Transcript Frontal Systems - meteonaa.esy.es

Frontal Systems
Lessons 35/36/37
Definition of a Front
• A front is a zone of transition between two contrasting air masses.
• Within the frontal zone exist strong horizontal gradients of temperature
and humidity.
• The interaction of two air masses at the frontal surface frequently gives
rise to a great deal of frontal cloud and weather.
Definition of a Front, cont’d
• A frontal surface separating two air masses slopes upwards over the
colder air. The surface itself is about 3000 feet in thickness.
• Typical frontal surface slopes are between 1:100 to 1:150.
Warm Front
• Warm air over
rides cold air
ahead.
• Gradual
lifting results
in layer cloud.
• NS,AS,CS,CI.
Warm Front Weather
Frontal slope
1:125
Vis. good but
reduces in ppn.
Poor Vis
•
•
•
•
•
Wind veers & increases slightly
Temperature/Dewpoint rise.
Pressure steadies
Ppn changes from RA to DZ or DZRA.
Cloud changes from NS to ST or SC.
Cold Front
• Cold air
undercuts the
warmer air
ahead
• Rapid lifting
causes
instability
and CB
clouds
Global Distribution of Fronts
•
•
•
•
Polar Fronts
The Arctic/Antarctic Fronts
Mediterranean Front
Inter-tropical Convergence Zone (ITCZ), or Inter-tropical
Front (FIT).
North Atlantic Polar Front
• The North Atlantic Polar Front is the boundary
between the cold polar air masses to the north and
the warmer tropical air masses to the south.
• In summer the NAPF lies further to the north
from about Newfoundland to North of Scotland.
• In winter it lies further south from Florida to
towards the SW of the British Isles.
• Lows form along the line of the front in
association with jet streams.
Classification of Fronts
• Generally classified as cold/warm and as kata/ana fronts.
• Ana-fronts:
– are active fronts associated with pronounced vertical motion (lifting) along
the frontal surface.
– pronounced temperature contrast across the frontal surfaces and
substantial cloud layers with associated precipitation.
Classification of Fronts con’t
• Katafronts:
– associated with subsidence aloft and a downward components at the
frontal surface.
– Thus any lifting is confine close to the surface and vertical cloud
development is curtailed
– Most of the cloud will be SC and ST with associated drizzle and patchy
light rain.
– The temperature contrast across katafronts are small and they are less
active and slow moving.
WARM ANAFRONT
Summary of Weather ahead of a Warm Front
• Increasing cloud cover (CI,CS,AS,NS and CU/SC
ahead with BKN ST in rain band) base gradually
lowering.
• Pressure falls then steadies on frontal passage.
• Wind gradually backs to SW and freshens ahead of
front, then veers on frontal passage.
• Temp and Dew point steady ahead, but rise rapidly
to meet each other on passage of front.
Summary of Weather ahead of a Warm Front,
cont’d
• Visibility good but moderate in rain with prob of
frontal fog just ahead of front.
• Low level wind shear just ahead of front
• Light to moderate continuous rain 100 - 150 nm
(200-400 km) ahead of warm front.
• Probability of rain ice below frontal inversion.
WARM KATAFRONT
Warm Kata-Front Cloud and Weather
• The weather hazards will be less severe due to the fact that it is less
active.
• Main precipitation is drizzle or a mix of drizzle and rain. Drizzle is far
more restrictive for visibility.
Warm Kata-Front Cloud and Weather
• Cloud layers have less vertical development giving thick SC and thus
• Icing is mostly light but on occasions may be moderate.
• Prolonged low level wind shear will also be less of a problem as the
wind shift at low level is less.
Cold Front Weather
Frontal
Slope 1:50
Good vis and SHRA.
•
•
•
•
•
Pressure rises rapidly
Wind veers and decreases slightly
Temp/Dew point fall.
Vis improves
Int Ra/Hvy Sh begin from NS/Embd Cb
COLD ANAFRONT
Summary of Weather Behind a Cold Front
• Low NS, AC/AS CC,CI with embedded CU/CB.
• Pressure rises on frontal passage.
• Wind veers to NW (squalls) on frontal passage
then decreases slightly.
• Temp and Dew point fall rapidly and spread on
passage of front.
COLD KATAFRONT
Cold Kata-Front Weather
• Occurs when there is a relative downward component along the
frontal surface.
• This happens when the air ahead has a greater velocity than the air
behind.
