Mid-Latitude Cyclones

and Fronts

Lecture 12

AOS 101

Homework 4

COLDEST TEMPS

GEOSTROPHIC BALANCE

Homework 4

L FASTEST WINDS

Rising Air

• Consider an air parcel rising through the atmosphere

– The parcel expands as it rises

– The expansion, or work done on the parcel causes the temperature to decrease

• As the parcel rises, humidity increases and reaches 100%, leading to the formation of cloud droplets by condensation

Rising Air

• If the cloud is sufficiently

deep or long lived,

precipitation develops.

• The upward motions

generating clouds and

precipitation can be

produced by:

– Convection in unstable air

– Convergence of air near a

cloud base

– Lifting of air by fronts

– Lifting over elevated

topography

Lifting by Convergence

• Convergence exists

when there is a

horizontal net inflow

into a region

• When air converges

along the surface, it is

forced to rise

Background on Cyclones

http://www.wunderground.com/hurricane/history/iop4_sat.jpg

•A cyclone is: An area of low

pressure around which the

winds flow counter-clockwise

in the northern hemisphere,

and clockwise in the southern hemisphere

• Hurricane (tropical cyclone)

• Mid-latitude cyclone

• Today, we’ll focus on mid-

latitude, or extra-tropical

cyclones, which have a life cycle and frontal structures.

Hurricanes, which we

discussed earlier, have no

fronts.

Background on Cyclones

Midlatitude cyclones are crucial in maintaining a

temperature equilibrium on our planet. This is

because in the northern hemisphere . . .

• . . . They advect warm air northward

• . . . And they advect cold air southward

• This helps maintain the radiative equilibrium on

our planet!

Background on Cyclones

• We already know that friction near the surface causes convergence into a low pressure center and that flow is counterclockwise around the low in the N. Hemisphere.

• So we end up with the cold air moving south and east and the warm air moving north and west

• Likewise, lifting by convergence forces parcels upward so we get clouds and precipitation in the vicinity of the low pressure

L

The figure to the right

represents a typical

midlatitude cyclone:

• Cold, dry air is advected

eastward behind the cold

front

• Warm, moist air is

advected north behind

the warm front

• The fronts move in the

direction the “teeth” point

Background on Cyclones

• Definition - boundary, transition zone between two different air masses

• The two air masses have different densities. Frequently, they are characterized by different temperatures and moisture contents

• Front has horizontal and vertical extent

• Frontal boundary/zone can be 1-100 km wide

• Types of synoptic-scale fronts: – warm fronts

– cold fronts

– stationary fronts

– occluded fronts

Background on Cyclones

Warm Front

• A transition zone where a warm air mass

replaces a cold air mass

• Drawn as a red line with red semi-circles

pointing in the direction of the front’s

movement

Warm Front

• Again, warm air is less dense than cold air.

• As the warm air moves north, it slides up the gently sloping warm front.

• Because warm fronts have a less steep slope than cold fronts, the precipitation associated with warm fronts is more “stratiform” (less convective), but generally covers a greater area.

Common Characteristics Associated with

Warm Fronts

Before Passing While Passing After Passing

Winds South-southeast Variable South-southwest

Temperature Cool-cold, slowly warming

Steady rise Warmer, then steady

Pressure Usually falling Leveling off Slight rise, followed by fall

Clouds Cirrus, Cirrostratus,

Nimbostratus

Stratus-type Clearing with scattered

Stratocumulus

Precipitation Light to moderate rain, snow, sleet or

drizzle

Drizzle or none Usually none, sometimes light

rain in showers

Visibility Poor Poor, but improving

Fair in haze

Dew Point Steady rise Steady Rise, then steady

• A transition zone where a cold air mass

replaces a warm air mass

• Drawn as a blue line with blue triangles

pointing in the direction of the front’s

movement

Cold Fronts

Cold Fronts

•Cold air is more dense than warm air.

• As the dense, cold air moves into the warm air region, it forces the warm air to rapidly rise just ahead of the cold front.

