December 18, 2011
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.