Tuesday, November 2, 2010

Curved Jet Streaks--They Do Work!

I figured I might as well start posting some of my thoughts on this blog as that's why I created it in the first case.  I understand some of this may be on a bit more technical level than a post geared toward the general public, but I'll try to provide a mix of different levels to satisfy all who are curious about random weather features.

Fig 1--GOES W Water Vapor Channel from 500Z, Nov. 2, 2010

Today we're focusing on satellite water vapor imagery over the central Pacific from this morning.  A shortwave trough is rather well-defined in this first images, where a quick streamline analysis highlights cyclonic flow around the base of the trough.  Note the relatively "dry" streak along the base of the trough.  This is indicative of a relative wind maximum in the upper-level winds whose moisture content is being sampled by this satellite image.  Such a maximum is commonly called a jet streak.
Fig 2--GOES W Water Vapor Channel from 900Z, Nov. 2, 2010
Fast forward four hours.  Figure 2 shows the same trough (whose axis is denoted in red) with a single streamline shown in yellow to remind us of the flow pattern. Let's clarify a few things from above.  This jet streak is "cyclonically" curved--what does this mean?  You can see in the above image that the wind through the jet streak is indeed curving. We can imagine that if it were to keep curving, it would eventually loop back on itself.  If we think about  that circle for a moment, we'd soon realize that the air moving around the curved circle would be moving counter-clockwise.  For reasons that are even more technical than I want to get into here, in the Northern Hemisphere, counter-clockwise rotation is termed "cyclonic" (i.e. "like a cyclone") and clockwise rotation is termed anti-cyclonic.  In this case, we do indeed have a cyclonically curved jet streak (note also in figure 2 that the "dryness" in the streak has intensified--an indication of strengthening wind speeds, for more complicated reasons...)
Fig 3--GOES W Water Vapor Channel from 1200Z, Nov. 2, 2010
 Now we move three hours later.  One way to to analyze jet streaks is to divide them into quadrants of halves.  In cyclonically-curved jet streaks, the most convenient way to analyze them is to split them in half by drawing a line through the center of the wind maximum and perpendicular to the flow.  In the case above, this line is about where the trough axis (the red, dashed line) is indicated.  We can then talk about the entrance and exit regions of the jet streak.  The entrance region is where air is entering the jet--pretty straightforward.  This would be to the west of the dividing line through the jet streak.  The exit region, where air is exiting the jet streak, would be to the right of this line.  Now for our weather adage of the day...

"Divergence aloft (and consequently rising motion) is enhanced in the exit region of a cyclonically curved jet streak."

By this point you should be gasping in awe at the wonderousness of this statement.  Or maybe not. I'm not going to go into the details of why this is so here, but there is strong support for this statement and patterns associated with other jet streak geometries.  (An early description of some other examples of jet streak-lift associations can be found in R. Lee (1955)). But, we can see this statement in action right now for ourselves in figure 3.  Among other things, remember that rising motion over the ocean should bring very moist air from near the ocean surface up into the upper troposphere (usually through thunderstorms, but there are other ways...).  And here, in the exit region of our cyclonically-curved jet, we do indeed see increases in moisture aloft now as indicated by the green arrow.  Move a few hours later, and it becomes even more obvious:
Fig 4--GOES W Water Vapor Channel from 1600Z, Nov. 2, 2010
Now we see lots of water vapor being lofted up in the exit region of the jet streak.  So, keep this adage in mind when looking at winds aloft and trying to identify areas of rising motion (which is the key to any good forecast).  I would note in figure 4 here that while the trough axis is still represented by the red dashed line, our perpendicular halving-line through the jet streak must move with the wind maximum.  Based on the "dryest" location, this location seems to have shifted eastward and is now indicated by the yellow line.  How jet streaks "move" is yet another fascinating question.  For an interesting early paper on jet streak analysis with regards to their motion, try Riehl and Janista (1952).  For a very nice description of how temperature gradients determine jet streak location, see Patrick Marsh's blog post here.

I apologize for the technical-ness of this discussion, but...I thought it was cool to see this particular theory played out so obviously on the WV channel.  More fun weather observations will come!

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