Atmos 1020 Lecture Slides 10/13,15

The electromagnetic spectrum:

While the gray model atmosphere provides conceptual understanding of the greenhouse effect in broad terms, to understand the nuances of climate change due to increasing CO2, we must consider the wavelength dependence of the IR transmission spectrum.

Just like white light from the sun is composed of all visible colors, infrared light emitted from the Earth’s surface is composed of a range of infrared colors

transmission=1All the light gets through

transmission=0.5half the light gets through

transmission=0None of the light gets through

Infrared ColorsLess red More red

Atmos 1020 – Lecture 2The Radiation Balance of the Atmosphere

7 microns wavelengthClouds near ocean are invisible

11 microns wavelengthSee clouds near ocean surface

Atmos 1020 – Lecture 2The Radiation Balance of the Atmosphere

Many of the trace gasses in the earth’s atmosphere are absorbing

Atmospheric Window

Interesting Fact: The Earth’s sfc temperature is ~288 K. This corresponds to most of the energy being emitted to space in the 8 to 12 micron wavelength ranges. This is no accident.

Atmos 1020 – Lecture 2The Radiation Balance of the Atmosphere

Many of the trace gasses in the earth’s atmosphere are absorbing in the IR.We call these greenhouse gasses – water vapor is the most important!

Atmospheric Window

An increase in CO2 causes the partially transmissive part of the atmosphereic window to become less transmissive – more opaque –effectively trapping more IR energy in the atmosphere and reducing the IR emission to space. i.e. Closing the Atmospheric Window!

Atmos 1020 – Lecture 2The Radiation Balance of the Atmosphere

• Increased CO2 causes the atmospheric window to close as the partially transmissive bands near 13 microns become more opaque.

• The surface then warms in a manner similar to our prior discussions• The warmer surface causes more water to evaporate into the air

causing the partially transmissive bands near 7 microns to become more opaque.

• The surface then warms more….

Window closes

From Kiehl and Trenberth (1997)

Atmos 1020 – Lecture 2The Radiation Balance of the Atmosphere

Ein = Eout

Top

Atm

Sfc

Surface Energy Balance

Input Output

Sunlight – 168 Sensible – 24Evap. Water – 78Net IR - 66

Atmosphere Energy Balance Sensible – 24Precip – 78Sunlight - 66

Net IR - 168

Atmos 1020 – Lecture 2The Radiation Balance of the Atmosphere

Ein = Eout

Absorbed Solar Radiation = Emitted IR Radiation

What can throw this equation out of Balance?

1. Changes to… 1. Changes to…

Atmos 1020 – Lecture 2The Radiation Balance of the Atmosphere

Ein = Eout

Absorbed Solar Radiation = Emitted IR Radiation

What can throw this equation out of Balance?

1. Changes to the sun2. Changes to aerosols3. Changes to clouds4. Changes to surface properties

1. Changes to greenhouse gasses (water vapor, carbon dioxide, others)

2. Changes to clouds

Atmos 1020 – Lecture 2The Radiation Balance of the Atmosphere

What of clouds?

Clouds have 2 competing influences on energy balance:

Ein = Eout

Absorbed Solar Radiation = Emitted IR Radiation

Cloud trap IR radiation which warms the system.

Clouds reflect solar radiation which coolsthe system.

Atmos 1020 – Lecture 2The Radiation Balance of the Atmosphere

How do clouds trap IR radiation?

Recall that temperature decreases with height….

Cirrus – Cold Tops

Stratus – warm TopsStratus – warm Tops

Stratus (low level) clouds emit IR radiation at about the same rate as the surface but they are very reflective to solar radiation

Cirrus (high level) clouds absorb the warm IR radiation from below and emit IR at MUCH colder temperatures (~factor of 4 less IR).

IR

Clouds and the Energy Balance

Clouds appear almost uniformly white in terms of reflected sunlight

The picture of clouds in the IR is much more variable with light and dark areas corresponding to cloud top temperatures.

Clouds and the Energy Balance

Low Level Stratus Clouds tend to cool the climate system.

Albedo is HighInfrared is about the same as the surface

Clouds and the Energy Balance

Thick Clouds in the Upper Troposphere

Reflect Solar – a Cooling Trap IR – A warming

The net effect depends on the details of the clouds…