L7_Energy budget_UHI

Urban Energy Budget and Heat Island Effect

• Urban Energy Budget: Refers to the balance between energy input, output, and storage in urban environments.

• Urban Heat Island (UHI): A phenomenon where urban areas experience higher temperatures than their rural surroundings (Oke, 1982).

Temperature Profiles and UHI

• Late Afternoon Temperature: Illustrates the temperature differentials between urban and rural areas.

  • Temperature Chart: Example Values

    • Urban: 92°F (33°C)

    • Rural: 85°F (29°C)

Energy Budget Components

• Inputs:

  • Energy Input: Primarily from the sun via short-wave radiation.

  • Factors affecting energy arrival at Earth's surface:

    • Angle of the sun

    • Reflection and absorption by clouds, water vapor, pollution

    • Properties of absorbing surfaces:

      • Reflectivity

      • Heat storage capacity

• Outputs:

  • Energy Loss: Occurs through long-wave radiation re-emission.

• Energy Transfers: Urban vs Non-urban:

  • Inputs:

    • Short-wave: 317 W/m² from the sun.

    • Long-wave: 246 W/m² from atmosphere.

  • Outputs: Include reflection (QR) and various forms of energy loss (QLout, QE, QH).

Urban and Forest Energy Budget Comparisons

• Human Activity Input (QF): Energy generated by combustion and devices like refrigerators.

• Albedo (Reflection) Values:

  • Varies by surface type:

    • Fresh Snow: 80-95%

    • Grass: 25-30%

    • Asphalt: 5-20%

    • Urban surfaces typically absorb more heat due to lower albedo.

Latent Heat Flux and Energy Loss

• Evapotranspiration (QE): Loss of energy through evaporation and transpiration, predominantly through forests.

• Long-wave Emissions and Sensible Heat Loss:

  • Heated urban surfaces lose long-wave energy and exchange energy through convection.

Urban Heat Characteristics

• Temperature Differences: Urban areas typically 3-4°C warmer, particularly during evening.

• Cooling Dynamics: Urban areas cool slower than rural areas due to various influencing factors.

Causes and Impacts of UHI

• Contributors to UHI:

  • Heat absorption/dissipation by materials and morphology.

  • Variations in air quality and energy generation (e.g. from vehicles).

• Examples of Material Temperature Changes:

  • Water: 3-4°C

  • Asphalt: 30°C

  • Grass: 20°C

Human Influence on UHI

• Increased Temperature at Night: Cities retain heat due to materials that trap thermal energy.

  • Urban landscapes show decreased cooling rates at night; this is pronounced on clear, windless nights.

• SkyView Factor: Influences the intensity of UHI; lower factors imply more severe UHI effects.

Consequences of UHI

• Impact on Weather Patterns: Alters local rainfall and fog frequency.

  • Warm air rising creates unstable weather conditions conducive to precipitation.

• Differential Fog Patterns:

  • Urban heat from building materials impacts local condensation, causing a decline in fog frequency (since 1950 in LA).

• Cherry Blossom Timing in Osaka: Urban heat directly affects the phenology of plant flowering dates, shifting timing based on proximity to urban centers.

Final Remarks

• Urban areas remain warmer due to anthropogenic energy sources, reduced albedo, and higher heat storage capacity of materials, leading to varied ecological impacts.