I'm trying to learn and understand the symbols on the significant weather charts and I found a symbol for "trowal". But I have no idea what trowal means. Can anybody explain what it is and how it's relevant to aviation?
(Source: Wikipedia)
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$\begingroup$ trowal $\endgroup$– user14897Commented Jul 31, 2016 at 11:58
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4$\begingroup$ I'm voting to close this question as off-topic because it better belongs on EarthScience.SE $\endgroup$– SMS von der TannCommented Jul 31, 2016 at 13:05
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7$\begingroup$ I have never heard of a trowal, but if it is on weather map used for aviation I am interested. What is it? Why is important enough to be on the significant weather map. Does a trowal affect aviation? If not, why is it on the map? If it does affect aviation, why is it off-topic? $\endgroup$– DeltaLima ♦Commented Jul 31, 2016 at 22:35
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2$\begingroup$ @DeltaLima: A trowal is a thin occlusion of warm air ahead of a cold front meeting cool air. In that small warm atmospheric valley the aft cold front and the warm front ahead are very close. Further ahead of the trowal there is a high risk of icing, due to rain falling from the trowal thru a mass of cool air. Behind the trowal there is a risk of thunderstorm. Trowals seem to be more frequent over Canada. $\endgroup$– minsCommented Aug 1, 2016 at 1:07
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2$\begingroup$ @mins thank you. So the trowal is basically the line where the three air masses of an occluded front meet. It is up in the air and, when projected on the map has a different location as the occluded front. A cold occluded frontline on the surface would be ahead of the trowal while a warm occluded frontline is trailing the trowal. $\endgroup$– DeltaLima ♦Commented Aug 1, 2016 at 17:36
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1 Answer
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As a retired Environment Canada meteorologist I used trowal symbols on surface map analyses. The theory of how a trowal was formed was never proven, but a few decades ago a detailed upper air analysis of the air masses surrounding an occluding frontal system/low showed no warm air aloft where the trowal was supposed to exist. Apparently, the occlusion process mixed the cold and warm air masses leaving no detectable original warm air signal around the low.
The trowal was purely a Canadian feature on surface weather maps because it was thought that the more traditional occluded front represented only a weak temperature discontinuity that did not fit the associated weather pattern too well. Some analyses showed trowals extending from the decaying frontal vertex for hundreds of miles when no temperature contrast existed at the surface where an occluded front was supposed to be.
More research into the upper air features around occluding systems should eventually lead to new symbols replacing both the occluded front and trowal to better represent the physics for the sharp discontinuities and weather observed on satellite pictures, radars, and predicted by numerical models. The weather around these frontal lows is a lot more complex than can be represented by simplistic occluded fronts and trowals. I've noticed lately that Environment Canada's surface analyses have been dropping the use of trowals.
