Wednesday, October 3, 2012

A Curious "Fropa"

Fropa is meteorological shorthand for frontal passage.  Today's is quite interesting, and some meteorologists would not call it a frontal passage at all because there was no concentrated drop in temperature.  Nevertheless, a change in airmass has occurred.  

The graphs below show the weather conditions over the past 12 hours at the University of Utah.  Since 0900, the temperature has risen only a couple of degrees.  This is an indication that the rate of heating from the sun is largely balanced by the advection of cold air from the north and northwest.  

Note, however, the aforementioned lack of a clear drop in temperature.  In the bottom graph one can find an increase in wind speed and gusts (red and green lines respectively) just before 1100 MDT, which was accompanied by a shift in the wind from southwest to west-northwest.

Some might use this to mark the front, but there is no coincident drop in temperature or dewpoint.  Curiously, the dewpoint drops later (green line in the top graph) when there is another burst of higher winds.  Throughout this period, there was barely a blip on the altimeter trace, which rose through the night.

Thus, there is no clearly defined front, despite the airmass change.   Not all airmass changes are concentrated like in the text books.

Another interesting aspect of this event is that the cold air is extremely shallow.  We are collecting upper-air observations over the Salt Flats today at at 1115 MDT the top of the cold air was only about 2000 ft above the ground.  It will, however, deepen this afternoon. 

1 comment:

  1. From the SLC sounding at 12Z (Oct 3) it looks like winds became northwesterly above the surface during the night, advecting the nocturnal lake boundary layer over a very shallow, colder/drier layer in SL Valley. So it took a little sunlight to mix that out, and around 0900 the wind shift was reflected at the surface. The dew point drop later on (1200) seems to mark the mixing out of the slightly deeper lake influenced boundary layer which extends up to about 6000 feet elev in the sounding.