Lecture 14Outline of Last Lecture I. What is the Jet Stream?II. Review of PressureIII. Changing the SurfaceIV. ConvergenceV. DivergenceVI. Convergence and DivergenceVII. Surface Low Pressure SystemsVIII. Surface High Pressure SystemsIX. Net DivergenceX. Net ConvergenceXI. Strengthening and WeakeningXII. Mid-Latitude Cyclone DevelopmentXIII. Jet Stream- 300 mb MapXIV. Upper Air Troughs and Surface LowsXV. Vertical MotionXVI. Hydrostatic BalanceOutline of Current Lecture XVII. ReviewXVIII. Upper-Air Troughs and Surface LowsXIX. Review XX.Stages of Mid-Latitude Cyclone DevelopmentATMS 100 1st EditionXXI. CyclogenesisXXII. Where do cyclones form?XXIII. Rocky MountainsXXIV. Mid-Latitude Cyclone LifeXXV. Review: Heights and TemperaturesXXVI. Mid-Latitude Cyclone TracksCurrent LectureXXVII. Reviewa. At the surface, air spirals counter-clockwise and inward toward center of lowi. surface convergenceii. causes column to gain air moleculesb. There must be net loss of air molecules to reduce pressurei. divergence aloft must be greater than convergence near the surface for low to strengthensc. Upper-Air Troughs and Surface Lowsi. Upper-Trough:1. Slows down (convergence) air sinks and speeds up (di-verges) 2. causes pressures to fall, and surface low d. Surface Low Pressure systems are typically found east of upper-level (jet stream) troughsi. divergence aloft occurs east of troughs in a jet streamii. surface pressure falls if divergence aloft greater than conver-gence near surfacee. Surface lows cannot strengthen directly beneath upper-level troughsi. no divergence aloft directly beneath upper-air troughXXVIII. Stages of Mid-Latitude Cyclone Developmentc. Frontal Wave (initial development)d. Open Wave (cyclone strengthens)e. Mature (cyclone strongest, begins to occlude)f. Occlusion (cyclone begins to weaken)g. Cut-off cyclone (cyclone dissipates)XXIX. Cyclogenesisc. Development (birth) of mid-latitude cycloned. There are favored regions for cyclogenesisi. east of mountain rangesii. along temperature gradients1. coastlines2. stationary frontse. upper-level divergence is required for cyclones to formi. look east of upper-level troughs in favored regionsXXX. Where do cyclones form?c. Two Regions:i. east of the Rockies1. Westerly winds flow over mountains2. Reduces troughing (counter-clockwise flow) on east sidea. lowsii. east of the Appalachians (along Atlantic/East Coast)XXXI. Mid-Latitude Cyclone Lifea. Stationary Fronts (or temperature gradient)b. surface low forms along stationary front owing to upper-level diver-gence from trough aloft. cyclone will not form without trough aloftc. southerly winds advect warm air northward east of low, northerly windsadvect cold air southward west of lowd. Upper-Level divergence to east of upper-air trough leads to cyclogene-sise. As surface low intensifies, cold air advection (surface cooling) west of low deepens upper-air troughf. Heights fall west of surface low because of colder temperaturesg. As heights fall within upper-level trough, the trough becomes more in-tensei. greater curfacture flowii. winds aloft become even slower relative to geostrophiciii. rememeber that winds ar eslowe than geostrophic around troughts aloft (at jet stream level)h. results in even greater upper-level divergence over surface low i. surface pressure at center of low continues to fallii. surface cyclone gets strongeri. as the surface low intensifies,its surface pressure becomes loweri. increase pressure gradient force around cycloneii. pressure far away from cyclone doesn’t changeXXXII. Review: Heights and Temperaturesa. High Heights are related to warm air below the pressure levelb. low heights ar related to cold air below pressure levelc. cold air advection (cooling temperatures) mean that heights fallXXXIII. Mid-Latitude Cyclone Life Cyclea. As heights fall within upper level trough, the trough becomes more in-tensei. greater curvature flowii. winds aloft become slower relative to geostrophiciii. remember that winds ar eslower than geostrophic b. Results in even greater upper-level divergence over surface lowi. surface pressure at center of low continues to fallii. surface cyclone gets strongerc. As the surface low intensifies, its surface pressure becomes loweri. increases pressure gradient force around cycloneii. pressure far away from cyclone doesn’t changed. Stronger PGF means stronger windsi. stronger winds transport more warm air northward on east side of low and more cold air southward on west side of lowe. Cold air advection near surface west of low intensifies upper-air troughi. increases upper-level divergence over surface low, surface pressure continue to fall, low becomes strongerf. This is feedback a mechanismXXXIV. Mid-Latitude Cyclone Tracksa. What countrols how a mid-latitude cyclone moves?i. Moves in diretion of greatest surface pressure decreases (falls)ii. warm advection (and divergence aloft) yield surface pressure fallsiii. rule of thumb: low will move parallel to, but just north of is warm frontXXXV. MId-Latitude Cyclone Lifea. Cyclone continues to intensify, reaches maturityb. intense cyclone wraps cold air completely around it. cold front appears to catch up to warm front, occluded front forms (cyclone occludes)c. cyclone weakens, surrounded by cold air (no temperature graident near low)d. after cyclone occludes, upper-level trough catches up to surface lowi. upper level trough moves faster than surface low because of stronger winds alofte. when cyclone becomes vertically stacked, there is no longer diver-gaence aloft above surface lowi. max divergence now east of surface lowf. winds at surface still flow CCW and inward toward center of lowi. surface convergenceii. convergence>divergenceg. surface pressure rises at center of lowh. low slowly weakensi. triple point is often favored location for new