What exactly is a Low Level Jet?

As I understand it, an LLJ is a low level wind, ie in the lower 1200 meters, that flows contrary to the prevaling flow. It's a term used by severe weather forcasters to understand shear setup's. It's usually found in the presence of low pressure system.

I have attached two two images showing what I understand to be LLJ's in my country on days when supercells and tornadoes were reported. Would these be LLJ's...?

skewt.gif


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I think Jason's links up there are about the most concise and quick answers to your question that you'll find. It's great to see questions like this being posed, as to understand the forecasting of nuances that can lead to tornado days you first of all have to understand the basics. Sometimes, I find myself forgetting the basics.

I think the following quotations from TheWeatherPrediction.com are the most concise:-

1. The low level jet is a high speed return of warm and moist air from the south or southeast; moisture source is the Gulf of Mexico
2. The low level jet occurs in the warm sector of a developing mid-latitude cyclone in the Central and Eastern U.S.; occurs generally ahead of the cold front boundary
3. Low level jet adds heat, mass and momentum to developing thunderstorm and produces low level speed and directional shear (results in very high Helicity values)
4. Produces abundant WAA (warm air advection) that may break a weak to moderate cap. WAA produces broad synoptic scale uplift
5. Strongest low level jet winds are generally at the top of Planetary Boundary Layer due to less friction than at the surface
6. Advection may well be over 65 miles per hour

It is this very phenomena (the LLJ) that many chasers remember with almost feverish clarity. Looking back on an amazing tornadic day - sometimes you find that your memories are not necessarily filled with visions of the huge vortices you saw, but rather of the evening before the event. Standing underneath a torn and shredded sky at midnight, watching the tops of the largest trees rustle and the orange city lights reflect off ragged, raging strato-cu as it tears north like high octane fuel to feed the hungry system that is approaching! :D

Sigh........

KL
 
Yes, the feeling of warmth and moisture as the LLJ cranks up return flow is great - Roger Edwards has perhaps summed it up beautifully here:

http://www.stormeyes.org/tornado/return.htm

Karen makes a good point about low-level shear - a strong LLJ can give excellent low-level shear, increasing the chances of tornadogenesis (if storms develop from the boundary layer).

Of course, low-level jets often occur at night about stable boundary laters, but strong mass convergence along frontal zones can see elevated severe storms developing overnight...this is often the situation as an upper trough approaches - severe nocturnal convection develops as the LLJ cranks up, usually on the nose of the LLJ, and then the following afternoon sees severe day-times at peak heating.
 
I actually wrote a paper comparing LLJ phenomena around the world my senior year at OU. Some of the common things I've found is that many of the recurring LLJs have a diurnal cycle. In fact, the Southern Plains of the U.S. typically experience a nocturnal LLJ during spring and summer.

The other common thing....Almost every LLJ I studied was the PRIMARY mechanism for low level moisture transport. For example, the Southern Plains LLJ is the primary means of transporting moisture into the eastern U.S. The South American LLJ is responsible for brining in moisutre from the Atlantic on the east side of the Andes helping with the Amazon rains. The Somali Jet across the Indian Ocean is big contributor to the India moonsoons.
 
Thank you for all the replies.


I gather then that an LLJ is a general term for any kind of low level flow that goes against or underneath the prevailing upper flow? Aside from the well known plains LLJ, a LL Jet is a common feature found in many parts of the world. My two skew-t diagrams shown above, for example?
 
As I understand it, an LLJ is a low level wind, ie in the lower 1200 meters, that flows contrary to the prevaling flow. It's a term used by severe weather forcasters to understand shear setup's. It's usually found in the presence of low pressure system.

I have attached two two images showing what I understand to be LLJ's in my country on days when supercells and tornadoes were reported. Would these be LLJ's...?


The low-level jet term is used rather generically as I've seen it, and can describe essentially any jet like flow (narrow stream of concentrated flow) within the lowest 2-3 km. This could be forced by diurnal effects of differential radiational cooling on sloping terrain (like the nocturnal LLJ in the southern plains of the US during the warm season, and other regions with similar terrain features adjacent to a large body of warm water), or the low-level branch of the ageostrophic flow on the nose of an upper level jet - essentially the warm conveyer belt in the warm sector of a mid-latitude cyclone. As for the two images you included, I don't see any signs of a nocturnal LLJ, and without a plan view of a low-level pressure surface (such as at 850 mb), it would be difficult to confirm the sampling of a low-level jet in those soundings.
 
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