Upper air circulation & winds • Friction is said to be insignificant in the upper air circulation. • Why? How does it affect the upper wind air circulation?
Upper wind circulation • Winds at the top of the troposphere are generally poleward and westerly in direction. • The next diagram describes these upper air westerlies along with some other associated weather features. Three zones of westerlies can be seen in each hemisphere. Why? • Each zone is associated with either the Hadley, Ferrel, or Polar circulation cell.
Upper wind circulation Simplified global three-cell upper air circulation patterns.
Rossby waves • What do you notice concerning the flight time of the return trip back to HK? • Why? • How about other application in history?
Rossby waves- >200 km/hr Satellite view of the atmospheric circulation at the South Pole. (Source: NASA).
Rossby waves & Jet streams • Causes? • Existence of large mountain barriers • Uneven distribution of land and sea • Steep thermal gradient in the upper atmosphere, esp. in the mid-latitudes Jet streams – the narrow bands of extremely fast- moving air within Rossby waves (>230 km/hr)
Rossby waves & Jet streams • Meandering Pattern? 1. Seasonal - winter vs summer 2. Different jet streams - significant vs seasonal a. Polar front jet stream PFJS b. Subtropical jet stream STJS c. Easterly equatorial jet stream (~summer monsoon system) vs ITCZ
The Polar Front & temperate cyclones • Division between the Ferrel and Polar cells • the boundary between warm tropical and cold polar air • In N-H, moving south: bringing cold air which descends in a clockwise direction (i.e. anticyclone) • Moving north: bringing warm air which rises in an anticlockwise direction (i.e. cyclone)
Temperate / Mid-latitude cyclones • The are the result of the dynamic interaction of warm tropical and cold polar air masses at the polar front. • This interaction causes the warm air to be cyclonically lifted vertically into the atmosphere where it combines with colder upper atmosphere air. • This process also helps to transport surplusenergyfrom the lower latitudes to the higher latitudes.
Temperate / Mid-latitude cyclones • They are rarely motionless and commonly travel about 1200 kilometers in one day. • The direction of movement is generally eastward . • Precise movement of this weather system is controlled by the orientation of the polar jetstream in the upper troposphere.
Temperate cyclones & Jet streams • Frontal cyclone development is related to polar jet stream processes. • Within the jet stream, localized areas of air outflow can occur because of upper air divergence. • Outflow results in the development of an upper air vacuum. To compensate for the vacuum in the upper atmosphere, surface air flows cyclonically upward into the outflow to replenish lost mass. • The process stops and the mid-latitude cyclone dissipates when the upper air vacuum is filled with surface air. • http://www.physicalgeography.net/fundamentals/7s.html
Others • Weather conditions related to anticyclones and cyclones • Weather conditions related to a temperate cyclone • “blocking anticyclones”