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Snow density from Bulk and Pit Samples during APLIS07 Ice Camp
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Robert Harris1, Cathleen A. Geiger2, Adrian Turner3, Katharine Giles3
1 Hartford
High School, White River Junction, VT, USA
University of Delaware, Newark, DE, USA
3 Centre for Polar Observation and Modelling, University College, London, UK
2 Geography,
B. Calibration Survey Lines
ABSTRACT
A basic set of in situ snow measurements were taken during the
SEDNA project (http://research.iarc.uaf.edu/SEDNA/) on an ice
camp in the Beaufort Sea (April 1- 15) at the start of IPY. Bulk
density, stratigraphy, and basic snow characteristics were
recorded as part of an integrated set of snow and ice thickness
measurements. These measurements are important because
snow density distribution is a critical parameter for hydrostatic
calculations of sea ice thickness from remotely sensed data.
Results show that the snow depth on sea ice ranged from a
dusting to 1m drifts on the multiyear floes and an average of 20
cm on level ice surfaces. Depth hoar accounted for up to half of
the snow pack depth and was half the density of the wind slab
snow. Several of the depth hoar samples included very large cup
crystals (1-2 cm) with broken capped bullet crystals in the wind
slab layer. Rime deposition was often present on the fresh snow
crystals. The largest depth hoar crystals were located over level
refrozen leads where the ocean heat flux and moisture could
permeate through the ice to the bottom of the snow pack beneath
the wind slab. One high school teacher, and two post doctoral
research fellows, received in-the-field training as a result of this
experiment.
A. Instrumentation
(a) Laying out survey lines
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Camp
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An array of six 1 km transects (numbered) were surveyed using 1 m high
flags placed every 25 m, a short 1 m stake every 100 m, and a long 2 m
flagged stake every 500 m. A seventh line was added at the end of the
survey and originated from the command hut parallel to line 1 to compare
with submarine tracks. Bulk density samples were collected every 100 m
along these lines. Snow pits were made every 500 m (red boxes) with
density samples collected from each snow layer. Snow pit crystalline
structure was also noted. Snow depth measurements were collected
every 10 m. Line 3 was only surveyed to 750 m beyond which an open
water crack (~1 m) prevented further access.
C. Density Calculations
Snow density ρ is calculated using
ρ=
(b) Snow tube for bulk density
m
π r 2d
where m is the mass of the snow (g), r is the radius of the tube
(cm) and d is the snow depth (cm).
Assuming that the errors in the measurement of the mass,
depth and radius are independent, the error on the snow
density (σρ ) is given by
(c) Snow pit
(d) Box sampler for layer density
2
2
⎛ 1 ⎞
⎛ −2 m ⎞
⎛ −m ⎞
σ ρ = σ ⎜ 2 ⎟ + σ r2 ⎜ 3 ⎟ + σ d2 ⎜ 2 2 ⎟
⎝π r d ⎠
⎝π r d ⎠
⎝π r d ⎠
2
2
2
m
where σm , σr , and σd are the errors of the snow mass
(± 0.5 g), tube radius (± 0.1 cm) and snow depth (± 1 cm). The
radius r is 4.15 cm. Variables d and m are measured directly.
Box samplers had a fixed volume with the weight uncertainties being
the dominant error source computed as
Harsh polar conditions require the use of simple but effective
instrumentation to collect representative samples.
D. Results
⎛1⎞
⎟
⎝V ⎠
σ ρ2 = σ m2 ⎜
s
2
Scatter and uncertainty assessment of bulk density samples. For samples
less than 6 cm, the scatter increases considerably. Box samplers were
used for some of these smaller samples with improved uncertainty but
increased scatter. Points indicated with zero density were snow samples to
small to retrieve given the snow depth/type and weather conditions.
SUMMARY
The SEDNA field campaign took advantage of a planned U.S.
Navy ice camp in the Beaufort Sea at the edge of the perennial ice
pack. The season and location of the camp occurred when the
dynamic component dominates the mass balance of sea ice
through the formation of leads and ridges in a snow and ice
cover that is near its maximum annual thickness. The design of
the field campaign focuses on integrating measurements over
connected scales: 1km-10km-100km-1000km. These snow
measurements are part of a larger measurement suite for
conducting an extensive inter-comparison of satellite, airborne,
in situ, and underwater measurements of snow and ice thickness.
Results shown here provide us with a range of snow properties
over most of the characteristic local sea ice types. The error
analysis provides an overview of the variability in the bulk
density samples including physical variations in the snow pack
and measurement uncertainty.