Question about low level jet

[Chris Hayes]

After years of reading forecast discussions, and SPC outlooks, it just occured to me that about the only time they talk about a "low level jet or LLJ" kicking in is in the evening and during the nighttime. I was curious as to why a low level jet kicks in only during the evening and nighttime? My best guess would be because of the air starting to cool off as the sun is going down, and the air becoming more dense because of it? Thanks for the help.

 

[Mike Hollingshead]

I don't have the perfect answer, but believe it's mostly related to vertical mixing dying off after the sun sets. Less rising and sinking motions from no more heating, so less resistance to flow. Also sits atop the boundary layer, since the top of the boundary layer is the height Earth's sfc no longer causes friction.

 

[Patrick Marsh]

Extremely quick response here:

The term LLJ is thrown about quite a bit to describe separate things. A strict definition of a Low-Level Jet is a river of fast moving air in lower levels of the atmosphere in which the magnitude of the wind decreases with height. A "true" low-level jet can form (and persist) during the daylight hours in and around intense low-pressure systems.

What is probably most commonly referred to as a LLJ on this forum (and I suspect in the SPC products) would be more accurately described as a "nocturnal boundary layer wind maximum". (One theory states that) This occurs after sunset when the thermal wind vector reverses direction (owing to a change in the differential heating on a sloping terrain) and now acts in the same direction as the true wind.

In simpler terms, the thermal wind (actually not a wind, but a shear vector) is oriented such that on a horizontal surface the warmer air is on the right and colder air is on the left.

Using OK as an example, the terrain in the TX panhandle is higher than the terrain in eastern OK. Assume a constant surface temperature across the region and a uniform south wind. If you move up to a certain low-level height above sea-level (so that you are on a constant horizontal surface with respect to sea-level) you would expect the temperature during the day to be warmer at this height in the higher terrain than in the lower terrain owing to the fact you are closer to the warmer surface in the higher terrain. In this event, the thermal wind would be oriented from north to south...while the true wind is oriented south to north. Thus, during the day the two "winds" oppose each other.

Now, at sunset, the ground cools faster than the air just above the ground which cools faster than the air just above that (and so on). So using the same example, you would expect the temperature at our constant height to be cooler after dark in the higher terrain than in the lower terrain. This means the thermal wind has reversed direction and is now acting in the same direction as the true wind. It is when the thermal wind shear vector and the true wind vector align that the "LLJ" (really the nocturnal boundary layer wind maximum) kicks in.

I'm sure I misspoke somewhere in there so someone smarter than I might want to chime in as well.

 

[Van DeWald]

The wind is also channeled into a smaller column than during the day when you have mixing. For example, during the day, the mixing height might be 700 mb, but at night, you lose that mixing, so instead of a column that is 10000' thick, it's now only 4 or 5000' thick. The wind, assuming the same speeds during the day/night would be faster at nighttime because of the narrower column to squeeze through. Coupled with the fact that the thermal wind reverses directions as Patrick stated and becomes aligned "with" the mean wind instead of against it, you get a low level jet.

 

[beaudodson]

LLJ

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

From Jeff Haby (see link above for images and sketches)

"The low-level jet is a common experience for forecasters in the Great Plains and Eastern US. As the name implies, it is a fast moving ribbon of air in the low levels of the atmosphere. It can rapidly transport Gulf moisture and warmer temperatures to the North at speeds ranging from 25 to over 70 knots. There are two primary classifications of low-level jets. They are the nocturnal low level jet and the mid-latitude cyclone induced low level jet. Both are described in this section along with sketches characterizing their formation."