Shortwaves

[Andrew Pastore]

I have been reading chase accounts today, and many reference "shortwaves"
What are they and how do they affect weather? What do they look like on a weather map?

 

[Tony Laubach]

Shortwaves are little dips in the flow. When someone refers to a shortwave, what they typically are saying (correct me if I'm wrong, gang) is that a small dip in the flow is rolling through.. Think of it as a mini-trough. These typically spark storms as the region downstream from this wave is ascending, creating upward vertical motion which is a key ingredient in storm formation. If moisture and instability are in place, these shortwaves can act as a focusing point for areas of severe weather. At the moment, I think it would be much easier to have a visual reference and comparison, so if anyone has those off-hand, they may clear up what I'm saying.

 

[Mickey Ptak]

Shortwave (or Shortwave Trough) - A disturbance in the mid or upper part of the atmosphere which induces upward motion ahead of it. If other conditions are favorable, the upward motion can contribute to thunderstorm development ahead of a shortwave.
NOAA:http://www.erh.noaa.gov/er/pit/branick2d.html


Here are a few other links to help guide you. Its so much fun learning about this stuff IMO.

http://www.theweatherprediction.com/habyhints/14/
http://www.theweatherprediction.com/habyhints/131/

Mick

 

[Aaron Kennedy]

Only addition I have to the above is that short waves move in the same direction as the flow they are embedded in. Long waves can propagate in either direction depending on the speed of the flow and the wavelength of the wave itself.

Aaron

 

[Nick Smith]

When a forecaster mentions a "shortwave", he/she is usually talking about a shortwave trough, although there are also shortwave ridges as well. I can see about finding more images of these if need be.

 

[Jon Miller]

Assuming you have a negative shortwave , on the 500mb chart, where would the vertical rising air generally be at relative to the shotrwave axis ?

 

[Tim Vasquez]

The rising air is usually ahead of the short wave trough and behind the short wave ridge. If you're going to try to pick it out based solely on the 500 mb chart, then an absolute vorticity overlay helps tremendously.

There are two basic assumptions with this for the omega equation to spit out synoptic-scale rising motion:
1. The vorticity ahead of the short wave must increase with height. This is usually assumed but is not always true.
2. The increasing vorticity must not be offset by cold air advection (typically associated with isentropic downglide).

There's an interesting paradox in that vorticity analysis has a stigma nowadays (no one really wants to touch it, and you never seem to hear about PVA in the discussions anymore, at least not like in the old days). But you DO see short waves mentioned extensively. I'm not sure what the cause of this disconnect is; perhaps it's part of the rush to toss out subjective forms of analysis/interpretation in favor of more "definitive" output like precipitation, RH, and vertical velocity (the latter of which I find is quite noisy and somewhat useless). I also think forecasters don't want to get labelled as relying on the PVA/NVA model too much (which certainly was beaten into the ground in the 1980s and early 1990s). It's too bad, as still think vorticity is a quite useful part of analysis.

Tim

 

[Tim Johnson]

When analyzing a shortwave, I assume it is going to be steered by upper level winds (correct me if I am wrong).....how do you judge or forcast the movment of this area of lift, both long range and short range? By the speed and direction of upper winds, or the use of models, or both?

Hopefully that made sense

 

[Gabe Garfield]

When analyzing a shortwave, I assume it is going to be steered by upper level winds (correct me if I am wrong).....how do you judge or forcast the movment of this area of lift, both long range and short range? By the speed and direction of upper winds, or the use of models, or both?

Shortwave systems (and likewise longwave systems) do not behave like feathers in a stream (they aren't advected). Rather, they propagate.

Essentially, a shortwave trough is synonymous with a vorticity maximum. Vorticity is comprised of two components; shear vorticity and curvature vorticity.

Now, you can think of shear vorticity like the proverbial paddle wheel example. Say, for example that the winds are stronger aloft at Norman, Oklahoma than they are at Wichita, Kansas. If you put a large paddlewheel between the two locations, the paddlewheel will spin counterclockwise (or cyclonically). This is an example of cyclonic (positive) shear vorticity.

Curvature vorticity is essentially what you would see on a 500 mb map. The more curved a 500 mb trough is, the more curvature vorticity that is present.

These two components can add up together, or they can cancel each other out (i.e. positive shear vort and negative curvature vort).

Generally speaking, most shortwave troughs can not be seen as curvature vorticity maxima. They are most readily seen as a local maximum (jet max) in the ambient upper level flow. This is why jet maxima are nearly synonymous with shortwave troughs (except in the case of strong anticyclonic curvature).

The physical explanation for why differential positive vorticity advection (DPVA) leads to rising motion is a little complicated. But trust me, it works.

As far as determining the motion of shortwave troughs, two things need to be kept in mind: vorticity advection and temperature advection. It's a little too lengthy to get in depth on this forum, but for a general rule of thumb, shortwave troughs will propagate in the direction of greatest cyclonic (positive) vorticity advection/warm air advection. This isn't always true (see the quasi-geostrophic equations for further information), but it is most often the case.

Gabe

 

[Bill Hark]

To one of the moderator's: This is a great discussion. Thanks! When it is completed, let's move the thread to the Newbies section. Otherwise it will eventually be lost in additional subjects.

 

[Tim Vasquez]

I agree.

Tim

 

[Glen Romine]

Originally posted by Tim Johnson
When analyzing a shortwave, I assume it is going to be steered by upper level winds (correct me if I am wrong).....how do you judge or forcast the movment of this area of lift, both long range and short range? By the speed and direction of upper winds, or the use of models, or both?

Hopefully that made sense

There is also a gross forecasting rule that shortwaves at 500 mb propagate at ~ 50% of the mean flow they are embedded within. But, there aren't a lot of practical situations where this can be applied, as there are often multiple waves of considerably different scale super-imposed on each other, and these waves are often changing scale with time. Roughly, as a wave is larger in scale it retrogrades against the mean flow it is embedded within more aggresively. So, with a large scale trough you would often see slow prograde motion of the system as a whole, with shortwaves travelling around the larger trough since they won't retrograde as strongly, and if there is a surface front underneath, the passage of the shortwaves can induce surface waves along the front that then leads to areas of warm and cold advection that acts to amplify the upper level disturbance increasing it's scale. This in turn modifies the upper level jet stream as the frontal zone position is disturbed, thus modifying the track of the short wave disturbance embedded within this flow. I prefer to look at the 850 mb temperature advection maxima for forecast track.

With a reasonable knowledge of the current flow and a little bit of math, you can make reasonable forecasts of what the flow might look like in 6-12 hours, but it is pretty tough to go much beyond that without the help of computer models.

Glen

 

[Tim Johnson]

Facinating!!!

Anyone know of or have a case study, or event, that can be tied to a shortwave playing a large role in the devlopment of severe weather? I would imagine that would be highly educational...for me at least :lol:

 

[Robert Dewey]

I am not positive, since I haven't done any studies on this event, but I think May 3, 1999 would be a good one. If I recall correctly, I believe the shortwave pulled out of the southwest and was actually stronger than predicted (which also enhanced the mid level jet), and resulted in the damaging tornado outbreak across OK...

 

[Gabe Garfield]

Robert is absolutely correct. The shortwave trough associated with the May 3rd outbreak had wind speeds that were 10-15 kts stronger than was forecast. For more information, refer to the following paper by SPC's Roger Edwards and Rich Thompson: http://www.spc.noaa.gov/publications/thomp.../3may99/waf.htm

Gabe