Please enable JavaScript.
Coggle requires JavaScript to display documents.
Fronts & mid-latitude cyclones (Norwegian cyclone model (inward flow…
Fronts & mid-latitude cyclones
Fronts
It is a region of significant horizontal gradients in temp or humidity
Fronts are typically 100-200km wide with possibility of v sharp transitions
Boundary between two characteristically different air masses
They are a dominant feature of mid latitudes, especially fronts associated w/ low pressure:
Mid-latitude cyclones
Extra tropical cyclones
Depressions
Frontal cyclones
Cold front can catch up to warm front to form an occluded front- where two fronts mix
Fronts are found embedded in
frontal cyclones
Warm air behind front (semi circles) is a warm front
Cold air behind front (triangles) is a cold front
Warm front
Inclination of frontal surface is v shallow: 0.5 to 1 degree
The front approaching can be seen as a high cirrus/ cirrostratus cloud- slowly lowers w/ nimbostratus dominating
Warm air flows up over denser cold air
Rain starts ahead of surface front- widespread and persistent
Skies clear quickly after passage of surface front
Cold front
Type 1
Around 2 degree steeper than warm front
Deep convective clouds form above surface front- heavy rain in narrow band
Dense cold are pushes forward into warmer air & forced upwards
Near surface, cold air may surge forward- producing very steep frontal zone
Type 2
Dense cold air pushes forward over warmer and over runs it
Deep convective clouds form ahead of upper front- heavy rain
Between upper and surface fronts, there is a shallower cloud
Stationary front
Stationary front=
when boundary between air masses doesn't move
Wind speed isn't zero as air masses do still move
Small difference between air masses either side of front- defined by direction of motion only
Occluded fronts
Warm
More common in UK
Cool air pushes warm air over cold air ahead
Cold
Very rare
Cold air moves into cool air, warm air pushes together and upwards
Cold fronts move faster than warm fronts so catch up and mix to form occluded
Both warm and cold occlusions are associated with layered clouds and precipitation
Mid-lat cyclones
Low pressure systems form in well defined zones associated w/ polar front
Provide a strong temp gradient & convergent flow resulting from global circulation
Low pressure systems are characteristic of mid-lat temperate zones
Norwegian cyclone model
inward flow compensated by large-scale lifting-> cooling -> cloud formation
Surface low is maintained due to divergence aloft > convergence at surface
Flow near surface is directed towards low centre due to friction
Warm front moves slower than cold, so cold front overtakes to form an occlusion- works out from centre
Low pressure forms at surface over polar front- rotation around low causes a wave to develop around polar front
Depression usually achieves max intensity 12-24 hours after start of occlusion
Low starts to weaken as inflowing air fills up low pressure- clouds break up as low weakens
Conveyor belts
Important in transport of atmospheric properties (heat, moisture & trace gases)
Cold belt at warm front
Originates in poleward side of warm front
Air becomes saturated from rain from WCB above
Belts of low level winds ahead of cold and warm fronts
Warm belt at cold front
Main source of warm, moist air that feeds cyclone
Originates in warm sector of cyclone