Monday, January 28, 2013

Cold Fronts and the Orographic Ratio

There have been a few events this year during which the enhancement of snowfall over the mountains has been limited.  Some have started to call these "upside down" storms, although being a powder skier I prefer to use the term upside down to describe storms in which the snow density increases with time.

Meteorologists typically refer to the ratio of mountain to lowland precipitation as the orographic ratio.  In a storm in which the mountain and lowland precipitation is the same, the orographic ratio is one.  If the mountain precipitation is four times the lowland precipitation, the orographic ratio is four.  In an upside down storm, the orographic ratio is less than one.

On average, the orographic ratio between Alta and Salt Lake City during January is nearly 5.5.  This is the highest average orographic ratio of any month during the year and illustrates that on average January features the greatest enhancement of precipitation by the Wasatch Range.  

This is, however, the average, and the orographic ratio can vary dramatically from storm to storm and even during storms.

Most research suggest that the smallest orographic ratios along the Wasatch Front occur during frontal passages.  In part, this is because the front provides most of the lift for precipitation formation and the mountains play a less prominent role.  In some instances, the front can be fairly shallow and the storm dynamics are actually better over the valleys than over the mountains.

For example, during the November 2001 Hundred Inch Storm, there were two storm periods.  In the second, the orographic ratio was smallest (and equal to 1.3) during the frontal passage, indicated by the frontal storm stage below.  Note how Alta received only 1.3 times as much precipitation as Salt Lake during the frontal passage.

Precipitation at Alta and Salt Lake City during the second storm period
of the Hundred Inch Storm.  Stable and UPF indicate pre-frontal precipitaiton
stages, frontal the passage of the cold front, and PF1, lakeband, and PFII
post-frontal storm stages that included lake-effect precipitation.
Source: Steenburgh (2004)
In contrast, larger orographic ratios typically occur during pre-frontal periods if there is a lot of dry air in the low levels (which causes precipitation to evaporate before reaching the valley floor) and during post-frontal periods when lifting by the mountains is essential for generating precipitation.

That being said, these are generalizations and there are a lot of factors that affect precipitation.  Producing models that can better predict the distribution of precipitation is an area of ongoing research and one in which we sorely need advances to help us anticipate everything from deep powder to commuter disruptions.

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