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“Coupling” up: A new tool for predicting snow squalls
Winter travelling in Atlantic Canada can be a tricky business. Freezing rain, blizzards and high winds can pose some challenges. Perhaps one of the most disliked winter weather phenomena is a snow squall: that little winter “storm” that seems to come out of nowhere.
Snow squalls happen when cold air blows over a warmer body of water such as the Gulf of St. Lawrence or Bay of Fundy. Traditionally, snow squalls are confined to a small geographic area – sometimes a short stretch on the highway – but they are intense and can often result in accidents.
No one understands better the challenges that snow squalls can pose than the meteorologists at Environment Canada’s storm prediction centres in Dartmouth, Nova Scotia and Gander, Newfoundland and Labrador.
“Predicting the cold winter air mass that will give rise to snow squalls is quite easy, but forecasting the location, duration and snowfall amounts associated with these clouds is very challenging and can change rapidly depending on changes to wind and ice conditions,” says Dr. Chris Fogarty, a past researcher with Environment Canada’s National Lab for Marine and Coastal Meteorology in Dartmouth.
Environment Canada forecasters now have access to new information to help them better predict where and when snow squalls may occur thanks to a new approach that combines an atmospheric model with a dynamic ice-ocean computer model. Originally created for use in the Gulf of St. Lawrence, the combination of these two separate pieces of forecast modelling is called a coupled atmosphere-ice-ocean forecasting system or a “coupled system” for short.
Serge Desjardins is a senior research meteorologist with the National Lab in Dartmouth. He and his colleagues – including Chris Fogarty – spent five years test-driving the coupled system and evaluating its usefulness for meteorologists in Dartmouth and Gander.
“We have compared the way we observed ocean temperature, atmospheric moisture and wind in the past with how the coupled system presents the same information, and the results show that the coupled system simulates reality more accurately. That results in real improvement to our forecast ability,” says Desjardins.
The coupled system allows forecasters to “see” the development and movement of sea-ice, allowing them access to a more complete and accurate simulation of the moisture and heat exchange between the atmosphere and the ocean. That’s important, since when the ice breaks up, the warmer ocean water is uncovered. This releases moisture and heat into the atmosphere creating conditions for snow squalls.
Chris Fogarty explains: “Atmospheric models that are “uncoupled” with the ocean – where the system does not account for changing ice cover and water temperatures while the forecast model is being run ─ can predict the presence of snow squalls, but typically fall short of predicting the amount of snow from them. When the atmospheric model is run in a coupled system it uses the extra moisture and heat, created in areas of open water, in its calculations. This allows forecasters to more accurately predict the intensity and actual snowfall amount from snow squalls.”
Serge Desjardins adds: “The coupled system also shows when ice is forecasted to move closer towards the coast, which can actually stop snow squall activity. The uncoupled model can’t do that.”
This “coupled system” has the potential for use on a much broader, Canada-wide scale. The Arctic is a great example where this advancement in forecasting ability – as it relates to the interaction between the ocean and the atmosphere and its effect on local weather patterns – will have tremendous implications.
“This model will be very beneficial for predictions in the Arctic,” says Desjardins. “With a coupled system, forecasters from the Canadian Ice Service will be able to better predict the thickness of sea-ice in shipping lanes for instance. That has implications for not only commercial traffic but future Arctic exploration as well.”
Desjardins and Fogarty do not hesitate to correct the perception that weather forecasting is an exact science. It is more accurately a combination of the right tools (radar, satellite imagery, 3-D modelling, etc.) and a forecaster’s knowledge and experience. That’s why the coupled system is so valuable. It gives forecasters one more tool to aid them in the prediction of weather events like snow squalls, allowing them to deliver constantly improving services to Canadians.
“That’s our goal in the National Lab,” says Desjardins. “We use science to help Canadians every day. That feels good.”
The coupled system was initially developed from a research collaboration between Environment Canada's Environmental Numerical Weather Prediction Research Division, the Department of Fisheries and Ocean’s Institut Maurice Lamontagne, the Institut des Sciences de la Mer from Université du Québec à Rimouski and the European Centre for Research and Advanced Training in Scientific Computation (France). Since its initial development, the coupled system has been tested at the Atlantic Storm Prediction Centre in Dartmouth, the Newfoundland and Labrador Weather Office in Gander, the Quebec Storm Prediction Centre in Montreal and the Canadian Ice Service in Ottawa.
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