• This results in deeper clouds and precipitation than we saw with a warm front. The clouds that form can be convective and can be

associated with more intense precipitation and thunderstorms

• Often, the precipitation along a cold front is a very narrow line of

thunderstorms

Common Characteristics Associated with

Cold Fronts

Before Passing While Passing After Passing

Winds South-southwest Gusty; shifting West-northwest

Temperature Warm Sudden drop Steadily dropping

Pressure Falling steadily Minimum, then sharp rise

Rising steadily

Clouds Increasing: Cirrus, Cirrostratus,

Cumulonimbus

Cumulonimbus Cumulus

Precipitation Short periods of showers

Heavy rains, sometimes with

hail, thunder, lightning

Showers, then clearing

Visibility Fair to poor in haze Poor, followed by improving

Good, except in showers

Dew Point High; remains steady

Sharp drop lowering

Occluded Fronts

• A region where a faster moving cold front has caught up to a slower moving warm front.

• Generally occurs near the end of the life of a cyclone

• Drawn with a purple line with alternating semicircles and triangles

Cold Occlusion

• The type most associated with mid-latitude cyclones

• Cold front "lifts" the warm front up and over the very cold air

• Associated weather is similar to a warm front as the occluded front approaches

• Once the front has passed, the associated weather is similar to a cold front

• Vertical structure is often difficult to observe

http://apollo.lsc.vsc.edu/classes/met130/

notes/chapter11/index.html

Warm Occlusion

• Cold air behind cold front is not dense enough to lift cold air ahead of warm front

• Cold front rides up and over the warm front

• Upper-level cold front reached station before surface warm occlusion

http://apollo.lsc.vsc.edu/classes/met130/not

es/chapter11/index.html

Stationary Front

• Front is stalled

• No movement of the temperature gradient

• But, there is still convergence of winds,

and forcing for ascent (and often

precipitation) in the vicinity of a stationary

front.

• Drawn as alternating segments of red

semicircles and blue triangles, pointing in

opposite directions

1. Find the region of

lowest sea level

pressure

2. Find the center of the

cyclonic (counter-

clockwise) circulation

How to Locate a Cyclone

1. Find the region of

lowest sea level

pressure

2. Find the center of the

cyclonic (counter-

clockwise) circulation

How to Locate a Cyclone

L

How to Locate a Front We know that we need to look for low pressure and a boundary of cold and warm air.

To pinpoint the parts of our cyclone, look for specifics in the observation maps

• Find the center of cyclonic rotation

• Find the large temperature gradients

• Identify regions of wind shifts

• Identify the type of temperature advection

• Look for kinks in the isobars

Polar Front Theory - Development and

Evolution of a Wave Cyclone

Also, referred to as Norwegian Cyclone Model

(NCM)

• The wave cyclone (often called a frontal wave) develops along the polar front

• When a large temperature gradient exists across the polar front - the atmosphere contains a large amount of Available Potential Energy

NCM cont.

• Northward moving warm air

and southward moving cold

air are forced around each

other, forming a bend in the

temperature gradient (b). This

forms the warm front and the

cold front. Now with a

counter-clockwise spin, winds

converge at the newly formed

low pressure minimum at the

center of rotation.

NCM cont. • (c)- A fully-developed cyclone is seen 12-24 hours from its inception. It consists of: – a warm front moving to the northeast

– a cold front moving to the southeast

– region between warm and cold fronts is the "warm sector"

– central low pressure (low, which is deepening with time)

– wide-spread precip. ahead of the warm front

– narrow band of precip. along the cold front

– Wind speeds continue to get stronger as the low deepens

– The production of clouds and precip. also generates energy for the storm as Latent Heat is released

http://apollo.lsc.vsc.edu/classes/met130/notes/chapte

r12/index.html

NCM cont.

