# SO441 Synoptic Meteorology

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SO441 Synoptic Meteorology

What is vorticity? Many meteorological web sites include some form of vorticity contours on the 500-mb charts Why? How does having contours of vorticity help a meteorologist understand the atmosphere?

What is vorticity? Vorticity is a measure of spin
First, some math: On a weather chart (like the 500 mb chart), want to measure spin about a vertical axis Like if you stuck a pencil or a pinwheel on the 500 mb chart, would it spin? And in what way? So isolate the “vertical vorticity” : This quantity is also often called “relative vorticity”

Relative (vertical) vorticity
Relative (vertical) vorticity is not a measure of “vertical spin”, but rather a measure of horizontal spin The word “vertical” comes from the fact that it’s the component of the vorticity vector Advantage: in one variable, both u and v wind components are used Example: calculate the relative vorticity for these two cases: 50 nm 50 nm 50 nm 50 nm

Relative vorticity Measure of local spin Sign convention:
Positive spin, or positive vorticity, is in the same sense as the counter-clockwise rotation of the Earth (the earth would spin counter-clockwise if you looked down on it from above the north pole) Negative vorticity: clockwise spin Two types of vorticity: curvature and shear Positive vorticity Negative vorticity

Earth’s vorticity The Earth is a rotating sphere.
It imparts vorticity to fluid parcels Earth vorticity along the vertical axis is called Coriolis: , where f is Coriolis, Ω is the angular speed of earth’s rotation, and θ is latitude In Southern Hemisphere, Earth’s vorticity is negative (why?) Adapted from

Geostrophic absolute vorticity
To examine the actual spin of an air parcel, need to look at vorticity contributions from both the Earth and the parcels themselves: Absolute vorticity: At 500 mb, flow is nearly geostrophic So re-name relative vorticity as “geostrophic relative vorticity” Also re-name absolute vorticity as “geostrophic absolute vorticity”

Importance of vorticity
Remember from Lesson 5 (Extratropical cyclones), vorticity is the quantity that “spins up” (or down) during the development of an extratropical cyclone Figure adapted from

Identify relative and absolute vorticity on a weather map

Relative & absolute vorticity in higher latitudes (Canada)
Note color scale differences

Relationship between relative vorticity and geopotential height
Use the fact that the 500 mb level is nearly geostrophic to derive another equation for vorticity: (Derivation)

Summary of relative vorticity
Geostrophic relative vorticity: average difference in geopotential height around a central point If average height is higher across the region than at the center point, geostrophic vorticity is positive (i.e. cyclonic) i.e., point is lower than its surroundings, such as in a trough If average height is lower than at center point, geostrophic vorticity is negative (i.e., anticyclonic) i.e., point is higher than its surroundings, such as in a ridge

Greatest advection of absolute vorticity occurs between ridges and troughs Why?

Physical relationship between vorticity advection and rising motion
Cyclonic vorticity advection produces height falls, proportional to the magnitude of the vorticity advection. Cyclonic vorticity is associated with low heights, so if vorticity advection increases with height (i.e., more vorticity advection at 250 mb than at 850 mb), heights will fall more aloft than at the surface Thus the depth of the atmosphere between the surface and upper levels is decreasing The depth of a column of air is proportional to its mean temperature, so a less-deep column means a colder column If no temperature advection (or any other change) occurs – i.e., the only thing taking place is vorticity advection – then the only way to cool the column of air is for rising motion to take place Vorticity advection occurs at various upper-level heights. More vorticity advection occurs higher in the atmosphere. Deeper trough in upper levels causes a smaller column of air in the trough. Smaller column of air must be associated with cooler temperatures, and the way to cool the air is to lift it This method is adapted from from C. Doswell and K. Crawford, Univ. Oklahoma School of Meteorology.

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