Chapter 10 Notes

Extratropical Cyclones are low pressure systems that cause wet, windy conditions, but are different from tropical cyclones.
They are born on the downwind side of tall mountain ranges, near warm ocean currents, and beneath strong jet-stream winds.
They are also associated with fronts, and often have comma-cloud shapes with dry slots (a mostly clear region that separates the comma cloud head from the comma tail on satellite images).
Norwegian Cyclone Model Life Cycle:

Frontal Wave- cyclone arises as this along a stationary front separating cold, dry cP air from warm, moist mT air. It is called a wave b/c the Warm Sector regions between cold air warm fronts looks like a gradually steepening ocean wave.
Open Wave- next step of development where strong warm and cold fronts with obvious wind shifts arise with the whole system moving to the east or northeast.
Occluded Cyclone- storm at full maturity in this stage, where an occluded front sprouts out of cyclone. Barometric pressure at center of storm reaches a minimum here and the winds are generally strongest as well. This is also a sign of the beginning of the end for an extratropical cyclone.
Cut-off Cyclone- final stage of cyclone where it slowly dies a frontless death, as clouds and precipitation around the low’s center dissipate.

Cyclogenesis- the development of a cyclone. Surface temperature gradients, strong jet streams, and tall mountain ranges or other surface boundaries can lead to extratropical cyclogenesis.
Baroclinic Instability- the process through which extratropical cyclones get energy for their growth. Warm air rising and cold air sinking in tilted frontal regions are the source of this energy.
Typical regions of cyclogenesis vary from season to season, however they generally move north in the summer b/c temperature gradients and jet streams move north with the Sun during the summer as well.
Cyclones and Angular Momentum

A squashed cyclone (high mountains squash cyclones) spins more slowly than a stretched cyclone (oceans and plains stretch them out).
The rate of spin increases when the height of the cyclone increases; spin decreases as cyclone height decreases.
So, Conservation of Angular Momentum says, Spin divided by the cyclone height must be a constant.
However, a cyclone is also affected by the Coriolis force, or the effects of the Earth spinning, which makes the cyclone have a double dose of spinning.
Vorticity is the spin around a vertical axis. Relative Vorticity is measured relative to the ground. Planetary Vorticity is the Earth’s spin. Relative Vorticity and Planetary Vorticity added together equal Potential Vorticity.
This creates a new equation, referred to as Conservation of Potential Vorticity:

(planetary vorticity + relative vorticity) / cyclone height = constant

So, as a cyclone movies over the Rocky Mountains the cyclone height decreases, thus the Potential Vorticity must also decrease which will, in turn, decrease the spin of the cyclone. Conversely, if the cyclone was traveling over the flat plains then the height would increase, and the spin would also have to increase to conserve Potential Vorticity.

Surface pressure drops when there is divergence of the wind in the column of air above the low.
Strong upper level divergence must exist in order to intensify the surface Low Pressure, since less air will be above the cyclone to exert pressure on the surface, making the surface having lower pressure.
Upper level divergence can occur in two ways:

An anticyclone is one big, slow, fairly calm and stable air mass.
They can create a “cut-off high”, or a “blocking high” which can trap and recirculate hot air around and around its center for weeks, which can lead to a heat wave, a drought, or an episode of air pollution.