Friday, July 22, 2016

Seasonality of accumulation and ablation: not simple!

Earlier posts here have discussed how snow accumulation at Quelccaya during the 2015-16 El Niño was considerably below normal. We verified AWS measurements during April and May fieldwork, measuring for example a mere 30 cm of snow at an elevation only 130 m below the ice cap's summit! On top, we made comprehensive measurements and obtained snow samples for frozen transport and analyses which are currently underway. However, preservation of this meager accumulation will depend upon the extent of both precipitation and ablation currently occurring -- during the 2016 dry season (i.e., approx. June - October).

The following sequence of Landsat 8 images were obtained from the USGS, corresponding to dates shown in pink/purple on the following timeseries of snow surface height at the summit of Quelccaya. Height increases are primarily the result of snowfall, while decreases mostly reflect melting.  In viewing the images below, it might be helpful to have this graph open in another tab or window; click here for a jpeg or here for a PDF.

A composite GIF of the images is shown first. For more detail on each image, scroll down. Captions for each relate snowcover in the scene to AWS measurements in the days and months prior. We will continue tracking accumulation at the summit and relating surface height to Landsat imagery as the season continues. Further information about accumulation in past years, and interpretation of AWS measurements, is detailed in our comprehensive paper in the Journal of Geophysical Research (Hurley et al., 2015). See our previous blog post for images of fieldwork during April and May.

Fig. 1 - Snow-covered glacier and snowy landscape (5 Feb.), after one of the final snowfall events of the 2015-16 "core" wet season. Although impossible to assess the depth of accumulation from this visual image, note high albedo of the entire glacier, raising the reflectivity of incoming shortwave radiation (solar).

Fig. 2 - Two weeks later (21 Feb.), glacier ice is becoming exposed around the margin, and snow has melted from the surrounding landscape. Most of the glacier remains bright (i.e., high albedo) due to a minor snowfall event several days prior (see graph above). The wet season continues through February, and as in figure 1, clouds are indicative of atmospheric instability.

Fig. 3 - During the month between this image (24 Mar.) and that in figure 2, a net lowering of glacier surface height occurred, despite a snowy interval (late Feb.) and a large snowfall event in early March. A larger area of glacier ice is visible around the margin, and albedo has decreased at all but the highest elevations. To the west of the ice cap a solid red circle indicates the location of our camp during April / May fieldwork.

Fig. 4 - Only slight changes in the month since figure 3. On this date (25 Apr.) we were at the glacier, camped at the location indicated in figure 3. Just 5 days prior, a precipitation event with heavy graupel was sufficient to collapse one of our tents. Snowcover blanketed the regional landscape far to the west the next morning; four days later (see above), new snow remains on the glacier but has melted from the landscape. Note location of AWS, which is not indicated on subsequent figures.

Fig. 5 - During our time at Quelccaya we observed a seasonal change in weather which we interpret to be the transition from wet to dry seasons. Between a few days prior to the time of figure 4 and our departure on 4 May, the atmosphere became considerably more stable. Particularly at lower elevations on the ice cap, the melt rate was tremendous, and with a thin cover of snow the transient snowline rapidly increased in elevation. This image from 11 May - only 2 weeks after that in figure 4 - demonstrates how rapidly mass can be lost from a glacier in years of low accumulation.

Fig. 6 - Rapid ablation earlier in the month was halted by a relatively minor amount of accumulation ending just a few days prior to this image (27 May). The receipt of net radiation decreased tremendously due to higher albedo of the fresh snowcover.
Fig. 7 - Two weeks later (12 Jun.) mid-May snow accumulation is ablating. Due to a problem processing telemetry data, snowfall in the days just prior to and following this date is not known.

Fig. 8 - This image of the same scene from 28 Jun. depicts a typical dry-season snowfall event which occurred the day before. This is the southern hemisphere winter, when air temperatures are lower and precipitation arrives in the form of snow at lower elevations than during the wet season. Although partially obscured by clouds, note widespread snowcover on the landscape.

Fig. 9 - Two weeks after Fig. 8, snow lingers on the landscape, yet albedo is decreasing at lower elevations of the glacier as the new snow melts and sublimates. Ultimately, the extent to which 2015-16 accumulation is preserved will depend upon whether additional winter events occur. In general, however, surface lowering at the summit (i.e., ablation) typically - but not always - accelerates during August into September, and sometimes into October. Net accumulation for the mass-balance year cannot be determined until the subsequent wet season begins.

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