Friday, December 31, 2010

IOP 5: The big one?

With the new year comes a new cold air pool, and this one could be the big one we've been hoping for. More on that in a moment, but first I'd like to reflect a bit on what we've learned thus far through one month of operations that involved 4 IOPs and numerous sub-IOPs.
Potential temperature deficit for the month of December as a function of height and time.
Values are calculated relative to a dry adiabat from the mountain top crest level.

I think the most significant realization that we've come to is that cold air pools are essentially the climatological base state during the heart of the winter when the sun angle is low. And to that end, it is more the lack of solar heating than the prominance of nocturnal cooling that seems to allow these events to persist for days at a time. I say this because December has been one of the cloudiest winter months in memory in Salt Lake... and the presence of clouds has severely limitted the number of nights with strong radiational cooling. Instead the synoptic-scale flow has frequently produced warming (advective and subsidence) in the lower mid levels (600-700 hPa) that has effectively capped the valley's of the great basin. The lack of surface heating then limits our ability to 'burn through the cap.' So it has been warming above, not cooling below that has generated most of our stability (although we have had a few nights were cooling played a big role).

Furthermore, we went into the project expecting most of our events to occur during strong ridging... well we haven't seen many strong ridging situations, with the month as a whole having anomalously strong SW and moist flow, and yet we've had some impressive cold air pools. So while the synoptic flow has exerted a strong control on the strength of the cold air pools, it has done so in ways that we may not have expected. Its not all about ridging!

We've also had a chance to observe two break up periods during strong southerly winds. Data seems to reveal a competition between advective terms (forcing the cold air pool north) and density current or thermally driven terms (trying to drive the cold air pool to the south). As the balance between these terms shifts in time so does the position of the interface between the cold air pool and the dry, warm and well mixed air. The lake has also been seen to be a 'resevoir' of cold and moist air, which can contribute to the density current nature of the cold air pools.

Okay so now to IOP 5.

Ridging?... not exactly. Clear skies?... maybe. Warming aloft?... definitely. Snow on the Ground?... yes sir. Cold air intrusion?... You betchya!

A pretty good recipe for a persistent cold air pool.

The precondition for IOP 5 may be the single most important factor for what looks to be a very long lived event. An extremely cold air mass (-22 C at 700 hPa) has moved across Utah for the last day of 2010 and a combination of lake effect and frontal snow has left the salt lake valley coated in high albedo white. As the upper level cold air shifts east this weekend warm air will begin to move in aloft trapping residual frigid air in the valleys below... in other words, a Cold Air Pool (CAP).

With some clear skies likely tonight and through the weekend, we could see additional cooling at the surface. The snow on the ground may allow the surface energy balance to remain near zero during the daylight hours... which are still very short.

The best news from our perspective though is that currently there are no strong systems forecast to impact the area in the next 5 or so days (and if you believe the long range models, and I don't, none for 7 or more days). As such we are anticipating a very significant persistent cold air pool for the first week of 2011 and will be iniating IOP 5 tonight. Air quality will likely become an issue and fog and clouds may be possible as the event matures. We'll probably start some additional observations in the coming week to focus on some of these sub-processes.

What a great start to the new year... for us at least.

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