Low Level Jet location

[ChristianTerry]

I know the Jet goes through the middle of the country in Spring and up into Canada in the summer. Does the jet go through the gulf coast states in the fall and winter when the majority of coastal tornadoes occurs? Does it ever come in the Spring or Summer?

 

[Jeff Duda]

The low-level jet (LLJ) most certainly shows up in the Gulf Coast states. That's close to its origin over the Gulf of Mexico and northern Mexico in many cases (especially during the cool season months of Nov - Mar). In fact, most severe weather/tornado outbreaks (if not every single one) will have a low-level jet present. LLJs cause enhanced low-level shear which is one major ingredient that differentiates normal severe storms/supercells that don't produce tornadoes from those that do. Climatologically speaking, during the mid-summer months the LLJ is favored to exist in a generally 10 degree longitude wide N-S oriented band from just east of the Front Range to about the Mississippi River. Thus it is not as likely to appear over AL, GA, FL, but it certainly could be found over LA and AR during the summer months.

 

[Tim Supinie]

To add to Jeff's post, technically, a "low-level jet" refers to any wind speed maximum in the low-levels of the atmosphere, usually below about 700 mb. The wind speed on either side of the maximum usually has to decrease by a certain amount, I can't remember what the number is. The nocturnal low-level jet on the Great Plains of the US is somewhat special because of the mechanisms by which it is generated and the regularity with which it is observed. That's not to say you won't see low-level jets over other parts of the country, such as the southeast.

By the strict definition of "low-level jet" I gave above, a low-level jet is not a requirement for a major outbreak. As an example, I've attached the 13Z sounding from Jackson, MS, on 24 April 2010 (the Yazoo City violent tornado). At least in the vertical, there's nothing I would call a low-level wind maximum, even though the shear profile is quite impressive, to say the least. Granted, this is kind of an extreme event, and structures that could be called "low-level jets" are observed frequently in association with major severe weather outbreaks.

 

[samuel stone]

can i add a new question in here? why does the low level jet ramp up so high during the night over the great plains? does it have something to do with the cooling tempertures tightening up the pressure gradient?

 

[Rob H]

Yes, that's pretty much the reason. Air wants to flow from high to low pressure, which can be roughly equated with warm to cold air masses. In between these masses there will be a pressure gradient. The air wants to flow between these masses (usually east to west in this case), but the Coriolis force redirects it 90degrees so you end up with south-north winds. Haby's Hints has a good page detailing this (with diagrams), and it also mentions why this is nocturnally enhanced:

http://www.theweatherprediction.com/severe/llj/

 

[Jeff Duda]

can i add a new question in here? why does the low level jet ramp up so high during the night over the great plains? does it have something to do with the cooling tempertures tightening up the pressure gradient?

One of the most well known and accepted theories is the "inertial oscillation". This theory assumes that the ageostrophic wind vector rotates through a 360 degree circle in a time period that is proportional to the local Coriolis force magnitude. For a southerly geostrophic wind vector, this ageostrophic vector becomes aligned with the geostrophic vector during the late night/overnight hours, which is when the LLJ accelerates and reaches its maximum southerly magnitude. Then, as the ageostrophic wind vector continues to rotate, it becomes less aligned with the geostrophic wind and the total wind reduces (and tends to veer).

ADD: I'll expand on the inertial oscillation theory here. The theory assumes there is a southerly component to the total wind, but especially to the geostrophic wind, at the level at which the LLJ occurs. This is quite a reasonable assumption. During the daytime when the PBL is well mixed, friction in the PBL will cause an imbalance between the pressure gradient and Coriolis forces, thus resulting in an ageostrophic wind vector that points perpendicular to the left of the total wind vector (which has a southerly component). Thus you have a northeasterly ageostrophic wind. When the sun sets and diabatic heating stops, the frictional force vector will weaken as the PBL decouples from the surface over a short period of time. The vector math results in the ageostrophic wind vector beginning to rotate clockwise during this decoupling process. The period of oscillation of the ageostrophic wind is 2*pi/f, where f = 2*omega*sin(phi), where omega = 7.27x10^-5 -s is the rotation rate of the earth and phi is latitude. For example, the period is one day at 30 deg. N and S latitudes and is about 17 hours at 45 deg. N and S lat. The ageostrohpic wind vector will rotate at a constant rate due to its inertia. As the ageostrophic wind becomes aligned with the geostrophic wind (which has a southerly component), the total wind will accelerate and reach a peak, then decelerate and veer as the ageostrophic wind continues to oscillate past the point of being aligned with the geostrophic wind. This is why the LLJ peaks during the overnight and veers as it dies during the early-mid morning.

On a separate note, you can get flow in the lower atmosphere that might appear to be an LLJ due simply to pure geostrophy in strong synoptic disturbances. This can result in southerly winds of up to 70 - 80 kts around 850mb. This wouldn't technically be a LLJ, though, because there wouldn't be a wind speed max with height there (unless there was a weak LLJ evolution superposed on top of this geostrophy). However, this kind of wind can certainly provide for sufficient low-level shear for tornadoes. The inertial oscillation theory only explains diurnal LLJ evolution. It does not explain LLJs that occur during the middle of the day. LLJs that occur during the middle of the day, especially in the Gulf coast region, are caused pretty much exclusively by synoptic scale dynamics (like the geostrophy I just mentioned, or also the impact of upper-level jets on the lower atmosphere).

 

[samuel stone]

thanks for that input robert, and jeff after a couple google searches i think i understand your theory, however, why does the ageostrophic wind vector happen to align with the geostrophic vector at night? why would its rotation never/not often line up with the geostrophic vector during the day? does that question make sense? thanks for the input

jeff: does the ageostrophic vector rotation only line up with the geostrophic vector at night because of the increased LLJ? (wich would be enhanced by a tightening of the preasure gradient at night?, like working in tandem with it sort of?)

 

[Jeff Duda]

thanks for that input robert, and jeff after a couple google searches i think i understand your theory, however, why does the ageostrophic wind vector happen to align with the geostrophic vector at night? why would its rotation never/not often line up with the geostrophic vector during the day? does that question make sense? thanks for the input

jeff: does the ageostrophic vector rotation only line up with the geostrophic vector at night because of the increased LLJ? (wich would be enhanced by a tightening of the preasure gradient at night?, like working in tandem with it sort of?)

Sam, I have replied to your post by editing mine above. Hopefully this answers your questions.