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MET 61 Introduction to Meteorology - Lecture 12
Midlatitude Cyclones
Dr. Eugene Cordero
San 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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MET 61 Introduction to Meteorology
Atmospheric Scales of Motion
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Scale
Time ScaleDistance Scale Examples
Macroscale
-Planetary
Weeks to years
1000-40,000km
Westerlies,
trade winds
-Synoptic
Days 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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MET 61 Introduction to Meteorology
Polar Front Theory
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 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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MET 61 Introduction to Meteorology
Fig. 13.2
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MET 61 Introduction to Meteorology
Divergence
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 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 nondivergent
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MET 61 Introduction to Meteorology
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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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MET 61 Introduction to Meteorology
Baroclinic Wave Theory
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 Upper air flow:
interrupted by waves
imbedded in the flow
– long waves and short
waves
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MET 61 Introduction to Meteorology
Baroclinic Wave Theory
 Barotropic
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 Baroclinic
– Isotherms (lines of
constant temperature)
are parallel with
isobars. If flow is
geostrophic (parallel
to isobars), no
temperature
advection can occur.
– Isotherms cross
isobars. Temperature
advection occurs (for
geostrophic flow)
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MET 61 Introduction to Meteorology
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MET 61 Introduction to Meteorology
Baroclinic Wave Theory
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 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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MET 61 Introduction to Meteorology
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Development of a Baroclinic Wave
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MET 61 Introduction to Meteorology
Vorticity Advection
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 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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MET 61 Introduction to Meteorology
Vorticity
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 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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MET 61 Introduction to Meteorology
Vorticity Advection
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 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).
NVA
 Negative vorticity advection
typically enhances a surface
high or diminish a surface
low.
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MET 61 Introduction to Meteorology
Vorticity Advection
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 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.
PVA
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MET 61 Introduction to Meteorology
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NVA
PVA
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MET 61 Introduction to Meteorology
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Relationship
to vertical
motion field
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MET 61 Introduction to Meteorology
Geostrophic Wind
1 p
 fv  
 x
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1 p
fu  
 y

1
ˆ
Vg  k 
P
f

  Vg  0
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MET 61 Introduction to Meteorology
Quasi-geostrophic (QG) theory
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 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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MET 61 Introduction to Meteorology
Quasi-geostrophic (QG) theory
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 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 threedimensional wind field can be determined by
the isobaric distribution of geopotential height
(x,y,p,t) alone.
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MET 61 Introduction to Meteorology
Quasi-Geostrophic Theory
Omega Equation
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Dp

  gw
Dt
1 2

 2 f 02  2 
f0   


  



V





f


g
2 


 p 
 p 
f0








A
B

  

  Vg   
 
p 


1
2
C
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MET 61 Introduction to Meteorology
Quasi-Geostrophic Theory
Vorticity Equation
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 g


 Vg   g  f   f 0
t
p
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Idealized Secondary Circulation Associated with a
Developing Baroclinic Wave from a Q-G Perspective
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