Velocity and Vorticity charts

[dave48]

I have been looking at these charts a lot recently but don't really understand what they mean. I have gone over all the wikipedia descriptions and others I can find on the internet but it is difficult to but the concepts into context. I would really appreciate it if anyone could give a brief definition of the concepts, how they relate to forecasting, particularly thunderstorms and also what are the rough sort of numbers to be looking out for on the charts. Many thanks.

[J07]

I'm guessing you're on NW Extra and that's where the charts are.

Vorticity is a fluid dynamical concept, it's actually a vector, which means that giving a purely numerical value for it is essentially meaningless. However, usually, values given for "vorticity" are actually "zeta" which is taken to be the vorticity about a vertically oriented axis.

It's kind of a "microscopic" view of the circulation of a fluid about an axis. It's related to velocity because it is the curl of velocity.

Vorticity can be split into two different types: Shear and Curvature. Curvature vorticity occurs when an isobar turns a corner. If it turns cyclonically then there is cyclonic vorticity. Shear is perhaps harder to visualise and relates to the change in speed of a fluid. For me the best way to picture it is to think of "water wheels". A slow velocity at the "bottom" of the wheel and a fast velocity at the "top" of the wheel will give rise to shear vorticity.

In terms of meteorology, someone else will answer a lot better than me, but forecasters are often interested in areas of cyclonic vorticity advection. In the Northern Hemisphere this is often called positive vorticity advection....which is just an arbitrary convention that makes cyclonic vorticity positive in the NH.

Basically it is concerned with how the thermal wind is advecting vorticity downstream. If you are in a region *between* an upper trough and an upper ridge (say, 500hPa) then you will likely be in a zone of CVA (PVA), because the wind will be advecting more cyclonic vorticity towards you. This is assuming an idealised baroclinic wave, so just imagine a roughly westerly surface wind where you are standing and the 500hPa trough to your west.

CVA is important because it directly implies upward motion in the atmosphere. The reasons for this are a bit hard to visualise and are contained in an idea called the quasi geostrophic theory.

So if you are downstream of an upper trough, you are in an area which favours upward motion in the atmosphere, which obviously can be a great help for convection in general.

My internet is being pathetically slow so I can't check if those charts are on free NW or not. I suspect not. I can only imagine that a field giving vorticity values will be giving you zeta, and it will not be splitting it into shear and curvature seperately. So in areas of cyclonically turning isobars you should see cyclonic vorticity values, assuming the shear is also acting in the same way.

My guess is that to find CVA you will have to put two-and-two together yourself and see where the wind is advecting these cyclonic vorticities into interesting locations.

[BrickFielder]

When you pull the plug out of the bath you get a little vortex of water going down into the hole. Depending on whether you are in the northern or southern hemisphere then the water will rotate around the plug in a particular dirction. What causes the vortex is the fast moving water going down through the hole. Now lets rotate the concept and suggest that we have a hose pipe which we can focus down to a small high speed jet of water and now lets supose we are under water. When we fire the jet of water we should expect a vortex to occur around the jet of water. Let us change water to air and instead of a stream of water we have a stream of fast moving air (a jet stream) and we get the same vortex around the jet.

The net effect is that for approaching vorticity you tend to get rising air ahead of the vorticty (PVA Positive Vorticity Advection) and sinking air behind (NVA Negative vorticity Advection). I have over simplified and you have to take into account other factors particularly whether air is converging or diverging and at what level.

We know by the way winds curve around long waves and jet streaks that we get particular areas of Convergence and Divergence of air associatd with these features.

In practice we look at high level winds to see where they diverge as this is a good indication of rising air and cloud formation with an area to investigate storm wise.

Lots of things actually combine to get a net lifting or sinking of air and the end result is a vertical velocity (speed of rising or sinking air) . On the charts you will find at netweather then a negative value means rising air. To get storms you need strongly rising air at all levels. perhaps the following chart helps a little with the concept of vertical velocities.

Hope that helps a little perhaps John or Nick will be along later with a clearer explaination. You can delve a lot deeper into this with the concepts of conservation of mometum and how each small parcel of air interacts with each other and then on into Quasi-Geostrophic theory and equations but conceptually the generalisations I have given you should be a good start.

Edited August 1, 2008 by BrickFielder

[johnholmes]

enough there Dave to keep you occupied reading and understanding it, tks Brick and J

[dave48]

Two very long and detalied responses ! Thanks so much for your help guys, I really appreciate it, that has really helped a lot.

Edited August 1, 2008 by dave48