Urban Climate

both features modify the atmosphere:

Urban form

• fabric: material (of buildings, roads, vegetation) determine radiative, thermal and moisture properties

  • -> absorb, reflect, emit radiation

  • -> accept, transfer and retain heat and water

• surface cover: built up, paved, vegetated, bare soil -> determine partioning of heat, water and albedo

• urban structure: dimensions of building, spaces inbetween, street width,… determine aerodynamic roughness and albedo

Urban function

• metabolic cycles: work day, work week, … influence pulses -> traffic, water use, heating and cooling

• anthropogenic emmisions

• periods of constructions, decay, destruction

• air pollutants interfere with radiation (-> can form condensation nuclei)

• radiative: geometry, refelctivitiy, absorptivity

• thermal: specific heat, heat capacity, thermal conductivity

• moisture: interception and storage capacity, stomatal characteristics, chemical nature

• aerodynamic: roughness, zero-plane displacement, porosity

• Facets: roofs, walls, lawns

• elements: building, tree

• urban canyon: street with flanking buildings

• urban block: grid pattern producing square or rectangular blocks -> road pattern

• neighbourhood: land use zones in urban planning (residential, industrial,..)

• City: largest unit

• QF

• heat released as a result of human activities in a city

• Surces:

  • combustion of fuels

  • consumption of electricity (heating, cooling)

  • lighting

  • cooking, heating water

• three categories:

  • buildings

  • transport

  • human/animal metabolic output

• Energy use by individuals depends on:

  • climate of the region

  • nature of the economy

  • modes of transport

  • available resources

  • behaviroural differences

• 20-100% of annual net allwave radiation (higher values in winter due to heating)

streets with flanking buildings

H/W = lambdas -> canyon aspect ratio (H=height of buildings, W=width of stree)

-> Impact of geometric configuratuib of urban canyon on radiation exchange

• increase of canyon aspect ratio leads to increase od absorption and decrease of albedo

• clustered by their typical fabric, land cover, structure and metabolism

• also entails natural ecosystems

• can be determined from aerial photographs, remote sensing and maps or from measurements

• suited for predicting climatic characteristics, not land use

• if no regional wind present, climatic influence of a city is restricted to a self-constaining urban dome

• with regional wind -> urban plume

  • plume contains thermal, mpositure and kinematic effects of the city

  • plume carries air pollutants (up to hundreds of kilometers)

  • possible that plumes from multiple cities combine (megalopolitan plume)

  • lowest 10% of UBL is aurface layer (here are effects of citiy most profound)

• site of intense human activity (and excahnge and transformation of energy, momentum and water)

• top of UCL is the height of urban elements (sometimes hard to definde specific height due to few unusually high buildings)

• different processes near roof-level and within canopy (e.g. street level)

• radiation exchange is disrupted by restricted view of sun and sky

• Sensible heat QH: driven by differences between surface and atmosphere (->minimizes temperature differences)

• Latent heat QE: energy requires to vaporise a certain water mass

• Bowen ratio:

if beta >1 -> lower atmosphere is warmed

if beta < 1 -> keeping surface cooler while adding humidity

(Ocean beta = 0.1; Crops beta = 0.3; Urban with much green beta = 1; Urban with few green beta = 4; sandy dessert beta = 10)

• at citiy scale: urban-rural thermal property differences are small

• seasonal changes can alter sign and size of differences

• if surrounding rural area has dry soils, fresh snow or vegetation -> favour heat storage in the city

• if surrounding rural area has wet soils (even flooded) or bare rock -> differences are much smaller (even reversed)

• Cities can sequester heat efficiently (due to geometry and great active area)

• Generally: the greater the density the greater the potential for storage