Wednesday, January 14, 2015

One Recipe for an Inversion

Many people think that inversions form because air in the valley cools, and this can be a mechanism for inversion formation or strengthening at night, but the primary mechanism for the formation of the persistent inversions that plague us during the winter is the warming of air aloft following an incursion of cold air at low levels.  This isolates a pool of cold air within the valley that is too dense to mix efficiently with the lower density air aloft.  Given the limited heating by the sun from November to February, we're stuck with a cold airmass that gets increasingly polluted with time.

There are two major mechanisms that can contribute to the warming aloft.  The first is the horizontal transport of warm air, something that meteorologists call horizontal temperature advection. The second mechanism, which can be very important but is often under appreciated, is the sinking of air aloft.  

As air sinks, its pressure increases and it is warmed by compression at a known rate of 9.8 ºC/km (5.5 ºF/1000 ft).  Meteorologists sometimes call this subsidence warming.  Because the air originates at upper levels, it also tends to have low water vapor content and thus a low dewpoint.

This morning's sounding shows the tell-tale signs of subsidence warming.  A relatively deep valley cold pool is surmounted by a 2ºC inversion from 750 to about 700 mb.  Within the inversion layer, the dewpoint temperature drops dramatically and is very low (< -40ºC) at and just above the top of the inversion.  This is usually a dead giveaway that the air at the top of the inversion air is of subsided origin.

However, we also have additional evidence of sinking air aloft in the computer model analyses.  The 500-mb analysis for 0600 UTC, about 6 hours before the morning sounding was taken, shows that northern Utah was located beneath an area of strong sinking motion aloft in the wake of the 500-mb trough that gave us that great storm on Monday.  The yellow contours (a bit hard to see) show the areas of sinking motion.

Due to a large degree to this subsidence, the inversion will both strengthen and lower over the next 24 hours, leaving us trapped in an increasingly shallow layer of cold, stable air.  This inversion, however, may have some different characteristics than the last as it is very moist already, with wide spread low clouds.  More on the significance of those clouds perhaps in a future post.

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