. Although July is solidly within the core winter dry season, the atmosphere was particularly dry at this time; the cloud visible in the background above was literally the largest we saw during our time at the ice cap.

Below are two oblique views of the glacier margin, on the day that we arrived in the area (20 July) and the day we departed (27 July). Due to funding constraints, this was a relatively short trip. After 8 days of clear sky and intense solar radiation, some might expect to see more change in the transient snowline than the images show. In other years of fieldwork this has certainly been the case. This year, the areas of greatest apparent change over the interval are highlighted by the red ellipses. Although these areas were determined subjectively from the images, rather than measurements of snow depth, two clear-sky satellite images roughly bracketing the period reveal a similar pattern (see below).

Why so little ablation of snow, within a zone which is both thinning and retreating rapidly in recent decades? Most likely, this was due to the combination of dry atmosphere and low wind speed during this interval. With a negative vapor pressure gradient due to the dry air, any available energy went toward sublimation, requiring ~8 times more energy per kilogram than melting. Furthermore, low wind speed suppressed turbulent transfer of latent heat. At the summit, relative humidity was typically ~10 percent (vapor pressure <2 hPa) and rarely rising to 50 percent during the afternoons.

Despite little ablation during this brief interval of the dry season, recession of the ice cap in this area is accelerating. A subsequent post and paper will quantify retreat we have been measuring since 2008, revealing the profound changes associated with the 2015-16 El Niño event.





Monday, June 19, 2017

dry season begins?


Beautiful clear weather at Quelccaya at the end of last week. The transient snowline appears to be rising, as evidenced by the darker areas of bare ice around the ice cap margins. Note that some snow still exists on high ridges around Quelccaya.

Great products coming from the European Space Agency (ESA) these days, and planned for the future. Hopefully NASA Earth observation will begin receiving more support and recognition, after the U.S. 2018 mid-term elections!

Monday, May 15, 2017

2016-17 accumulation [updated]



The current accumulation season began ~8 months ago, almost immediately after a Landsat image was acquired on 16 September. Last year was thus an atypical situation in which the ELA can be known with some confidence, which for 2016 was ~5600 m.

Excepting the first half of November, accumulation steadily increased into early April. Although the rate of accumulation then decreased, telemetry reveals that snowfall continues, as illustrated by the image above - depicting the area yesterday through high clouds. Note that fresh snow is visible on the landscape even at elevations below the glaciers. Since the mid-September minimum, 2.05 m of snow has accumulated at the AWS.

[UPDATE 06/01:  Two weeks later, snowcover on Quelccaya and the surrounding landscape shows little change, on a Landsat 8 image acquired on 30 May. Quite a contrast to last year, following reduced accumulation during the 2015-16 El Niño.]