Download Extratropical Systems

ATMS 310
Extratropical Systems
This section serves as an introduction to the idea of quasi-geostrophic (QG) flow. QG is
a fancy term for “almost geostrophic”. We will talk more about this later, but the
advantage of using QG analysis is that it simplifies things.
Extratropical synoptic-scale motions (e.g. mid-latitude cyclones), QG analysis is
generally valid. These systems are responsible for our daily weather in the mid-latitudes
and are thus an important component of the atmospheric circulation. This section will
examine the observed structure of these systems. This will provide a basis for the
development of the QG equations later on.
Environment of Extratropical Development
Extratropical (ET) storms are generally highly baroclinic and asymmetrical. The
strongest winds and temperature gradients are typically found along frontal areas. To
understand these systems we first examine the mean flow (general circulation) in which
the ET storms develop.
The figure below (Fig. 6.1 from Holton) shows a meridional (all longitudes averaged for
each latitude circle) cross section (chop the atmosphere in half and look at it from the
side):
Some important things to notice:
1) The Equator-Pole temperature gradient for the Northern Hemisphere is much stronger
during the winter months (a)
2) The meridional temperature gradient in the Southern Hemisphere is generally smaller
due to having less land mass (since oceans do not change temperature as rapidly from
season to season).
3) Recall from thermal wind arguments that a stronger thermal gradient leads to a
stronger geostrophic wind shear. Geostrophic winds (“jet stream”) in the upper
troposphere (around 300 hPa) are much stronger in the winter season. The jet stream
core is typically located just below the tropopause.
4) ET systems tend to develop near jet stream cores and move downstream, steered by the
jet stream current.
Stages of Development of ET Systems
Examine the figure below. It shows the mean zonal winds at 200 mb for the northern
hemisphere winter months.
The maximum areas that you see (off eastern coasts of Asia, North America) are
climatological locations of jet stream maximas. A jet stream is a strong, narrow band of
winds in that are found near the troposphere-stratosphere boundary. Jets tend to form
above the polar frontal zone, which divides the cold polar air from the warm tropical air.
Of course this boundary moves north and south with the seasons, and jet streams are
stronger in the winter season due to the stronger north-south temperature gradients found
in the mid-latitudes during that time of year.
The top panel in the figure below shows isentropes (potential temperature lines) and
isotachs (dotted) in a cross section.
Note that the jet stream forms above the polar frontal zone, which is indicated by the
sloping, high gradient potential temperature lines from 500 mb down to the surface near
40N. The stratosphere is shown where the isentropes are stacked very closely together
∂θ
and is ultra-stable, since
>> 0 . In the bottom panel, PV is analyzed for the same
∂p
region. Note that at the polar frontal zone, high PV air is displaced down to the surface.
This is a result of the strong cyclonic vorticity created on the poleward side of the jet as
∂θ
well as the strong stability (
>> 0 ) created on the cold air side of the frontal zone.
∂p
A disturbance introduced into a jet stream region will tend to grow by drawing energy
from the jet itself. In the mid-latitudes, most ET systems are thought to develop as a
result of baroclinic instability, which is essentially an instability of the jet stream. This is
caused by a meridional temperature gradient, particularly at the surface. This condition is
found near the polar frontal zone, which divides warm tropical air from the cool, dry
polar air.
We frequently observe that the vertical structure of a developing ET system “tilts” with
height. In other words, the center of the system at 500 mb is usually well west of the
surface center. This is due to cold air advection (CAA) behind the surface cold front.
This lowers the heights west of the surface low, creating an upper level low in that
region. Note in the figures below how the 500 mb trough trails the surface low over NW
Illinois.
When an ET system becomes “stacked” – that is, when the lower and upper level lows
are on top of each other, it means the system has become occluded and dissipation will
soon follow.
500 mb – Height contours in black
Surface – isobars in black