Meteo 3: Chapter 12/13 The Cyclone Model: Common characteristics and evolution of mid- latitude lows Read pages , ,

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 poleward

Terminology  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

Self-development  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

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 weights/pressure  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 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 nimbostratus

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 front….instability!  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 belt  Drawn east into cyclone’s circulation  Leads to clearing & end of precipitation  Easily tracked on water vapor imagery

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