Jökull - 01.01.2005, Side 140

L. A. Rasmussen
Figure 1. Seasonal variation of mean conditions over
1958-2003 at NCEP-NCAR Reanalysis gridpoint at
65◦N, 17.5◦W. (a) wind direction at 850 hPa, (b) wind
speed at 850 hPa, (c) temperature interpolated at 800
m, (d) relative humidity at 850 hPa, (e) precipitation
flux F calculated from direction 180◦ and snow flux
f , the part of F when T<+2◦C at 800 m. Monthly
resolution. –
The five glaciers considered here have slightly dif-
ferent periods of record of mass balance observations.
To reduce the effect of sampling error due to this, dif-
ferences in mean balances between two glaciers are
computed over their common period of record. Dif-
ferences between Brúarjökull and each of the other
glaciers are shown in Table 1 for each component.
Brúarjökull was used as the basis of comparison be-
cause its record gives a greater common period (n
years) with the other glaciers than if any of the other
glaciers were used as the basis.
Standard deviations of the seasonal balance com-
ponents are shown in Table 1. Although σs is gener-
ally much larger than σw, the correlation rns of net
balance bn with summer balance bs is only slightly
larger than rnw with winter balance bw, because the
correlation rws between bw and bs is so large; see
Equation (13) of Rasmussen and Conway (2001). The
rws are very large compared with glaciers elsewhere
in the world; see Figure 6 of Dyurgerov and Meier
(1999) and Figure 5 of Braithwaite and Zhang (1999).
Glacier to glacier correlation of bs is much more co-
herent spatially than of bw (Table 2).
UPPER-AIR MODEL
Precipitation at the glacier is assumed to be propor-
tional to the precipitation flux F at the 850-hPa level
at the nearest NCEP-NCAR gridpoint, which is esti-
mated from measurements of the wind and humidity
by the relation
F =
{
U RH (U ≥ 0)
0 (otherwise)
(1)
Here 0 ≤ RH ≤ 1 is the relative humidity, and U is
the component of the 850-hPa wind in the empirically
determined critical direction φ′. That is,
U = |~V850| cos(φ850 − φ
′) (2)
in which φ850 is the wind direction and |~V850| is its
speed in meters per second. Although precipitation
indeed might occur with wind in the opposite direc-
tion to φ′, empirical results show it correlates most
strongly with the U component. Moreover, the simple
model described here obtained better results (Hayes et
al., 2002) in a direct comparison with a sophisticated
mesoscale precipitation model.
Precipitation is assumed to fall as snow if the tem-
perature at elevation z1 =800 m is less than the criti-
cal temperature T ′
f =
{
F (T (z1) ≤ T
′)
0 (otherwise)
(3)
140 JÖKULL No. 55