Geology Reference
In-Depth Information
9
Retrieval of Sea Ice Surface Features
Sea ice surface features refer to mechanically generated
aspects such as surface deformation or thermally gener-
ated aspects such as surface melt or frost flowers. Remote
sensing data can be used to identify most of these features.
Surface deformation is manifested in different forms.
When ice sheets/floes converge or collide, pressure ridges
and rubble surfaces form. On the other hand, when they
diverge, leads and cracks form. Thermally generated sur-
face features are manifested in the forms of onset of melt
ponds in the summer and refrozen leads in winter.
This chapter covers methods of identification and
characterization of four surface features using remote
sensing techniques. These are (1) surface deformation
caused by compression and crushing of ice sheet, (2)
leads/cracks caused by divergence of ice sheets, (3) sur-
face melt (onset of melt, melt pond, and advanced melt),
and (4) frost flowers that cover a young ice surface for a
short time when atmospheric conditions are appropriate.
The first two are caused by mechanical forcing (motion
generated). They can be identified and analyzed using
ice displacement or vector motion maps when produced
regularly. A notable work on this subject is the ice motion
map products from Alaska SAR Facility (ASF) using
the Radarsat Geophysical Processor System (RGPS).
Maps are produced at time intervals between 3 and 6
days from sequential SAR (mainly Radarsat) images.
This subject is covered in more detail in section 10.7. The
last two features are caused by thermodynamic forcing.
The surface melt onset is an operational product from a
few ice centers such as the Canadian Ice Service. Frost
flowers are caused by the release of water vapor from the
thin ice surface (a few centimeter thick), followed by
immediate condensation under very cold atmospheric
temperature. Their presence is not important from an
operational ice monitoring point of view, but they affect
the energy balance at the ice surface.
While the chapter is mainly concerned with using
remote sensing to detect and characterize the four afore-
mentioned forms of ice surface features, it also covers
briefly a few geophysical aspects of some features. An
example is the role of the Beaufort Sea Gyre on lead for-
mation and dynamics in the vast ice cover across the
Beaufort Sea.
9.1. Surface Deformation
In this section, the term surface deformation is restricted
to forms caused by convergence of mobile ice sheets/floes
when they collide against each other. Although cracks and
leads, caused by divergence of the ice sheets, are equally
considered as forms of deformation, they are addressed in
the following section. All forms of surface deformations
play an important role in determining the aerodynamic
coupling between the ice and the atmosphere on the one
hand and the hydrodynamic coupling between the ice and
the ocean on the other hand.
Collision of ice sheets result in rafting if the sheets are
thin enough or broken ice blocks if at least one of the
sheets is thick enough (section 2.4). If crushing of the two
sheets is limited to their edges, the broken blocks will
mount up to heights between a few tens of centimeters to
a few meters. Small heights (a few tens of centimeters)
characterize what is known as raised edges (Figure 2.45).
Large heights characterize ridges. Pressure ridges are usu-
ally higher than shear ridges (see section 2.4 for defini-
tions). Ridges usually appear within an agglomerated
mass of ice floes (Figure 2.47). If, on the other hand, the
impact of the crushing extends deep into the ice pack,
then a rubble field is formed. These are extensive distri-
bution of ice blocks without vertical accumulation
(Figure 2.40). The presence of ridges and rubble ice can
be used as an indicator of the presence of relatively thick
