General Circulation
 
1. Definition
 The average hemispheric circulation for a period of one month or
 longer.
2. Surface or lower atmosphere
 (1). 4 Pressure zones
 A. The Equatorial Low: The equator, a thermal Low (the 
  Rising of warm air produces the low)
 B. The Subtropical High (the horse latitude): 30 oN, the
 dynamic High (warm High, subsidence produces the surface
  divergence, compressional heating) 
 C. The Arctic Low: 60 oN, the dynamic Low (cold Low, the
 surface convergence of the air currents from the
 north Pole and from the subtropical High)
 D. The Polar High: the North Pole, the thermal High (cold
 High, dense cold air sinks, producing the high).
 (2). 3 Prevailing (most frequent) wind zones
 A. The tropical easterlies or trade winds (the winds with an
 east component, NE, E, SE winds)
 B. The temperate (middle latitude) westerlies.
 C. The polar easterlies.
 (3). 3-cell meridional circulation
 A. Hadley cell: the tropic, the low latitude, equator to 30
 degree latitudes.
  B. Ferrel cell: the temperate, the middle latitude, 30 to 60
 degree latitudes
 C. Polar cell: the high latitude, 60 degree latitudes to 
  poles.
 (4). 4 Centers of Action
 A. The North Pacific Subtropical High (Hawaiian High)
 B. The North Atlantic Subtropical High (Bermuda High)
 C. The Aleutian Low
 D. The Icelandic Low
 (5) The cause of the formation of 4 centers of action: differential
  heating between land and ocean.
 A. Winter
 The ocean surface is relatively warm compared to the nearby
 land at the same latitude. The warmer ocean surface
 air rises, intensifying the Arctic Lows and weakening the
  subtropical Highs.
 B. Summer
 The ocean surface is relatively cool compared to the nearby
 land at the same latitude. The cooler ocean surface
 air sinks (subsides), intensifying the subtropical Highs 
  and weakening the Arctic Lows.
 (6) The permanent centers of action
 A. The subtropical Highs (both Hawaiian High and Bermuda High).
 B. The subtropical Highs exist year round.
 C. The subtropical Highs intensify in summer due to the 
  Relatively cooler ocean surface air that subsides, 
 intensifying the surface divergence (net loss of air).
 (7) The semi-permanent centers of action
 A. The Arctic Lows (both Aleutian and Icelandic Lows)
 B. The Arctic Lows weaken and even disappear in summer.
 (8) The seasonal variations of the strength of the centers of 
  action
 A. The subtropical Highs
 (A) Summer: strengthen, expand, and their centers move
 toward north (about 40 oN), following the northward
 (poleward)migration of the sun.
 (B) winter: weaken, shrink, and their centers displace
 toward the equator (about 20 oN), following
 the southward migration of the sun.
 B. The Arctic Lows
 (A) Summer: weaken or disappear, relatively cool ocean
 favors the formation of thermal Highs, and thus
 weakens the Lows (thermal effect).
 (B) Winter: intensify and expand. 
3. Middle and Upper Atmospheres (700 mb or 10,000 feet and higher)
 (1) 3 pressure or elevation (on an isobaric surface) zones
 A. The equatorial Low
 B. The subtropical High
 (A). The location of the center of the subtropical High
  slopes upward toward the equator as the elevation
  increases.
 a. The mean location of the surface subtropical High:
  30 oN.
 b. The mean location of the 500 mb subtropical High: 
 25oN.
 (B). The subtropical High is a continuous zone, instead of
 separating into the Hawaiian and the Bermuda Highs: 
  not affected by the surface differential heating 
  between land and ocean. 
 C. The polar Low (the circumpolar vortex center)
 The surface polar High is replaced by a Low at the 700 mb 
  And higher levels because the cold air is associated with 
   an upper level low (the hydrostatic equation). 
 (2). 2 prevailing wind zones
 A. The tropical easterlies.
 South of the subtropical High to the equatorial Low
 B. The polar westerlies
 (A). The circumpolar vortices.
 (B). From the subtropical High to the North Pole.
 (C). The low-level cold air in the north establishes an
 upper-level polar Low. The low-level warm air in 
  the south (the subtropical warm Highs) establishes 
  an upper-level High (the hydrostatic equation). 
 The pressure gradient force makes the air currents
  move from south (High) to north (Low) but the 
  Coriolis force turns the air currents to the right, 
  Resulting in the westerly flows (the westerlies).
 (D). Stronger westerlies in winter than in summer: the
 low-level horizontal temperature gradient is 
   greater in winter than in summer.
 (E). The westerlies expand toward the equator in winter 
  And shrink toward the North Pole in summer,
 following the seasonal migration of the sun.
 (3). The subtropical ridgeline
 A. A line connecting the centers of the subtropical Highs
 at the different elevations.
 B. The ridgeline slopes upward toward the equator (the
 hydrostatic equation states that warm air is 
 associated with an upper High).
 Mean location of the surface subtropical High: 30 oN.
 Mean location of the 500 mb subtropical High: 25 oN.
 Mean location of the 100 mb subtropical High: 10 oN.
 (4). The Arctic Low.
 A. Slopes upward toward the North Pole.
  Cold air column is associated with an upper Low (the
 hydrostatic equation).
 B. Mean location of the surface Arctic Low: 60 oN. 
 C. Location of the 700 mb (or higher) Arctic Low: the
 North Pole.
 (5). The Rossby wave (Long wave or Plannetary wave)
 A. Wavelength of thousands of miles.
 Wavelength: the distance between two subsequent ridges 
  Or troughs, determined by the contours on an isobaric
 surface.
  B. 3 mean winter Rossby-wave troughs
 (A). The eastern North American seaboard: 80 oW.
