Topic_2_Ch_6_Energy_Balances

Topic 2: Energy and Radiation Balances

Chapter 6: Energy Balances Introduction

• Energy is conserved globally; pathways and uses may vary but total energy remains constant

• Imbalances occur as scale increases

Key Components of Earth's Energy Balance

• Primary components:

  • Inputs/outputs of solar (shortwave) radiation

  • Inputs/outputs of terrestrial (longwave) radiation

• Temperature changes driven by the balance of inputs vs. outputs:

  • When inputs exceed outputs, temperatures rise leading to heat transfer to atmosphere

  • Heat exchange depends on which (Earth/atmosphere) is hotter

  • Heat energy flow is termed a flux

  • Changes in energy flows due to temperature shifts are termed forcings

Introduction Continued

• Examples of Radiative Forcing:

  1. Urbanization alters surface characteristics, creating local radiative forcing through raised surface temperatures and increased energy emission to atmosphere (radiative heat flux)

  2. Greenhouse gas emissions change atmospheric composition, causing global radiative forcing and increased energy transfer to Earth's surface (radiative heat flux)

• Local and global nature of these changes is significant

Effective Radiating Temperature

• Defined as the temperature at which a system radiates energy received:

  • Solar radiation absorbed = terrestrial radiation emitted

• Balance is maintained by negative feedback mechanisms preventing continuous temperature escalation

  • Increased solar output leads to increased Earth output to retain balance

Solar Radiation Interception

• Earth intercepts solar radiation as a circular area:

  • Amount intercepted: S × π × r^2

  • Solar radiation intensity on flat surface: 1361 W/m²

  • On sphere's surface, intensity is one-quarter due to the greater area

Effective Radiating Temperature Continued

• Average albedo of Earth system: 30%

• Results in effective radiating temperature: 255K (approximately -18°C)

• The greenhouse effect adds ~33°C, leading to average surface temperature: 15°C

Flows of Solar (SW) Radiation

• Analysis of solar radiation dynamics necessary for understanding energy balance

Flows of Terrestrial (LW) Radiation

• Important for understanding how energy is radiated back to the environment

Planetary Energy Balance and Clouds

• Clouds and solar radiation:

  • Reflect solar radiation, causing cooling

  • Absorb and emit longwave radiation, causing warming

  • Effect depends on cloud type

Radiative Properties of Clouds Continued

• Daytime Effects:

  • Low thick clouds cool surface

  • High thin clouds warm surface

• Night-time Effects:

  • All clouds generally warm surface

Latitudinal Radiative Imbalances

• Understanding the redistribution of energy imbalances across latitudes

Surface Energy Balance

• Variations in surface conditions generate microclimates

Radiative Heat Transfer

• Net Solar Radiation (K):*

  • Difference between incoming solar radiation and reflected radiation

• Net Longwave Radiation (L):*

  • Affected by atmospheric and surface temperature/emissivity

  • Influence of clouds and obstructions

Non-Radiative Heat Transfer

• When Q* (net radiation) is positive, there is a radiation surplus leading to energy release into the environment:

  • QH: convective sensible heat flux into air

  • QE: convective latent heat flux into air

  • QG: conductive sensible heat flux into ground

• When Q* is negative, flow reverses toward the surface, cooling air and groundwater

Non-Radiative Heat Transfer Continued

• Dry Conditions:

  • Energy surplus divided into QH and QG, raising ground and near-surface air temperatures

  • Partitioning depends on temperature gradients, surface conductivity, and wind speed

• Moist Conditions:

  • Additional evaporation component (QE) affecting temperature gains

  • Increased QE leads to less temperature rise

  • Plant transpiration regulates QE via water movement from roots to stomata

  • Bowen Ratio (H/LE): Higher ratios indicate more energy used for atmospheric heating

Desert Surface Characteristics

• Lower Q* due to high albedo

  • High L↑; hot surface, clear skies, dry conditions

  • Significant daytime temperature increases due to low specific heat and moisture availability

Ocean Surface Characteristics

• Higher Q* due to low albedo

  • Low L↑; minor temperature decreases at night

  • Lower daytime temperature increases due to high specific heat and convective mixing

Urban Heat Island Effect

• Urban areas exhibit higher net radiation due to anthropogenic alterations in surface and atmospheric properties