Geoscience Reference
In-Depth Information
Figure 12.6
Energy balances (W m
-2
) over four terrain types in the polar regions. M = energy used to melt snow.
Source
: Weller and Wendler (1990). Reprinted from
Annals of Glaciology
, by permission of the International Glaciological Society.
C VEGETATED SURFACES
12.7). In this example of snow melt at Bad Lake,
Saskatchewan on 10 April 1974, the value of
R
n
was
kept low by the high albedo of the snow (0.65). As the
air was always warmer than the melting snow, there was
a flow of sensible heat from the air at all times (i.e.
H
negative). Prior to noon, almost all the net radiation
went into snow heat storage, causing melting, which
peaked in the afternoon (
From the viewpoint of energy regime and plant canopy
microclimate, it is useful to consider short crops and
forests separately.
1 Short green crops
M
maximum). Net radiation
accounted for about 68 per cent of the snow melt and
convection (
H
∆
Short green crops, up to a metre or so in height,
supplied with sufficient water and exposed to similar
solar radiation conditions, all have a similar net radiation
(
R
n
) balance. This is largely because of the small range
of albedos, 20 to 30 per cent for short green crops
compared with 9 to 18 per cent for forests. Canopy
structure appears to be the primary reason for this
albedo difference. General figures for rates of energy
dispersal at noon on a June day in a 20-cm high stand
of grass in the higher mid-latitudes are shown in Table
12.1.
LE
) for 31 per cent. Snow melts earlier
in the boreal forests than on the tundra, and as the albedo
of the uncovered spruce forest tends to be lower than
that of the tundra, the net radiation of the forest can be
significantly greater than for the tundra. Thus, south of
the arctic treeline the boreal forest acts as a major heat
source.
