In Argentina they’re called zonda; in Europe they’re known as foehn; New Zealanders call the phenomenon the Nor’west arch.
In southern Alberta, the warm, dry wind that blows down the Rockies’ eastern slope is called chinook.
And while the weather pattern associated with the chinook is well known to inhabitants of southern Alberta, very little is known about the effect chinooks have on the region’s freshwater resources. The only Chinook research was conducted in the city during the 1980s and ‘90s by University of Calgary professor Lawrence Nkemdirim.
“There has never been an intensive study of what happens to snowpacks during chinooks,” says Canmore’s Dr. John Pomeroy, Canada research chair in water resource and climate change with the University of Saskatchewan’s geography department.
“Water resource managers have long had trouble closing the water balance for the chinook zone of southwestern Alberta. It’s difficult to relate the amount of snow accumulation to streamflow generation in this part of the province. The chinook wind is credited with removing this missing water through evaporation (sublimation) of snow on the ground.
“We need to test to see if this snow is really evaporating, or if it is melting and entering the soil and groundwater.”
Since mid-January, Matt MacDonald, a PhD candidate with the University of Edinburgh’s School of GeoSciences, has been investigating these questions. Located in a clearing in Bow Valley Provincial Park, MacDonald’s research site consists of a 5.2-metre high tower equipped with several instruments.
Securely attached to the tower are a 3D sonic anemometer, which measures air temperature and wind speed from three directions, and a krypton hygrometer which measures rapid fluctuations in water vapour (evaporation and sublimation). Combined, the two instruments can estimate vapour transfer and heat transfer between the air and the ground/snowpack.
A snow stake records snowpack temperatures at different levels using thermocouple wire. Other instruments measure temperature and relative humidity, incoming and outgoing longwave radiation, incoming and outgoing shortwave radiation, and wind velocity. Below the ground surface, three ground heat flux plates measure the heat transfer between the ground and the atmosphere, or between the ground and the snowpack.
Soil moisture and temperature are measured at three levels below ground, between two and 30 centimetres; the one at two centimetres’ depth records evidence of ground thaw or infiltration of snow melt water. MacDonald also measures snow depth and density manually the day before each chinook arrives.
“This is the perfect location to study chinooks,” MacDonald said. “It can be very cold and calm, but then very windy and warm during chinooks. And the eddy covariance technique instruments require a couple of hundred metres of unobstructed, fairly level terrain.”
A second site, accessible by a three-hour snowshoe hike at the Barrier Lake fire lookout, monitors the upper air meteorological conditions. Setting up his sites in November and December presented several challenges, including frostbite concerns as MacDonald and research technician May Guan installed metal instruments in cold temperatures. Writing computer programs for logging data for the first time presented MacDonald a new set of challenges.
Essential ingredients for a chinook include a large water supply -– an ocean - coastal mountains and winds that blow perpendicular to the mountains. During a chinook, the wind blows eastward from the Pacific, replacing the region’s usual arctic air mass with a Pacific air mass. As the Pacific air mass rises over the mountains, it cools. Once cool enough, it reaches its dew point (determined by the temperature and humidity), and precipitation falls. When it falls, the air gains sensible heat and the air mass starts descending on the Rockies’ eastern slope.
“During a chinook, we’re not sure what’s happening to the water,” MacDonald explained. “How much snow drifts to other locations? How much snow is sublimating? How much snow is melting and infiltrating the ground? Does the ground thaw? Does the meltwater pond? How much meltwater evaporates?”
These occurrences, he explained, can significantly affect southern Alberta and Saskatchewan’s freshwater supply, which originates in the Rockies.
“If snow is melting and infiltrating, then that water may reach rivers eventually. Snowpack left on the ground, including drifts, can also eventually be a water supply. But if it’s sublimating or meltwater is evaporating, that water supply is back in the atmosphere.”
By March 4, MacDonald’s highest temperature recorded was 11 C on Jan. 27; the lowest was -35 C on March 1. The greatest fluctuation in a 15-minute period was four degrees on Feb. 20 at 6 p.m., the greatest one-hour increase was 11 C on Feb. 19. Also on Feb. 19, the temperature rose 21 C in three hours. The highest wind speed was 54 kilometres per hour.
This winter’s roller coaster weather has provided lots of data, which MacDonald will spend the summer analyzing. He’ll also use numerical computer models to simulate snow melt, blowing snow, snowpack sublimation, evaporation and infiltration, and the development of patchy snow cover by building or modifying algorithms to predict these things.
“This winter, it seems we’ve had a chinook, then it’s been cold and snowed a few days, then another chinook, and the pattern repeats,” MacDonald said. “The typical pattern is that at the onset of a chinook it is still cold, below zero, but warming. There are very high wind speeds, and dry snowpack results in lots of blowing snow and sublimation.
“Later on, as it becomes warmer, there is melt and more sublimation. At the end of a chinook there is a patchwork of snow and exposed ground or grass - a mangy-looking snowpack. If it’s quite a severe chinook, the patches have high liquid water content. Then, as the chinook passes, these patches freeze and become very icy and hard. That is stored water. These hard snow patches persist from chinook to chinook. When snow melts, it can become liquid water on the ground, while some may evaporate. Some may re-freeze within the snowpack.”
As for the subsurface, MacDonald said he’ll study that element next winter, but thus far, there doesn’t appear to be much infiltration.
“Matt’s study is a highly sophisticated study of the transport of heat and water vapour between the atmosphere and the snowpack during chinook,” said Pomeroy, MacDonald’s co-supervisor. “He’s learned that blowing snow sublimation is an important part of the snow removal that was not really recognized before.”
Everyone in the region should be interested in the results of this research, Pomeroy added.
“We need to better understand Alberta’s water resources because we are using close to the maximum that we can in the Bow River without damaging the aquatic ecosystem,” Pomeroy said. “This means that we must precisely manage our water resources, and to do this we have to understand how the hydrology of our headwater watersheds functions.
“In southwestern Alberta, that means understanding the chinook and how it influences our available water supply. This will also help us to predict future water availability under climates that have been changed by humanities’ emission of greenhouse gases.
“And it will help us to better model the hydrology and water supply of the region and to better predict floods, droughts and the impacts of forest management and climate change on water resources.”
