peratures reach near the melting point at any level,
These melt features may never reach the ice sur-
melt features and their commensurate problems
face (always retaining a thin ice cover) but they
may be an issue at the site. Even if no melt fea-
still may be deep and massive in extent.
tures are apparent at the site in its natural condi-
Radiometers are the best tool for measurement
tion, if the ice temperature profile shows values
of solar radiation. A number of varieties exist; it is
over 10C at any time during the season, the
best to use a model that records input from an
effect of construction may change the site enough
entire hemisphere (half-globe), since most glacial
to allow melt features to develop in the heat of
ice sites are located at high latitudes where the
the summer.
sun angle will be low, but a large portion of the
Thermocouple or thermistor strings are the easi-
horizon can be swept by the sun. The radiometer
est means of monitoring the ice temperature pro-
used should be capable of measuring radiation
within the wavelength range from 0.3100 m.
file for long time periods. At the Pegasus site, we
used Cu-Co thermocouple strings with sensors
We advocate installing radiometers in pairs,
located at 1 cm above the natural ice surface, and
one facing straight up (base plane parallel to the
at distances below the ice surface of 1, 5, 10, 25,
ice surface) and the other directly toward the ice
50, 75, and 100 cm. We completely enclosed the
surface. The pair should be held about 1 m above
thermocouple strands in a single segment of heat
the ice surface by a secure system that has set-
shrink tubing. The leads and housing extended
back leg(s) that cast little or no shadow within a
for 1.5 m beyond the top of the ice surface. Instal-
3-m radius of the centerline of the radiometers.
lation of the thermocouple strings was completed
By using radiometer pairs in this manner, one can
by hand-auguring a 5-cm-diam. vertical hole in
determine the total incoming radiation, the
the ice, inserting the string, and backfilling the
amount reflected from the terrain surface, and the
hole with snow, followed by slow filling with
net absorbed radiation (the latter is the difference
cold fresh water. Once the thermocouple string
between the two measured values). Radiometer
was frozen-in, the lead was run along the ice
pair measurements should be taken over each
surface 1.5 m away from the hole and connected
surface type to gain a clear picture of the amount
to a weather-tight box containing a Campbell Sci-
of absorbed radiation over bare and snow-cov-
entific data acquisition system. We covered the
ered ice, and in areas where natural contaminants
thermocouple lead and the data logger box with
exist (e.g., mineral dust, incorporated rocks). These
data will be valuable in assessing whether melt
snow to protect it from excessive solar heating.
will be a problem, whether natural materials can
Temperature sensor strings should be placed
be used to minimize or eliminate melting, and if
at locations in the area where it is most likely the
certain localities within the region are less prob-
runway would be sited. It may also be wise to
lematic and thus more suitable from the outset.
install a string in the ice in any areas within the
We feel that it is best to take radiometer mea-
locale that have distinctive characteristics (e.g.,
surements very near the sites where ice tempera-
bare and snow-covered ice, in obvious melt fea-
ture profiles are being monitored. This allows a
tures). The goal should be to get a representative
good understanding of the site-specific balance of
picture of the range of ice temperature regimes
air temperature, solar input, and ice temperature
within the area of potential runway siting.
response. Unfortunately, radiometers are usually
It should be recognized that the presence of the
very expensive. This may limit the number that
thermocouple string will alter the thermal regime
can be placed in the field. If regular site visits are
of the ice at that spot, but, if properly installed,
feasible, a single radiometer pair could be used to
solar effects can be minimized.
continuously record data at a single location at
the site, but, upon each visit, the radiometers could
Solar radiation
be moved around to each ice temperature mea-
In areas where the air temperature reaches or
surement site to obtain at least a series of single-
nears the melting point, the effect of solar radia-
point correlations.
tion will need to be well understood. Of primary
At the Pegasus site, we used Eppley pyranom-
importance is the potential for radiation to be
eters on several occasions during the peak of the
absorbed into the ice and act as a heat source at
austral summer. Unfortunately, we have never
some depth in the ice (Brandt and Warren 1993).
had the opportunity to connect them to a data
With air temperatures near, but often below, freez-
acquisition system and thus only have spot read-
ing and intense sun, the near-surface ice may heat
ings. We used a tripod system for mounting the
to the point of melting at certain times of the year.
23
Previous Page
