Jökull - 01.12.1972, Page 47

an isothermal state with no appreciable heat
conduction in the surface layer. Engergy trans-
ferred by precipitation falling on the glacier is
consiclered to be negligible. Sources or pro-
cesses conveying energy to the snow surface are
defined positive ancl sinks are taken to be
negative.
Net radiation budget
The net radiation budget can be expressed
as the sum of the short-wave and the long-wave
radiation, as follows
Qr = (Rg-Rí) + (Ra-Rs)
= (1 -«) Rg + R, (2)
where
Rg is the global radiation
Rr is the short-wave radiation reflected frorn
the glacier
a = (Rr/Rg) is the albedo of the surface
Ra is the long-wave radiation from the atmos-
phere
Rs is the long-wave radiation from the giacier
surface.
The net long-wave radiation of a melting
giacier surface varies mostly with the atmos-
pheric long-wave radiation and is therefore a
function of cloudiness. In lack of registrations
daily values of the long-wave radiation balance
were estimated by the relation
Ri = Ro (! -k(c/8)2) ly/min (3)
given by Hoinkes and Untersteiner (1952). Here
c is the mean cloudiness in octas of the sky.
Tlie constants were taken as R0 = —0.085 ly/
min (or 59.3 W/m2) and k = 1.4. The equation
which gives positive values for c between 7 and
8 has given useful estimates on several glaciers
(LiesUþl 1967, Miiller and Keeler 1969).
The transfer of latent and sensible heat
When dealing with the transfer of moment-
um and heat in the boundary layer of the
earth’s atmospliere one usually considers physic-
al models for a stationary and horizontally
homogeneous air flow over a horizontal aero-
dynamically rough surface (see e.g. Lumley a,nd
Panofsky 1964 ancl Calder 1966).
Analysis of the equation of motion and the
energy equation shows then that the effect of
Fig. 1. A view to south in the Bægisárdalur
valley. From right to left the mountains Trölla-
fjall (1471 m), Tröllatindur, Steinsfell (1343 m),
Snorragnúpur, Jökulborg and Lambárhryggur.
Mynd 1. Séð suður Bagisárdal. Tröllafjall er
lengst til vinstri og Tröllatindur, Steinsfell,
Snorragnúpur og Jökulborg upp af jöklinum,
en I.ambárhryggur lengst til hægri.
the turbulence on the mean motion is express-
ecl in the following three statistical quantities:
1. the vertical eddy-flux density of the hori-
zontal impulse, — Fu. In the direction of
the mean wind we write the Reynolds stress
r = -Fu = pu'w' (4)
where
p is the air density (assumed constant)
u' is the fluctuation of the wind velocity
in tlie direction of the air current
w' is the corresponding quantity in the
vertical direction
2. the vertical eddy-flux density of the “dry”
enthalpy (sensible heat) given by
Hd = cp P w'T' (5)
where
cp is the specific heat capacity of dry air
at constant pressure
T' is the fluctuation of the air tempera-
ture
3. tlie vertical eddy-flux density of latent heat
given by
H1 = LV pw'm' (6)
JÖKULL 22. ÁR 45