How does New Zealand’s snow ‘stack up’ internationally?
We're about to find out
NIWA is participating in a World Meteorological Organization (WMO) experiment, involving 16 countries, to compare methods and instruments for measuring solid precipitation (snow and ice).
NIWA's Snow and Ice Network (SIN) station at Mueller Hut, near Aoraki-Mt Cook, has been continuously recording snow data since March 2010. Located 1800m above sea level (about half as high as Aoraki-Mt Cook's summit), Mueller will become a field reference station for the WMO Solid Precipitation Inter-Comparison Experiment (SPICE).
Precipitation gauges go on trial
One objective of SPICE is to characterise two differently configured automated reference gauges (Geonor T-200) for the continuous measurement of solid precipitation.
These snow gauges continuously weigh the precipitation collected in a weighing bucket suspended within the gauge.
Historically, these types of gauges have proven reliable: they have no moving parts, are almost unaffected by temperature and have negligible drift. They accurately measure changes as small as 0.1mm of precipitation.
To assess the effectiveness and accuracy of an instrument, it must be compared against a superior (reference) instrument exposed to the same conditions.
We'll compare data from the Geonor snow gauges against that recorded by the co-located SIN reference station. SIN data is regularly calibrated with real physical snow samples, collected manually during routine snow surveys.
How the reference SIN station works
The Mueller SIN reference station measures snow depth and density.
The reference snow depth sensor transmits ultrasonic pulses downward towards the snow surface, measures the time it takes the echoes to return, and uses this measurement to determine the distance to it. As snow falls, this distance decreases and the data logger calculates the depth of the snow as it accumulates.
The reference station uses a snow pillow to weigh the snow. A pillow is three metres in diameter and filled with a food-grade glycol solution that remains liquid at very cold temperatures. It transmits the pressure, from the weight of the snow, to an electronic pressure sensor. We record this pressure and convert it to weight.
From the depth and weight of the snow, we can calculate its density. This indicates how much water is stored in the surrounding snowpack and its potential capacity to generate hydroelectric power or cause flooding and, over the long term, will indicate the effects of climate change.
Adding SPICE brought challenges
Integrating SPICE with the Mueller SIN station was challenging.
Firstly, we had to mount the instruments above the highest-expected snow level. We mounted the three items of SPICE equipment on 2.5m-high lattice pedestals bolted into the bedrock.
Secondly, we needed lots of power for heating. The snow gauges have heaters to prevent the gauge-opening from becoming capped with snow. These need much more power than is available from the existing solar supply. Unlike the heaters, the SIN station itself requires very little power.
We turn the snow gauge heaters on for two minutes, every 15 minutes, when the air temperature falls below 2oC. We'll investigate how gauge heating affects precipitation catch efficiency and measurement accuracy.
Generating solar or wind power in an alpine environment is difficult, so we opted for a more sophisticated solution – a compact methanol-powered fuel-cell.
Clean power for 'green' snow-measurement
The fuel cell converts the chemical energy stored in methanol fuel into the electrical energy needed to power the snow gauge heaters and SPICE electronics.
Two sealed 28-litre cartridges of methanol will power SPICE for around two months, but will depend on how much heating is used. Because of the uncertainty, we monitor the methanol level to indicate when we need to visit the site to refuel.
The fuel cell uses methanol (the simplest alcohol) and oxygen (from the air) to produce electricity. It's efficient and environmentally friendly; its exhaust contains only a small amount of warm water vapour and carbon dioxide.
Some pre-commissioning trial data
This pre-commissioning graph provides a glimpse of how the Geonor snow gauge tracks the reference snow depth during a late August snowfall event. Orange line = Geonor, Blue line = reference.
The next step
The site will be formally commissioned following a rigorous end-to-end process to verify the installation, methodology, data quality and security of the data storage process.
After collecting data over at least two full winters, NIWA will assess how well the equipment worked, providing data sets to WMO to show how closely the snow gauges agreed with the reference.
This will be a significant step forward in the world-wide standardisation of practical and effective instrumentation and processes for collecting high-quality solid precipitation data.