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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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ENVI 1400 : Meteorology and Forecasting : lecture 4 3
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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ENVI 1400 : Meteorology and Forecasting : lecture 4 4
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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ENVI 1400 : Meteorology and Forecasting : lecture 4 5
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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ENVI 1400 : Meteorology and Forecasting : lecture 4 6
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
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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
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cold air
warm air
cool air
Cold Occlusion
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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
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31-08-2000 : 1310 UTC
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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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ENVI 1400 : Meteorology and Forecasting : lecture 4 13
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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ENVI 1400 : Meteorology and Forecasting : lecture 4 14
Low starts to weaken asinflowing air fills up the lowpressure
Low continues to weaken,clouds break up
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ENVI 1400 : Meteorology and Forecasting : lecture 4 15
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ENVI 1400 : Meteorology and Forecasting : lecture 4 16
A
BA
B
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ENVI 1400 : Meteorology and Forecasting : lecture 4 17
A
BA
B
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ENVI 1400 : Meteorology and Forecasting : lecture 4 18
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ENVI 1400 : Meteorology and Forecasting : lecture 4 19
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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ENVI 1400 : Meteorology and Forecasting : lecture 4 20
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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ENVI 1400 : Meteorology and Forecasting : lecture 4 21
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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ENVI 1400 : Meteorology and Forecasting : lecture 4 22
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500mb surface height (dm)
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ENVI 1400 : Meteorology and Forecasting : lecture 4 24
upper wind
A
B
C
D
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ENVI 1400 : Meteorology and Forecasting : lecture 4 25
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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ENVI 1400 : Meteorology and Forecasting : lecture 4 26
0
30
60
Polar Front
Mid-latitude
Jet Stream
Tropical
jet
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ENVI 1400 : Meteorology and Forecasting : lecture 4 27
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)