Geoscience Reference
In-Depth Information
Table 4.3 HIGHTSI model parameters
Parameter
Value
Source
1.5 - 17 m 1
Light attenuation coefficient of
lake ice
Heron et al. (1994), Arst et al. (2006), Lei
et al. (2011)
6 - 20 m 1
Light attenuation coef cient of
snow
Patterson et al. (1988), Arst et al. (2006), Lei
et al. (2011)
910 kg m 3
Lake ice density
Corresponds to 1 % gas content
330 kg m 3
Initial snow density
Assumed
Surface emissivity
0.97
Vihma (1995b)
model has no active water layer beneath the ice, but once the ice growth has started the
model catches up well with the nature.
The results were good (Figs. 4.12 and 4.13 ). Modelled ice climatology showed growth
by 0.5 cm day 1 in December
March and 2 cm day 1 melting in April. Tuned heat
-
fl
ux
from the water to ice was 0.5 W m 2 . The diurnal weather cycle gave signi
cant impact
on the thickness of ice in spring. Ice climatology was highly sensitive to snow conditions,
and surface temperature showed strong dependency on thickness of thin ice (<0.5 m). The
lake ice season responded strongly to air temperature: a level increase by 1 or 5
C
decreased the mean length of the ice season by 13 or 78 days (from 152 days) and the
thickness of ice by 6 or 22 cm (from 50 cm), respectively.
In fact, the
°
fit of such model simulations largely depends on the quality of the forcing
data and the sub-model for air
ice interaction. The annual cycle of the change in ice
thickness is well represented by the model and demonstrates that the model can be used to
simulate the evolution of the ice sheet under a range of meteorological conditions. The
weakest components of the model appear to be snow formulation together with the onset
of melting, which need further testing with winter seasons of varying snow conditions.
Internal melting is an important process in the melting season. The implications are that
the physical properties of the ice sheet change,
-
first of all the strength of the ice strongly
decreases, and liquid water pockets form serving as habitats for biota. To properly treat the
internal melting necessitates the inclusion of the ice porosity as a dependent model var-
iable. This approach was taken by Lepp
ranta (2009b), who considered a two-phase
approach with liquid phase and solid phase portions given for each grid cell in addition to
the temperature (Fig. 4.14 ). When the temperature reaches the freezing point, additional
heat adds on the porosity. At freezing point and zero porosity, heat loss takes the tem-
perature below the freezing point. A realistic structural pro
ä
le was obtained with locations
of liquid water containing layers. In the case of brackish or saline ice, the same approach
works but then the porosity changes at any temperature due to equilibrium requirement for
the salinity of brine (see Maykut and Untersteiner 1971).
In the two-phase model the phase proportions are simulated for each grid cell in
addition to the temperature. The melting season begins after the heat balance has turned
positive. Once the temperature of the ice-sheet reaches the melting point, melting takes
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