Chapter 7
General Circulation and Secondary Circulations
2. Describe the key pressure and wind features in the idealized circulation model.
Answer: In the idealized circulation model there are alternating regions of high and low surface pressure in each hemisphere at 30o intervals of latitude from the poles to the equator. Specifically, low surface pressure exists in a broad belt near the equator, known as the ITCZ, with high surface pressure near 30 degrees north and south latitude (the subtropical highs), low surface pressure near 60 degrees north and south latitude (the subpolar lows), and high surface pressure at the poles (the polar highs). These pressure differences set the atmosphere in motion and create the planetary wind systems.The circulation between the equator and 30o latitude is largely convective, with rising motion near the equator and sinking motion near 30o latitude.The trade winds in each hemisphere flow toward lower pressure at the equator and form the ITCZ.The sinking air near 30o latitude forms the subtropical highs.The weak descent of air in these regions provides very calm conditions.Air flowing on the poleward side of the subtropical highs in each hemisphere forms the midlatitude westerlies in response to pressure gradient force and Coriolis deflection.The polar easterlies are formed in both hemispheres as air moves from the polar high toward lower latitudes.Areas of relatively low pressure, termed subpolar lows, are centered on 60o latitude in each hemisphere due to the convergence of the midlatitude westerlies and the polar easterlies
4. Does the idealized global circulation model adequately describe upper-atmospheric circulations? If not, why not?
Answer: Although the idealized global circulation model does provide a simplified description of upper atmospheric circulation, the model lacks detail. The model does not allow for changes in the air flow over all portions of the globe as influenced by vorticity and changes in geopotential heights due to fluctuating temperatures. In the middle latitudes, the idealized model indicates primarily westerly flow. However, reality shows that considerable latitudinal amplification is possible. This meridional flow creates the ridges and troughs of the Rossby circulation.
6. Discuss the annual migration of the semipermanent pressure cells.
Answer: The polar highs weaken and contract during the summer hemisphere and the Intertropical Convergence Zone (ITCZ) moves poleward in response to the migration of the sun’s vertical rays. The subtropical anticyclones over the ocean basins are strongest in the summer when the ocean surface is colder than the continental surface in the subtropics. The polar highs strengthen and expand equatorward during the winter hemisphere while the subtropical anticyclones weaken and retreat equatorward.
8. Discuss the role of positive and negative vorticity in association with ridges and troughs.
Answer: As air moves from a ridge to a trough the vorticity of the air becomes increasingly positive. the maximum positive vorticity can be found near the trough axis. Air acquiring positive vorticity encourages the formation of low pressure and cyclogenesis. The vorticity of an air parcel moving from a trough to a ridge decreases, with the air near the ridge axis experiencing the most negative vorticity. Strong negative vorticity is associated with the development and support of anticyclones.
10. Why is absolute vorticity important? Of what is its function?
Answer:Absolute vorticity is important because it is the sum of relative vorticity and the planetary vorticity. Where planetary vorticity is at a minimum, relative vorticity is at a maximum and vice versa. Therefore, absolute vorticity remains constant throughout the atmosphere. By remaining a constant, the absolute vorticity of an air parcel in motion is conserved.
12. What are teleconnections and teleconnection action centers and why are they important to upper-atmospheric flow patterns?
Answer: Teleconnections refer to the atmospheric flow pattern that exhibits a “see-saw” pattern of ridging and troughing in the atmosphere. Teleconnection action centers are locations where ridging and troughing, or opposite pressure anomalies occur simultaneously. Any adjustment in a ridge will initiate a similar, and opposite adjustment in the teleconnected trough. Because teleconnections are associated with middle latitude ridge and trough systems, they are important drivers of both weather systems and longer-range climatic variations at the surface.
14. What is the polar front jet stream, how does it form, and why is it an important feature of midlatitude upper air flow?
Answer: The polar front jet stream is a core of fast-moving air that meanders through the upper atmosphere. It occurs at the boundary between the cold, dry air of the polar regions and the warmer, moister, equatorially-derived air. This meeting place produces a thermal-moisture boundary with a steep thermal gradient throughout the vertical profile of the atmosphere. As air flow across this gradient it accelerates. The Coriolis deflection turns the air and allows a geostrophic balance to be reached with the pressure gradient force pulling the air towards the poles. The result is a rapidly moving stream of air flowing generally from west-to-east above the cold/warm boundary. The jet stream is useful in identifying ridges and troughs in the upper atmosphere, and it tends to govern the development and movement of midlatitude storm systems.
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