Jökull - 01.12.1959, Page 4

(3)
used to derive an expression similar to that by
Ángström (Brunt 1939 p. 144)
- K 9V
- p Km ~zT
Rm = 0.14 (1 - 0.089 m)
where Rm = outgoing radiation, and
m = cloud amount in octas.
The outgoing radiation found from this ex-
pression was subtracted from the net incoming
radiation to find the net racliation available at
the surface.
The wincl speed at 2 metres height was ge-
nerally more than 8 knots, and the plotted pro-
files coulcl then be fitted by a logarithmic law
which, for the velocity distribution in an air
layer near the ground, w'as expressed bv Prandtl
in c. g. s. units (Brunt 1939 p. 246):
vz = -l— V-Áo_ log e (1)
k p z0
where vz = wind speed at height z
k = von Karman’s constant = 0.4
rB = surface horizontal shear stress
p = fluid density
zo = Parameter of surface roughness =
the height above the surface at
which v = 0.
V2 Zo
Using wincl ratios —— at height —— gives
vi ‘ Zl
log z0
V2 log Zl — Vi log Z2
V2 — Vl
(2)
This roughness coefficient evaluated in cm
for each of the three areas is: I 0.2; II 1.61;
III 0.31.
The vertical flux (of momentum, heat and
water vapour) is proportional to the product of
the coefficient of turbulent transfer and the
vertical gradient (of momentum, heat and water
vapour). In a near neutral atmosphere (i. e. tem-
perature gradient near to adiabatic) and over
a homogeneous surface, many workers have
shown that at a specific height, the coefficients
of turbulent transfer (of momentum KM, of
heat Ku, of water vapour IvE) are approxima-
tely equal (Rider & Robinson 1951, p. 388).
The vertical transfer of horizontal momentum,
the shearing stress
is assumed constant in the first few metres of
atmosphere (Sheppard 1947, p. 209), which com-
bined with equation (1), gives:
K,
v, k2 z
log e -
zo
(4)
To check the similarity of behaviour of the
momentum, heat ancl water vapour, it is at least
necessary to superimpose the vertical profiles
(of wind speed, temperature and vapour pres-
sure) for each test to check that they follow
similar laws of distribution with height (Pas-
quill 1949 p. 250). Figure 1 shows an example,
with scale adjustment, of superposition and
divergence of the profiles. There is good agree-
ment with wind and temperature profiles but
less agreement with vapour pressure profiles.
Some of these show change of direction of slope,
indicating water vapour feeding up from the
glacier surface and down from the atmosphere
into the 2 metre air layer. Such a condition re-
quires horizontal transfer which is possible,
particularly over the varied glacier surface. To
measure this advection requires at least two sta-
tions operating simultaneously. This was not
possible so the profiles showing pronounced ad-
vection were abancloned. Superposition of the
SUPERPOSIIION NONE
TEST NO 22
POOR COOD
12 28
FIG I SUPERIMPOSED PROflLES OF WIND SPEED, TEMPERATURE
ANO VAPOUR PRESSURE.
2