Decoupling Boundary Layer

[Brian Press]

Hey All,

I keep seeing this term and I have no idea what it means. Below is a excerpt form the latest SPC Day 2 forecast. So what does this "Decoupling Boundary Layer" mean and how does it affect Tornado / severe storm formation?

Thanks so much,

Brian

"HODOGRAPH SIZE IN THE 0-1 KM LAYER WILL INCREASE SUBSTANTIALLY AS THE LOW LEVEL JET STRENGTHENS DURING THE EVENING RESULTING IN A SMALL WINDOW FOR ISOLATED TORNADOES BEFORE THE BOUNDARY LAYER DECOUPLES.

 

[Peter Wharton]

Heres a reference - http://www.theweatherprediction.com/habyhints2/420/ - good source for quick basic understanding - you can explore from there

 

[Chris Nuttall]

Boundary layer decoupling can mean a lot of things with regards to thunderstorm formation. If the BL can decouple, you'll have cooler air at the surface and warmer air aloft. This would result in the formation of a stable layer (often an inversion) above the surface, which would prevent any convection from surfaced based air parcels. Now, you could still get thunderstorms to develop, but they would be "elevated" and their main source of unstable air is above the inversion. This is not really conducive for tornadoes. However, you can still get severe hail and wind from elevated convection.

 

[Chris Gullikson]

I spend a lot of time in the boundary layer while flying in an open-cockpit light aircraft...albeit when the weather is nice. It is very cool to experience this decoupling first hand as the sun angle gets low in the evening and radiational cooling at the surface shuts down vertical thermal advection. Landing after sunset on a calm, warm day that had lots of sunshine can be a shocker when you hit the cold air at the surface. The ground cools the air at the surface creating an inversion that is usually only 10-20 feet thick in the early evening. If the night is clear allowing strong radiational cooling, this inversion can be several hundred feet thick by morning. It is fun to fly in the morning and climb in the cold stable air, through the inversion to find yourself in a much warmer adiabatic layer mixed from the prior days thermals. There is often times an area of shear as you go through the inversion level - the air is moving along faster above the inversion because it is less dense then the cold air below and not subject to friction at the earth surface...the nocturnal jet. As the sun angle gets higher, thermals begin to mix out the inversion and the smooth air that you had been flying in gets turbulent from the rising air.

Tornadoes are "usually" the product of surface based storms...a cool boundary layer is stable and as this layer becomes deeper later in the evening, storms become elevated and the tornado threat "usually" diminishes.
I say "usually" because there are plenty of cases of very late and early morning tornadoes that can be explained by various reasons including strong moisture/thermal advection over a surface inversion that is shallow and/or being mixed out from above.

 

[Paul Knightley]

If powerful supercells are occurring, the mesocyclone can lift stable boundary layer parcels providing the cap is not too strong - such storms will usually have developed in the late afternoon/early evening, and then keep going after sunset, if instability/shear allow. As the boundary layer cools a little, the decoupling means a strengthening low-level jet just around and for a (usually fairly short) time after sunset...this increases 0-1km shear/hodographs, and can enhance tornado potential, and also help strong-violent tornadoes form, as at Greensburg.