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