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Temperature Structure of the Atmosphere Temperature Structure of the - - PowerPoint PPT Presentation

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Temperature Structure of the Atmosphere Temperature Structure of the Atmosphere EES 3310/5310 EES 3310/5310 Global Climate Change Global Climate Change Jonathan Gilligan Jonathan Gilligan Class #6: Class #6: Friday, January 17 Friday,

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Temperature Structure of the Atmosphere Temperature Structure of the Atmosphere

EES 3310/5310 EES 3310/5310 Global Climate Change Global Climate Change Jonathan Gilligan Jonathan Gilligan

Class #6: Class #6: Friday, January 17 Friday, January 17 2020 2020

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Review Question Review Question

What is the “atmospheric window”? What is the “atmospheric window”?

  • 1. Regions where there are few clouds to block radiation.
  • 2. Desert regions with very little water vapor.
  • 3. Tropical regions with low CO2 concentrations.
  • 4. A range of wavelengths where no greenhouse gases absorb much.
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Measuring Band Saturation Measuring Band Saturation

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Set up MODTRAN: Set up MODTRAN:

Go to MODTRAN, set CO2 to 0.25 ppm, and set all other gases to zero. Set altitude to 20 km and location to “1976 US Standard Atmosphere”. Press “Save this run to background” Note Iout Double CO2 and note the change in Iout Keep doubling CO2 until you get to 1024 ppm. Do you notice anything about the changes in Iout?

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0 ppm CO 0 ppm CO2

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0.25 ppm CO 0.25 ppm CO2

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0.5 ppm CO 0.5 ppm CO2

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1 ppm CO 1 ppm CO2

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2 ppm CO 2 ppm CO2

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4 ppm CO 4 ppm CO2

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8 ppm CO 8 ppm CO2

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16 ppm CO 16 ppm CO2

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32 ppm CO 32 ppm CO2

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64 ppm CO 64 ppm CO2

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128 ppm CO 128 ppm CO2

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256 ppm CO 256 ppm CO2

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512 ppm CO 512 ppm CO2

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1024 ppm CO 1024 ppm CO2

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2048 ppm CO 2048 ppm CO2

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Band Saturation (I Band Saturation (Iout

  • ut)
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Iout

  • ut (CO

(CO2 on log scale)

  • n log scale)
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ΔIout

  • ut
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Change in Iout from no CO2

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Increments of Increments of Iout

  • ut
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Change in Iout from previous Iout

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Measuring Greenhouse Effect: Measuring Greenhouse Effect:

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Measuring Greenhouse Effect: Measuring Greenhouse Effect:

Go to MODTRAN, set CO2 to 0 ppm, and set all other gases to zero. Set altitude to 70 km and location to “1976 US Standard Atmosphere”. Press “Save this run to background” Note Iout Set CO2 to 400 ppm and note the change in Iout Adjust the temperature offset to make the difference in equal zero.

(New − BG) Iout

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No Greenhouse Gases No Greenhouse Gases

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400 ppm 400 ppm

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Adjust temperature Adjust temperature

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Calculating Global Warming Calculating Global Warming

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Calculating Global Warming Calculating Global Warming

“Climate sensitivity” = Temperature rise for doubled CO2. Uncertain (because of feedbacks) Best estimate: 3.2K (range 2.0–4.5 K) Every time you double CO2, rises by . For arbitrary change in CO2:

ΔT2x Δ ∼ T2x T ΔT2x ΔT = Δ × T2x ln( )

new pCO2

  • ld pCO2

ln 2

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Global Warming Potential Global Warming Potential

Absorption by CO2 and water vapor are very saturated Absorption in the atmospheric window is not saturated Therefore, molecule-for-molecule, gases that absorb in the window have a much bigger effect on the climate than adding more CO2. One chlorofluorocarbon molecule = thousands of CO2 molecules Global Warming Potential (GWP) of x = how many CO2 molecules cause the same warming as one molecule of x

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Evolving theory of greenhouse effect Evolving theory of greenhouse effect

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Greenhouse effect Greenhouse effect

  • 1. Purely radiative (no convection)

Each layer has uniform temperature

  • a. Single-layer, uniform spectrum (Mon. 1/13)

Absorbs 100% longwave light

  • b. Multi-layer, uniform spectrum (Lab #2)

More layers greater greenhouse effect.

  • c. Realistic spectrum (Wed. 1/15 & today)

More realistic Harder to do calculations (need computer)

  • 2. Introduce convection (Today & Wednesday)

Temperature changes with height Convection moves heat up and down Radiative-convective models are very accurate But require computers

⇒

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The Vertical Structure of the Atmosphere The Vertical Structure of the Atmosphere

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Greenhouse effect Greenhouse effect

  • 1. Purely radiative (no convection)

Each layer has uniform temperature

  • a. Single-layer, uniform spectrum

Absorbs 100% longwave light

  • b. Multi-layer, uniform spectrum

More layers greater greenhouse effect.

