Coriolis Force, Vorticity, Rossby Waves and General Circulation

Formation of wind
What is Coriolis force?
How many forces are there acting on the moving air (or wind)?
Barometric pressure and isobars
Pressure gradient force
Frictional force
Geostrophic wind
What are cyclone and anticyclone
Jet Stream
Divergence and convergence; Cold and warm front; Hurricane; Monsoon
Global and regional circulations; General Circulation Models

What is vorticity?

Conservation of (potential) vorticity
What is Rossby waves?
Air pressure is simply the weight of air above a given level. As we climb in elevation above the earth's surface, there are fewer air molecules above us; hence, atmospheric pressure always decreases with increasing height. At sea level, "standard atmospheric pressure" is

1013.25 millibar (mb) = 29.92 inches (in) of mercury (Hg) = 76 centimeter (cm)

1 hectopascal (hPa) = 1 mb
Isobars: Lines on a map joining points of equal atmospheric pressure.

The pressure gradient force is always directed from higher pressure toward lower pressure and it is the pressure gradient force that causes the air to move.

Steep pressure gradients (tightly packed isobars) produce strong pressure gradient forces and high winds; gentle pressure gradients (widely spaced) isobars) produce weak pressure gradients forces and light winds.
Once the wind starts to blow, the Coriolis force causes it to bend to the right of its intended path in the Northern Hemisphere and to the left of its intended path in the Southern Hemisphere.

Geostrophic wind: Horizontal wind in the upper atmosphere that moves parallel to isobars. Results from a balance between pressure gradient force and Coriolis force.

Additional Readings
Cyclone: Area of low pressure in the atmosphere that displays circular inward movement of air. In the Northern Hemisphere circulation is counterclockwise, while Southern Hemisphere cyclones have clockwise wind patterns.

Anticyclone: An atmospheric pressure system consisting of an area of high pressure and outward circular surface wind flow. In the Northern Hemisphere winds from an anticyclone blow clockwise, while Southern Hemisphere systems blow counterclockwise.

When something spins, it has vorticity. The faster it spins, the greater its vorticity. Vorticity is a measure of the spin of small air parcels. When viewed from above, air that spins cyclonically (counterclockwise) has positive vorticity and air that spins anticyclonically (clockwise) has negative vorticity.

Because the earth spins, it has vorticity. In the Northern Hemisphere, the earth's vorticity is always positive because the earth spins counterclockwise about its vertical North Pole axis. Any object on the earth has vorticity simply because the earth is spinning, and the amount of this earth vorticity increases from zero at the equator to a maximum at the poles.
Moving air will generally have additional vorticity relative to the earth's surface. This type of vorticity is called relative vorticity.
Why so many wave cyclones form on the leeward side of the Rocky mountains, especially in eastern Colorado where the crest of the mountains reaches its maximum elevation? [using the conservation of potential vorticity theory meaning that the absolute vorticity (earth vorticity plus relative vorticity) of the column divided by its depth is equal to a constant.]

"Hence, any rotating air mass that is shifted from its original latitude tends to trace out a path of alternating cyclonic and anticyclonic loops, downwind." [Bryant, 1997, in page 35 end of the 1st paragraph]  In the upper atmosphere, these waves are termed Rossby waves (or longwaves). At any one time, there are usually between three and six of these waves looping around the earth. Imbedded in longwaves are shortwaves, which are small disturbances, or ripples.

Rossby wave: A large wave (or longwave) in the polar jet stream and the westerlies that extends from the middle to the upper troposphere. Often associated with the formation of a mid-latitude cyclone at the ground surface.

Convergence: An atmospheric condition that exists when the winds cause a horizontal net inflow of air into a specified region.

Divergence: An atmospheric condition that exists when the winds cause a horizontal net outflow of air into a specified region.

Cold front: A transition zone in the atmosphere where an advancing cold air mass displaces a warm air mass.

Warm front: A transition zone in the atmosphere where an advancing warm air mass displaces a cold air mass.

Occluded front: A transition zone in the atmosphere where an advancing cold air mass sandwiches a warm air mass between another cold air mass pushing the warm air into the upper atmosphere.

Hurricanes: An intense cyclonic storm consisting of an organized mass of thunderstorms that develops over the warm oceans of the tropics. To be classified as a hurricane, winds speeds in the storm must be greater than 118 kilometers per hour.

Would it be possible for a hurricane to form over land? Explain.

What factors tend to weaken hurricane?

In what ways is a hurricane different from a mid-latitude cyclone (also called extratropical cyclones in the Bryant book)? In what ways are these two systems similar?

Why do most hurricanes move westward over tropical waters?

Suppose, in the North Atlantic, an eastward-moving ocean vessel is directly in the path of a westward-moving hurricane. What would be the ship's wisest course -- to veer to the north of the storm or to the south of the storm? Explain.

What is a monsoon? Where is the largest monsoon system on earth?

General Circulation Model: Computer-based climate model that produces future forecast of weather and climate conditions for regions of the Earth or the complete planet. Uses complex mathematical equations and physical relationships to determine a variety of climate variables in a three-dimensional grid. The need for accurate simulation of past and modern climate and for reliable prediction of future climate is driving the development of very sophisticated models that incorporate greenhouse gases, aerosols, and biogeochemical processes.

Additional Readings:

More Readings about Vorticity

Last updated on 12/08/09 02:52 PM