Monday, February 7, 2011

The Puget Sound Convergence Zone and Convection

Ok--I was going to do a post about the large scale pattern in upper-air features over the past two weeks and how that will hopefully start breaking down by the end of this week.  But then on the way back from the University today, I started getting text messages from people...

"Was that hail that just fell?"  "Hail?  Graupel?"  "Is it supposed to snow today?"

Hmm...we were in the upper 40s today in terms of temperature at the surface...I was pretty sure we weren't expecting any kind of frozen precipitation...

But sure enough, when I turned on this evening's news, their lead story was about the downpour of hail that hit Seattle this afternoon.  Interesting...  So I fired up the radar and cycled back to the time of the report.  The radar showed this:
Fig 1 -- KATX 0.5 degree base reflectivity at 0048Z, Feb. 8, 2011.
The little cell I point out with the red arrow is the closest I could come to something looking remotely like convection going on.  A cross-section through the radar returns in that cell is also not very impressive:
Fig 2 -- KATX base reflectivity cross section through the cell identified in figure 1.
There is a small core in the middle of the cell with some vertical coherency.  The reports said the "hail" was only pea-sized or smaller (hardly worth calling "hail" in my opinion...) and as such we wouldn't expect to see THAT large of a reflectivity return from the hail compared with the surrounding rain.  The top of the core only makes it up to an estimated 5000-6000 feet too--not very tall.  But apparently this was vigorous enough convection to produce some small hail.  It's too bad our local sounding site is way out at Quileute out on the Pacific coast so our upper-air profile over Seattle is a bit of a mystery...

So how do we get convection in Seattle?  This place usually does not get the strong fronts and temperature gradients nor the instability aloft needed to produce deep convection.  So we live with shallow convection.  Still...there has to be a mechanism to provide the lift necessary to get convection going.  This is where the Puget Sound convergence zone comes in.

To describe this phenomenon, we first need to focus on the geography of the Puget Sound area.  To the east of the city of Seattle and Puget Sound lie the Cascade Mountains, running north to south.  However, to the west of Puget Sound lies the Olympic Peninsula and the relatively high, but isolated Olympic Mountains.
Fig 3 -- Topographic map of western Washington.  Higher elevations are shown in the tans and browns.  The orange colors indicate urban areas.  To the west of Seattle lie the Olympic Mountains.
Puget Sound convergence zones set up when the winds in the low levels of the atmosphere have a strong westerly component.  Let's consider what happens to low level winds out of the west once they reach the western shore of Washington.  Immediately they are confronted by the isolated high terrain of the Olympic Mountains.  All that air rushing in has two choices--it can either rise up over the mountains or go around them.  Now, we do see a fair amount of lift on the windward side of the Olympics--the Hoh rainforest lies on the western side of the Olympics for that very reason.  But generally the atmosphere suppresses large-scale rapid vertical motions like that (there are many reasons for this, but we'll just accept that for now).  Instead, much of the air is forced to go around the Olympic Mountains.
Fig 4 -- Schematic of westerly flow separating to go around the Olympic Mountains.  A lee-side low is formed.
However, when all that air splits to go around the mountains, what happens on the eastern side of the mountains?  All the air flowing around the mountains creates a sort of void in the atmosphere immediately behind the mountain range.  This is signified by a lowering of the pressure there.

But what does the atmosphere do in response to a lowering of pressure?  Air will rush in to try and fill this "void" that has been left on the eastern side of the mountains.  As a result, the wind that split to go around the mountains will be sucked back toward the low pressure that has formed on the lee side of the mountains.
Fig 5 -- The lee-side low draws the winds inward on the lee side of the mountains.  As the winds from the north and the south meet, a convergence zone is formed.
Of course, air is being sucked in toward the low pressure from both the north and the south.  When these two wind streams meet, there's a region of rather strong convergence (indicated by the dotted line in the figure above).  This convergence can provide enough lift to get shallow convection over Seattle and form small hailstorms like we saw today.  So that's the Puget Sound convergence zone in a nutshell.

So did we have those kinds of conditions today?  You bet we did.  But with a slight twist.  Here's the forecast 925 mb (low-level) chart for 00Z this evening (near the time when the hail over Seattle occurred):

Fig 6 -- UW 4km WRF 12 hour forecast of 925 mb  temperature (shaded) and winds (barbs) at 00Z, Feb. 8, 2011.
We can see here that off the Washington coast, the winds were out of the northwest.  This isn't straight out of the west, but you can imagine that a similar effect occurs.  In this case, the convergence zone would form further south than indicated in the diagram above and would also be oriented in a northwest-to-southeast direction.

Fig 7 -- Same as in figure 5 but adjusted for more northwesterly winds.  The convergence zone is located further south and oriented from northwest to southeast.
So we'd expect to see a convergence in our low-level winds south of Seattle somewhere, though the location tends to meander with time.  What I showed above is a model forecast for 925mb winds.  Do we see this convergence reflected in the observations?  Here's the surface map of observations from 0100Z this evening (about the time of the hail in Seattle):
Fig 8 -- Surface METAR observations from western Washington at 0100Z, Feb. 8, 2011. 
I've added several blue arrow roughly paralleling the wind barbs in those areas.  We see a clear flow of wind around the Olympic Mountains and decent convergence at the surface right through the southern part of Puget Sound (the dashed red line).  Also, the lowest pressure in the area (1022.8 mb) is the observation at Shelton, which I circled in orange.  This is pretty close to where we would expect see that lee-side low pressure form.  So--this is a classic convergence zone case.

Furthermore, we can look at the radar radial velocities from this time to see convergence there.
Fig 9 -- KATX 0.5 degree radial base velocities at 0101Z, Feb. 8, 2011.  Arrows showing the rough direction indicated by these colors demonstrate convergence.
The KATX radar is in the northern part of this image.  If we remember that green colors indicate air moving toward the radar and red colors indicate air moving away from the radar, we can see convergence right along that line we were expecting to find it in figure 8.  So the radar velocities also show some convergence there.

It gets more complex though--a second convergence zone seems to be forming across the northern Puget Sound area and the Strait of Juan de Fuca.  Can we have dual convergence zones?  Interesting possibility.  The surface winds don't show as clear of convergence there.  Could this be convective instability released in a direction parallel to a jet over open water (similar to a lake-effect snow band coming off of the Strait of Juan de Fuca)?  Perhaps.  More investigation would be needed.  But that's about all I can cover in one blog without going too long... 

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