Geology Reference
In-Depth Information
required large temperature gradient within the air layer
adjacent to the ice surface. The substantially cold air tem-
perature in the polar regions enhances the chance of the
formation of frost flowers. It is not uncommon to find
frost flowers covering the surface of refrozen leads in the
Arctic. When the ice grows too thick its surface tempera-
ture decreases and so does the difference between the sur-
face and air temperatures. Frost flowers no longer form
under this condition.
The mechanism of frost flower formation is illustrated
graphically in Figure 9.19. Profiles of atmospheric tem-
perature (
T
) and vapor density (
ρ
) above the ice surface
are shown, along with the saturation vapor density pro-
file (
ρ
sat
), which is a nonlinear function of temperature as
given by the following equation [
Style and Worster
, 2009]:
Ehlert
[2012] presents a credible study on growth and
melt of frost flowers and their interaction with the under-
lying sea ice. It included detailed measurements of the
morphology, temperature, and salinity evolution and
their impact on the sea ice during both field and labora-
tory experiments. The study was conducted in a small
Greenlandic Bay (72.79°N, 56.06°W) in March 2010. A
photograph of almost one‐day old flower is shown in
Figure 9.20. Once formed, the flower grows by accumula-
tion of stellar‐shaped crystals. The base of the flower
“old” crystals appear darker (hence thicker) than the
“young” crystals at the top. It is interesting to note the
sea ice surface below the flower, which features milky
appearance and much less porosity than the surrounding
surface.
Ehlert
[2012] recorded and presented a complete
life cycle of a frost flower. It started to grow on a protru-
sion at the young ice surface. Its height increased from
0.5 to 4.8 cm within 3 h. The melting process was
induced after 16 h and the flower collapsed after
approximately 22 h. After 42 h the flower had melted
completely, leaving a brine pool at the ice surface.
According to the study, the source of nucleation of a
frost flower is not snow or diamond‐dust crystals but
only ice platelets protruding at the surface. The author
T
T
ML
RTT
11
() () exp
T
T
(9.22)
sat
sat
where
M
is the molar mass of water,
L
is the latent heat
of vaporization per unit mass,
R
is the gas constant, and
T
∞
is the temperature of the far‐field atmosphere. When
water vapor from ice reaches the relatively cold air above
the ice, a region of supersaturation can develop above
the surface if the vapor density exceeds the saturation
density (Figure 9.19). This is where frost flowers crys-
tals can grow around appropriate nuclei. Once formed,
they usually wick up brine from the surface and there-
fore can have an extremely high salinity that may reach
100‰ or more [
Martin et al
., 1995]. Frost flower crys-
tals are similar to the hoar ice crystals at the snow base
[
Style and Worster
, 2009].
ρ
T
ρ
sat
Frost flowers
Figure 9.20
Photograph of a frost flower that grew on natural
sea ice in a bay in the Greenland Sea on the night of 16 March
2010. Note the different gray shade between the base and the
top of the flower, representing thicker and thinner ice crystals,
respectively. Note also the small single crystals circled in the
lower left corner from which other flowers can grow. The sea
ice below frost flowers appeared milky and was almost free
of air bubbles (courtesy of I. Ehlert, Max Planck Institute of
Meteorology, Hamburg, Germany, from
Ehlert
, 2012, Fig. 2.14].
Young sea ice
Figure 9.19
Schematic diagram of the temperature
T
and
humidity (vapor density)
ρ
in the air above ice surface. The
dashed curve indicates the saturation vapor density
ρ
sat
(
T
). A
region of supersaturation can develop above the ice when
ρ
>
ρ
sat
This is where frost flowers can grow.
