04 Fronts and Mid Latitude Cyclones

8/12/2019 04 Fronts and Mid Latitude Cyclones

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Fronts and Mid-latitude

Cyclones

ENVI 1400 : Lecture 4


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ENVI 1400 : Meteorology and Forecasting : lecture 4 2

Fronts

The boundary between twodifferent air masses is called afront.

It is a region of significanthorizontal gradients in

temperature or humidity.Typically 100 to 200 km widevery sharp transitions areuncommon.

Fronts are a dominant feature ofmid-latitudes. In particular frontsassociated with low pressuresystems(mid-latitude cyclones,extra-tropical cyclones,

depressions).The movement of fronts isresponsible for much of the day-to-day variability in weatherconditions.

Northwest Europe receives manydifferent air mass types, withfrequent frontal passagesresults in very variable weather.


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Warm Front

warm air

cool air

movement

of front

cool air

warm airnimbo-stratus

alto-stratus

cirro-stratus

cirrus

~300 km ~500 km

Warm air flows up over denser

cold air Inclination of frontal surface is very

shallow: 0.5 to 1

Approach of front signalled by high

cirrus or cirrostratus, cloud base

lowering as surface front

approaches.

Rain starts ahead of surface front,

is widespread and persistent

Skies clear quickly after passage

of surface front

~10km


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Cumulo-

nimbus

Cold Front

cold air

warm air

movement

of front

cold airwarm air

~70 km ~200 km

Dense cold air pushes forward into

warmer air, which is forced upward Steeper than warm front: ~2

Deep convective clouds form

above surface front, heavy rain in

narrow band along surface front

Behind front cloud base lifts,

eventually clearing

Near the surface the cold air

may surge forward, producing

a very steep frontal zone

~10

km


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Stationary Fronts

There is no fundamental difference

between the air masses either sideof warm and cold frontsthe front

is defined by the direction of

motion

When the boundary between air

masses does not move it is called

a stationary front Note that the wind speed is not

zerothe air individual masses

still move, but the boundary

between them does not

cold air

warm air


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Occluded Fronts

movement

of front

In general cold fronts move faster

than warm fronts, and may thuscatch up with a warm front ahead

the result is an occluded front

There are two types of occluded

fronts: warmand cold, depending

on whether the air behind the cold

front is warmeror coolerthan theair ahead of the warm front

Cold occlusions are the more

common type in the UK

Occlusion is part of the cycle of

frontal development and decaywithin mid-latitude low pressure

systems


7/28ENVI 1400 : Meteorology and Forecasting : lecture 4 7

cold air

warm air

cool air

Warm Occlusion

In both warm and cold occlusions,

the wedge of warm air isassociated with layered clouds,

and frequently with precipitation

Precipitation can be heavy if warm

moist air is forced up rapidly by the

occlusion



cold air

warm air

cool air

Cold Occlusion



Mid-latitude Cyclones

Low pressure systems are a

characteristic feature of mid-

latitude temperate zones

They form in well defined

zones associated with thepolar frontwhich provides a

strong temperature gradient

and convergent flow resulting

from the global circulation

31-08-2000



31-08-2000 : 1310 UTC



Low pressure forms at surfaceover polar front due todivergence aloft

As rotation around initial lowstarts, a wave develops on thepolar front

Friction effects cause surface

flow around low to converge Mass balance: inward flow

compensated by large-scalelifting cooling cloudformationcloud


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Surface low is maintained (ordeepens) due to divergencealoft exceeding convergence atsurface

Flow is super-geostrophic: coldsector air pushes cold frontforward; warm sector air flowsup warm front warm frontmoves slower than cold

Cold front overtakes warm frontto form an occlusion, whichworks out from centre

Depression usually achieves

maximum intensity 12-24 hoursafter the start of occlusion


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Low starts to weaken asinflowing air fills up the lowpressure

Low continues to weaken,clouds break up


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A

BA

B


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A

BA

B


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Ana-Fronts

Air is rising with respect to bothfrontal surfaces

Clouds are multi-layered and

deep, extending throughout the

troposphere

tropopause

cold

warm

cold


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Kata-Fronts

tropopause

ScSc

subsidence inversion

Air aloft in the warm sector issinking relative to the fronts

Restricts formation of medium &

high-level clouds. Frontal cloud is

mainly thick stratocumulus, its

depth limited by the subsidence

inversion

Precipitation is mostly light rain ordrizzle.

cold

warm

cold


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Ana-cold fronts may occur

with kata-warm fronts, and

vice-versa.

Forecasting the extent of

rain associated with fronts is

complicated

Most fronts are not ana- or

kata- along whole length, or

at all levels within thetroposphere

Some general guidance

may be obtained from

charts of vertical velocity (eg

from NCEP)

For short-term forecasts

(periods of hours) &

nowcasts, rainfall radar

provide the best estimates

of rainfall.


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500mb surface height (dm)


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upper wind

A

B

C

D


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Crossed-Winds Rule

If an observer stands with their back to the surface wind

and estimates the direction of the upper-level winds from

motion of high-level clouds, they can a) estimate their

position within a low pressure system, and hence b)

make a rough forecast: If upper wind from your LEFT (position A), the weather is likely to

deteriorate

If upper wind from you RIGHT (position B), the weather is likely

to improve

If upper wind is BEHIND or AHEAD of you (positions C, D), there

is likely to be little change in the weather


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0

30

60

Polar Front

Mid-latitude

Jet Stream

Tropical

jet


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30

60

80

Major Frontal Zones Northern Hemisphere Winter

Atlantic

Polar Front

Pacific Polar Front

Canadian

Arctic Front

Atlantic/Asiatic

Arctic Front

Mediterranean

Front


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ENVI 1400 M t l d F ti l t 4 28

500 hPa height (m), and temperature anomaly (C)

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04 Fronts and Mid Latitude Cyclones