Lecture 11: Mid-Latitude Cyclones

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Transcript Lecture 11: Mid-Latitude Cyclones

Meteo 3: Chapter 12/13
The Cyclone Model: Common
characteristics and evolution of midlatitude lows
Read pages 488-489, 492-494,
Mid-latitude cyclones
 Recall weather is caused by uneven radiative
heating of the earth
 These cyclones (low-pressure centers) are
another mechanism by which the atmosphere tries
to mitigate temperature contrasts
 Cold air brought equatorward, warm air brought
How highs and lows strengthen,
despite surface wind flow
• For lows to strengthen, upper-level divergence
must exceed surface convergence
• For highs to strengthen, upper-level
convergence must exceed surface divergence
 Advection: The horizontal transport of
some atmospheric quantity by the wind (i.e.
temperature, moisture)
 Vorticity: Measure of amount of rotation
Origins of a mid-latitude cyclone
 Upper-trough must approach and bring
upper-level divergence for pressure to lower
(column weight to decrease)
 This begins cyclogenesis- formation of low
pressure center
Cyclonic (positive) vorticity in troughs,
anticyclonic (negative) vorticity in ridges
 Cyclonic = counterclockwise
 Anticyclonic = clockwise
 Assumption: Air parcels moving faster than trough-ridge system
Stationary front: A storm’s breeding ground
More on cyclones- Advection
 Warm sector: Lies between cold and warm front,
mild with nearly uniform temperature and moisture
 Cold advection behind cold front
 Warm advection ahead of warm front
– Warm air forced to rise over denser, cold air =>
overrunning…lifting mechanism supports widespread
clouds and precipitation
Fronts as 2D surfaces: Dense cold air wedges
under warm air
 Low pressure systems help themselves strengthen
by concentrating temperature gradients along
fronts and promoting temperature advection
 Temperature advection sharpens the upper-level
trough, increasing the vorticity maximum,
increasing the divergence aloft to the east of the
trough, thereby lowering the surface pressure
Cyclone evolution
500 mb heights
At the surface, a stationary front has
cold air to the north, warm air to the
south, and a wind shift across it. The
500 mb winds are approximately zonal,
but a disturbance (a vorticity maximum)
is approaching from the west.
surface winds
Cyclone evolution
The divergence ahead of the vorticity maximum aloft
causes a low to develop at the surface along the front. The
circulation around the low at the surface causes the front to
develop an inflection, with warm air moving northward east
of the low pressure center, and cold air moving southward
west of the low pressure center.
Cyclone evolution
The temperature advections
induced by the surface low cause
the 500 mb wave to amplify. Cold
(warm) advection west (east) of
the surface low causes the 500
mb heights to fall (rise) west (east)
of the surface low. Because the
500 mb trough is more amplified,
the vorticity maximum in the 500
mb trough axis increases, thereby
increasing the divergence east of
the trough axis, thereby causing
the surface pressure to drop
further, thereby intensifying the
cyclonic circulation at the surface,
thereby increasing the
temperature advections and
further amplifying the 500 mb
Cyclone evolution
Eventually the 500 mb trough becomes
colocated with the surface low, the
vorticity advection at 500 mb ceases.
Thus, the surface low is no longer
situated beneath divergence aloft, and
the pressure within it starts to rise
(there is still convergence at the
surface, therefore there is a net
increase in the weight of the
atmospheric column). As the pressure
within the low rises, the pressure
gradient weakens, the wind circulation
around it therefore weakens, and the
storm eventually dies!
Occlusion: Low reaches its strongest point, but begins to
decay…cold front overtakes warm front…no more warm advection
over low
Motion of mid-latitude cyclones
 Pressures lower the most (a low moves in the
direction) where upper-level divergence and warm
advection ahead of a low sum to create the largest
pressure falls
– WAA lowers air density, lowering column
 Lows usually move toward area of negative
pressure tendencies via these two processes
 Highs move toward positive pressure tendencies
behind cold front
Today’s surface analysis
Weather ahead of a warm front
Patches of cirrus
Then cirrostratus (with halo?)
Lowering/thickening clouds
Falling pressure
 Steady, long duration (stratiform) precipitation from
Warm front cross section
Conveyor Belts
Conveyor belts…view from satellite
A little on the cold conveyor belt
 Initially, a cold, dry, low-level flow of air moving
west to the north of a warm front
 Moistens via falling precipitation evaporating
 Begins to ascend upward as it approaches low
 In winter, produces heavy snow to NW of low
Weather associated with cold fronts
 Convective precipitation (showers/thunderstorms)
 Decreasing pressure as it approaches…rising
pressure after its passage
 Increasingly warm and humid air ahead of cold
 Wind shift at frontal passage
 Drier, cooler air behind front
 Sinking air
 Decreasing clouds
Cold front cross section
Dry slot- Creating the comma shape
 Generated by sinking air motion
west of cyclone
 Stream of dry, cold air from
higher in atmosphere sinks as it
flows southward = dry conveyor
 Drawn east into cyclone’s
 Leads to clearing & end of
 Easily tracked on water vapor
Mixing of cold, dry air with warm air…a
cyclone’s demise