Topic 2_Ch 6_"Energy Balances"

Chapter 6: Energy Balances

Introduction

• Energy is conserved globally, irrespective of pathways or usage.

• However, imbalances can occur as scale increases:

  • Earth’s primary energy balance components:

    • Inputs and outputs of solar (shortwave) and terrestrial (longwave) radiation.

    • Temperature rises when inputs exceed outputs, causing heat transfer to the atmosphere and vice versa.

    • Temperature determines heat flow direction between Earth and atmosphere.

    • Heat flow is termed 'flux'.

    • Changes in energy flow due to temperature changes are referred to as 'forcing'.

Urbanization and Radiative Forcing

• Urbanization changes surface characteristics, leading to:

  • Local radiative forcing and increased surface temperatures.

  • Enhanced radiative heat flux to the atmosphere.

• Greenhouse gas emissions change atmospheric composition, resulting in:

  • Global radiative forcing affecting energy transfer to Earth's surface.

  • Both urbanization and greenhouse gas effects can be local or global.

Effective Radiating Temperature

• Definition: Temperature at which a system radiates as much energy as it receives.

• Balance maintained through negative feedback mechanisms:

  • An increase in solar output results in increased Earth output to maintain energy balance.

Solar Radiation Input

• Solar radiation intercepted by Earth over a circular area:

  • Calculated as SπrE².

• Solar radiation on flat surface: about 1361 W/m² (solar constant).

• On a spherical surface, the solar intensity averages to one-quarter of the solar constant due to area differences.

Effective Radiating Temperature Calculations

• Average albedo (α) of Earth system: 30%.

• This yields an effective radiating temperature (TE) of:

  • 255K or -18°C.

• Greenhouse effect raises surface temperature by approximately 33°C, resulting in an average of 15°C.

Radiative Properties of Clouds

• Clouds have dual effects:

  • Reflect solar radiation, tending to cool the Earth.

  • Absorb and emit longwave radiation, tending to warm the Earth.

  • Effects depend on cloud type:

    • Low thick clouds cool during the day.

    • High thin clouds warm during the day.

    • All cloud types generally warm during night.

Surface Energy Balance and Heat Transfer

• Surface energy conditions create microclimates:

• Radiative Heat Transfer

  • Net solar radiation (K*): Difference between incoming radiation and reflected amounts.

  • Net longwave radiation (L*): Depending on temperature and atmospheric emissivity, influenced by clouds.

• Non-radiative Heat Transfer

  • Positive Q* indicates radiation surplus, with energy flows as follows:

    • QH: Convective sensible heat flux into the air.

    • QE: Convective latent heat flux into the air.

    • QG: Conductive sensible heat flux into the ground.

  • Negative Q* signifies flows toward the surface, cooling ground and air.

    • Condensation reduces water vapor; formation of dew or frost occurs.

Energy Partitioning in Different Conditions

• In dry conditions, surplus energy is split between QH and QG.

• Temperature of ground and near-surface air rises due to energy surplus:

  • Influenced by surface conductivity and wind speed.

  • Ground's low heat conductivity means most surplus allocated to QH.

• In moist conditions, additional partitioning to evaporation (QE) reduces temperature increase.

  • The partitioning depends significantly on moisture availability.

• Bowen Ratio: Ratio of QH to QE, indicating more heating or cooling.

Effects of Surface Types on Energy Balance

• Desert Surface:

  • Lower Q* due to high albedo.

  • High longwave emission (L) yields large daytime temperature increases due to low heat retention and little moisture.

• Ocean Surface:

  • Higher Q* due to low albedo.

  • Small nighttime temperature decrease attributed to high specific heat, convective mixing, and abundant evaporation, leading to low QH and high QE.

Urban Heat Island Effect

• Illustrates localized temperature variations due to urban development.