Transcript Air Masses, Fronts, and Mid

Fronts and Mid-latitude
Cyclones
SOEE 1400 : Lecture 8
Fronts
A boundary between two
characteristically different ‘air
masses’ is called a front.
It is a region of significant
horizontal gradients in
temperature or humidity.
Fronts are typically 100 to 200 km
wide – but very sharp transitions
are possible over a few km or
even hundreds of metres.
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.
SOEE1400 : Meteorology and Forecasting : lecture 8
2
Potted history
• In the 19th century
forecasters believed that
cyclone were symmetrical
(like tropical cyclones).
• Margules (1905) first
showed how a front could
be sustained by the winds
blowing along the frontal
surface.
• The ‘frontal cyclone’
model was pioneered by
the Bergen school around
and just after WWI.
• This was the beginning of
major improvements in
quantitative weather
prediction – timing of
events and prediction of
their development.
SOEE 1400 : Meteorology and Forecasting : lecture 8
3
Fronts are found embedded in the
“frontal cyclone”
ENVI 1400 : Meteorology and Forecasting : lecture 8
4
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
ENVI 1400 : Meteorology and Forecasting : lecture 8
5
Cold Front - 1
• Dense cold air pushes forward into
warmer air, which is forced upward
• Steeper than warm front: ~2 or
1/50
• 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
ENVI 1400 : Meteorology and Forecasting : lecture 8
6
Cold Front - 2
~10 km
• Dense cold air pushes forward into
warmer air, and over-runs it.
• Deep convective clouds form
above upper front, heavy rain in
narrow band along upper front
• Between upper and surface fronts,
there is shallower cloud.
cold air
cold air
warm air
movement
of front
Cumulonimbus
warm air
~70 km
~200 km
ENVI 1400 : Meteorology and Forecasting : lecture 8
7
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
ENVI 1400 : Meteorology and Forecasting : lecture 8
warm air
8
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
• Warm 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 8
movement
of front
9
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, unstable air
Cold, stable air
ENVI 1400 : Meteorology and Forecasting : lecture 8
10
Cold Occlusion
Regarded to be very rare. Some
believe that they do not exist
(see Schultz and Mass, 1993, Mon.
Wea. Rev., 121, 918-940).
warm air
cold air
cool air
ENVI 1400 : Meteorology and Forecasting : lecture 8
11
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
ENVI 1400 : Meteorology and Forecasting : lecture 8
12
ENVI 1400 : Meteorology and Forecasting : lecture 8
13
31-08-2000 : 1310 UTC
ENVI 1400 : Meteorology and Forecasting : lecture 8
14
• 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
• Mass balance: inward flow
compensated by large-scale
lifting  cooling  cloud
formation
cloud
ENVI 1400 : Meteorology and Forecasting : lecture 8
15
• 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
ENVI 1400 : Meteorology and Forecasting : lecture 8
16
• Low starts to weaken as
inflowing air ‘fills up’ the low
pressure
• Low continues to weaken,
clouds break up
ENVI 1400 : Meteorology and Forecasting : lecture 8
17
ENVI 1400 : Meteorology and Forecasting : lecture 8
18
B
B
A
A
ENVI 1400 : Meteorology and Forecasting : lecture 8
19
B
B
A
A
ENVI 1400 : Meteorology and Forecasting : lecture 8
20
ENVI 1400 : Meteorology and Forecasting : lecture 8
21
An alternative to the Norwegian
model of occlusion
Incipient broad-baro-clinic phase
(I), frontal fracture (II), bent back
front and frontal T-bone (III), and
warm core frontal seclusion (IV).
Upper: sea level pressure (solid),
fronts (bold), Figure and cloud
signature (shaded). Lower:
temperature (solid), and cold and
warm air currents (solid and
dashed arrows). Shapiro and
Keyser, 1990
Shapiro, M.A. and Keyser, D. (1990) .On the structure and dynamics of fronts, jet streams and
the tropopause. Extratropical Cyclones: The Erik Palmén Memorial Volume, C. W. Newton and
E. O. Holopainen, Eds., Amer. Meteor. Soc., pp.167-191.
ENVI 1400 : Meteorology and Forecasting : lecture 8
22
Conveyor belts
• Since the 1970’s, coherent belts of low
level winds have been identified ahead of
cold and warm fronts.
• These are important in the transport
(“advection”) of atmospheric properties
such as heat, moisture and trace gases.
ENVI 1400 : Meteorology and Forecasting : lecture 8
23
Warm conveyor belt at a cold front (“JET” directed into the page here)
ENVI 1400 : Meteorology and Forecasting : lecture 8
24
Warm conveyor belt at a split cold front
(“JET” - directed into the page here)
ENVI 1400 : Meteorology and Forecasting : lecture 8
25
Cold conveyor belt at a warm front (“JET” directed into the page here)
ENVI 1400 : Meteorology and Forecasting : lecture 8
26
Warm and cold conveyor belts (WCB and
CCB) in a frontal cyclone
ENVI 1400 : Meteorology and Forecasting : lecture 8
27
ENVI 1400 : Meteorology and Forecasting : lecture 8
28
Infra-red image of the cyclone
ENVI 1400 : Meteorology and Forecasting : lecture 8
29
D
A
B
upper wind
C
ENVI 1400 : Meteorology and Forecasting : lecture 8
30
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
ENVI 1400 : Meteorology and Forecasting : lecture 8
31
The “sting in the tail”
• Highest winds are often
observed to the SW of a
secluded Keyser-Shapiro
cyclone (bottom right
panels).
Graham Parton PhD and
QJRMetS paper: statistics of
UK windstorms relative to
frontal cyclones.
ENVI 1400 : Meteorology and Forecasting : lecture 8
32
The CCB and
the sting jet
• Shapiro-Keyser
cyclone, with
warm seclusion.
• High winds in the
CCB (“CJ” here)
in the SW part of
the cyclone, on
the cold side.
• Sting Jet on the
warm side –
short-lived, very
intense winds.
SOEE 1400 : Meteorology and Forecasting : lecture 8
33
Examples of ... Sting
Jets in European
Windstorms
Tim Baker, Peter Knippertz &
Alan Blyth
Cyclone Friedhelm
08 December 2011
Cyclone Friedhelm
Cyclone Friedhelm
Cyclone Friedhelm
• Cyclone Friedhelm shows
obvious banded structure in the
cloud ahead of the main cloud
head.
• Evidence from analysis of the
Met Office UM suggests
numerous sting jets may have
occurred over Scotland.
Baker et al, 2013
Cyclone Zafer
01 December 2011
Cyclone Zafer
Cyclone Zafer
VIDEO
Cyclone Zafer
• Sections from the dropsonde leg
show two maxima in the wind
speed.
• This could be evidence of a
sting jet similar to that seen in
Cyclone Ulli.
• More analysis and modelling is
needed to confirm this.
Courtesy of Geraint Vaughan, NCAS/Uni. of
Manchester
The “sting in the tail”:
references
• Observational investigation of the
Great Storm of October 1987 (Browning
K.A., 2004, Q.J.R.Meteorol. Soc.)
• Investigation of the3-D Structure of the
Sting Jet with a high-resolution NWPModel (Clark P.A., K.A. Browning and C.
Wang, 2005, Q.J.R.Meteorol. Soc.)
ENVI 1400 : Meteorology and Forecasting : lecture 8
43