Friday, September 27, 2002
Quasi-geostrophic Theory: Vorticity advection
By the end of the class, students should be able to:
identify regions of cyclonic and anticyclonic geostrophic vorticity advection
from 500 mb maps;
link these regions to observations of precipitation and cloud;
establish where vorticity advection supports cyclogenesis
Omega and divergence profile
Quasi-geostrophic theory links divergent winds and vertical motion to
patterns in pressure and geopotential height fields.
Certain geoptential patterns induce the following vertical motion and
divergence profile. In general, vertical motion is capped at the tropopause
by the stable stratosphere and at the ground by the earth's surface.
Vorticity can be thought of as the amount of "spin" that the planet
imparts upon an air parcel; relative vorticity is imposed by the atmospheric
flow, while planetary vorticity is imparted by the Earth's rotation (omega).
There are two types of relative vorticity. Since the upper atmosphere is
in quasi-geostrophic balance, these correspond to patterns in the
NB: Positive (negative) vorticity is anticyclonic (cyclonic) in
Vorticity advection (troughs, ridges and curvature)
Quasi-geostrophic theory and practical experience predicts that we can
diagnose the vertical wind field from the 500 mb vorticity advection. The
following are common patterns associated with curvature brought on by troughs
These patterns can be found in the 500 mb
charts in top left panel of standard four panel analyses and forecasts.
A string of short waves resides over the western plains of the us. The light blue
circle shows a zone of NVA. This is associated with a
ridge in the SLP field (see top right panel of chart). The light pink circle shows
a zone of PVA. This is associated with a cyclone in the SLP pressure field.
Vorticity advection (jet streaks)
Jet streaks (maxima in the retstream) generate ascent in right exit and
left entrance regions:
The 300 and 250 mb pressure level is generally closer to the jet streak maximum
at the tropopause, so that jet streaks are best identified on
250 and 300 mb maps.
Satellite Servers: (satellite instruments and applications)
Realtime Satellite Data