Lake Breezes Linked to Severe Weather

The seemingly benign breezes that blow inland from lakes and rivers can have a considerable influence on the formation of thunderstorms, hail and even tornadoes, according to scientists with Environment Canada's Meteorological Service of Canada. A field study carried out by the Department and Toronto's York University this summer investigated an apparent link between breezes from the Great Lakes and severe weather in southwestern Ontario's "Tornado Alley".

The experiment on the "Effects of Lake Breezes On Weather", known as ELBOW, was a follow-up to a pilot study carried out in 1997 near London, Ontario, that was the first to look closely at the effect of lake breezes on severe weather.

Lake breezes occur because air over land warms up faster than air over water, creating an imbalance in pressure that causes low-level air from the lake to blow inland at a right angle to the shoreline, forcing the air over the land upward. In places where the shoreline juts outward toward the lake, the breezes themselves converge—creating an even stronger updraft.

These breezes, however, are not usually the only winds present. Normally, larger-scale high- and low-pressure systems in the atmosphere cause prevailing winds that blow at the same time. The lake breeze circulation and the prevailing winds converge along lines that can extend over 100 kilometres inland. Since there is an updraft along these convergence lines, there is also a strong tendency for clouds to form.

In southern Ontario, prevailing winds in the summertime often blow from the southwest—bringing the warm, moist air needed to form thunderstorms. Under these conditions, convergence lines develop primarily where the shoreline runs parallel to the direction of the prevailing wind—for example, along the northwest shore of Lake Erie and the northwest shore of Lake Ontario between Hamilton and Toronto. These convergence lines are not only capable of triggering storms on their own, but also of interacting with a cold front or colliding with other convergence lines to create particularly severe weather.

An examination of past tornado events shows that most significant twisters have occurred near where convergence lines form when the prevailing wind is from the southwest. This theory is illustrated by the absence of tornadoes on the north shore of Lake Erie east of St. Thomas, where the shoreline does not run southwest-northeast, and a proliferation to the west, where it does.

To get a better picture of the forces at work in this process, a number of observation platforms were deployed this summer between lakes Erie and Huron centred around Exeter, northwest of London. A network of 14 surface weather stations was set up along lines perpendicular to the lakes' shores. Temperature, humidity and wind speed and direction measurements were also taken from permanent weather stations at London, Sarnia and Windsor, as well as from upper-air radiosondes and mobile sources. Additional data were collected from two Doppler radars, two wind profilers and a research aircraft, and forecasts were issued using special ultra-high-resolution versions of the Global Environmental Multi-scale (GEM) numerical weather prediction model with 10- and 2.5-kilometre resolutions.

Despite drier-than-normal conditions, several storms yielded important data. On July 4, satellite imagery showed multiple interactions between a cold front and lake breeze convergence lines on the American side of Lake Huron. Outflow from this storm interacted with a convergence line on the Canadian side to form a supercell storm. Doppler radar captured a strong "hook echo"—a sign of a probable tornado—but no tornado was observed. On July 16, a rotating thunderstorm formed near Melbourne west of London, when convergence lines from Lake Erie and Lake St. Clair merged. Past occurrences in the same area support the theory that tornadoes may be more likely to form in such cases.

A closer analysis of the data from ELBOW 2001 will provide scientists with a better three-dimensional view of the atmospheric conditions leading up to lake-breeze-induced severe weather, and enable them to verify and improve models and techniques for forecasting this phenomenon. A related project is proposed to take place in 2003 in the foothills of the Rockies northwest of Calgary, where the mountainous topography causes small-scale air circulations that interact in a similar manner with prevailing winds.

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