Monday, December 6, 2010

IOP1: Cold Fronts and Warm Fronts

We've been quite busy with PCAPS operations for the past few days so sorry for a lack of updates.

Weather conditions within cold air pools are frequently described as quiescent. This cold pool apparently hasn't read the published literature! Dense fog fronts, strong winds, rapid temperature variations, freezing rain and drizzle, horrendous air quality have all been observed since my last posting... not really quiescent if you ask me. And that is why we've been so busy... and why I'll post a few installments to get you up to speed.

Lets start with some of the interesting events on Thursday night and early Friday morning (Dec 2nd-3rd). A weak short wave trough moved just north of the region, clouds overspread northern Utah, and some light rain fell across the Salt Lake Valley, especially up on the east bench. Accompanying the shortwave trough aloft was a regional pressure gradient that accelerated low altitude winds from the South (low pressure to the north, high pressure to the southeast). Terrain channeling may have helped to orient these winds along valley. The net result was that strong south winds penetrated to the surface at the south end of the valley, effectively 'mixing out' the inversion and advancing a 'warm front' like structure north through the valley. At 06 UTC on Dec 3rd (11 PM local) the advance of the cold pool edge accomplished its northern most extent. Some of the PCAPS team members who live in the south end of the valley observed this transition in person at their homes and described a sudden whoosh and temperature warm up.

However this localized 'mix out' was relatively short lived. In the predawn hours of Friday morning a weak pressure trough swung south across the valley, and effectively reversed the local pressure gradient. The cold pool responded by running back to the south, now resembling a cold front, rapidly engulfing all of the valley.

This evolution is summarized in the following figures:

06 UTC 12 UTC

13 UTC 14 UTC

The temperature/humidity and vector wind traces from around 9000 S and I-15 show both the retreat and return of the cold pool at that location.

Later during Friday afternoon another interesting transition took place. The incredibly hazy air trapped within the cold pool ascended in altitude engulfing locations that had been clear earlier in the day. It is unclear without more detailed analysis what caused this vertical expansion but one hypothesis is that a very shallow cold front moved across the region, gliding over the top of the colder air within the cold pool. This may have knocked some of the strength off of the capping portion of the cold pool and allowed for the upward expansion. Some evidence for this can be seen by comparing the morning sounding with the evening one:

It is readily apparent that nearly all of the cooling in the lower atmosphere took place near the inversion top and that overall the stability in the lowest levels has decreased. While there was no clear indication at the surface of a true cold frontal passage, analysists at the Hydrometeorological Prediction Center (HPC) seem to believe that a cold front passed during this time.

Overall it was an exciting 24 hour period and one that will be examined with more detail in the future.

Tomorrow I'll bring you all (if anyone actually reads this blog) up to date on some of the more recent changes in the cold pool including dense fog and possible break up.

No comments:

Post a Comment