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Mid-latitude dynamics and weather systems 1. Mid-latitude westerly and jet stream Thermal wind Caused by horizontal temperature gradient 2. Rossby Waves 3. Rotation, Circulation, and Vorticity f Planetary vorticity Latitude : 0 90 0 Planetary vorticity f : 0 max effect : Planetary vorticity changes with latitude The direction of f is defined as positive f Relative vrticity 0 0 Cyclone Anti-cyclone Absolute vorticity=Planetary vorticity + Relative vrticity a f 4. Potential vorticity conservation Absolute vorticity Potential vorticity Depth f constant H is conserved 5. Westward propagation of Rossby waves P-1 H h f constant H P S (warm) N (cold) Northward motion: 0 anti-cyclonic circulation Southward motion: 0 cyclonic circulation Westward propagation Westerly winds 6. Atmospheric instability Static instability The atmosphere is statically unstable if a parcel at equilibrium is displaced slightly upward and finds itself warmer than its environment and thus continues to rise spontaneously away from its starting equilibrium point due to its own buoyancy. The atmosphere is statistically stable if a parcel at equilibrium is displaced slightly upward and finds itself colder than its environment and therefore sink back to its original equilibrium point. 7. Helmholtz instability --- dynamic instability Vertical wind shear can generate vorticity. 9. Tilted atmospheric system --- baroclinic 10. Baroclinic instability density surface . warm S cold N Release of potential energy stored in a tilted system to kinetic energy cold L H warm The energy for the growth of cyclone and anti-cyclone is from the potential energy stored in a tilted system. 11. Heat transport by transient weather systems Heat flux unit : J2 W2 H ms m Specific heat at constant pressure Kinematic sensible heat flux, sh sh H , C p J m 2s kg J kgK 3 m ms K Velocity X Temperature C p 1004 kgJK Mid-latitude: mainly through the transient weather systems. North warm northward cold southward warm northward v' t' 0 v' 0 v' 0 v' 0 t' 0 t' 0 t' 0 v' t' 0 East North cold southward v' 0 t' 0 L v' t' 0 warm northward H cold southward v' 0 v' 0 t' 0 t' 0 v' t' 0 East 12. air mass An immense body of air, usually thousand kilometers or more across and perhaps several kilometers thick, which is characterized by relatively homogeneous physical properties (in particular temperature and moisture conetent) at any given altitude. Movement of a Cold and Dry Air Mass Brings Winter Weather Air Mass Source Regions Two criteria for an ideal source region: 1. It must be an extensive and physically uniform area. 2. The atmospheric circulation must be relatively stagnant so that air can stay over an area long enough to come to some measure of equilibrium with the surface. Air Mass Modification 13. Fronts The interface zone between different air mass. Warm Front 1:200 Cold Front 1:100 Cold Fronts and Warm Fronts Stationary Front The surface position of the front does not move, or moves very slowly. Dryline Oklahoma Occluded Front Midlatitude Cyclones Stages in the Life Cycle of a Mid-latitude Cyclone
