Nature of the Measurements
In the dry snow zone of the Greenland Ice Sheet, accumulation is relatively
easy to measure by digging a pit or by drilling to retrieve a core. The span
of the sample is measured by counting annual layers. The core or the pit
wall is sampled to measure the accumulated mass, which is reported in kg
m-2 a-1. At lower elevations, melting during summer
blurs the distinction between annual layers. The meltwater percolates from
the surface down into older layers, but the total accumulation can still be
measured. There are between 300 and 400 usable published measurements of
this kind, unevenly distributed over the ice sheet and having variable
record lengths. At still lower elevations some or even all of the snowfall
may be lost by melting and runoff during summer, and there are very few
reliable measurements of accumulation.
To estimate accumulation over the whole ice sheet it is necessary to
extrapolate from the small number of points with measurements. This exercise
is doomed in the ablation zone, where the nearest measurements may be
hundreds of kilometres away, unless information is taken from coastal
weather stations. At these stations the snowfall is measured regularly, and
can be assumed to be proportional to the "accumulation" which would be
measured if there were a glacier there. These indirect estimates must,
however, be corrected by subtracting an estimate of sublimation.
Data and Results
The dataset being used in Trent's work on accumulation in Greenland is based
on the work of earlier compilers, but includes a small number of additional
GR.ACCUM.ZIP is a set of three files. GR.ACCUM.V01.DAT contains accumulation
measurements from the ice sheet itself or from nearby ice caps.
GR.PRECIP.V01.DAT contains estimates of "accumulation" from coastal weather
stations. GR.ACCUM.V01.DOC is a Word document which describes the structure
of the two data files and lists the data sources.
2003JD004449.PDF is the electronic version (© 2004, American Geophysical
Union) of Cogley, J.G., 2004, Greenland accumulation: an error model, Journal of
Geophysical Research, 109(D18), D18101, doi: 10.1029/2003JD004449. 15p.
The main results to date of work on spatial interpolation of accumulation are
which show accumulation and its standard error.