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EOS 340 Atmospheric Science
EOS 340 Atmospheric Science
Rossby Waves
Synoptic Scale Weather
The general poleward pressure gradient should produce a pattern
of uniform eastward (zonal) winds. This is certainly the case for
the sub-tropical and polar jet streams which exist near the
boundaries between the Hadley and Ferrel, and Ferrel and Polar
cells respectively. But the Earth’s surface has large and
imposing contrasts associated with continental and ocean
boundaries. These boundaries introduce meridional (northsouth) undulations into the zonal flow.
In has proven useful to describe a regions weather in terms of
the re-occurring movement of near permanent air masses, fronts,
highs and lows. Any locations “climate”, can usually be well
defined by the proximity and movement of these dominant
features.
Air Masses
There are six types of air mass, defined by combinations of their
humidity and temperature at their source,
Rossby Waves.
The conservation of angular momentum across regions of the By Humidity:
maritime (moist) and
planet with different Coriolis (f) values, results in an
continental (dry)
environment that can support waves. Undulations in the zonal By Temperature: Tropical (warm)
winds, (i.e. jet stream) can be described in terms of Rossby
Polar (cold)
Waves, with wave speeds given by,
Arctic (very cold)
c = U − βl 2
Marine Arctic: mA
where U is the zonal wind speed, β=df/dy is the meridional
Marine Polar: mP
gradient in f, and l is the zonal wavelength. Often, these
Marine Tropical: mT
undulations appear to be “stationary (arrested) waves”, when
c=0. This occurs when
Fronts
ls =
U
β
Continental Arctic: cA
Continental Polar: cP
Continental Tropical: cT
The largest and more significant atmospheric fronts develop
when these major air masses meet, and advance on one another.
Since humans are most interested in a change of weather, fronts
are named for the advancing air mass.
Long waves (l>ls) travel westward, while shorter waves (l<ls),
travel eastward. The stationary waves give rise to near
permanent ridges and troughs in the 500 mb pressure field.
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EOS 340 Atmospheric Science
EOS 340 Atmospheric Science
Cold Front
When Arctic or Polar air masses advance on Polar or Tropical
air masses, respectively, then the front is a Cold Front. These
fronts are usually steep and have strong uplift and winds
associated with them. Precipitation may often start out as rain,
only to turn to sleet and snow as the front advances. Weather
maps sow these as a line with triangles pointing in the direction
of the advancing cold air mass.
Warm Front
Produces when warmer, Tropical or Polar air advances onto
colder Polar or Arctic air, respectively. Usually the uplift/slope
is more gradual, and there may be shallow temperature
inversions limiting convection. Identified by a line with semicircles, extending from the warm into the colder air mass.
Mid-Latitude Cyclones
An instability is a dynamical phenomena, akin to the random
perturbations associated with turbulence, and typically develop
along shear lines. When a zonal pressure gradient separating
Polar and Tropical air masses becomes unstable, then the
instability can grow, and a mid-latitude cyclone can develop.
The Basic Model:
1) A kink (instability) develops in the polar front.
2) The kink grows as the westward cold, and eastward warm
fronts begin to advance.
3) A low pressure regions develops at the kink, becoming the
center of rotation, and a storm in born (cyclogenesis).
4) clouds and winds continue to develop along both fronts.
5) If the cold front catches the warm, and occluded front may
develop.
6) The low pressure region may get cut off from the warmer,
tropical air, limiting the source
for
further
convection/pressure reduction, and the storm will dissipate.
Occluded Front
Because cold fronts may travel faster than warm front, they may
catch and overtake/join the two fronts: an occluded front. This
results in three air masses in close proximity: an advancing cold
air mass, a slower warm air mass, and the departing cold air
mass. A Cold occlusion occurs when the advancing cold air
mass is colder than the departing cold air mass, otherwise it’s a
warm occlusion. Identified by both triangles and semi-circles Two sources of energy exist for the storm. First is the release of
with the warmer air on the side of the triangles.
potential energy, as dense, cold air sinks underneath lighter,
warm air. Second is the release of latent heat, as the warmer air
is pushed up and water vapour condenses in to liquid water
Stationary Front
A front separating air masses, but neither is advancing on the droplets.
other. Winds may still blow along frontal region. Triangles and
semi-circles appear on opposite sides of front.
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