7. urban climates - From Thompson, 1998 (textbook) for 2018 lecture PDF

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From Thompson, 1998 (textbook) for 2018 lecture...

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8 CONTENTS Urban Heat Island (UHI) Effect 1. Factors determining the effect’s magnitude 2. Spatial variations Causes of UHI Effect 1. Higher Thermal capacity 2. Sensible + latent heat transfer 3. Urban structure reduces insolation and outgoing terrestrial radiation loss and 4. Heat production via human activities 5. Pollutants trap outgoing terrestrial radiation and reemit it back to earth 6. Lower wind speeds discourage ventilating effect Urban Induced Precipitation 1. Causes of higher urban precipitation levels 2. Case studies - St Louis and Paris Other Aspects of Urban Climates 3. Solar radiation 4. Relative humidity 5. Generation of country breeze

Words in italics have been paraphrased/added from the notes and may not be necessary to write.

Temperature Changes in Urban Climates: Urban Heat Island Effect Definition Increase in temperature of an area’s microclimate caused by the creation of a city (Urban area temperatures are 0.5 to 1.5 °C higher than surrounding areas) 1

Determinants of UHI magnitude Main Factors 1. Size of city: larger size means greater effect 2. Population density: higher means greater effect Micro-scale Variations ● Presence of industries: GREATER EFFECT ○ This is why LDCs may have an equal effect as DCs ● Open spaces (high wind), rivers, canals, parks (evapotranspiration, shade): SMALLER EFFECT ● Time of day: GREATER EFFECT in late afternoon - early evening ● Season: GREATER EFFECT in winter (usage of artificial heating) ● Latitudinal region: GREATER EFFECT in temperate than tropical cities ○ Tropical regions have fewer human-generated heat sources

2

Spatial variation of UHI effect The intensity of the effect varies spatially across a city.

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Highest temperatures ○ Zones of greatest building densities and industrialisation ○ Zones of highest population densities Lowest temperatures ○ Parks

UK ● 3

Kew, London, has 70 more frost-free days than nearby rural Wisley

Causes The UHI effect occurs due to modifications to energy balance that occur when

natural surfaces are paved and built upon, and when human activities release heat into local environments. 3a

Changes to Thermal Capacity Urbanisation increases the thermal capacity of an area 1. Thermal capacity is the ability to store heat. 2. Materials used in building construction (cement, asphalt) have higher thermal capacities than most natural surfaces (thin soil). 3. This affects the amount of heat transferred in the following process a. Building surfaces are warmed in the day, and a temperature gradient results that conducts heat towards building interiors. b. When cooling occurs in the late afternoon, heat stored in buildings is released to the lower atmosphere, in higher quantities than for natural surfaces, and nocturnal temperatures rise. 4. Thus city surfaces cool slower than the countryside, gradually releasing additional heat accumulated during the day

3b

3c

Transfer of Sensible Heat Urbanisation increases sensible heat transfer at the expense of latent heat ● Arising from the transfer of surplus insolation received in city areas to the atmosphere 1. City streets and sidewalks are almost impervious to water unlike natural surfaces. 2. During precipitation, most water runs off the surface and flows out of flood-control systems 3. This reduces available water, raising the input of sensible heat to the atmosphere while reducing latent heat, as heat that would have been used to evaporate water is now used for raising surface temp

Latent heat is associated with changes in the state of water.

Relationship between moisture content of surface and sensible heat 1. Near the sea a. When moisture is available near the surface, the transfer of energy as latent heat exceeds sensible heat transfer (implying that most insolation surplus is consumed by evaporation) 2. In dry surfaces a. Surplus insolation raises surface temperatures far above air temperatures, so sensible heat is created 3. Ceteris paribus, the higher the ratio of sensible heat to latent heat, the greater the UHI effect.

“Water runs off surface” = Horton overland flow

Urban Structures Urban structures decrease urban areas’ albedos ● Changes surface albedo, as urban surfaces (asphalt streets, roofs) are less reflective and have lower albedos than natural surfaces they replace ● Changes in surface albedos further affect amounts of heat absorbed and reflected ● Process of multiple reflections in cities’ complex 3D structures 1. Buildings’ vertical walls tend to trap radiation more than rural areas with flat surfaces, due to their lower flat surface areas 2. As insolation hits the side of a building, some of it is absorbed, and

Sensible heat is heat you can feel radiating from the surface.

some is scattered back as diffuse radiation, some of which strikes adjacent buildings, where once again a portion is absorbed and likewise partially scattered. 3. This process repeats. Thus as the sides of urban structures emit stored heat, a portion is reradiated between buildings and dissipated slowly. 4. This increases the total absorption such that overall urban area’s albedo is lower than the albedo of individual surfaces. Urban structures increase longwave radiation transfer ● Process similar to multiple reflection of insolation 1. At night, open rural surfaces can emit longwave radiation that travels upwards to space, losing heat from surfaces, without impediment 2. Urban surfaces however impede the escape of longwave radiation to space as walls absorb a portion of outgoing radiation and scatter the rest to other buildings. 3. The process repeats. Outgoing radiation is reradiated between buildings and dissipated slower than in open rural areas. 4. This reduces the amount of longwave radiation lost and thus, the rate of nocturnal cooling. 5. Thus daily minimum temperatures at night rise. 3d

