A few model-reading questions

Now that we're heading into severe weather season, I've been paying more attention to the model data concerning severe weather development and found that I'm a bit lacking in a few areas.

1. I have an ok idea how wind shear plays into Severe Weather and basically how to read shear comparing like the 850mb and 700mb layer winds, but I know places like SPC's mesoanalysis and TwisterData have dedicated level-level shear such as This panel from TwisterData. I'm not sure how to read this. If Shear is a difference in either speed or direction over height, how can there be a single vector? I can understand a number, or colored-in-field, but not a vector.

2. I'm still not sure how to see low pressure systems, particularly upper level lows. I've looked at the 500mb wind and vorticity charts, and I know the signature of a low pressure system, but am not sure I'm interpreting them right.

3. This sort of goes with 2 above. What exactly is vorticity telling us?

Thanks for taking the time to answer these questions. I've read on theweatherprediction about this, but I'm still just not getting some of this.

KG4GUF, on 06 March 2010 - 10:17 PM, said:

1. A vector is a representation of something with both a magnitude and direction. With a shear vector it is the difference in the wind vector between two heights. The cross product of the two vectors is calculated and a third vector is found that is known as the shear vector. This product is handy for knowing the difference in wind vectors through a critical layer like 0-1km 0-3km 0-6km etc where there are some prior benchmarks for what is favorable for severe thunderstorm production.

2. Posted Image

In upper air charts whenever you have isoheights that are closed surrounding an area of lower heights relative to those around it you have a "closed low" , same but opposite true for highs although you don't hear them called "closed highs" but usually just upper ridges, not sure why thats the common vernacular but it seems to be. For troughs and ridges when you have an area of lower heights spread across a broad area but no closed contours you have a trough, the opposite is true for a ridge. There will also be a counterclockwise (lows) or clockwise (highs) kink in the winds in the open systems, or a circulation of such in a closed system.

3. Simply speaking vorticity is the amount of spin in the atmosphere. Counterclockwise spin is "positive" vorticity in the northern hemisphere while clockwise spin is "negative" vorticity. The movement of higher or lower values of vorticity into an area is called vorticity advection. Vorticity advection, specifically layer differential vorticity advection is a common tool used in the diagnosis of vertical motion. Through a layer positive differential vorticity advection suggests upward vertical motion while NDVA suggests downward vertical motion. All of that is tied together to the source of divergence/convergence causing the motions through a series of very critical equations in dynamic meteorology.