TO CATCH A SNOWFLAKE
Bert Davis speaks about the snow lab on Mammoth Mountain
The 2019 SNARL (Sierra Nevada Aquatic Research Laboratory) seminar series opened on Tuesday night.
Bert Davis spoke. “A staple of SNARL,” Director of Valentine Eastern Sierra Reserves Carol Blanchette called him. He’s also the Senior Scientific Technical Director, Cold Regions Research & Engineering Laboratory, and Chief Scientist, Geospatial Research and Engineering at the US Army Engineer Research and Development Center.
He’s studied snow in the eastern Sierra for almost 40 years.
“We can tell what’s going at the top [of the snow]. We can tell what’s going on at the bottom but there are still mysteries of what’s going on inside without having to dig,” said Davis.
He stood beside the lectern. He walked in front of it, leaned on it, but never went behind it. He looked at graphs and slides like old friends. Notes … he didn’t need them. He implored the audience to look. He acknowledged a graph being confusing. It felt like a conversation. It was a lesson.
The lesson began with an overview of how energy exchange affects snowpack and how a snowflake forms and changes, called snow flake metamorphism.
Snowflake coarsening, snow densification, water moving through the snowpack are all factors that effect the density of a snowpack. To calculate snow water equivalent, the snowpack’s density must be known.
Solar radiation, long wave radiation, latent heat and sensible heat affect snowpack.
From above, solar radiation, sunlight, and long wave radiation hit the snowpack and are reflected back into the atmosphere. Wind, rain, and snow also affect the pack. Dirt on the surface affects the reflectivity of the snow and makes it melt faster. Sublimation, the process of a solid becoming to a gas, occurs at the surface.
From within, vapor diffusion occurs across temperature gradients. Grains of snow conduct heat.
From below, energy is conducted between the soil and the snow.
Snow science 101 concluded and history started.
In the late ‘70s, Carl Martin, Pat Armstrong, Jeff Dozier, Danny Marks and Davis met with Dave McCoy to ask if they could get power to the area next to the pond next to McCoy Station.
“We didn’t have a lot of money but we had a lot of do-it-yourselfness,” Davis said.
They had two towers one 16 feet, one 20 feet that they could crawl down.
They were pioneers. They wanted to know everything.
A tent was shelter.
The goal of that first station was to measure all the energy exchanges that affect the snowpack.
“And that’s where we failed,” Davis spoke. The scope of the project was huge.
First on the list was solar radiation. A slope’s aspect, the cardinal direction a slope faces, affects how much sun hits the snow.
They wanted to figure out the snow’s water content which is called snow water equivalent. Snow water equivalent is the depth of water snow would produce if it was melted all at once.
“That’s the money term for water resources,” Davis said.
Measuring energy exchanges on the surface was relatively easy. Radiometers measured, and still measure, solar radiation. They measured temperature and humidity. They blew air over those sensors to keep them from overheating from the sun. They measured wind with an anemometer.
Measuring inside the snowpack proved difficult. They used snow pillows to weigh snow. It’s a big rubber pillow filled with antifreeze. It acts like a water bed.
Lysimeters in the tower measured water loss through evapotranspiration.
They wanted to look inside the snowpack. They built a tower from metal pipe from which the sensors would hang out to the side and into the snowpack.
“The first year in 1979 it got squashed flat.”
Catching snow proved difficult. Snow sticks to itself and covers tube openings. It was easier to measure snow that fell on the ground. It wouldn’t come to them, so they went to it. They used snow depth sensors measure snow depth by emitting a ‘Chirp’ and measuring the time it takes for that sound to return to its source.
They adapted. The instrument that let them see inside the snowpack was the shovel. They dug and dug. The holes and pits allowed them to remove samples, which they weighed to help them calculate density. They multiplied the density of the sample by the thickness of the layer from which the sample was taken to calculate the snow water equivalent.
They tried measuring snowpack temperature by hanging strings with temperature sensors on them into the pack. The sensors measured air temperature of the holes created by the string hanging down and not the snowpack temperature.
They waved the white flag. “That was pretty much our last attempt at continuously monitoring much of what’s going on in the pack with things in the pack.”
Despite the failures, they found out that solar radiation accounts for 60-90% of the energy to melt the snowpack. Evaporation removes energy; wind adds energy; they pretty much offset. But in some years, evaporation can remove up to 20% of snow water equivalent from a pack.
They wanted to measure snow remotely, using microwaves. They measured microwaves emitted from ground with a radiometer. Snow blocks the microwaves from the atmosphere. To figure out how much snow is on the ground, figure out the amount of energy, microwaves, coming from the ground without snow. Then figure out how much energy (microwaves) are coming through snow and they could calculate the depth of snow by measuring energy.
In 1987, year McCoy told them that he was going to build a lift through the site and they’d have to move it. The site moved to the knoll behind McCoy Station. It was an upgrade.
They sunk a shipping container halfway into the ground. They stuck a lift tower tube out of it that goes two stories up to a platform.
Nowadays, satellites can map snowmelt of the entire mountain ranges.
They added ability to measure depth of snow without digging by shooting a laser that can shoot through snow.
“Our big break.” “The range finder.” It is accurate to within two millimeters.
The Mammoth snow study site has pioneered remote sensing of the snowpack.
