The Low Level Jet

[Austin Jones]

I'm trying to get a grasp on the Cyclone Induced Low Level Jet but I'm struggling with some concepts. Using http://www.theweatherprediction.com/severe/llj/ as a resource I have gathered that the jet moves from east to west across the warm sector because of a pressure gradient between the warm sector and the cold front. This seems strange to me has I know cold air to be denser than warm air and for wind to move from high pressure to low pressure. So I would expect an area of cooler air to have a higher pressure and therefore move towards an area of warmer air and lower pressure.....In writing all this out I think I just figured it out for myself, so just correct me if what I say below is false.

From the picture below I gather that my general understanding of cold to hot / high to low can be overcome by a change in surface elevation causing a change in constant pressure height with respect to the constant height. Even thought the air is cooler in the west it as at higher elevation and so the atmosphere above is "thinner" and the 850mb line is lower. Causing a lower pressure in the west at a constant height, compared to the east.





One more question, at the bottom of Habby's page there is an example of a cyclone induce LLJ, however it appears to come more from the west , but from everything on the page I would assume it to come more from the east?

 

[Caleb Grunzke]

Pressure gradients result from a temperature gradient. Wind is made to try restore the temperature balance in the atmosphere. A stronger temperature gradient means a stronger pressure gradient which in turn means stronger winds.

Jets move west to east due to the overall circulation in the northern hemisphere. Jets are often times found near the warm sector and the cold front but this is a concept of mid-latitude cyclone formation that I don't want to get into as it would take much too long to explain.

Cyclone induced low-level jets don't really come from a particular direction. Cyclone induced LLJs are a secondary circulation of an upper-level jet (ULJ) formed by the ageostrophic wind. I'm going to share some notes of the Synoptic class I'm currently TA'ing for as they explain the circulation of the ULJ without going too much into the equations. (Pages 4-8 will be most helpful.)

http://derecho.math.uwm.edu/classes/SynI/Jets.pdf

The exit region of the ULJ will typically be located in the vicinity of a severe weather event. There are exceptions but for this instance I will stick to this as it's easy to diagram and can be flipped for the entrance region of the jet.



The figure above shows that the ageostrophic wind is pointing from north to south in the exit region of the ULJ. This creates a convergence of air to the south of the jet and a divergence of air to the north. In order to balance this, the air must go downward in the southern portion and upward in the northern portion. As you may have guessed from the figure, the air must move from south to north near the ground to balance the whole circulation. This air moving from south to north is the LLJ.

I would be careful using haby hints as some of what is on that website is very rule of thumb or just plain wrong.

 

[Austin Jones]

That was very helpful Caleb, thank you. Up Until now I had heard only good things about Haby's Hints. I'm surprised to hear that it has wrong information, but I will very careful using it in the future.