Geography 103

Weather

Lecture 9: General Circulation (Quiz 3, final)

1. 3-cell meridional circulation (standing cell or thermal cell, North to south or south to north direction).
    Assumption: The earth is not rotating around its axis.
    (1). Hadley cell (tropic)
          A. Equator to about 30 ^oN.
          B. Surface: Northerly flow (north wind).
          C. Upper -level: Southerly flow.
    (2). Ferrel cell (middle latitudes or temperate)
          A. 30 ^oN to 60 ^oN
          B. Surface: Southerly flow.
          C. Upper level: Northerly flow (more accurately, southerly flow).
    (3). Polar cell (high latitudes).
          A. 60 ^oN to the north Pole.
          B. Surface: Northerly flow.
          C. Upper-level: Southerly flow.
          D. The circulation is similar to the Hadley cell.
2. Four pressure zones
    (1). The equatorial Low (ITCZ or doldrum):
          A. The equator.
          B. A thermal or heat low (warm air rises, inducing the surface convergence).
    (2). The subtropical High (Horse latitudes):
          A. About 30 ^oN.
          B. The upper air originating from the equator subsides (sinks) and induces the surface divergence.
          C. A dynamic High :
              (A). A warm High due to compressional heating of the subsiding air.
              (B). Subsidence induces the surface divergence creating a surface High.
    (3). The Arctic Low (the subpolar low or the polar front):
          A. About 60 ^oN.
          B. A dynamic Low
              (A). The surface convergence between the cold air from the North Pole and the warm air from
                     the subtropic resulting in a rising air, a surface Low.
              (B). Not a heat Low.
    (4). The polar High
          A. The North Pole.
          B. A thermal High:
              The cold air has a high density and subsides creating a surface High.
3. Prevailing (the most frequent) winds
    (1). Tropic (Hadley cell)
          A. Surface: The easterlies or trade winds (winds with east component, NE, E, SE winds): very deep.
          B. Upper level: The westerlies.
    (2). Temperate (middle latitudes or Ferrel cell)
          Surface: The westerlies.
          Upper level: The westerlies.
    (3). High latitudes (the Polar cell)
          Surface: The easterlies (very shallow, below 3 km or 700 mb).
          Upper level: The westerlies (The circumpolar vortices).
4. Centers of action
    Both the subtropical High and the Arctic Low are not continuous zones but broken into isolated cells due
    to the differential heating between continent and ocean (Thermal effect).
    (1). The subtropical High (permanent centers of action)
          The North Pacific subtropical High (Hawaiian High) and the North Atlantic subtropical High
          (Bermuda High).
          (A). Winter
                  a. Weakens, shrinks and displaces toward the equator following the migration of the sun.
                  b. The warm air over the relatively warm ocean surface suppresses the High (thermal effect).
          (B). Summer
                  a. Intensifies, expands, and moves toward the higher latitudes following the northward migration
                      of the sun.
b. The North Pacific subtropical High (Hawaiian High)The cold air over the relatively cold ocean
                      surface promotes the High.
    (2). The Arctic Low (semipermanent centers of action)
          The Aleutian Low and the Icelandic Low.
          (A). Winter
                  a. Deepens and expands.
                  b. The warm air over the relatively warm ocean surface promotes the Low.
          (B). Summer
                 a. Weakens or disappears.
b.The cold air over the relatively cold ocean surface suppress the Low.
5. Jet streams
A zone of strong upper-level winds (30,000 to 40,000 feet high).
    (1). The Subtropical jet:
           Near the tropopause gap between the Hadley and Ferrel cells.
    (2). The polar front jet:
          A. Near the tropopause gap between the Ferrel and polar cells.
          B. The polar front extends from the jet downward to the ground surface.
6. Causes of surface High and Low
    (1). Thermal effect (air temperature difference)
           cold High and warm Low
    (2). Dynamic effect (motion of air , divergence, convergence, subsidence, and ascent)
          warm High and cold Low.