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What’s with the rain? Precipitation expert explains.

August 25, 2014 — 

Winnipeg and surrounding areas recently received biblical torrents of rain, much of the city flooding on Aug. 21.

Precipitation extremes is the research focus of Ronald Stewart, department of environment and geography at the University of Manitoba. UM Today asked him to explain the recent downpour through his research paradigm, and if this may be, perhaps, linkable to climate change.

He says:

We experience all kinds of extreme weather here, and it can occur in any season. This extreme rain event was linked with a shift in atmospheric forcing from our prolonged hot and dry conditions to one with somewhat cooler temperatures and more rainfall.

It is common for extreme precipitation to occur in such shifts. In this case, a slow-moving frontal feature led to substantial rainfall all along its extensive length, but there is always a great deal of variability in the actual amounts of rain. This variability is linked with pockets of embedded convection and associated circulations that preferentially produce large amounts of rainfall in particular locations along the frontal feature. Such convective pockets can develop very quickly. Rainfall amounts on Thursday evening were accentuated because of the slow moving nature of the front, and its associated pockets of convection that happened to pass over some parts of the city.

This storm is somewhat reminiscent of a torrential rainfall event in Toronto during the summer of 2013.  A slow-moving storm system and an embedded small convective pocket were the key factors in that event as well. We intend on examining the similarities and differences between our Thursday evening case and the Toronto situation.

It is not yet clear as to whether the Thursday night event could be linked with our changing climate. However, the increased likelihood of slow moving systems is one expected consequence of climate change. But there is considerable uncertainty as to the factors leading to pockets of much heavier rainfall and the degree to which these may be changing.


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