Transcript Air Masses, Fronts, and Mid

Fronts and Mid-latitude
Cyclones
ENVI 1400 : Lecture 4
Fronts
The boundary between two
different air masses is called a
front.
It is a region of significant
horizontal gradients in
temperature or humidity.
Typically 100 to 200 km wide –
very sharp transitions are
uncommon.
Fronts are a dominant feature of
mid-latitudes. In particular fronts
associated with low pressure
systems (mid-latitude cyclones,
extra-tropical cyclones,
depressions).
The movement of fronts is
responsible for much of the dayto-day variability in weather
conditions.
Northwest Europe receives many
different air mass types, with
frequent frontal passages –
results in very variable weather.
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Warm Front
• 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
warm air
cool air
movement
of front
cirro-stratus
warm air
alto-stratus
nimbo-stratus
cool air
~300 km
~10 km
cirrus
~500 km
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Cold Front
• 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
~10 km
Cumulonimbus
cold air
warm air
movement
of front
• Near the surface the cold air
may surge forward, producing
a very steep frontal zone
cold air
warm air
~70 km
~200 km
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Stationary Fronts
• There is no fundamental difference
between the air masses either side
of warm and cold fronts – the 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
zero – the air individual masses
still move, but the boundary
between them does not
cold air
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warm air
5
Occluded Fronts
• In general cold fronts move faster
than warm fronts, and may thus
catch up with a warm front ahead
– the result is an occluded front
• There are two types of occluded
fronts: warm and cold, depending
on whether the air behind the cold
front is warmer or cooler than the
air 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 decay
within mid-latitude low pressure
systems
ENVI 1400 : Meteorology and Forecasting : lecture 4
movement
of front
6
Warm Occlusion
• In both warm and cold occlusions,
the wedge of warm air is
associated with layered clouds,
and frequently with precipitation
• Precipitation can be heavy if warm
moist air is forced up rapidly by the
occlusion
warm air
cool air
cold air
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Cold Occlusion
warm air
cold air
cool air
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Mid-latitude Cyclones
31-08-2000
• Low pressure systems are a
characteristic feature of midlatitude temperate zones
• They form in well defined
zones associated with the
polar front – which provides a
strong temperature gradient –
and convergent flow resulting
from the global circulation
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31-08-2000 : 1310 UTC
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• Low pressure forms at surface
over polar front due to
divergence aloft
• As rotation around initial low
starts, a ‘wave’ develops on the
polar front
• Friction effects cause surface
flow around low to converge
cloud
• Mass balance: inward flow
compensated by large-scale
lifting  cooling  cloud
formation
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• Surface low is maintained (or
deepens) due to divergence
aloft exceeding convergence at
surface
• Flow is super-geostrophic: cold
sector air pushes cold front
forward; warm sector air flows
up warm front – warm front
moves slower than cold
• Cold front overtakes warm front
to form an occlusion, which
works out from centre
• Depression usually achieves
maximum intensity 12-24 hours
after the start of occlusion
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• Low starts to weaken as
inflowing air ‘fills up’ the low
pressure
• Low continues to weaken,
clouds break up
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B
B
A
A
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B
B
A
A
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Ana-Fronts
• Air is rising with respect to both
frontal surfaces
• Clouds are multi-layered and
deep, extending throughout the
troposphere
tropopause
warm
cold
cold
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Kata-Fronts
• Air aloft in the warm sector is
• Precipitation is mostly light rain or
sinking relative to the fronts
drizzle.
• Restricts formation of medium &
high-level clouds. Frontal cloud is
mainly thick stratocumulus, it’s
depth limited by the subsidence
inversion
tropopause
warm
subsidence inversion
Sc
Sc
cold
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 the
troposphere
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)
L
L
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D
A
B
upper wind
C
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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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Mid-latitude
Jet Stream
60°
Polar Front
Tropical
jet
30°
0°
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Pacific Polar Front
Canadian
Arctic Front
Atlantic/Asiatic
Arctic Front
80
Atlantic
Polar Front
60
Mediterranean
Front
30
Major Frontal Zones
Northern Hemisphere Winter
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500 hPa height (m), and temperature anomaly (C)
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