1. Fronts and Mid-latitude Cyclones
16/04/2017
Fronts and Mid-latitude Cyclones
ENVI 1400 : Lecture 4
ENVI 1400 : Meteorology and Forecasting : lecture 4

2. ENVI 1400 : Meteorology and Forecasting : lecture 4
16/04/2017
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 day-to-day variability in weather conditions.
Northwest Europe receives many different air mass types, with frequent frontal passages – results in very variable weather.
Term front coined in WW1 (by Norwegians – analogy to battle fronts: lines of conflict between different air masses - results in turmoil of bad weather
ENVI 1400 : Meteorology and Forecasting : lecture 4
ENVI 1400 : Meteorology and Forecasting : lecture 4

3. ENVI 1400 : Meteorology and Forecasting : lecture 4
Warm Front
warm air
cool air
movement of 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
cirrus
warm air
cirro-stratus
alto-stratus
~10 km
nimbo-stratus
cool air
~300 km
~500 km
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Cold Front
cold air
warm air
movement of 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
Cumulo- nimbus
Near the surface the cold air may surge forward, producing a very steep frontal zone
~10 km
cold air
warm air
~70 km
~200 km
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5. ENVI 1400 : Meteorology and Forecasting : lecture 4
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Stationary Fronts
cold air
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
warm air
In practice a front is defined as stationary if it’s motion is negligible on a synoptic chart, speed of front < 5 knots / 2.5 m/s
ENVI 1400 : Meteorology and Forecasting : lecture 4
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6. ENVI 1400 : Meteorology and Forecasting : lecture 4
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
movement of front
ENVI 1400 : Meteorology and Forecasting : lecture 4

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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
ENVI 1400 : Meteorology and Forecasting : lecture 4

9. Mid-latitude Cyclones
Low pressure systems are a characteristic feature of mid-latitude 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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: 1310 UTC
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Low pressure forms at surface over polar front due to divergence aloft L
As rotation around initial low starts, a ‘wave’ develops on the polar front
Friction effects cause surface flow around low to converge
Mass balance: inward flow compensated by large-scale lifting  cooling  cloud formation L
cloud
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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 L
Cold front overtakes warm front to form an occlusion, which works out from centre
Depression usually achieves maximum intensity hours after the start of occlusion L
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Low starts to weaken as inflowing air ‘fills up’ the low pressure L
Low continues to weaken, clouds break up L
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15. ENVI 1400 : Meteorology and Forecasting : lecture 4W
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16. ENVI 1400 : Meteorology and Forecasting : lecture 4B W C A B C A W
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17. ENVI 1400 : Meteorology and Forecasting : lecture 4W B
Cool C A B C A W
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18. ENVI 1400 : Meteorology and Forecasting : lecture 4W
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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 sinking relative to the fronts
Restricts formation of medium & high-level clouds. Frontal cloud is mainly thick stratocumulus, it’s depth limited by the subsidence inversion
Precipitation is mostly light rain or drizzle.
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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24. ENVI 1400 : Meteorology and Forecasting : lecture 4B
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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26. Mid-latitude Jet Stream0°
30°
60°
Polar Front
Tropical jet
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Pacific Polar Front
Canadian Arctic Front
Atlantic/Asiatic Arctic Front 80 60
Atlantic Polar Front
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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