Mass balance
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Mass balance |
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temporal evolution of the mass balance of different ice categories was
monitored at 10 sites that were carefully selected to represent the
widest possible range of ice types and conditions. These sites typically
consisted of a thermistor string and several
thickness gauges. At two
sites (Quebec 2, Seattle) there was an above-ice acoustic sounder that
measured surface positionevery hour to within 5 mm. A water-level recorder was
used at Quebec 2 and the Doghouse to measure the hydrostatic equilibrium of the floe during
the melt season. These readings are valuable since changes in hydrostatic
equilibrium represent integrated changes in the mass of the entire floe.
To reduce error, the water level reading can be adjusted for changes in
the density of the underlying water.We also made
measurements of individual melt ponds and pressure
ridges. There were over 120 mass balance points where an ablation stake measured mass loss at the surface and a hot-wire thickness gauge measured bottom accretion or ablation. Data was manually collected every 1-2 weeks during winter and every other day during the summer melt season. Thickness gauge results showed a general decrease in ice thickness during the SHEBA year.
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Instrumentation at mass balance site: A) Campbell Scientic CR-10 datalogger, B) ablation stake, C) thickness gauge, D) thermistor probe, and E) acoustic sounder.
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Mass balance sites Pittsburgh - SHEBA column site, undeformed multiyear; thermistors, gauges The Ridge - Thick second-year ridge, gauges, thermistors Quebec 1 - Thinner, multiyear; gauges, thermistors Quebec 2 - Thick multiyear; gauges, thermistors, water level recorder, acoustic Seattle - Ponded area, hummocks nearby; thermistors, gauges, acoustic sensor Tuk - Mature multiyear ridge; thermistors, gauges Baltimore - First-year ice; thermistors, gauges Mainline - Multiyear ice adjacent to snow Mainline; gauges only Atlanta - Multiyear ice; gauges only Doghouse - Thick multiyear ice; gauges, water-level recorder Sarah's Lake - First-year ice near a lead; thickness gauges. |
Reading gauges along the Mainline in August. |
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The building block of the strings was a 1-m-long PVC rod with 10 thermistors spaced every 10 cm. These rods could easily be connected to assemble strings that extended from the air through the snow and ice into the upper ocean. The default spacing was 10 cm, though occasionally strings were overlapped to provide 5-cm spacing. Temperatures were automatically logged every hour. YSI thermistors, with an accuracy better than 0.1 C, were used. Thermistor strings appeared to work quite well during the cold portion of the year, but there were some problems during the melt season. In general, during summer, solar heating of the strings resulted in measured air temperatures that were too high. In addition, ponding resulted in a couple of the strings melting out during summer (Quebec 1, Baltimore). |
Measuring
ice growth and decay was decidedly a
low-tech
operation. We used a combination of an ablation stake and a hot-wire
thickness gauge. The ablation stake was a 3-m-long wooden stake, painted
white with metric tape. The stakes were typically installed with 1.5 m
frozen in the ice and the other 1.5 m in the air. Adjacent to the ablation
stake was a hot-wire thickness gauge. This gauge consisted of stainless steel wire with a steel rod attached on one end for ballast and a wooden handle on the other. The stainless steel wire was hooked to a generator that was also connected to a copper wire grounded in the ocean. The current would melt the wire free and the handle was pulled upward until the steel rod hit the bottom of the ice. The handle position was read off the ablation stake, giving the position of the ice bottom. Accuracies of stake and gauge readings were typically 1 cm. In some cases, gauges gave erratic reading, due to ice blocks on the ice bottom. A few gauges were crushed in pressure ridges, and in some cases the rod was frozen into the ice. During summer, several of the ablation stakes in ponds melted through the ice. The thickness gauge results are beta data - refinement is underway.
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