Frontal SystemsLessons 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

• 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.

Vis. good but

reduces in ppn.

Poor Vis

Frontal slope 1:125

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

• Pressure rises rapidly• Wind veers and decreases slightly• Temp/Dew point fall.• Vis improves• Int Ra/Hvy Sh begin from NS/Embd Cb

Good vis and SHRA.

Frontal Slope 1:50

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 anti-clockwise around the primary low.

Occluding Frontal Depression

• The point of occlusion ‘O’ (triple point) now moves to the SE.

Quasistationary Front• Is a front whose

position is almost unchanged on successive synoptic charts.

• There is a strong tendency for wave like disturbances to form due to cyclonic windshear.

Cyclonic W

indshear

L

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.