Friday, September 16, 2011

Extratropical Transition

Hurricane Maria is currently up near Newfoundland and is undergoing a period of its life known as the "extratropical transition".
GOES-E wide view infrared satellite image from 1615Z Sept. 16, 2011.  Maria is in the top right corner.
What does this mean?  In the classification of large-scale cyclones, we usually think of two broad classes: tropical and extratropical (that is, "outside the tropics").  Alternatively, you could consider these warm-core cyclones and cold-core cyclones.  If we go way back to my fourth blog post ever, I talked about hurricanes as warm-core cyclones.  Right now, Maria is transitioning from a tropical, warm-core cyclone to an extratropical, cold-core cyclone.

How can we see this?  Well, for one thing, the structure of the cyclone on satellite has changed dramatically.  Notice how Maria (in the upper right corner of the image above) no longer looks like a symmetrical storm with an eye in the center.  It looks far more comma-shaped, with a long, trailing band of convection stretching down to the south.  That band marks the position of a front, which is our first bit of evidence.

Extratropical cyclones have fronts.  Tropical cyclones do not have fronts.

As the hurricane moved north, it moved away from the tropics--where the temperature is basically warm everywhere. Instead, it moved up north and began encountering the polar front -- the divide between the cold polar air to the north and the warm subtropical air to the south.  As the swirling winds around the cyclone encountered this temperature gradient, they started wrapping cold air around the western side of the storm.  And, with that, a front was born.

Here's the GFS analysis of 850mb virtual temperature (just think low-level temperatures) over the region.
GFS analysis of 850mb virtual temperature valid 12Z, Sept. 16, 2011.
Keep in mind Newfoundland off the east coast of Canada as the location of Maria.  You can see how this tropical cyclone has brought up a lot of warm air from the tropics as it moved northward (it also helps that the Gulf Stream current is right there along the east coast, too).  But the storm is now running into much colder air to the north and west.  In fact, the formation of the strong cold front in the Atlantic that we saw on satellite imagery isn't exclusively due to the hurricane--that front actually started out as the leading edge of the very cold air that has sunk down over the central part of the US this week.  As the cold front at the leading edge of the cold air moved eastward out over the Atlantic, it was picked up by the circulation surrounding Hurricane Maria, helping to convert that cyclone into a cold-core, extratropical type storm.

We can also see that Maria is in an intermediate phase by looking at upper-air soundings.  Here's this morning's sounding from Stephenville, Newfoundland:
12Z sounding from CYJT on Sept. 16, 2011.
This sounding has some tropical cyclone characteristics and some extratropical cyclone characteristics.  Notice how high the tropopause height is--that's the height where the temperature stops cooling with height and abruptly starts warming.  On this sounding it's at just above 200mb--unusually high for some place that far north (for comparison, this morning's sounding in central Quebec had a tropopause at about 250mb).  Furthermore, in extratropical cyclones, the tropopause tends to drop closer to the ground as the trough moves overhead--not remain high. 

Also, notice how the temperature profile is nearly moist adiabatic all the way up to the tropopause--the air is nearly saturated all the way through the troposphere.  Once again, this is far more typical of tropical cyclones than extratropical cyclones, where usually only the near-surface layer(s) tend to be saturated with moisture.

However, the wind structure tells a different story.  In a mature or maturing tropical cyclone, the lower part of the storm rotates cyclonically while the outflow in the upper part of the storm rotates anticyclonically.  It's a high pressure area on top of a low pressure area, basically.  As I explained in that blog post I liked to above, this is a characteristic of a warm-core cyclone.  However, if this were happening, then the winds in the top half of the storm would be blowing in the opposite direction as the winds in the bottom half of the storm.

But we don't see that above!  Instead, we see that the winds are all blowing in pretty much exactly the same direction.  This is more typical of a dying extratropical cyclone than a tropical cyclone.  So, the wind field is starting to show evidence of this extratropical transition as well.

The exact mechanisms involved in extratropical transition are still not well-understood--how does a storm flip its structure like that while not tearing itself apart? It's an active area of research, and hopefully we'll understand it better some day soon.

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