Meteo 3: Chapter 12/13Meteo 3: Chapter 12/13

The Cyclone Model: Common The Cyclone Model: Common characteristics and evolution of mid-characteristics and evolution of mid-

latitude lowslatitude lowsRead pages 488-489, 492-494,Read pages 488-489, 492-494,

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Mid-latitude cyclonesMid-latitude cyclones

Recall weather is caused by uneven radiative Recall weather is caused by uneven radiative heating of the earthheating of the earth

These cyclones (low-pressure centers) are These cyclones (low-pressure centers) are another mechanism by which the atmosphere tries another mechanism by which the atmosphere tries to mitigate temperature contrasts to mitigate temperature contrasts

Cold air brought equatorward, warm air brought Cold air brought equatorward, warm air brought polewardpoleward

How highs and lows strengthen,despite surface wind flow

• For lows to strengthen, upper-level divergencemust exceed surface convergence

• For highs to strengthen, upper-levelconvergence must exceed surface divergence

tropopause

TerminologyTerminology

Advection: Advection: The horizontal transport of The horizontal transport of some atmospheric quantity by the wind (i.e. some atmospheric quantity by the wind (i.e. temperature, moisture)temperature, moisture)

Vorticity: Vorticity: Measure of amount of rotationMeasure of amount of rotation

Origins of a mid-latitude cycloneOrigins of a mid-latitude cyclone

Upper-trough must approach and bring Upper-trough must approach and bring upper-level divergence for pressure to lower upper-level divergence for pressure to lower (column weight to decrease)(column weight to decrease)

This begins This begins cyclogenesiscyclogenesis- formation of low - formation of low pressure centerpressure center

Cyclonic (positive) vorticity in troughs, Cyclonic (positive) vorticity in troughs, anticyclonic (negative) vorticity in ridgesanticyclonic (negative) vorticity in ridges

Cyclonic = counterclockwiseCyclonic = counterclockwise Anticyclonic = clockwiseAnticyclonic = clockwise Assumption: Air parcels moving faster than trough-ridge systemAssumption: Air parcels moving faster than trough-ridge system

Stationary front: A storm’s breeding groundStationary front: A storm’s breeding ground

More on cyclones- AdvectionMore on cyclones- Advection

Warm sector:Warm sector: Lies between cold and warm front, Lies between cold and warm front, mild with nearly uniform temperature and moisturemild with nearly uniform temperature and moisture

Cold advection behind cold frontCold advection behind cold front Warm advection ahead of warm frontWarm advection ahead of warm front

– Warm air forced to rise over denser, cold air => Warm air forced to rise over denser, cold air => overrunning…lifting mechanism supports widespread overrunning…lifting mechanism supports widespread clouds and precipitationclouds and precipitation

Fronts as 2D surfaces: Dense cold air wedges Fronts as 2D surfaces: Dense cold air wedges under warm airunder warm air

Self-developmentSelf-development

Low pressure systems help themselves strengthen Low pressure systems help themselves strengthen by concentrating temperature gradients along by concentrating temperature gradients along fronts and promoting temperature advectionfronts and promoting temperature advection

Temperature advection sharpens the upper-level Temperature advection sharpens the upper-level trough, increasing the vorticity maximum, trough, increasing the vorticity maximum, increasing the divergence aloft to the east of the increasing the divergence aloft to the east of the trough, thereby lowering the surface pressuretrough, thereby lowering the surface pressure

Cyclone evolution

At the surface, a stationary front hascold air to the north, warm air to thesouth, and a wind shift across it. The500 mb winds are approximately zonal,but a disturbance (a vorticity maximum)is approaching from the west.

500 mb heights

surface winds

The divergence ahead of the vorticity maximum aloftcauses a low to develop at the surface along the front. Thecirculation around the low at the surface causes the front todevelop an inflection, with warm air moving northward eastof the low pressure center, and cold air moving southwardwest of the low pressure center.

