Geosystems: Atmospheric Energy and Global Temperatures

Atmospheric Energy and Global Temperatures: An Introduction to Physical Geography

Atmosphere and Surface Energy Balances

• Energy Essentials:

  • Energy Pathways and Principles:

    • Shortwave energy in from the Sun: Includes ultraviolet (UV), visible light, and near-infrared (near IR) radiation.

    • Longwave energy out from Earth: Primarily thermal infrared (thermal IR) radiation.

  • Insolation input: Refers to all solar radiation (both direct and indirect) received at Earth's surface.

  • Energy Pathway Terms:

    • Transmission: The passage of energy through the atmosphere or water, allowing direct radiation to reach the surface.

    • Scattering (Diffuse radiation): The process where light's direction of movement is changed without altering its wavelengths. This results in diffuse radiation.

    • Refraction: The change in both speed and direction of light as it passes from one medium to another (e.g., through the atmosphere).

  • Energy Principles:

    • Albedo: The reflective quality of a surface.

      • It is directly related to the surface's color and texture.

      • High albedo: Indicates a highly reflective surface (e.g., light-colored, smooth surfaces).

      • Low albedo: Indicates a less reflective surface (e.g., dark-colored, rough surfaces).

      • Earth's overall albedo is 31\%: This average is composed of 21\% from clouds, 3\% from the ground, and 7\% from the atmosphere.

      • Examples of Albedo values (% reflected):

        • Fresh snow: 80\% - 95\%

        • Grass: 25\% - 30\%

        • Forests: 10\% - 20\%

        • Crops, grasslands: 10\% - 25\%

        • Asphalt (black top): 5\% - 10\%

        • Concrete, dry: 17\% - 27\%

        • Moon: 6\% - 8\%

        • Water bodies: 10\% - 60\% (varies with Sun altitude)

        • Dark roof: 8\% - 18\%

        • Light roof: 35\% - 50\%

        • Brick, stone: 20\% - 40\%

    • Clouds, Aerosols, and Albedo (Conflicted role of Clouds):

      • Cloud-albedo forcing: Typically associated with lower stratus clouds.

        • These clouds reflect shortwave energy from the Sun.

        • Leads to atmospheric cooling.

      • Cloud-greenhouse forcing: Typically associated with higher cirrus clouds.

        • These clouds act as insulation, trapping longwave radiation.

        • Leads to atmospheric warming.

• Absorption and Heat Transfer:

  • Absorption: The assimilation of radiation by matter and its conversion into heat.

  • Conduction: Molecule-to-molecule transfer of heat energy (e.g., heat transfer from the ground surface to deeper soil).

  • Convection: Energy transferred by vertical movement of fluids (e.g., warm air rising).

  • Advection: Energy transferred by horizontal movement of fluids (e.g., wind carrying warm air across a landscape).

  • Radiation: Energy traveling through air or space in the form of electromagnetic waves.

Energy Balance in the Troposphere

• The Greenhouse Effect and Atmospheric Warming:

  • The atmosphere absorbs heat energy.

  • Similar to a real greenhouse, which traps heat inside by restricting air movement, the atmosphere delays the transfer of heat from Earth into space.

  • It's important to note that a real greenhouse primarily works by restricting convection, while the atmospheric greenhouse effect involves certain gases absorbing and re-emitting longwave radiation.