  From the Hudson Bay (Canada) to the east coast 
  of the USA)
  (B). The eastern Asiatic coast: 140 oE.
  East of Japan.
  (C). The east Mediterranean sea: 50 oE.
 C. Causes of the three troughs
  (A). The effect of the Tibet Plateau and the Rocky 
  mountains.
  (B). Strong horizontal temperature gradient between 
 Large ocean and continent.
 (6) Long wave
 A. Stationary (slow-speed)
 B. Responsible for weekly or monthly weather variations.
 Examples: heat wave, flood, or drought lasts for a 
  week or longer.
 (7) Short wave
 A. Wavelength of hundreds of miles.
 B. Fast speed: 600 miles/day.
 C. Responsible for daily weather variations.
 (8) Jet Stream
  A. A fast-moving upper-level air current (strong wind).
 The wind speed exceeds 75 knots at the 300-mb level,
 denoted by an isotach (line of equal wind speed) of 75 
  knots.
 B. The thermal wind theory
  (A). The upper-level pressure gradient is determined 
  By the low-level horizontal temperature gradient
  (the hydrostatic equation).
  (B). A strong low-level horizontal temperature 
 gradient creates a strong upper-level pressure 
 gradient, and hence strong wind (a jet stream). 
 C. A westerly jet separated the cold air to the north and
 the warm air to the south (similar to an upper-level 
  front).
  D. An easterly jet separates the warm air to the north and
 the cold air to the south (in the tropic).
 E. Confluence Theory (proposed by Namias and Clapp).
  (A). The cold air current from northwest around a Low.
  (B). The warm air current from southwest around a 
  High.
  (C). Both cold and warm air currents flow side by 
  Side (confluence) between the High and the Low,
 creating a strong horizontal temperature gradient, 
 and resulting a strong wind zone in the upper 
  level. 
 F. Types of Jet
 (A). The polar front Jet.
  Mean location: 47 oN (35 to 60 oN).
  Jet core (the elevation of the highest wind 
 speed): 300 mb.
  Occurs all year round.
  Llarge meandering.
  Associated with a surface polar front.
  South of an upper-level Low and north of an upper 
 High.
  Between the gap of the middle-latitude tropopause 
 and the polar tropopause.
   (B). The subtropical jet
  Mean location: 27 oN (20 to 35 oN).
   Jet core: 150 mb.
  Ooccurs in winter only (lack of strong horizontal
  Temperature gradient in the lower atmosphere in
  summer). north of a subtropical High (the gap 
 Between the tropical and the temperate 
 tropopauses).
  Southern section of a polar front jet.
 (9). Palmen and Newton's Genral Circulation Model
 A. The tropopause is broken into 3 sections.
 (A). Tropical tropopause (18 km).
 (B). Middle latitude tropopause (12 km).
 (C). Polar tropopause (6 km).
 B. The subtropical jet: 
 The gap between the tropical and middle-latitude 
  tropopause.
 C. Polar front jet;
 (A). The gap between the middle-latitude and polar
 tropopause.
 (B). The polar front extends to the ground surface
 from the jet core.
 (10). Conservation of absolute vorticity (wave patterns of air 
  flows)
 A. Absolute vorticity = (relative vorticity + Coriolis
 effect) = constant for a given air parcel.
 B. Vorticity: the trndency of an air parcel to rotate
  clockwise (negative vorticity) or countercloskwise
  (positive vorticity).
 C. Relative vorticity (shear vorticity): the vorticity
 caused by horizontal wind speed differences.
 D. North wind (northerly flow):
  (A). Decrease in Coriolis effect leads to increased
 relative vorticity.
 (B). An air parcel tends to change from clockwise
 circulation to counterclockwise circulation
  (The air parcel changes the direction, moving 
  Toward north).
 E. South wind (southerly flow)
 (A). Increase in Coriolis effect leads to decreased
 relative vorticity.
 (B). An air parcel tends to change from countercloc-
 Kwise circulation to clockwis (The air parcel 
  changes the direction, moving toward south)
 (11). Missing Storms over the Rocky Mountains.
 A. Theory of the conservation of the potential vorticity.
 (Absolute vorticity/Depth of an air column) = constant 
 for a given air column.
 (A). A ridge tends to generate or intensify over a
   large mountain due to the shrinking of the air
  column. Absolute vorticity decreases (promoting
  anticyclonic circulation) due to the 
  decrease in the depth of the air column in order
  to keep potential vorticity (a ratio)unchanged.
 (B). A trough tends to generate or intensify in the 
 Lee of a large mountain due to the stretching of 
 the air column. Absolute vorticity increases 
 (promoting cyclonic circulation)because depth of 
 air column increases in order to keep the 
 potential vorticity unchanged (conservation)
  B. The Great Plains (Colorado): cyclogenesis (cyclone
 formation or intensification). In the lee of the Rockies.
4. Trade wind Inversion
 (1). Most intense in the east part of a subtropical high (Hawaiian
 High for example)
  A. Los Angeles:
 (A). Average inversion base height: 500 m (1500 feet).
 (B). Average inversion magnitude (inversion top 
  Temperature - inversion base temperature) = 12 oC.
 B. Honolulu:
 (A). Averge inversion base height: 2000 m (6000 feet).
 (B). Average inversion magnitude: 6 oC.
 (2). Causes
 A. Compressional heating due to the subsidence (sinking)
 of the upper level air originating from the equator.
 B. Cool surface marine layer
 (A). Cold ocean current: travels from north to south.
 (B). Upwelling: 
 a. Rising of ocean water from below.
  b. Ekman spiral: Ocean surface water flows to
 the right of north or northeast winds away
  from the continent inducing the rising of
  ocean water from below to replace the water
  that flows away from the land.