Friday, April 26, 2019

Tirolian Battle of Airmasses

The Tyrol has long been an important strategic region due to the Brenner Pass south of Innsbruck being the lowest pass across the Alps.  The mountains themselves served as an important strategic defense for centuries against invaders from other regions, and today they still serve as major barriers to the penetration of cold airmasses into the Tirol.

Today provides a great example.  As of 1000 UTC (1200 CET), cold air has surged across the Alpine Foreland in southern Germany where surface stations (upper left of figure below) are reporting temperatures of 5–7˚C.

Map source: University of Innsbruck
This airmass, however, is fairly shallow.  A sounding collected four hours earlier in Hohenpeissenberg, the easternmost of the two sites shows shallow northwesterly flow with strong southwesterly flow aloft.  A strong inversion caps a shallow pool of cold air between about 875 and 825 mb.  Basically, there is a shallow layer of post-cold-frontal air capped by very warm air that is downsloping off the Alps.  Such downsloping flow is known as Föhn.

Source: University of Innsbruck
In contrast, temperatures at Innsbruck, which is still under the influence of the Föhn moving down the Wipp Valley, remain very mild (22˚C).  Similarly, temperatures at sites southeast of Innsbruck (within red circles above) are also very mild.

Nevertheless, the cold air is attempting to penetrate through low-elevation valleys and saddles in the terrain of the northern Alps.  For example, there is a saddle in the terrain near Seefeld (identified above) through which cold air is currently moving, resulting in temperatures of only 9˚C at Seefeld, which is only about 500 meters higher than Innsbruck (thus, this air is colder than one would expect solely from altitude effects in a well mixed airmass).  One can also see cold northerly flow in the Achen Valley northeast of Innsbruck.

To the east of Innsbruck, it is also clear that there is terrain channeled flow moving up the Inn Valley.  Time series below from Kufstein (top) and Jenbach (bottom) show the development of NE flow at Kufstein followed by Jenback as cooler air moves up the Inn Valley.  This air, however, is not as cold as that moving through the Seefeld saddle since it is from further east.  However, it is clearly cooler than the air at those sites yesterday.

Source: University of Innsbruck

Source: University of Innsbruck
One of the challenges forecasting for Innsbruck is predicting when the Föhn will weaken and whether or not cold air will reach the city from the Seefeld Saddle, the Inn Valley, or directly over the Nordette immediately north of the city.  In addition to the importance for the public, the timing of these wind transitions is absolutely critical for aviation into and out of the Innsbruck airport.  During Föhn, turbulence is high and planes typically fly very close to the southern slope of the Nordkette where the Föhn flow tends to move upslope and provide lift.  However, this is the last place you'd want an aircraft if the cold air pushes over the crest of the Nordkette and plunges down the south slope.  

Today, it appears we won't see cold air pushing over the Nordkette, but reaching Innsbruck through the Seefeld Saddle or up the Inn Valley.  

Lo' and behold, while I was writing this, we had frontal passage.  Note below the temperature drop and the wind shift (dotted line) from S or SW flow to something near W.  

Source: University of Innsbruck
That indicates the cold air got here first through the Seefeld Saddle.  Seefeld Saddle 1, Inn Valley 0.

No comments:

Post a Comment