Update to melt and ablation season onsets
One week ago, we launched a new and improved surface mass balance data product on the Polar Portal. The improvements in the underlying model include changes in the details of how water from melted snow percolates into the snow layers below, where it either refreezes within the snowpack or runs off as liquid water. Ice lenses within the snowpack are also treated more realistically. These changes and the fact that we were able to extend the data series back to 1980 (so that we can use the more common reference period 1981-2010) have consequences for the onset of the melting season and the onset of ablation season on the ice sheet.
New improved model
As described previously, researchers from DMI and GEUS have in an article in the scientific journal “Frontiers in Earth Science” described advances in the model that is used to calculate the surface mass balance product on the Polar Portal. These include a better estimate of the glacier-covered area as well as improvements to the firn model, which deals with the different pathways of melted snow which may either refreeze or run off to lower elevations.
Our recent update on the Polar Portal included these improvements. A consequence is that the reference surface mass balance for the whole annual cycle is at 370 gigatons (Gt), compared to about 290 Gt in the previous version. Dr. Ruth Mottram, a climate scientist at DMI, says: “The changes mean that the reference curve is different from what it was before the update. The average annual total SMB is higher due to higher accumulation and lower summer melt in the longer reference period.”
However, the changes in the treatment of runoff and refreezing also have another consequence. Dr. Martin Stendel, also a climate scientist at DMI, explains: “Not all melt seasons are alike. Some, like 2016, have lots of melting at low elevations, but little change at high elevations. Others, for example 2012, showed widespread melting up to the highest elevations. The difference is caused by the different wind regimes during the melting season. Thus, the model improvements in percolation, refreezing and runoff have different effects for different years.”
Changing onsets of melting and ablation
This means that the onset of the melting and ablation seasons, which depend on the delicate balance of these height-dependent factors, have taken place at other dates than in the old version, because an individual warm spell may just have (or may just not have) reached the threshold as we have defined it. The threshold itself is unchanged from the previous version.
Martin Stendel continues: “If we sort individual years for the dates of the onset of melting and ablation, the consequence is that we see some changes in the ordering of years. In addition, we now also see years from the 1980s that were not included in the previous version.”
Below, we have listed the ordering of years in the recent, improved model version. To make comparisons easier, we have also given these dates as they were determined using the old model version.
The table below shows the onset of the ablation season, defined as the first day of a period of at least three consecutive days where the surface mass balance (SMB) is negative and below –1 Gt/day (1 Gt corresponds to 1 cubic kilometer of water).
Martin Stendel says: “In most of the years, changes are only minor and on the order of one or two days. However, two years stick out (marked by arrows). The warm spell in mid-May 2016, which remained just under the threshold in the old model version, just fulfilled the criterion in the improved version, thus making it the earliest onset of ablation since 1980. A relatively large change can also be noticed for the warm spell in May 2010, which just didn’t cross the threshold in the improved version. As said before, these changes are due to the fact that not all melting seasons are alike, and there are changing contributions to ablation from different heights and areas.”
The table below shows the onset of the melting season, which is defined as the first day of a period of at least three consecutive days where more than 5% of the ice sheet experiences melting. In our model, we have defined melting to occur when the melt rate is greater than 1 mm/day.
As for the onset of the ablation season, changes are rather small for most of the years. But also for melting, there are two notable exceptions (marked by arrows). The late April melting event in 1996 was just too small to pass the threshold, and the next melt event (which was large enough) occurred only three weeks later. And the very early melting event beginning on 11 April 2016 remained just below the threshold in the old model version, as discussed here, but just passed the threshold in the updated version.
Dr. Peter Langen, also a climate researcher at DMI, comments: “In part these thresholds were designed to specifically exclude early extreme melting events, as these are not generally representative for the remainder of the thawing season. Nevertheless, the 2016 event came across that threshold. As there were four rather cold weeks in Greenland following the melt event of 11 April, the overall effect is that the onset of melting in 2016 is one month earlier in the updated version.”
2017 has so far had an unusually positive surface mass balance, and there have been no widespread melting events so far. However, with the buildup of a large blocking high over Greenland, some widespread melt might occur this weekend.
Polar Portal is a collaboration between DMI, GEUS, DTU-Space and DTU-Byg with funding from the Danish Energy Agency (Energistyrelsen).