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MET 61 Introduction to Meteorology - Lecture 12
Midlatitude Cyclones
Dr. Eugene CorderoSan Jose State University
Reading: Chapter 13 (Ahrens); Pg. 313-320 (W&H)
Class Outline:
Polar front theory Cyclone development QG Theory
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Atmospheric Scales of Motion
Scale Time Scale Distance Scale Examples
Macroscale -Planetary Weeks to years 1000-40,000km Westerlies,
trade winds
-SynopticDays to weeks 100-5000km Cyclones, anticyclones and hurricanes
Mesoscale Minutes to days 1-100km Land-sea breeze,thunderstorms and tornadoes
Microscale Seconds to minutes <1km Turbulence, dust devils and gusts
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Polar Front Theory
Low pressure or cyclones are the principal weather makers at midlatitudes.
Development of a low pressure begins with a small perturbation or disturbance along the polar front.
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Fig. 13.2
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Divergence
In order for a low pressure to develop upper level divergence must exceed surface convergence.
At upper levels, the flow is parallel to the isobars, and thus non-divergent
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Upper air divergence required for low to develop at the surface
Notice tilting with altitude of high and lows
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Baroclinic Wave Theory
Upper air flow: interrupted by waves imbedded in the flow– long waves and short
waves
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Baroclinic Wave Theory
Barotropic – Isotherms (lines of
constant temperature) are parallel with isobars. If flow is geostrophic (parallel to isobars), no temperature advection can occur.
Baroclinic – Isotherms cross
isobars. Temperature advection occurs (for geostrophic flow)
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Baroclinic Wave Theory
Warm advection – Movement of air from a warm region to a colder region.
In upper level flow this typically occurs along the downstream side of a low - air typically heading NE
– Warming air causes air to expand and diverge - divergence region - induces upward movement of air, intensifies surface low
Cold advection Opposite occurs - air moves in from cold region - air
cools, contracts, sinks - intensifies a high pressure
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Development of a Baroclinic Wave
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Vorticity Advection
Vorticity– Is a measurement of an object's circulation.
Counterclockwise (cyclonic, low) is defined as positive vorticity, clockwise (anticyclonic, high) is negative vorticity.
Planetary vorticity– The earth's rotation gives every object some vorticity
which is the Coriolis parameter, f. – f is positive for all northern latitudes. f is zero at the
equator and maximum in magnitude at the poles.
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Vorticity
Relative vorticity – This is an object's
local rate of circulation, ignoring planetary vorticity, eg. a skater spinning.
Absolute vorticity
– The sum of planetary and relative vorticity.
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Vorticity Advection
Vorticity Advection– Air that flows from a high
to a a low moves from a low vorticity environment (the high) to a region of high vorticity (a low).
This is called negative vorticity advection (NVA).
Negative vorticity advection typically enhances a surface high or diminish a surface low.
NVA
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Vorticity Advection
PVA
Vorticity Advection– Air that flows from a low
to a high moves from a high vorticity environment (the low) to a region of low vorticity (a high).
This is called positive vorticity advection (PVA).
Positive vorticity advection typically enhances a surface low or diminish a surface high.
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PVANVA
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Relationship to vertical motion field
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0 gV
Geostrophic Wind
x
pfv
1
y
pfu
1
Pf
kVg 1ˆ
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Quasi-geostrophic (QG) theory
Quasigeostrophic theory—A theory of atmospheric dynamics that involves the quasigeostrophic approximation in the derivation of the quasigeostrophic equations.
Quasigeostrophic theory is relatively accurate for synoptic scale atmospheric motions in which the Rossby number is less than unity.
However, it cannot accurately describe some atmospheric structures such as fronts or small strong low pressure cells.
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Quasi-geostrophic (QG) theory
Aimed to help diagnose observational structures and predict future developments.
QG analysis is great simplification over full primitive equations.
The result of this analysis is that for flow is hydrostatic and nearly geostrophic, the three-dimensional wind field can be determined by the isobaric distribution of geopotential height (x,y,p,t) alone.
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C
g
B
g
A
pV
ff
Vp
f
p
f
2
2
0
02
2202
1
1
Quasi-Geostrophic TheoryOmega Equation
gwDt
Dp
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p
ffVt ggg
0
Quasi-Geostrophic TheoryVorticity Equation
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Idealized Secondary Circulation Associated with a Developing Baroclinic Wave from a Q-G Perspective