(d) - As the cold front moves swiftly eastward, the systems starts to occlude. – Storm is most intense at this stage

– have an occluded front trailing out from the surface low

– triple point/occlusion - is where the cold, warm, and occluded fronts all intersect

http://apollo.lsc.vsc.edu/classes/met130/no

tes/chapter12/index.html

Final Stage (e) - the warm sector

diminishes in size as the systems further occludes. – The storm has used most all of

its energy and dissipates

– cloud/precip production has diminished

– The warm sector air has been lifted upward

– The cold air is at the surface - stable situation.

• The temperature contrast which drove this whole situation from the surface perspective is no longer near the center of the wave of low pressure

http://apollo.lsc.vsc.edu/classes/met130/not

es/chapter12/index.html

Another view

Weather associated with a typical late fall to

early spring mid-latitude cyclone

Figure courtesy of Jon Martin

Precipitation Around a Cyclone and its Fronts

http://weather.unisys.com

To the right is a major cyclone

that affected the central U.S. on

November 10, 1998.

Around the cold front, the

precipitation is more intense, but

there is less areal coverage.

North of the warm front, the

precipitation distribution is more

“stratiform”: Widespread and

less intense.

Precipitation Around a Cyclone and its Fronts

Again, in this radar and surface

pressure distribution from

December 1, 2006, the

precipitation along the cold front

is much more compact and

stronger.

North of the warm front, the

precipitation is much more

stratiform.

Also note the kink in the isobars

along the cold front!

• We have all seen cyclones on weather maps,

but how do we know if it will strengthen or

weaken? The key to cyclone development is

in the upper level flow

So let’s look at Upper Levels…

Similarly to lower levels, at upper levels of the atmosphere,

there is often a series of high pressures (high heights) and

low pressures (low heights)

Typical 500 mb height pattern

Upper Levels

Ridge Trough Ridge

Why Do These Patterns Occur?

• The patterns of convergence and divergence

have to do with vorticity advection

• If there is positive vorticity advection,

divergence occurs

• If there is negative vorticity advection,

convergence occurs

• Let’s explain vorticity …

Vorticity

Vorticity is simply a

measure of how much the

air rotates on a horizontal

surface

Positive vorticity is a

counterclockwise (i.e.

cyclonic) rotation

Negative vorticity is a

clockwise (i.e. anticyclonic) rotation

Therefore, troughs contain positive vorticity, and

ridges contain negative

vorticity

Trough Ridge

Let’s Revisit …

Vorticity < 0 Vorticity < 0

Vorticity > 0

Diagnosing Vorticity Advection

• To determine vorticity advection, first find the locations of maximum (positive) vorticity and minimum (negative) vorticity

• Then, determine what direction the wind is moving

• Areas of negative vorticity advection (NVA) will be just downstream of vorticity minima, and areas of positive vorticity advection (PVA) will be just downstream of vorticity maxima

Positive

vorticity

advection

Negative

vorticity

advection

Vorticity Advection and Vertical Motion

* Positive vorticity advection (PVA) results

in divergence at the level of advection

* Negative vorticity advection (NVA)

results in convergence at the level of

advection

Vorticity Advection and Vertical Motion

Remember that convergence at upper levels is associated with

downward vertical motion (subsidence), and divergence at upper

levels is associated with upward vertical motion (ascent).

Then, we can make the important argument that . . .

Upper Tropospheric Flow and Convergence/Divergence

• Downstream of an upper tropospheric ridge, there is convergence,

resulting in subsidence (downward motion).

• Likewise, downstream of an upper tropospheric trough, there is

divergence, resulting in ascent (upward motion).

Upper Tropospheric Flow and Convergence/Divergence

• Downstream of an upper tropospheric ridge axis is a favored

location for a surface high pressure, and of course, downstream of

an upper tropospheric trough axis is a favored location for a surface

low pressure center.

Upper Tropospheric Flow and Convergence/Divergence

• Surface cyclones also move in the direction of the upper

tropospheric flow!

• The surface low pressure center in the diagram above will track to

the northeast along the upper level flow