Storm Warnings
YORK RESEARCHERS CHASE THE WORST MOTHER NATURE CAN DISH OUT

Story by Lydia Dotto
Photos By Horst Herget

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David Sills has been fascinated by severe weather for most of his life, but Mother Nature is playing coy with him.

                      The York University graduate, now a severe weather scientist with Environment Canada, spends part of his time chasing storms around the back roads of Ontario in his Neon, but he has yet to "capture" a tornado, although he saw his first one in Sydney, Australia last year. "I like to get out there to see the science in action and nature in its finest form," he said. "I've been interested in storms since I was a little kid. This is my dream job."

Sills, who received a BSc in '93 and his PhD in '98 in Earth and Atmospheric Science at York, now works at Environment Canada's Meteorological Research Branch, based at the King City Radar Facility north of Toronto. He and a colleague, Patrick King, have focused their research on an atmospheric phenomenon known as "lake breezes" that can trigger the development of thunderstorms and occasionally even tornadoes in southern Ontario. The work is aimed at improving the forecasting of these damaging storms. (Of the nine F4 tornadoes that have occurred in Canada, seven have been in Ontario. The Fujita tornado scale goes from F0 to F5. Canada has never recorded a F5 tornado.)

Lake breezes-winds blowing inland from the Great Lakes-are caused by temperature differences between lake water and land surfaces. "At the edge of this wind blowing inland, there's a boundary or front and you can often see clouds along this line, a line of clouds moving inland," said Sills.

If conditions are right, these lake breeze fronts can act as a "triggering mechanism" for thunderstorms and tornadoes. They provide the "lift" needed to push air high into the atmosphere where thunderstorms develop. "You need three ingredients for thunderstorm formation: atmospheric moisture, instability and lift," said Sills. "Sometimes you get the first two but not the last one. A lot of days in this area, the lake breeze provides the last one and gets the thunderstorm going." He said lake breezes occur frequently-about every other day between late spring and early fall-and they can penetrate more than 100 kilometres inland. Most occur in July and August. The size of the lake is a factor; the bigger it is, the stronger the effect.

Southwestern Ontario has the dubious distinction of having a "mini tornado alley", a wedge-shaped region extending from Windsor and Sarnia up to Barrie. "It's mainly Lake Huron and Lake Erie that cause this focusing of thunderstorms and tornadoes," said Sills. "Lake Ontario also creates lake breezes but, on days when hot, moist and unstable air streams up from the Gulf of Mexico, they tend to focus storms along a zone north of the lake. Toronto is often protected from some of the worst severe weather in these situations since the lake breeze passes north of the city."

One of the more intriguing aspects of recent research by weather scientists is the evidence that tornadoes may form by being "sucked up" from the ground instead of being "dropped down" from thunderstorms, as has been believed. "There's been a real rethinking of what's happening," Sills said. "The tornado research community is finding that things are not happening in the way we thought-they seem to be happening from the bottom up."

He explained that when lake air meets land air, winds start moving in different directions and create eddies or vortexes along the front. If a thunderstorm moves over the eddies, "it can stretch that vortex and spin it up to tornadic intensity."

Sills said these findings have "huge implications" for forecasting because the boundaries created by the mixing of lake air and land air can be detected at the surface with radar.

Sills is currently working on a training document for forecasters to help them understand when and where lake breezes can be expected to occur, how far inland they'll move and where the boundary is likely to form. This information is particularly valuable when the environment is "primed for big thunderstorms," he said.

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