PDA

View Full Version : What is lift?



Rockwell Schrock
01-05-2005, 02:45 PM
I realize that one of the things to consider when forecasting a target area is an area's lift, but I don't exactly understand the concept of lift and how it helps (or hinders) storm formation. So, what is lift?

Also, what maps or tools would one use to diagnose lift?

Thanks a lot.

Robert Dewey
01-05-2005, 06:57 PM
Originally posted by Rockwell Schrock
I realize that one of the things to consider when forecasting a target area is an area's lift, but I don't exactly understand the concept of lift and how it helps (or hinders) storm formation. So, what is lift?

Also, what maps or tools would one use to diagnose lift?

Thanks a lot.

Lift is basically the vertical motion within the atmosphere. This veritcal motion can be induced by vorticity maximums in the mid levels, jet streaks, warm/cold fronts/dry lines/outflow boundaries (basically, SFC convergence zones), and geographically induced.

One way to diagnose lift, is by looking at low and mid level wind fields to analyze jet streaks/jet max's. Vorticity fields are also very helpful, especially if the vorticity is located in the mid levels, as a mid level circulation would indicate rising air (lift). You can also use vertical velocity fields, as well as frontogenesis, to diagnose areas of lift. If your really good, and find some decent forecasting maps, you can use isentropes, which are really good for large scale ascent/descent patterns, such as those found in winter storms.

One thing to remember, lift is pretty much useless without moisture. If you have lift within a very dry environment, you will just be pushing warm air upwards, with little in the way of moisture to condense. For severe thunderstorms, you would also want to look at other factors such as:

1) Instability/thermodynamics (best visualized using skew-t's and hodographs)
2) Shear/wind fields
3) Boundary locations

That's just a basic "to the point" description...

Rockwell Schrock
01-05-2005, 07:28 PM
Originally posted by rdewey
One way to diagnose lift, is by looking at low and mid level wind fields to analyze jet streaks/jet max's. Vorticity fields are also very helpful, especially if the vorticity is located in the mid levels, as a mid level circulation would indicate rising air (lift).

So, from what I understand, you're saying to use 850mb to 500mb charts? And what am I looking for on these maps that indicate jet streaks? Is that just where the pressure contours are close together, or...?

Robert Dewey
01-05-2005, 07:52 PM
Originally posted by Rockwell Schrock+--><div class='quotetop'>QUOTE(Rockwell Schrock)</div>
<!--QuoteBegin-rdewey
One way to diagnose lift, is by looking at low and mid level wind fields to analyze jet streaks/jet max's. Vorticity fields are also very helpful, especially if the vorticity is located in the mid levels, as a mid level circulation would indicate rising air (lift).

So, from what I understand, you're saying to use 850mb to 500mb charts? And what am I looking for on these maps that indicate jet streaks? Is that just where the pressure contours are close together, or...?[/b]

Here is a basic breakdown of how I do it:

1) Look at the vorticity field at 500mb

2) Look at the wind fields at 250mb, 500mb, and 850mb (that pretty much represents the high level (250mb), mid level (500mb), and low level (850mb). Stronger winds indicate where jet streams are... For the 250mb and 500mb levels, the front left quadrant and rear right quadrant have maximized lift. At 850mb, the leading edge (or "nose") of the low level jet tends to have maximized lift.

3) Use model data and look at 850mb, 700mb, and 500mb vertical velocity. Vertical velocity will tend to show up as high values along very strong cold fronts and warm fronts, as well as drylines. They will also show up under regions of strong vorticity.

Also, vorticity and jet max's are usually associated with shortwaves (another topic), which can provide just the right amount of lift to break the CAP, and that's typically why severe thunderstorms will tend to break out, provided the other ingredients are in place.

For more information, and probably a better explanation, see this site: http://www.theweatherprediction.com/advanced/ (I also recommend surfing around that site a bit, it's interesting).

Rockwell Schrock
01-05-2005, 07:57 PM
Ah-ha, that is great; thank you. I'd visited that site before but never explored it in depth. Now I most certainly will. :)

Rockwell Schrock
01-05-2005, 08:52 PM
Quick question: Is "vertical motion" (such as found on this plot (http://www.rap.ucar.edu/weather/model/ruc12hr_925_mxr.gif)) the same as vertical velocity?

Robert Dewey
01-05-2005, 08:56 PM
Yes, but that plot of "vertical motion" is in different units than standard vertical velocity, though. It works just the same...

Here is another example, with higher resolution:

http://weather.cod.edu/forecast/ETA/GL/eta..._700_vvel_0.gif (http://weather.cod.edu/forecast/ETA/GL/etaGL_700_vvel_0.gif)

Joe Nield
01-06-2005, 12:43 PM
Originally posted by rdewey
You can also use vertical velocity fields, as well as frontogenesis, to diagnose areas of lift.

I really like to use frontogenesis, especially when considering snow situations.

