500mb Vort maps???

[Jared Farrer]

I was hoping someone could explain to me in very simple terms and maybe even paint some areas on the map or maps to show me how to read a 500mb vort map. I still do not have a good grasp of how to read a map very well at all right now. I have read alot on the subject but still fail at reading them. I cant make sense of the features. Could someone please explain this to me?

 

[Derek Weston]

Good question. I haven't taken the time to learn exactly what the significance of this data is as applied to forecasting.

 

[rdale]

You might get a more thorough explanation by posting this in the educational forum...

 

[Brandon Sullivan]

here ya go. take this link.. can sure describe it better than me..

http://www.google.com/custom?domains=www.theweatherprediction.com&q=500mb+vorticity&sa=Search&sitesearch=www.theweatherprediction.com&client=pub-2296614687681548&forid=1&ie=ISO-8859-1&oe=ISO-8859-1&cof=GALT%3A%23008000%3BGL%3A1%3BDIV%3A%23336699%3BVLC%3A663399%3BAH%3Acenter%3BBGC%3AFFFFFF%3BLBGC%3A336699%3BALC%3A0000FF%3BLC%3A0000FF%3BT%3A000000%3BGFNT%3A0000FF%3BGIMP%3A0000FF%3BFORID%3A1&hl=en

 

[Michael P. Morris]

Without getting into a full-blown discussion of QG theory, the thing that you want to look for severe-weather-wise is areas of positive vorticity advection at 500 millibars (i.e. downstream of a vorticity max, such as southern Iowa/northern Missouri on the map you posted). This region will be favored for synoptic-scale uplift, which can prime the environment for severe weather by weakening the cap or generating/intensifying a surface low.

For completeness, it should be noted that the exact degree of vertical motion depends on the vertical profile of vorticity advection, not its value on a single pressure surface. The strongest area of vertical motion will occur where the positive vorticity advection increases with height (i.e. no PVA at the surface, some PVA at 850/700, and a lot at 500). There is also a thermal advection component as well, such that warm air advection on a constant pressure surface is associated with vertical motion.

The exact opposite is true for downward motion - negative vorticity advection increasing with height, decreasing positive vorticity advection with height, and cold air advection on a pressure surface are all correlated with downward motion.

 

[Darrin Rasberry]

You might get a more thorough explanation by posting this in the educational forum...

Not meaning to stray too off topic, but Jared is a full member and thus not allowed to post in the educational forums (only junior members are, and approved regular members to answer them).

 

[Jared Farrer]

Thanx Mike very helpful, I am starting to pick things up here and there!

 

[rdale]

Not meaning to stray too off topic, but Jared is a full member and thus not allowed to post in the educational forums (only junior members are, and approved regular members to answer them).

Thanks for clarifying, I wasn't aware of the different levels.

 

[Darrin Rasberry]

Thanks for clarifying, I wasn't aware of the different levels.

No problem. I personally think that the educational forums should be open to everyone, given e.g. that equipment already has its own forum, but that's a thread of its own.

Related to this thread, I see only the 500mb being discussed here. Since I am personally curious about this topic also, is there one for, say, 700 and 850, and how important are those in relation to 500mb vort?

 

[Michael P. Morris]

The rules are identical at lower levels, but sometimes it is a bit more difficult to diagnose, especially at 850 since the mandatory levels below that pressure are below sea level over parts of the country. In an ideal situation, you would have a cyclone tilted to the west with height above a nearly circular surface pressure system, so that:

a) surface vorticity advection is negligible
b) low-level vorticity advection exists above or near the surface low (shortwave slightly to the west)
c) mid-level vorticity advection to the west of that

Since wind speeds tend to become stronger and more westerly with height in the Northern Hemisphere, the magnitude of vorticity advection will increase with height as well as it is dependent both on the gradient of vorticity and the magnitude of the wind vector. In this case, you will have positive vorticity advection increasing with height, inducing upward vertical motion on the synoptic scale in the region below.

You can have too much of a good thing though, such as what happens when there is a weak cap and strong cyclone aloft. I don't know exactly how to determine if the forcing is TOO strong, but perhaps one of the SPC or NWS folks can elaborate a bit on if they have a particular threshold on the omega field.

 

[Jeff Snyder]

Michael's done a very good job of explaining most of this, so I won't mention much else.

Chuck Doswell has a web-essay that discusses the link between vorticity advection and vertical motion in context of the QG approximation at --> http://www.flame.org/~cdoswell/PVA/PVA_disc.html ... Note that you're really looking for differential positive vorticity advection, though it's usually a pretty good assumption that the maximum in vorticity advection occurs somewhere in the middle layer of the troposphere. In addition, note that cyclonic vorticity advection increasing with height is equivalent to anticyclonic vorticity advection decreasing with height in regards to the QG omega equation. In addition, the QG omega equation is a diagnostic equation (i.e. there is no d/dt term). As such, it technically is not a cause-and-effect relation in the way that something happening causes something else to happen; in other words, you can't really say that DPVA causes upward motion. Rather, DPVA is associated with upward motion (neglecting thermal advection altogether). In a final word, the differential vorticity advection field is directly affected by the vertical motion field, and the vertical motion field is directly affected by the differential vorticity advection field.

 

[Jared Farrer]

OK so lets apply this to this map I am providing which is time sensitive but oh well! http://www.rap.ucar.edu/weather/model/eta30hr_500_vrt.gif

Lots of thermal advection going on below these features. Is it safe to say then, or rather to assume upward motion in the MO, NE, KS, IA areas?