Geology Reference
In-Depth Information
0
0
σ
W
σ
VH
σ
HH
σ
W
σ
VH
σ
HH
-5
-5
-10
-10
-15
-15
-20
-20
-25
-25
-30
-30
-35
-35
-40
-40
-45
-45
-50
-50
10
20
30
40
50
60
70
10
20
30
40
50
60
70
Incidence angle (deg.)
Incidence angle (deg.)
Figure 9.23
Angular variation of normalized radar cross section measured using a ship‐based C‐band scat-
terometer. Measurements were from Arctic thin ice with (a) frost flowers at the surface and (b) after removal
of snow and frost flowers. Third‐order polynomials are fitted to the data points. [
Isleifson et al
., 2010, Fig. 6,
with permission from IEEE].
by 4 cm during the same period. This is attributed to the
insulating effect of frost flowers (caused by their high
salinity) as mentioned before.
In another recent study
Isleifson et al
. [2014] measured
backscattering from laboratory‐grown frost flowers in an
outdoor pool using a surface‐based polarimetric C‐band
scatterometer. The study presents a detailed description
of the growth stages of frost flowers and a synopsis of
the polarimetric signature at each stage. The stages are
defined as initial formation, surface brine expulsion,
frost flower growth, and decimation. The initial forma-
tion stage lasted between 6 and 10 h. By the end of that
period the surface was completely covered with frost
flowers of an average height of 5 mm and the salinity of
the ice surface was 10‰. Backscattering from all polari-
metric channels was low at the beginning of this stage but
increased sharply when frost flowers covered most of the
area. For example,
nearly the entire ice surface. The authors interpreted this
as an area where brine was expelled upward to penetrate
the frost flowers. The surface salinity was doubled after
about 15 h, and polarimetric backscattering measure-
ments underwent a rapid decrease for all incident angles
until a local minimum occurred shortly after most of the
frost flower surface was covered with brine. This is per-
haps the same stage at which
Nghiem et al
. [1997] observed
a slush layer between the frost flowers as mentioned
above. In the third stage of frost flower growth (hour
15-38) the flowers developed to an average height of
15 mm with a dendritic appearance. Their salinity reached
a peak of 52‰ at hour 22. Polarimetric backscattering
measurements showed stable values that varied with the
incidence angle. For 25° incidence angle,
0
were almost equal (−16 and −19 dB, respectively) while
hv
0
and
hh
vv
0
was around −20 dB. The fourth stage of the deci-
mation is characterized by a reduction of the frost
flower height from 15 to 5 mm, occurring after hour 38.
Warm air temperature and increasing solar radiation
were responsible for the eventual elimination of the
flower crystals.
0
0
and
increased from about −25
hh
vv
0
increased from −37 to −29 dB, all
measured at an incidence angle of 25°. During the second
stage that lasted between hours 10 and 15, dark patches
were observed on a few spots and they grew to cover
to −10 dB while
hv
