Jökull - 01.01.2005, Side 126

Sverrir Guðmundsson et al.
Figure 5. Energy budget maps averaged over six days. M C : total energy supplied for melting as the sum of the
net radiation (R) and the turbulent heat fluxes (H). – Orkubúskapur á Brúarjökli.
ing. Thus, as was obtained by two independent meth-
ods: direct ablation measurements and energy budget
calculations (Figure 4). A good consistency was ob-
tained between calculated and measuredmelting, with
a daily correlation of 0.91 at all the three stations.
Since 1992, measurements of annual summer (b s)
and winter (bw) mass balance have been conducted at
several points on Brúarjökull (Figure 1). Each year
in April-May, cores for measuring winter mass bal-
ance and snow density have been drilled through the
winter layer. The summer mass balance has been de-
rived in September-October from readings at stakes
and on wires that were drilled into the glacier in April
or May and left there during the summer (Björns-
son et al., 1998, 2003). The AWS data, along with
the measured bw, was used to infer energy balance
maps (EBMs) for the summer of 2004 (Figure 5). The
weather parameters TG, r, u,Qi, and Ii were assumed
to vary only with elevation on the gently sloping out-
let glacier (which is up to ∼53 km wide from east
to west and ∼48 km long from north to south) and
estimated for the whole outlet glacier by a linear in-
terpolation between measurements at the three AWSs.
The long-wave radiation emitted from the glacier sur-
face (Io) was taken as 315Wm−2 (from a melting sur-
face) and the outgoing solar radiation as Qo = Qi ·α.
The observed albedo data (Figure 6) were used to in-
fer changes in albedo (α) for the whole outlet glacier
when melting a) the winter snow (bw), b) ice/firn after
removal of the winter snow and c) snow that falls and
melts during the ablation season.
The total ablation calculated from EBMs was gen-
erally consistent with the measured bs at the lower
elevations, but considerably higher than measured at
the two highest stakes (Figure 7a). This can be ex-
plained by frequent snowfall within the 2004 ablation
126 JÖKULL No. 55