• Earth–Atmosphere Radiation Balance:

  • Shortwave portion of the budget (Incoming Solar Energy: +100 units):

    • Reflected by clouds: -21 units

    • Diffuse reflection and scattering by atmospheric gases/dust: -7 units

    • Reflected by surface: -3 units

    • Earth's total albedo: -31 units (sum of reflected components)

    • Absorbed by clouds: +18 units

    • Absorbed by atmospheric gases and dust: +20 units

    • Stratospheric absorption by ozone: +3 units

    • Direct and diffuse radiation absorbed by Earth's surface: +45+21 = +66 units

  • Longwave portion of the budget (Outgoing Terrestrial Radiation):

    • Radiated by ozone layer to space: -3 units

    • Energy radiated to space (direct heat loss): -19 units

    • Energy radiated to space (from greenhouse effect): -66 units (sum of components (21+23+14+8=66), representing atmospheric window, cloud window and greenhouse gas absorption)

    • Outgoing Longwave from Atmosphere to Space: -96 units

    • Outgoing Longwave from Surface to Atmosphere: -110 units

  • Energy gained and lost by Earth's surface:

    • Surface heat input (absorbed shortwave): +66 units

    • Latent heat transfer (evaporation): -19 units

    • Convective (turbulent) transfer: -8 units

    • Infrared radiation (lost to atmosphere/space): -110 units

    • Infrared radiation (gained from atmosphere - Greenhouse effect): +96 units

    • Net surface balance: 66 - 19 - 8 - 110 + 96 = +25 units (This value should ideally be close to zero for long-term balance when all components are considered).

  • Energy gained and lost by the atmosphere:

    • Atmospheric heat input (absorbed shortwave): +41 units (18+20+3)

    • Latent heat transfer (from surface): +19 units

    • Convective (turbulent) transfer (from surface): +8 units

    • Infrared radiation (from surface): +110 units

    • Infrared radiation (lost to space): -96 units

    • Net atmospheric balance: 41 + 19 + 8 + 110 - 96 = +82 units (Again, this sum needs to be zero when considering emission to space from atmosphere itself. The slide's diagram is a simplified budget).

• Energy Budget by Latitude:

  • Equatorial and tropical energy surplus: These regions receive more incoming solar energy than they emit longwave radiation.

  • Polar energy deficit: These regions emit more longwave radiation than they receive incoming solar energy.

  • Poleward transport of energy: Atmospheric and oceanic circulation redistributes this energy from the tropics towards the poles, preventing extreme temperature differences.

Energy Balance at Earth’s Surface

• Daily Radiation Patterns:

  • Air temperature typically lags behind absorbed insolation.

  • The warmest time of day usually occurs in the mid-afternoon, even though peak insolation is at local noon.

  • The coolest time of day is typically just after sunrise.

  • A surplus of absorbed insolation occurs during the day, leading to temperature increases.

• Simplified Surface Energy Balance:

  • NET R (Net All-Wave Radiation) = +SW{in} - SW{out} + LW{in} - LW{out}

    • +SW (insolation): Incoming shortwave radiation from the Sun.

    • -SW (reflection): Outgoing shortwave radiation reflected by the surface (albedo).

    • +LW (infrared): Incoming longwave radiation from the atmosphere (greenhouse effect).

    • -LW (infrared): Outgoing longwave radiation emitted by the Earth's surface.

• Dissipation of NET R:

  • Latent heat of evaporation (LE): Energy stored in water vapor as water evaporates. This cools the surface.

  • Sensible heat (H): Back-and-forth transfer of energy between the air and the surface through conduction and convection. This directly warms or cools the air.

  • Ground heating and cooling (G): Energy that flows in and out of the ground by conduction. There is a positive gain during summer which is equalized by energy loss during the winter.

• The Urban Environment (Urban Heat Island):

  • Urban areas tend to be warmer than surrounding rural areas, especially at night.

  • Causes:

    • High sensible heat release: Much of the net radiation in urban areas is converted to sensible heat due to less vegetation and evaporation.

    • Lower latent heat: Less evaporation from impervious surfaces (concrete, asphalt) and less vegetation.

    • Low albedo surfaces: Dark roofs, roads absorb more shortwave radiation.

    • Trapped longwave radiation: Buildings obstruct wind flow and trap longwave radiation.

    • Anthropogenic heat sources: Heat from vehicles, industrial processes, air conditioning.

    • Reduced air circulation/turbulence: Buildings interfere with airflow.

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