METEOROLOGIST JEFF HABY
Synoptic scale vorticity is analyzed and plotted on the 500-mb chart. Vorticity is a clockwise or counterclockwise
spin in the troposphere. 500-mb vorticity is also termed vertical vorticity (the spin is in relation to a vertical
axis). This vorticity is caused by
troughs and ridges and other embedded
waves or height centers (speed and directional wind changes in
relation to a vertical axis). A wind flow through a vorticity gradient will produce regions of
PVA (Positive Vorticity Advection) and NVA (Negative Vorticity Advection). PVA contributes to rising air.
Vorticity caused by a change in wind direction or wind speed with height is termed horizontal vorticity (the spin
is in relation to a horizontal axis). Horizontal vorticity is most important in the
PBL (low-levels of atmosphere).
i.e. If the wind at the surface is southeast at 30 knots and the wind speed at 700 mb is west at 60 knots, there
will be a large amount of speed and directional (veering) shear with height and therefore a large amount of
Streamwise vorticity is the amount of horizontal vorticity that is parallel to storm inflow.
Storm inflow is the velocity of the low-level wind moving toward a thunderstorm.
Helicity is the amount of streamwise
vorticity that is available to be ingested by a thunderstorm. Helicity is a great chart to use to assess horizontal
vorticity and the threat for rotating thunderstorms.
In summary, vertical positive vorticity contributes to
upper level divergence in the PVA region and thus rising air while horizontal vorticity is important to
severe weather (large values of horizontal
vorticity lead to large values of Helicity, which increases the likelihood of
tornadoes in association with
supercell thunderstorms). Both vorticity
types are a clockwise or counterclockwise rotation, but one is in relation
to a vertical axis and the other a horizontal axis.