Heat Production by Combustion (anthropogenic heating) Urban areas produce waste heat from power generation ● For activities, e.g. transport, home heating, industrial activities ● Anthropogenic heating greatest during winter, thus UHI greater in winter Sheffield, UK and Berlin, Germany ● Studies showed these cities’ annual heat production was approximately one-third of that received from insolation Japan ● Combustion is believed to be the most significant factor causing UHI USA ● In Manhattan, heat production from combustion in winter was 2.5 times greater than the amount received from insolation Canada ● In Vancouver’s winter, anthropogenic heat released was approximately almost 4 times that received as insolation

3d

Presence of Pollutants Urban air pollution traps outgoing terrestrial radiation and then reemits it, enhancing the UHI effect ● “Pollution domes” (made of particulate matter, carbon dioxide, water vapour) over cities absorb a portion of outgoing longwave radiation emitted by the surface, then reemit a portion back to the ground ● This results in a ‘localised greenhouse effect’ that raises the temperature as outgoing radiation is prevented from leaving the atmosphere as readily as insolation was able to enter it

3e

Alteration of Wind Speed Urban structures lowers average wind speed, reducing heat loss rates ● Building surfaces are rougher than natural ones and increase frictional drag upon near-surface winds while reducing average horizontal wind

UHI (Urban Heat Island effect)

●

speeds Lower wind speeds lower the city’s ventilation by impeding the movement of cooler outside air that, if allowed to enter and mix with warmer air, would reduce higher temperatures in city centres

Urban structures cause local turbulence (and channeling) ● Winds between and above buildings increase in speed and become variable in direction and speed, reducing the number of calm areas ● Strong regional winds can counteract the UHI effect ○ Studies show when regional wind speeds exceed a critical value (the larger the city, the larger this critical value needed), the heat island is eliminated Urban Induced Precipitation Definition Urban areas have about 10% more precipitation than in rural areas 1

Causes of Higher Urban Precipitation UHI effect creates thermally induced upward air motions that increase atmospheric instability Urban industrial activities release more particulate matter that increase the availability of condensation nuclei, increasing the production of clouds ● This depends on the concentration of hygroscopic nuclei of the appropriate sizes in the atmosphere ○ Hygroscopic nuclei attract water and trigger early onset of condensation Rougher city surfaces slow down air moving laterally, but cause low-level air convergence and increased upward air motion ● Rougher surfaces impede progress of weather systems ● Thus when rain is being created, the processes may linger over cities and increase the rainfall amount Hygroscopic nuclei attract water and trigger early onset of condensation even if the air is not yet saturated ● For instance, fogs develop prematurely and stay longer in industrial areas as water droplets adhere strongly to industrial pollutants

2

Case Studies

2a

St. Louis, USA Hypothesis: cities are responsible for precipitation modifications ● Increases in precipitation at downwind locations that grew with industrial development ● Increases are greater on weekdays when urban activities are most intense, than weekends when activity ceases

2b

Paris, France

Hygroscopic: tending to absorb moisture from the air

Similar findings as studies in St. Louis, USA ● Precipitation is greater Monday to Friday than Saturday - Sunday Changes in Other Climate Aspects Reduction in Solar Radiation The pollution dome (blanket of particulates, created by urban pollution) over cities reduces the amount of solar energy reaching the ground by 15% ● Depends on angle of incidence from sun ○ Lower angle reduces insolation more: the length of the path through the pollution dome increases as the sun angle drops ● Depends on effectiveness of ventilation ○ Poorer ventilation reduces insolation more as more particles stay within the atmosphere, scattering or reflecting insolation back to the upper atmosphere

Solar energy is the same as insolation

Lowered Relative Humidity Main Causes 1. Higher temperatures (UHI effect) 2. Lower evaporation due to absence of moisture a. Fewer water bodies b. Rainfall is removed swiftly via flood-control systems c. Less vegetation cover which lowers evapotranspiration and thus, absolute humidity However, fogs and cloudiness are more frequent ● Due to the increase in quantities of condensation nuclei produced by human activities. When hydroscopic nuclei are plentiful, water vapour readily condenses on them even if the air is not saturated. Generation of Country Breeze Country breeze refers to a circulation pattern of winds blowing into the city from the surrounding countryside. Develops on relatively clear, calm nights when UHI effect is greatest ● Heating creates upward air motion, creating a low pressure area above cities, setting up a pressure gradient between the countryside and city, which initiates country-to-city airflow. Link to Hydrology Urban Induced Precipitation Higher precipitation increases the likelihood of Horton overland flow and thus floods, given the low infiltration capacity and rate of impervious urban surfaces Alteration of Wind Speed Overall urban areas encounter more atmospheric instability due to the lack of calm areas caused by local turbulence. Thus precipitation formation is more likely Lower Evapotranspiration

Urban areas have 10% more precipitation

Channel inputs in urban areas may exceed outputs due to the lack of evapotranspiration caused by an absence of moisture - depending on the veracity of flood control measures in place. Overall, the likelihood of floods increases in urban areas due to the changes to the microclimate....