Cold Kata-Front Weather.cont’d
• Extensive layers of medium and upper layer cloud are absent and
• Mostly stratocumulus and low altocumulus layers up to about 3 to 4
km altitude.
• The shallow cloud tends to spread out on either side of the frontal
zone rather than being mostly in the warm air mass.
Cold Kata-Front Weather (contd)
• Any precipitation is light and patchy.
• Temperature contrasts may still be high across the front even if rain
is slight.
• Only a small change in wind direction across the front.
• Turbulence and icing are much less due to the lack of strong vertical
motion in the cloud structures.
Summary of Weather Behind a Cold Front
• Visibility moderate in heavy rain but good
behind front.
• Low level wind shear just behind front
• Mod/heavy intermittent rain or rain showers 50
- 100 NM wide.
• Probability of rain ice below frontal inversion.
• Clear slot behind rain band.
• TCU/CB and SHRA follow about behind cold
front.
PLAN VIEW OF WARM SECTOR POLAR FRONTAL
DEPRESSION
Summary of Weather in the Warm Sector
• Low ST or SC
• Pressure steady.
• Surface wind generally steady westerly.
• Temp and Dew point steady, little or no
spread.
• Visibility poor especially in DZ, possibility of
hill fog.
• Light to moderate intermittent DZ or DZRA.
Typical Cross Section Through a
Frontal System
Mature Frontal Depression
• The frontal wave now
has an extensive N/S
depth as:
• The cold front begins
to catch up with the
warm front.
Occluding Frontal Depression
• The cold front over
takes the warm
front forming
either a:
• Warm Occlusion or
• Cold Occlusion.
• Section BA will
show typical
profiles through
these.
Cross section through occlusions
Warm Occlusion
• Less cold air over-rides colder air ahead.
Cold Occlusion
• Colder air under-cuts less cold air ahead
Additional Frontal Facts
• Secondary lows
may form at triple
point of an
occlusion.
• Weather in these
may be worse than
primary low.
Additional Frontal Facts
• Secondary lows
may also form on
trailing cold fronts.
• Secondary lows
tend to rotate anticlockwise around
the primary low.
Occluding Frontal Depression
• The point of
occlusion ‘O’
(triple point) now
moves to the SE.
Quasistationary Front
Developing Low
• Pressure disturbances
are caused by the
cyclonic windshear
along the line of the
quasi-stationary front.
• These pressure
disturbances may
develop into frontal
lows (depressions) if
conditions are
favourable.
Why do surface lows deepen?
• Lows are convergent at the surface which produce:
– Wide spread ascent within the low.
– An area of divergence must exist aloft.
• If upper level divergence exceeds surface
convergence then the low will deepen.
– If not then the low will quickly fill.
• The upper airflow, mainly the PF Jetstream provides
the upper level divergence and convergence to
sustain the surface pressure systems.
Deepening Low
• If the upper level flow is
favourable then the
surface low will:
• continue to deepen and,
• will follow the direction
of the jetstream aloft.
Development of Surface Pressure
• Upper winds largely
dictate the development
of weather systems.
– Upper winds may take away
more air than they bring,
leading to ascent of air to
replace it and falling surface
pressure.
– Upper winds can bring more
air than they remove,
leading to descending
(subsiding) air and rising
surface pressure.
L
H
Cyclonic
Development
• The strong upper level
(300mb) flow usually
the jet stream provides
areas of:
• Divergence aloft to
deepen surface lows
and,
• Convergence aloft to
sustain surface highs
(anti-cyclones).
How do fronts form?
• A front is a boundary between air masses of
different temperature properties.
• The warm air to the east of the low moves
northward over-riding the cold air to the
north causing a:
– Warm Front.
• The cold air to the west of the low moves
southward undercutting the warm air ahead
causing a:
– Cold Front.
Developed Frontal Wave
• At this stage the fronts are
now well defined, and
• the distinctive cloud
formations, and
• associated weather patterns
are now in evidence.
• Section AB will show a
typical profile through a
frontal depression
Typical Cross Section Through a
Frontal System
Tracks of North Atlantic Lows
Families of Depressions
Typical trough through a frontal depression
Squall line
Gust Front
Mountain effects
‘Stau’
Mountain Effect (contd)
Warm
Warm
Additional Frontal Facts
• In winter a front exist in the Mediterranean.
Between the cold polar air to the north and the
warmer tropical continental air to the south.
• Arctic Maritime air usually advances behind an
Arctic Front.