Mernild et al (2013) is a new paper that has authors from several countries that I am co-author on with Knudsen, Malmros in Denmark, Hanna from UK, Yde currently in Norway and Mernild in Chile. The key items here are using the snow line observed on any particular melt season day (transient snowline=TSL) as input for mass balance assessment. This paper examines how similar the migration of the TSL is from year to year, and how ablation rate can be determined using it, when field data can be used for validation. The first two images are figures from the paper of Lemon Creek Glacier and Mittivakkat Glacier illustrating the TSL at various dates. A second key is that if the progression is relatively repeatable towards the end of the melt season, than the equilibrium line altitude (ELA) can be determined, snowline at the end of the melt season, which is a key mass balance variable. Clouds often obscure the ELA from satellite image assessment, and this allows appropriate extrapolation. The figure below needs more data to determine the consistency and nature of the TSL variation at the end of the melt season, the ELA is the top of the parabola.
Base map of Lemon Creek Glacier in 2003 with colored lines indicating various dates of the TSL.
Base map of Miitivakkat Glacier in 2012 with colored lines indicating various dates of the TSL.
Progression of the TSL approximated with a second order Polynomial, to help derive the ELA.
A good example of the utility is an examination of the Landsat 8 imagery from this summer. Alaska had a warm and relatively clear weather period that provided a rare chance to examine the TSL in three consecutive satellite passes on June 14, June 21 and June 30. This period began with the glacier almost completely snow covered, red dots indicate TSL, red arrow indicates the 6/30 TSL. On June 14 the TSL was at 775 m within a couple of hundred meters of the terminus. By 6/21 the TSl had moved up the northwest side of the glacier 1.5 km to an altitude of 900m. On June 30th the TSL was at 975m two kilometers from the terminus. This progression up the northwest side of the glacier is typical. At the start of July the glacier is still 90% snowcovered. The Juneau Icefield Research Program is on this glacier in early July and field work will be critical to identifying snow depths above the TSL, that the TSL will transect later in the summer identifying ablation. The yellow arrow indicates the formation of Lake Linda, a meltwater lake that forms on the glacier, the expansion from June 14 to June 30 is evident. Pictures of the lake from JIRP during self arrest practice are gorgeous. More detailed examination of the longer term change of Lemon Creek Glacier and Mittivakkat Glacier has been completed.
June 14 2013 Landsat image
June 21 2013 Landsat image
June 30 2013 Landsat image
This post examines in simpler terms and more images the paper published this week in The Cryosphere on “Utility of late summer transient snowline migration rate on Taku Glacier, Alaska”. The transient snowline (TSL) is the point of transition from snow to older glacier firn and ice. The TSL rises during the course of the summer melt season and at the end of the melt season is the equilibrium line altitude (ELA). This paper represents a concept that occurred to me while skiing and probing snow pack on the Taku Glacier in 1998 with the Juneau Icefield Research Program (JIRP), something I have spent six months doing over the years. There simply was not enough consistent satellite imagery to apply the model until recently, we also needed field data-ground truth-to quantify and verify the TSL model. This meant probing snowpack along a 5 km transect near the TSL during several summers, following my 1998 probing, Matt Beedle completed the probing in 2004 and 2005 with JIRP and Chris McNeil did so in 2010, 2011 and 2014. Below is the transient snowline in August 2014 on the Juneau Icefield.
Landsat Image: T=Taku, G=Gilkey, H=Herbert, M=Mendenhall and N=Norris. Black arrows indicate the snowline which was quite high at over 1000 m with a month left in the melt season.
The ELA is the point at which accumulation equals melting on temperate alpine glaciers this is where snow transitions to bare glacier ice. Mass balance for non-calving glaciers is the difference between snow accumulation on a glacier and snow and ice loss from the glacier. The easiest to observe and most useful estimate of mass balance without detailed measurement is the equilibrium line altitude (ELA). Today the TSL can be observed frequently in satellite imagery. There are two ways the TSL is useful in assessing mass balance. First the rise of TSL during the melt season provides an assessment of the rate of melting. Second the TSL rate of rise can be used near the end of the melt season to determine the ELA, when imagery at that point is not available due to cloud cover. This allows widespread assessment of melt rate on glaciers. On Taku Glacier which is fairly typical we found a very consistent gradient of snowpack change with elevation from year to year. This allows determination of melt rate simply from rate of TSL rise. We use Landsat Imagery of which there are typically only two-four useful images during the melt season, barely enough and more recently MODIS imagery from GINA, which is obtained daily for the entire globe and provides the most frequent point of observation. However, the resolution of MODIS makes it inaccurate on glaciers less than 1 km wide or 1 km long. Taku Glacier is 55 km long and 5 km wide at the ELA. As the melt season begins the snow cover extent is large on Taku Glacier. The key is how rapidly the TSL rises during the melt season. On the ground the JIRP measures the snow depths and snow melt during July and August on Taku Glacier. This program was led by Maynard Miller, U Idaho for more than 50 years and is currently under the direction of Jeff Kavanaugh U Alberta. The Taku Glacier mass balance measurements allows validation of the melt rate, note snowpit locations on map below. For example in 2004 the TSL was at 850 meters on July 15, first image below. At this time the snowpack was 1.6 meters at 1000 meters. On September 1 the snowline was at 1030 m, second image below. The TSL had risen at an average rate of 3.9 meters per day, all 1.6 m of snow had melted. Below are images from May 26, 2006, then July 29, 2006 and then Sept. 15 2006. Indicating the rise of the snowline.
The below images from May 26, July 29 and Sept. 15 2006 indicate the rise of the ELA during the course of the melt season, from 370 m to 800 m to 975 m. Snow depths at the the Sept. 15 ELA, where snowpack=0, was 2 m on July 22. Thus, we had 2 meters of snow melt at 975 m between July 22 and Sept. 15. In 2004 the melt rate was 0.036 meters per day and in 2006 0.038 meters per day. All of the TSL images above are from Landsat> For Sept. 14, 2009 (top), Sept. 20, 2010 (middle) and Sept. 11, 2011 (bottom) MODIS images are used, resolution not as good as with the Landsat images. Note the similarity in the end of the year snowline on Taku Glacier for those three years. . The next task is to apply the TSL to other glaciers and to carefully compare results from MODIS and Landsat. Through 2010 there were only four days with good coverage from both. Below is the Landsat imagery from Sept. 11, 2011, same as the MODIS date above. Noted is the TSL, in this case the ELA for Lemon Creek and Taku Glacier.