Sunday, December 12, 2010

IOP 3: How much cold air can we produce in one night?

IOP 3 is now underway, but it won't be for long as we are expecting a short lived event. But that is part of the point. We want to see how quickly the Salt Lake Valley can form a cold air pool under clear and calm conditions.

A strong ridge built across the region on Sunday, a scenario that we typically associate with cold pools during the winter months. However, with no cold air in place and thick clouds the preceding night, only modest stability was generated in the lower atmosphere leading into today. The clear skies today lead to significant surface heating, and the the atmosphere became nearly dry adiabatic through the mountain top level during the day. As a result temperatures in town spiked into the low 50s, which was a welcome change.

Tonight however, we expect to develop a substantial nocturnal inversion under clear and calm conditions. In fact we can already see this cooling in progress. The net radiation measurements from the evening are strongly negative, especially as compared to last night. The balloon launched at 06 UTC from the ISS facility now shows a strong inversion near the surface, which is very different than the sounding from the airport shortly after sunset.

One interesting observation is the weakening of the subsidence inversion near 700 hpa during the past 6 hrs. Loosing a strong cap aloft could have a big impact on the evolution of this event.

We're also launching additional balloons tonight and tomorrow morning from Antelope Island in order to observed the cooling that occurs in the boundary layer over the lake.

Tomorrow with clear skies again expected for at least part of the day we'll have a look at how much of the surface inversion is removed by surface heating, how the lowering subsidence inversion (if it still exists) may couple with the nocturnal inversion, and how the diurnal wind reversal advects air from over lake into the valley. We'll have the motorized gliders out for those operations!

On Tuesday if conditions warrant we'll then be looking at the potentially rapid break up of the cold air pool by strong winds associated with an approaching vigorous trough.

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