ReviewAt the surface, air spirals CCW and inward toward the center of a lowSurface convergenceCauses column to gain air moleculesThere must be net loss of air molecules to reduce pressureDivergence aloft must be greater than convergence near the surface for low to strengthenSurface low pressure systems are typically found east of upper-level (jet stream) troughsDivergence aloft occurs east of troughs in jet streamSurface pressure falls if divergence aloft greater than convergence near surfaceSurface lows cannot strengthen directly beneath upper-level troughsNo divergence aloft directly beneath upper-air troughReview: Heights and TemperaturesHigh heights are related to warm air below the pressure levelLow heights are related to cold air below the pressure levelCold air advection (cooling temperatures) mean that heights FALLStages of Mid-Latitude Cyclone DevelopmentFrontal wave (initial development)Open wave (cyclone strengthens)Mature (cyclone strongest, begins to occlude)Occlusion (cyclone begins to weaken)Cut-off cyclone (cyclone dissipates)CyclogenesisCyclogenesis – Development (birth) of mid-latitude cycloneThere are favored regions for cyclogenesis!East of mountain ranges (e.g., the Rockies)Along temperature gradientsCoastlinesStationary frontsUpper-level divergence is required for cyclones to form!Look east of upper-level troughs in favored regionsMid-Latitude Cyclone Life Cycle(a) – Stationary front (or temperature gradient)(b) – Surface low forms along stationary front owing to upper-level divergence from trough aloft. Cyclone will not form without trough aloft(c) – Southerly winds advect warm air northward east of low, northerly winds advect cold air southward west of lowUpper-level divergence to east of upper-air trough leads to cyclogenesisAs surface low intensifies, cold air advection (surface cooling) west of low deepens upper-air troughHeights fall because colder temperatures mean even lower heightsAs heights fall within upper-level trough, the trough becomes more intenseGreater curvature of flowWinds aloft become even slower relative to geostrophicRemember that winds are slower than geostrophic around troughs aloft (in jet stream)This results in even greater upper-level divergence over surface lowSurface pressure at center of low continues to fallSurface cyclone gets strongerAs the surface low intensifies, its surface pressure becomes lowerIncreases pressure gradient force around cyclonePressure far away from cyclone does not changeStronger PGF means stronger windsStronger winds transport more warm air northward on east side of low and more cold air southward on west side of lowCold air advection near surface and west of low intensifies upper-air troughIncreases upper-level divergence over surface low, surface pressures continue to fall, low becomes even strongerThis is a feedback mechanism(d) Cyclone continues to intensify, reaches maturity(e) Intense cyclone wraps cold air completely around it. Cold front appears to catch up to warm front; occluded front forms (cyclone occludes)(f) Cyclone weakens, surrounded by cold air (no temperature gradient near low)After cyclone occludes, upper-level trough catches up to surface lowUpper-level trough moves faster than surface low because of stronger winds aloftWhen cyclone becomes vertically stacked, there is no longer divergence aloft above surface lowMax divergence now east of surface lowWinds at surface still flow CCW and inward toward center of lowSurface convergenceCONV > DIVSurface pressure rises at center of lowLow slowly weakensTriple point is often favored location for new cyclogenesisMid-Latitude Cyclone TracksMoves in direction of greatest surface pressure decreases (falls)Warm advection (and divergence aloft) yield surface pressure fallsRule of thumb: Low will move parallel to, but just north of its warm frontMid-Latitude Cyclones: Life Cyclone 10/22/2012Review-At the surface, air spirals CCW and inward toward the center of a lowoSurface convergenceoCauses column to gain air molecules-There must be net loss of air molecules to reduce pressureoDivergence aloft must be greater than convergence near the surface for low to strengthen-Surface low pressure systems are typically found east of upper-level (jet stream) troughsoDivergence aloft occurs east of troughs in jet streamoSurface pressure falls if divergence aloft greater than convergence near surface-Surface lows cannot strengthen directly beneath upper-level troughsoNo divergence aloft directly beneath upper-air troughReview: Heights and Temperatures-High heights are related to warm air below the pressure level-Low heights are related to cold air below the pressure level-Cold air advection (cooling temperatures) mean that heights FALLStages of Mid-Latitude Cyclone Development-Frontal wave (initial development)-Open wave (cyclone strengthens)-Mature (cyclone strongest, begins to occlude)-Occlusion (cyclone begins to weaken)-Cut-off cyclone (cyclone dissipates)Cyclogenesis-Cyclogenesis – Development (birth) of mid-latitude cyclone-There are favored regions for cyclogenesis!oEast of mountain ranges (e.g., the Rockies)oAlong temperature gradientsCoastlinesStationary fronts-Upper-level divergence is required for cyclones to form!oLook east of upper-level troughs in favored regionsMid-Latitude Cyclone Life Cycle(a) – Stationary front (or temperature gradient)(b) – Surface low forms along stationary front owing to upper-level divergencefrom trough aloft. Cyclone will not form without trough aloft(c) – Southerly winds advect warm air northward east of low, northerly winds advect cold air southward west of low-Upper-level divergence to east of upper-air trough leads to cyclogenesis-As surface low intensifies, cold air advection (surface cooling) west of low deepens upper-air trough-Heights fall because colder temperatures mean even lower heights-As heights fall within upper-level trough, the trough becomes more intenseoGreater curvature of flowoWinds aloft become even slower relative to geostrophicoRemember that winds are slower than geostrophic around troughs aloft (in jet stream)-This results in even greater upper-level divergence over surface lowoSurface pressure at center of low continues to falloSurface cyclone gets stronger -As the surface low intensifies, its surface pressure becomes loweroIncreases pressure gradient force around cycloneoPressure far away from