Vertical velocity and relative vorticity

[Paul]

Could someone please explain what vertical velocity and relative vorticity means please , ive been looking at these charts for some time on NW Extra and its baffling me to understand them ..

cna someone please help

nigel

[full_frontal_occlusion]

Vorticity

The measure of rotation of a finite air packet i.e the packet spins as it moves along its path with the axis of rotation extending in any direction. Meteoriologically, the rotational motion of the air about an axis is primarily perpendicular to the earth's surface.

No spin = zero vorticity. In the Northern Hemisphere, +ve vorticity is anticlockwise (cyclonic).

Relative Vorticity

The sum of the rotation of an air packet about the axis of the pressure system together with the rotation of the packet about its own axis.

Vertical Velocity

Applied to vorticity is the perpendicular velocity (rate of change of distance) of the air packet relative to the plane parallel with the earths surface.

The maths gets complex requirig a knowledge of div, grad and curl.

Hope this helps!

ffO

[multi cellular thunderstorm]

While I was looking for something easier to understand I found this Powerpoint presentation which is far more complicated.

It could be helpful for some on here but it goes straight over my head

www.atm.ox.ac.uk/user/mpalmer/lectures/l8_mike.ppt

[BrickFielder]

Where air of one temperature meets air of another temperature then you will tend to get wind moving along the boundary. These winds are part of the explanation behind vorticity. Lets take a step back though and think about something a little simpler like a merry go round in a park. If you run past the merry go round and give it a quick shove with your hand then it will rotate. If we then start to think about lots of people running past on one side and giving the merry go round a shove and others walking past on the other side and giving a lighter shove then the merry go round will rotate according to the shoves of the faster moving people. It is the same with a parcel of air with wind moving faster on one side of the parcel giving the air parcel a spin in a particular direction. Winds can differ at both different levels and in a horizontal plane and the amount of relative spin (Compared to earth spin) determines whether air is likely to rise or sink at a synoptic level. Generally low pressure systems have areas of higher relative vorticity and high pressure has lower relative vorticity which equates rising air and cloud, to higher relative vorticity.

Vorticity charts are looked at in a number of ways. Firstly in conjunction with jetstream charts where areas of divergence (jetstreak left exit and right entrance) are likely to increase vorticity. Here we are looking for a general lifting of the atmosphere due to approaching higher vorticity (PVA) or the oposite sinking air (NVA). Thunderstorms and clouds are less likely to form in areas of NVA or receeding vorticity.Put simply approaching vorticity increases the risk of cloud and showers.

Secondly we are looking at vorticity to give an indication of the boundaries between one air mass and another. Quite often cooler air coming in aloft will be preceeded by increased vorticity due to the increased wind at the air boundary. Here we are looking at vorticity to mark a trough or possibly changes in the stability of the atmosphere due to temperature differences between different levels in the atmosphere. Put simply vorticity can mark cooler air and increased instability(depending on height - upper cool air/vorticity) with again a greater risk of cloud and showers.

Low level vorticity (850hPa ,700hPa) also marks the boundary between different air masses and these charts are used more to detect frontal systems. Put simply increased relative vorticity can show the location of cold fronts.