Tuesday, November 9, 2010

Snowy, Stacked Low

Today I'm going to look at a weather setup from yesterday over the northeastern US that was labeled a "surprise" by some of the news outlets.  A weak cyclone that followed a typical nor'easter track moved into New England yesterday and brought some light snow to cities like New York and Boston.
Fig 1--Snow accumulations from November 7-8, 2010 from NOAA's NCDC.
We can see that many places in southern New England have seen about an inch of snow over the past few days.  Remember that long frontal boundary that had pushed off the east coast last week and was directing then-Hurricane Tomas away from the coast?  This low formed as a wave along that pre-existing frontal boundary, which is part of why it was able to spin up so quickly.

However, last night I was watching a news channel that was describing this storm and they mentioned that it was "strengthening" and "picking up steam".  Really?  This was around 00Z last night, so lets take a look at four soundings from the northeast at that time (click on the pictures for a larger version).

Fig 2--Sondings from 00Z, November 9, 2010, from Albany, NY (ALB); Chatham, MA (CHH); Portland, ME (GYX); and Upton, NY (OKX).  From the HOOT website.

One thing immediately clear, particularly in the Portland and Chatham soundings, is the low tropopause heights with this cyclone--around 550 mb in Chatham.  So this is indeed a rather deep low.  But is it strengthening or not?  Here's another adage from Synoptic Lab days that's good to remember:

A strengthing cyclone will exhibit a westward tilt with height.

This has to do with the thermal structure and jet-stream orientations surrounding the cyclone at various levels.  When we get a good example of this that isn't halfway out over the ocean, I'll elaborate more.  But for now, we'll just accept this adage and see how it applies to this situation.

We'll focus on the wind profiles in the above soundings.  The most obvious feature of these wind profiles is that there is very little change in wind direction with height.  There are varying amounts of speed shear, but directionally (in each individual location) there is not much change as we go up in height.  This is NOT consistent with a westward-tilting storm with height, for then you would expect the winds to be changing direction with height as well.  This is more indicative of what is called a "stacked" low, one where the center of the cyclone/trough is nearly over the same place at all levels of the troposphere.  Because the center of the storm is vertically coherent, the wind directions at all levels will also follow this same coherency.  Thus the soundings above represent a "stacked" low.

So if this low is stacked and not tilting westward with height, that seems to violate our adage above and we conclude that the storm must not be strengthening at this point.  Is there any more evidence for this, beyond this adage?  Yes--in the thermal wind (*moan**groan*...I know, it's the thermal wind AGAIN, but it's powerful!) (Even if it is a fictitious thing!).
Fig 3 -- Yet another thermal wind construction, this time with winds that change speed, but not direction, with height.  No temperature advection can be inferred from this arrangement.
Remember our thermal wind construction, where the thermal wind is the change in winds with height.  In all of our soundings above there is very little directional shear, but there still is speed shear.  Winds are generally increasing in speed with height, which does imply a thermal wind pointing in the same direction as the winds themselves.  But this brings up an important point: the mean wind throughout the troposphere is oriented in the same direction as the thermal wind. And, since the thermal wind is oriented parallel to the temperature gradient, there is no net temperature advection occuring.  Since an extratropical cyclone draws on temperature gradients for its source of energy (it requires a baroclinic environment, if we want to use a fancier term), then if there are no temperature gradients being advected around, there is no continuing source of energy.  Therefore the storm can't be strengthening anymore.

So there you have it--a minor comment by a news organization that wasn't quite correct at the time that they made it.  A stacked cyclone is not a strengthening cyclone.  Was the cyclone really stacked?

Fig 4 -- Upper-air objective analyses for 00Z, November 9, 2010.  At 300 mb, 500 mb, and 850 mb.  From the HOOT website.

You can see that all the way through the troposphere (and into the stratosphere, even), the center of the cyclone at every level was right about over Nantucket Island.  Very stacked indeed.

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