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Meteo 3: Chapter 12/13 The Cyclone Model: Common characteristics and evolution of midlatitude lows Read pages 488-489, 492-494, 519-532 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 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 tropopause 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 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 wave! 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! THE END https://www.e-education.psu.edu/courses/meteo101/Images/Section9/MLcyclone_0905.swf 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 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