Cyclone evolution

The temperature advectionsinduced by the surface low causethe 500 mb wave to amplify. Cold(warm) advection west (east) ofthe surface low causes the 500mb heights to fall (rise) west (east)of the surface low. Because the500 mb trough is more amplified,the vorticity maximum in the 500mb trough axis increases, therebyincreasing the divergence east ofthe trough axis, thereby causingthe surface pressure to dropfurther, thereby intensifying thecyclonic circulation at the surface,thereby increasing thetemperature advections andfurther amplifying the 500 mbwave!

Cyclone evolution

Eventually the 500 mb trough becomescolocated with the surface low, thevorticity advection at 500 mb ceases.Thus, the surface low is no longersituated beneath divergence aloft, andthe pressure within it starts to rise(there is still convergence at thesurface, therefore there is a netincrease in the weight of theatmospheric column). As the pressurewithin the low rises, the pressuregradient weakens, the wind circulationaround it therefore weakens, and thestorm eventually dies! THE END

Cyclone evolution

https://www.e-education.psu.edu/courses/meteo101/Images/Section9/MLcyclone_0905.swf

Occlusion: Low reaches its strongest point, but begins to decay…Occlusion: Low reaches its strongest point, but begins to decay…cold front overtakes warm front…no more warm advection over lowcold front overtakes warm front…no more warm advection over low

Motion of mid-latitude cyclonesMotion of mid-latitude cyclones

Pressures lower the most (a low moves in the Pressures lower the most (a low moves in the direction) where upper-level divergence and warm direction) where upper-level divergence and warm advection ahead of a low sum to create the largest advection ahead of a low sum to create the largest pressure fallspressure falls– WAA lowers air density, lowering column WAA lowers air density, lowering column

weights/pressureweights/pressure

Lows usually move toward area of negative Lows usually move toward area of negative pressure tendencies via these two processespressure tendencies via these two processes

Highs move toward positive pressure tendencies Highs move toward positive pressure tendencies behind cold frontbehind cold front

Today’s surface analysis

Weather ahead of a warm frontWeather ahead of a warm front

Patches of cirrusPatches of cirrus Then cirrostratus (with halo?) Then cirrostratus (with halo?) Lowering/thickening cloudsLowering/thickening clouds Falling pressureFalling pressure

Steady, long duration (stratiform) precipitation Steady, long duration (stratiform) precipitation from nimbostratusfrom nimbostratus

Warm front cross sectionWarm front cross section

Conveyor BeltsConveyor Belts

Conveyor belts…view from satelliteConveyor belts…view from satellite

A little on the cold conveyor beltA little on the cold conveyor belt

Initially, a cold, dry, low-level flow of air moving Initially, a cold, dry, low-level flow of air moving west to the north of a warm frontwest to the north of a warm front

Moistens via falling precipitation evaporatingMoistens via falling precipitation evaporating Begins to ascend upward as it approaches lowBegins to ascend upward as it approaches low In winter, produces heavy snow to NW of lowIn winter, produces heavy snow to NW of low

Weather associated with cold frontsWeather associated with cold fronts

Convective precipitation (showers/thunderstorms)Convective precipitation (showers/thunderstorms) Decreasing pressure as it approaches…rising Decreasing pressure as it approaches…rising

pressure after its passagepressure after its passage Increasingly warm and humid air ahead of cold Increasingly warm and humid air ahead of cold

front….instability!front….instability! Wind shift at frontal passageWind shift at frontal passage Drier, cooler air behind frontDrier, cooler air behind front Sinking airSinking air Decreasing cloudsDecreasing clouds

Cold front cross sectionCold front cross section

Dry slot- Creating the comma shapeDry slot- Creating the comma shape

Generated by sinking air motion Generated by sinking air motion west of cyclonewest of cyclone

Stream of dry, cold air from Stream of dry, cold air from higher in atmosphere sinks as it higher in atmosphere sinks as it flows southward = dry conveyor flows southward = dry conveyor beltbelt

Drawn east into cyclone’s Drawn east into cyclone’s circulationcirculation

Leads to clearing & end of Leads to clearing & end of precipitationprecipitation

Easily tracked on water vapor Easily tracked on water vapor imageryimagery

Mixing of cold, dry air with warm air…a Mixing of cold, dry air with warm air…a cyclone’s demisecyclone’s demise