General Atmospheric Circulation
V1003 - Science and Society
Take away concepts and ideas
The Earth is heated unevenly.
Why there are seasons.
Global-scale wind patterns.
Geostrophic flow: Balance of forces that act on atmospheric flow (Pressure gradients & gravity balanced by Coriolis force)
Wind belts and climate zones.
Monsoonal climate.
1. As a sphere, the Earth is unevenly heated,equator to poles
2. As a tilted sphere, the Earth has seasons
Seasonal changes in solar energy
winter summer winter
summer winter summer
The equation of state (air)
Atmospheric Pressure is a function of temperature and density
Ideal Gas Law: P = R T
if pressure is constant, then what happens to as T increases?
Concept of horizontal motion:Sea Breeze
Pressure imbalance = flow
1. Pressure imbalance is a FORCE (F = m x a)
2. Law of mass continuity (empty spaces not tolerated in fluids)
Earth’s uneven radiation
1. Earth is a sphere (uneven radiation)
2. Tilted (there are seasons)
Sphere means equator gets more radiation than poles.
Tilt mean the amount of radiation at one location changes with time (seasons).
Incoming (and outgoing) radiation
Heating imbalance (∆T) = Pressure imbalance (∆P) = Winds
Q1: Knowing uneven heating and P = R T
a) Poles have low density, equator has high density?
b) Equator has low density, poles have high density?
BUT - The Earth rotates
Apparent (inertial) forces control the flow of air in response to these pressure gradients.
Centripedal force is the force acting on body by string
Centrifugal force is the opposite force acted on by the ball (by its own inertia)
Imagine a satellite in orbit
Centripetal force = Gravity
Centrifugal force = mass and acceleration of satellite
Exact balance sets the angular velocity of the satellite
The Coriolis Force
Artifact of Earth’s rotation
An apparent force which acts on motion on the earth’s surface
Conservation of angular momentum
Fcoriolis = 0 on equator, increases poleward
Coriolis Force Cons. of Angular Momentum
Geostrophic Balance
… the pressure gradient force (∆P) is balanced by Coriolis force.
Applies to atmosphere and ocean circulation where friction can be ignored.
Geostrophic balanceBalance between pressure and coriolis forces
Geostrophic Balance
NORTHERN HEMISPHERE
Geostrophic flow in the Northern Hemisphere
Counter-clockwise around low pressure cells
Clockwise around high pressure cells
Opposite Coriolis deflection for opposite hemispheres
An example of Geostrophic flow
Low
High
Q2: What direction is the wind coming FROM in Chicago, IL today?
a) North
b) Northwest
c) West
d) Southwest
e) Northeast
Chicago
Application to Earth
Geostrophic flow gives rise to banded circulation zones
Trade Winds and the Intertropical Convergence Zone (ITCZ)
ITCZ
Hadley Cell Tropical convection cells
Tradewinds
Hadley Cell
westerlies
Tradewinds
Global vegetation patterns
Rising limb of Hadley Cell (ITCZ) = Rainforest
Descending limb of Hadley Cell = Deserts
Global Vegetation
Incoming (and outgoing) radiation
A note on the mid-term
Rough guide:
A/A+ >85
B/B+ 73
C/C+ 61 0
2
4
6
8
10
12
0 20 40 60 80 100
Count
Grade
73 ± 12