Physics of the Climate System

Question 1

A note on Units…

Answer 1

• Force: (F = m a) Newton (N) =
  kg⋅m/s2
• Pressure: (Force/Area) Pascal (Pa) = N/m2
• Other common units:
• Millibars (mb)
• Hectopascal (hPa)
• Energy: Joule (J) = N⋅m
• Work: Watt (W) = J/s
• Energy Flux: W/m2
• Energy passing though an area per unit time

Question 2

Forms of Energy

Answer 2

• Kinetic Energy = energy of motion
• KE = 1
  /2 mv2 (m = mass, v = velocity)
  ‣ Energy in use
  ‣ Examples?
• Potential Energy = stored energy
• PE = mgh (g = gravity, h = height)
  ‣ Energy not y

Question 3

Examples of Kinetic Energy
in the atmosphere

Answer 3

Nearly all this kinetic energy is associated with the synoptic% and planetary%scale horizontal wind field, which has a globally averaged root mean square velocity of about 12%15 m s%1. 2. Motions driven by convective instability account for the remainder of the atmospheric kinetic energy.

Question 4

Energy Transfer

Answer 4

• Radiation
• Conduction
• Convection

Question 5

Types of Radiation

Answer 5

ultraviolet light from the sun
heat from a stove burner
visible light from a candle
x-rays from an x-ray machine
alpha particles emitted from the radioactive decay of uranium
sound waves from your stereo
microwaves from a microwave oven
electromagnetic radiation from your cell phone
ultraviolet light from a black light
beta particle radiation from a sample of strontium-90
gamma radiation from a supernova
microwave radiation from your wifi router
radio waves
a laser beam
Near Infrared (NIR)

Question 6

Electromagnetic Radiation

Answer 6

• Radiation as waves…
  – Wavelength = length of a wave
  from crest to crest
  – Frequency = the number of waves
  that pass a particular point per
  second

Question 7

Radiation in the Earth’s Climate

Answer 7

• Shortwave radiation
• Radiation received from the sun
• UV, visible and near-infrared light
• Longwave radiation
• Radiation emitted by the Earth
• Infrared light (heat)

Question 8

Blackbody radiation

Answer 8

• A blackbody is a perfect absorber and a
  perfect emitter
• absorbs all incident radiation
• emits the maximum possible radiation at a
  given temperature
• emits radiation with a characteristic pattern

Question 9

Solar Radiation

Answer 9

Sun’s surface temperature = 6000 K
Sun’s emission at surface:
I = 7.35x107W/m2
* Intensity of radiation decreases with
distance from the sun
* Solar constant = 1368 W/m2

Question 9

Wein’s Law:

Answer 9

The blackbody curve
at any temperature has essentially the same shape as
the curve at any other temperature, except that each
wavelength is displaced, or moved over, on the graph.

Question 10

Earth’s radiation budget

Answer 10

• Average Solar radiation received @ TOA:
  342 W/m2
• Earth radiates energy (Stephan-Boltzmann)
• must emit energy to space to balance incoming
  energy => Longwave radiation
• “Radiating temperature” = –16°C
• Earth’s surface temperature = 15°C

Question 10

Earth’s Energy Balance

Answer 10

The earth’s temperature is determined by the balance of
incoming and outgoing radiation

Question 11

Atmospheric Structure

Answer 11

• Density and pressure
  structure
• Layers of the atmosphere
• Tropo: lower ~12 km at mid-latitudes (20 km
  @ tropics, 8 km @ poles)
• Strat: ~12-50 km
• Meso: 50-80 km
• Thermo: 80-500 km
• Gas Composition: Permanent,
  Variable, and Trace Gases

Question 12

Sea-level pressure:

Answer 12

1013.25 mb or hPa (= 101,325 Pa) => Near the surface, pressure decreases ~10 mb per 100 m altitude

Question 12

Ice-Albedo Feedback

Answer 12

Initial change —- Climate cooling — Increased snow and ice: higher reflectivity — less solar radiation — absorbed on surface —- greater cooling

Question 12

The Greenhouse Effect

Answer 12

Greenhouse gases
- Water Vapour (H2O)
- Carbon Dioxide (CO2)
- Methane (CH4)
- Nitrous Oxide (N2O)
- Ozone (O3)
* Absorb most of the longwave radiation
emitted by the Earth

Question 13

Absorption of Solar
radiation

Answer 13

• What determines how much solar
  radiation is absorbed in a given area?
• Latitude => beam spreading
• Season
• Surface Albedo
• Sun angle (can affect albedo)
• cloud cover

Question 13

Composition of the Atmosphere

Answer 13

Permanent Gases of the Atmosphere
Constituent Formula % by Volume
Nitrogen N2 78
Oxygen O2 21
Argon Ar 1
Variable and Trace Gases
Water vapour H2O variable
Carbon dioxide CO2 0.038
Methane CH4 0.00018

Question 13

Summary part 1

Answer 13

Solar energy and gravitational energy are the fundamental sources of energy for
the Earth’s climate system.
In the ideal case (referred to as “black body”) matter will absorb all the energy
impinging on it in the form of electromagnetic waves and as a result will warm
up and itself become a radiation source. This “give and take” of energy leads to a
state of equilibrium, where the outgoing radiation balances the incoming one.
The energy radiated from a black body is distributed over all wavelengths, in a
“bell-shaped” dependence on the wavelength. Maximum energy is radiated at a
wavelength proportional to the inverse of the absolute temperature.
The total (integral over all wavelengths) energy radiated from a black body is
proportional to the fourth power of its absolute temperature.
The energy flux radiating from a point source falls of as the square of the
distance from it. This is why light dims fast as one moves away from its source.

Answer 14

Using these fundamental laws and knowing the Sun’s temperature, we can calculate the socalled “effective” or “emission” temperature of any of its surrounding planets. This is the
temperature that the planet will appear to have when viewed from outer space. From space,
Earth has a global average temperature of -31C. The 15C global average temperature of the
Earth’s surface is due to the greenhouse effect.
The Earth and other planets are not perfect black bodies, as they do not absorb all the
incoming solar radiation but reflected part of it back to space. The ratio between the reflected
and the incoming energies is termed the planetary albedo.
The Earth’s atmosphere contains many trace (or minor) components
(see composition of the atmosphere). While the major atmospheric components (Nitrogen
and Oxygen) absorb little or no radiation, some of the minor components are effective
absorbers. Particularly effective is water vapor, which absorb in the IR wavelength range
Because of its spherical shape incoming solar radiation is not equally distributed over the
planet. At each instant, only the sun lights only half of the planet’s surface, with maximum
radiation coming in at local noon and less in other times of the day