For example, during the pre-Christmas storm, the southeastern half of Indiana received some fairly significant snowfall, independent of the low, which was still wrapping up near the Mississippi Delta. A very strong frontogenetical region set up over the area and the resulting snowfall totaled 8-10 inches in some areas. The GFS did an excellent job of diagnosing this several days out.

Glen Romine
01-06-2005, 02:00 PM
Originally posted by rdewey
Vorticity fields are also very helpful, especially if the vorticity is located in the mid levels, as a mid level circulation would indicate rising air (lift). You can also use vertical velocity fields, as well as frontogenesis, to diagnose areas of lift. If your really good, and find some decent forecasting maps, you can use isentropes, which are really good for large scale ascent/descent patterns, such as those found in winter storms.

One thing to remember, lift is pretty much useless without moisture. If you have lift within a very dry environment, you will just be pushing warm air upwards, with little in the way of moisture to condense. For severe thunderstorms, you would also want to look at other factors such as:


Trying not to pick too much here - but the notion of cyclonic vorticity advection at mid-levels leading to rising motion only is true with increasing cyclonic vorticity advection with height (cyclonic vorticity advection decreasing with height leads to sinking motion, anti-cyclonic vorticity advection decreasing with height = rising motion, etc....). Further, there is no mention here of warm and cold air advection (though suggested in the isentropic analysis - which is ideal for looking for just this forcing), which again it is the vertical gradient of temperature advection that is important. Like vorticity advection, it is often true that low-level warm air advection produces rising motion. There are other conceptual models for describing vertical motion - such as jet streaks, curvature effect, etc... - which have to also be considered, and often there is overlap and disagreement.

Finally, the comment about lift being meaningless without moisture I don't entirely agree with for the case of priming a severe weather environment. In fact, dry mid-levels leads to the lifted mid-level air cooling at the dry adiabatic lapse rate (not warming), and cooler mid-levels (all else equal) increases the potential instability. This is often the case in the plains where large scale ascent (we are talking about very small vertical motions here - not like in convection) cools the mid-levels and often the top of the capping inversion, increasing the likelihood a surface forcing mechanism, such as a thermal boundary, can push air up through the cap. Too much lift can eliminate the cap everywhere - leading to widespread precip, whereas too little may not allow convection anywhere, such that a critical balance has to occur for discrete convection to be favored. Knowing which mode to expect follows from understanding the amount of lift that can be expected from the shortwave, the amount of capping that will be present beneath the lift, and the strength of forcing to lift parcels, assuming that instability is present (another topic).

Glen

Robert Dewey
01-06-2005, 02:20 PM
Originally posted by Glen Romine+--><div class='quotetop'>QUOTE(Glen Romine)</div>

Trying not to pick too much here - but the notion of cyclonic vorticity advection at mid-levels leading to rising motion only is true with increasing cyclonic vorticity advection with height (cyclonic vorticity advection decreasing with height leads to sinking motion, anti-cyclonic vorticity advection decreasing with height = rising motion, etc....).[/b]

Sounds good, I will have to add that to my current knowledge (always up for learning something new).


Originally posted by Glen Romine@
[Further, there is no mention here of warm and cold air advection (though suggested in the isentropic analysis - which is ideal for looking for just this forcing), which again it is the vertical gradient of temperature advection that is important. Like vorticity advection, it is often true that low-level warm air advection produces rising motion. There are other conceptual models for describing vertical motion - such as jet streaks, curvature effect, etc... - which have to also be considered, and often there is overlap and disagreement.

Sounds reasonable... I think strong low level warm advection is associated with a strong low level jet, which can enhance lift (and usually results in heavy precipitation events).

<!--QuoteBegin-Glen Romine

Finally, the comment about lift being meaningless without moisture I don't entirely agree with for the case of priming a severe weather environment. In fact, dry mid-levels leads to the lifted mid-level air cooling at the dry adiabatic lapse rate (not warming), and cooler mid-levels (all else equal) increases the potential instability. This is often the case in the plains where large scale ascent (we are talking about very small vertical motions here - not like in convection) cools the mid-levels and often the top of the capping inversion, increasing the likelihood a surface forcing mechanism, such as a thermal boundary, can push air up through the cap. Too much lift can eliminate the cap everywhere - leading to widespread precip, whereas too little may not allow convection anywhere, such that a critical balance has to occur for discrete convection to be favored. Knowing which mode to expect follows from understanding the amount of lift that can be expected from the shortwave, the amount of capping that will be present beneath the lift, and the strength of forcing to lift parcels, assuming that instability is present (another topic).

That's pretty much what I was thinking as far as severe thunderstorms (which is the main topic), but I kind of changed subject without notice, and headed into the winter storm aspect of lift. If you have lift in a winter storm (large scale isentropic ascent, orographic lifting, etc.), and the air is bone dry, as it often is when the temperatures get cold, not much will happen... But, as far as the main discussion -- severe thunderstorms -- you are right.