MET 61

1 MET 61 Introduction to MET 61 Introduction to MeteorologyMeteorology

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

MET 61

2 MET 61 Introduction to MET 61 Introduction to MeteorologyMeteorology

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

MET 61

3 MET 61 Introduction to MET 61 Introduction to MeteorologyMeteorology

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.

MET 61

4 MET 61 Introduction to MET 61 Introduction to MeteorologyMeteorology

MET 61

15 MET 61 Introduction to MET 61 Introduction to MeteorologyMeteorology

Fig. 13.2

MET 61

16 MET 61 Introduction to MET 61 Introduction to MeteorologyMeteorology

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

MET 61

18 MET 61 Introduction to MET 61 Introduction to MeteorologyMeteorology

Upper air divergence required for low to develop at the surface

Notice tilting with altitude of high and lows

MET 61

19 MET 61 Introduction to MET 61 Introduction to MeteorologyMeteorology

Baroclinic Wave Theory

Upper air flow: interrupted by waves imbedded in the flow– long waves and short

waves

MET 61

20 MET 61 Introduction to MET 61 Introduction to MeteorologyMeteorology

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)

MET 61

21 MET 61 Introduction to MET 61 Introduction to MeteorologyMeteorology

MET 61

22 MET 61 Introduction to MET 61 Introduction to MeteorologyMeteorology

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

MET 61

23 MET 61 Introduction to MET 61 Introduction to MeteorologyMeteorology

Development of a Baroclinic Wave

MET 61

24 MET 61 Introduction to MET 61 Introduction to MeteorologyMeteorology

MET 61

26 MET 61 Introduction to MET 61 Introduction to MeteorologyMeteorology

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.

MET 61

27 MET 61 Introduction to MET 61 Introduction to MeteorologyMeteorology

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.

MET 61

29 MET 61 Introduction to MET 61 Introduction to MeteorologyMeteorology

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

MET 61

31 MET 61 Introduction to MET 61 Introduction to MeteorologyMeteorology

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.

MET 61

32 MET 61 Introduction to MET 61 Introduction to MeteorologyMeteorology

PVANVA

MET 61

33 MET 61 Introduction to MET 61 Introduction to MeteorologyMeteorology

Relationship to vertical motion field

MET 61

34 MET 61 Introduction to MET 61 Introduction to MeteorologyMeteorology

MET 61

36 MET 61 Introduction to MET 61 Introduction to MeteorologyMeteorology

0 gV

Geostrophic Wind

x

pfv

1

y

pfu

1

Pf

kVg 1ˆ

MET 61

38 MET 61 Introduction to MET 61 Introduction to MeteorologyMeteorology

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.

MET 61

40 MET 61 Introduction to MET 61 Introduction to MeteorologyMeteorology

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.

MET 61

42 MET 61 Introduction to MET 61 Introduction to MeteorologyMeteorology

C

g

B

g

A

pV

ff

Vp

f

p

f

2

2

0

02

2202

1

1

Quasi-Geostrophic TheoryOmega Equation

gwDt

Dp

MET 61

43 MET 61 Introduction to MET 61 Introduction to MeteorologyMeteorology

p

ffVt ggg

0

Quasi-Geostrophic TheoryVorticity Equation

MET 61

44 MET 61 Introduction to MET 61 Introduction to MeteorologyMeteorology

Idealized Secondary Circulation Associated with a Developing Baroclinic Wave from a Q-G Perspective