Geology Reference
In-Depth Information
Figure 9.21
Frost flowers that bloom on sea ice Nilas surface in the Barents Sea. The bluish background is partly
due to image processing as well as the reflection of the sky on the ice surface. The inset is a magnification of part
of the area, yet at a different location (image courtesy of S. Kern, University of Hamburg).
suggests that the physical processes driving the formation
of frost flowers are still not fully understood. A photo-
graph of frost flowers on Nilas ice surface in the Barents
Sea is shown in Figure 9.21.
From a geophysical point of view, the extremely high
salinity of frost flowers modifies the thermodynamic
properties of the underlying ice surface greatly. They act
as an insulator layer, making the ice surface warmer by
1-2 °C than the bare surface [
Martin et al
., 1995]. The
size of the flowers and their concentration determine
the average temperature of the underlying ice surface.
Ehlert
[2012] studied the morphology of sea ice under
frost flowers by carefully sawing out the piece of sea ice
on which the flower was grown. Optical and infrared
pictures of a slab (2.5 cm width and about 4 cm depth)
were taken to examine the structure and temperature
fields as shown in Figure 9.22. The sea ice area immedi-
ately under a frost flower is delineated by the blue line in
the photograph. The morphology of the ice in this area
looks different than the surrounding ice. While the latter
shows signs of associated columnar crystallographic
structure (roughly speaking), the structure below the
flower shows a random configuration with bright areas
that probably indicate the presence of liquid. The size of
the funnel in the upper part coincides with the former
size of the flower on top of the ice while the narrow
channel to the bottom of the ice marks a possible pres-
ence of a brine channel. The infrared photograph reveals
that the temperature within the funnel was lower by
0.6 °C than the surrounding temperature. This suggests
that the area under the frost flower was filled with salty
liquid that remained at lower temperature because it
had a lower freezing point than the surrounding less
saltier ice.
The impact of frost flowers on ice thermodynamics
is justifiably neglected in weather and climate models.
The flowers may persist for only a few hours or few days
on thin ice surface before any wind or rise in air tempera-
ture wipes their fragile structure. The impact of frost
flowers on atmospheric chemistry is more important.
Frost flowers have been recognized as the dominant
source of sea salt aerosol in the Antarctic, containing
bromine compounds such as bromine monoxide. The
ozone depletion is correlated with high correlation of
these compounds. It has been proposed that frost flowers
are part of the cause of tropospheric ozone depletion
events during the polar sunrise [
Rankin et al
., 2002;
Kaleschke et al
., 2004].
In relation to remote sensing, the interest in frost
flowers follows from their modulation of the measured
microwave emission and radar backscatter. In passive
microwave imagery the highly saline frost flowers cause
the brightness temperature to decrease (since the saline
flowers are lossy medium). However, the polarization
ratio (PR) is maintained within the same range of
flower free surface [
Grenfell and Perovich
, 1994]. This
means that the structure of the flower crystals does not
depolarize the signal any more than the depolarization