  • c. Realistic spectrum

More realistic Harder to do calculations (need computer)

  • 2. Convection:

Temperature changes with height Convection moves heat up and down Radiative-convective models are very accurate But require computers

⇒

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Radiative-Convective Equilibrium Radiative-Convective Equilibrium

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Normal Atmosphere: Normal Atmosphere:

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Vertical Structure Vertical Structure

Positive lapse rate: Air overhead is cooler (normal for troposphere) Negative lapse rate: Air overhead is warmer (abnormal, “inversion”)

Lapse rate = −ΔT Δheight

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Air vs. Water Air vs. Water

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Air vs. Water Air vs. Water

Pressure = weight of everything

  • verhead.

Air is compressible, water isn’t. 1 meter height of water weighs 1000 kg/m2 1 meter height of dry air at sea-level density weighs 1.3 kg/m2 1 m height of dry air 10 km above sea level weighs 0.4 kg/m2

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Air Pressure Air Pressure

Pressure at height : Half the air is below 5.5 km. 3/4 is below 11 km 7/8 is below 16.5 km NOTE: The number 5.5 km is not exact, but it’s consistent with the textbook.

h P(h) = P0 e−h/8.0km = P0 2−h/5.5km = P0( ) 1 2

h/5.5km

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Why is the air cooler higher up? Why is the air cooler higher up?

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Terminology Terminology

Environmental Lapse Measured temperature of actual atmosphere Compares one bit of air at one height with another bit at another height. Changes from one time and place to another. Adiabatic Lapse Change in a single parcel of air as it moves up or down “Adiabatic” means no heat flowing in or out Adiabatic changes are reversible Heat flow is irreversible

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Overview of Convection Overview of Convection

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Overview of convection Overview of convection

Closer to vertical = smaller lapse rate (vertical = zero) Closer to horizontal = larger lapse rate

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Stable Atmosphere Stable Atmosphere

Initial State Initial State

green = adiabatic lapse blue = environmental lapse < adiabatic

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Stable Atmosphere Stable Atmosphere

Parcel is heated Parcel is heated

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Stable Atmosphere Stable Atmosphere

Rises to new equilibrium Rises to new equilibrium

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Stable Atmosphere Stable Atmosphere

Parcel is cooled Parcel is cooled

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Stable Atmosphere Stable Atmosphere

Sinks to new equilibrium Sinks to new equilibrium

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Unstable Atmosphere Unstable Atmosphere

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Unstable Atmosphere Unstable Atmosphere

Initial State Initial State

green = adiabatic lapse blue = environmental lapse > adiabatic

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Unstable Atmosphere Unstable Atmosphere

Parcel is heated Parcel is heated

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Unstable Atmosphere Unstable Atmosphere

Rises without stopping Rises without stopping

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Summary of Stability Summary of Stability

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Summary of stability: Summary of stability:

Stable conditions: Adiabatic Lapse > Environmental Lapse Unstable conditions: Adiabatic Lapse < Environmental Lapse Why is stability important? A stable atmosphere does not move heat around An unstable atmosphere undergoes convection: Hot air rises, cold air sinks Redistributes heat

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Moist Convection Moist Convection

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Moist Convection Moist Convection

Dry air rises and cools Cooling water vapor condenses to liquid Condensation releases latent heat Latent heat warms air

⇒

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Moist Convection Moist Convection

Latent heat warms air Reduces adiabatic cooling Moist adiabatic lapse < Dry adiabatic lapse Smaller lapse = less stable Humid air is less stable than dry air

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Perspective Perspective

Stable: Environmental lapse adiabatic lapse Unstable: Environmental lapse > adiabatic lapse Adiabatic lapse: Dry: 10 K/km Moist: 4-8 K/km (depends on humidity) Pure radiative equilibrium: Would produce lapse of 16 K/km: unstable Radiative-Convective equilibrium: Convection modifies environmental lapse Normal environmental lapse is roughly 6 K/km (typical moist adiabatic lapse rate)

≤

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Greenhouse effect Greenhouse effect

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Greenhouse effect Greenhouse effect

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Greenhouse effect Greenhouse effect

1. 2.

  • 3. Increase greenhouse gases
  • 4. Skin height rises by

5. rises by

= 254 K Tskin = + lapse rate × Tground Tskin hskin Δhskin Tground lapse rate × Δhskin

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Vertical Structure and Saturation Vertical Structure and Saturation

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Set up MODTRAN: Set up MODTRAN:

Go to MODTRAN ( Go to MODTRAN ( )

Set altitude to 70 km and location to “1976 U.S. Standard Atmosphere”. Set CO2 to 1 ppm, all other gases to zero. Now increase by factors of 10 (10, 100, 1000, …)

http://climatemodels.uchicago.edu/modtran/ http://climatemodels.uchicago.edu/modtran/

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0.1 ppm CO 0.1 ppm CO2

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1 ppm CO 1 ppm CO2

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10 ppm CO 10 ppm CO2

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100 ppm CO 100 ppm CO2

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1000 ppm CO 1000 ppm CO2

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10,000 ppm CO 10,000 ppm CO2

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Question Question

Why do we see the spike in the middle

  • f the CO2 absorption feature?
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Question Question

Water vapor absorption is completely saturated. Why does water vapor emit at warmer temperatures than CO2?