This paper presents the results of a series of laboratory experiments
aimed at understanding
the processes associated with surface freezing of a two-layer fluid. The
flow configuration consists of a layer of cold, salty water overlying a
relatively deep
bottom layer of warm, saltier water. This situation is common in high-latitude
oceans
during periods of rapid ice formation. The experiments were conducted in
a tank
with well-insulated side and bottom walls, placed in a walk-in freezer
with air
temperatures from −12 to −20°C. A system of thermocouples
was used to measure
the temperatures at fixed levels in water, ice and air. Microscale conductivity
and
temperature probes were used to obtain vertical profiles of temperature
and salinity
in the water. In general, when external uxes of heat and salt are absent,
such a
system enhances static stability, in the sense that the net density difference
between
the layers increases with time. When external uxes of heat (because of
surface cooling)
and salt (rejected during ice formation) are applied, however, this fluid
system
may become unstable and overturning of fluid layers is possible. In addition,
heat
transport from the warmer bottom layer to the colder upper layer may be
important,
possibly leading to a reduction in the rate of ice formation compared to
that
of a homogeneous fluid with temperature and salinity identical to the upper
layer.
Descriptions of such physical processes are given using laboratory experiments,
and
quantitative measurements of salient parameters are compared with the predictions
of a theoretical model developed to explicate the flow evolution.
The first frost in the Pipe Nebula
