Circulation Systems Flashcards
The Hadley Cell System:
- Equator receives most of the sun’s energy, warming air and forcing it to rise.
- Air diverges in the upper atmosphere toward the poles.
- As air moves polewards it gradually cools, sinking and then is drawn back to the equator by a PGF to create the circulation cell.
- LP system at equator and HP systems at 30 degrees N/S
The Ferrell Cell
Not thermally driven but by polar & hadley cell motion.
- Calm and stable weather at the limits of the hadley cell due to high pressure.
- North/South moving air become westerlies as the coriolis effect deflects them.
- Less persisent winds than the trade winds due to barriers such as topography.
Hadley Circulation: Why?
Air pressure is directly proportional to air density, which is inversely proportional to temperature, hence there is a pressure gradient between the top of a warm air mass and a cold one. This leads to movement of air from high pressure to low.
What are Jet Streams?
Areas of fast winds in the upper troposphere, at around 10km altitude they attain speeds between 45 and 65 m/s
Where does the polar front jet form?
At the junction of cold polar air in the north and warm subtropical air from the south, and has an important influence on our weather in the UK.
Where does the sub-tropical jet form?
Where the hadley and ferell cells meet, thought to be due to conservation of angular momentum and related to temp gradients.
Why is upper atmospheric flow fast?
Because strong temperature gradients create the ‘thermal wind’
Is hotter air thicker or thinner?
thicker (more dense).
Thermal Wind Balance
Latitudes with high pressure systems have stronger thermally driven zonal winds at greater altitude.
Why do we see stronger winds in winter?
Greater temperature differences between the equator and poles mean we have a stronger thermal wind balance.
Names for stationary eddies?
Rossby waves, long waves or planetary waves
What are stationary eddies?
persistent waves in the upper atmosphere in waves and troughs that are induced by orographic barriers or heat sources (ocean currents in winter or land masses in summer) and due to the conservation of vorticity.
What is vorticity?
The tendency to spin about an axis, tighter rotation has a larger magnitude of vorticity.
What causes vorticity on Earth?
Planetary vorticity is from Earth’s spin and local vorticity is due to cyclonic or anticyclonic behaviour (relative vorticity)
What is absolute vorticity?
The sum of relative and planetary vorticity. This is conserved as is angular momentum, hence it is a constant but is still dependent on latitude due to the coriolis parameter.
Why does conservation of Vorticity result in stationary eddies?
Because absolute vorticity is constant, if planetary vorticity decreases then local vorticity must decrease, and vice versa. We can only change planetary through latitude, so we see individual parcels in rossby waves get more stable as they go poleward as there is less vorticity in the system.
How do Mountains influence stationary eddies?
Air is compressed as it flows over a mountain, so we get vertical eddies in the system, as air is compressed and deflected around a mountain it’s relative vorticity is greater so we say vorticity/depth of air column is a constant as it relys on the underlying topography.
What are transient eddies?
These are much more short lived than stationary eddies, and exist as cyclones and anticyclones.
What cause transient eddies?
Localised temperature gradients causing localised instability in the atmosphere.
What makes up zonal and time mean northward transport of energy/momentum?
- Mean meridional circulation (hadley circulation)
- Contribution by stationary eddies
- Contribution by transient eddies
What is Energy?
The ability to do work, cannot be destroyed or created but only transformed.
Internal Energy
Energy stored within atoms or molecules
Latent Energy
Energy absorbed or released upon phase changes of water
Potential Energy
Energy yet to be used such as elevation.
Kinetic Energy
Energy associated with motion.
Energy in the Atmosphere
Kinetic only constists 0.5% of total atmospheric energy but concerns the distribution of it.
Meridional circulation consists of latent heat release and sensible heat flux. Upwar transport converts LE to PE.
What happens to the angular velocity of an air parcel moving north?
It must increase, as its distances from Earth’s rotational axis decreases, and angular momentum is conserved, it must accelerate in a westerly direction assuming no exchange of momentum with other parcels.
Angular Momentum Equation
= mass * angular velocity * radius^2
or
= mass * linear velocity * radius
