Urban climate - lectures 16-19 Flashcards
Landscapes modified by human agency, buildings
Urban environment
The localised warming exerted by an urban area
Urban Heat Island
Heat added to the atmosphere by human activities
Qf
Road transport, power generation, domestic heating, air conditioning
Examples of Qf
Heat stored within the urban canopy
Qs
Advection (from surrounding areas)
Qa
Is Qe important in UHI
less important - little surface water available for transpiration and evaporation. For local climate with parks or irrigated areas may cause oasis effect!
Is Qg important in urban areas
Yes, an enhanced role
Why is Qg more important in urban areas
Increased heat storage during the day caused by urban materials, increased heat emission in evening and night, low sky view factors
Where is Qg less important
Mediterranean settlements
What heat flux is most important in suburbs
Qh
Suburban Qh lags rural Qh
diurnal variation
Urban radiation & energy fluxes lead to the generation of…
urban boundary layer
Multiple reflective surfaces (urban canyon), changed albedo, particulate matter - reflects and absorbs incoming SW radiation
SW radiation
Particulate matter reflects and absorbs incoming SW radiation, as a result is a source of downwards LW radiation, reduced LW emission (sky view factor)
LW radiation
Additional source of energy Qf, reduced Qe, increased Qg and Qs, reduced Qa
Surface energy balance
High Qh and Qs, low Qe
City centre energy balance
High Qe, low Qh and Qs
Rural energy balance
Slower to warm up in mornings (Qg) and low sky view factors mean reduced receipt of SW radiation
Urban cool island
Urban areas often have slower heating and cooling rates than rural areas
Diurnal evolution of UHI
Why are warming & cooling rates different?
Qg, outgoing LW radiation
How are parks different from urban surroundings?
colour - albedo
water availability
High proportion of available water
possible oasis effect Qe > Q*
Only occurs when synoptic-scale pressure gradients light, more common at night, can be of great significance for pollutant dispersal
Urban breeze
Increased drag and turbulence from urban surface results in a relatively deep zone of friction - reduced windspeeds in comparison to equivalent rural atmosphere
Urban Boundary Layer
if wind is oblique to street =
corkscrew effect
if wind is parallel =
no sheltering by buildings - channelling of wind may result in jetting effect so enhanced wind speed
Speed increases with height, so tall buildings will deflect faster moving upper air down to ground level, so can be up to 3 times windier than over open ground
Impacts of tall buildings
Weak synoptic flow / pressure gradient, clear skies, more prominent at night
Favourable synoptic conditions for UHI
City located on a high mountain ridge, prone to UHI development - UHI > 11°C recorded. Entrainment of warm urban air into katabatic flow makes some valleys warmer than valley heads.
Johannesburg, South Africa
Since mid 1970s, greater diurnal temp on _____ and ____ compared to rest of the week
Sunday and Monday
small town east of Chicago, downwind of steel mills, with increased annual precipitation from 990 mm to 1450 mm in 1930-1945. at nearby stations precipitation decreased from 990 to 900mm. rise in precipitation kinked to rise in steel production - but little investigation
The La Porte Anomaly
positive correlation between UHI intensity and vapour pressure - so warmer nights associated with more moisture in air, difference in vapour pressure
Urban moisture excess (UME)
Urban Plume - meso or micro?
Meso
Canopy Layer - meso or micro?
Micro
Line & point pollution sources
Canopy Layer/ microscale
Areal pollution source
Boundary layer/ mesoscale
Warm air overlying cold air
inversion
occurs in a stable atmosphere, vertical motion is suppressed causing a relatively thin, concentrated plume
fanning
occurs during the development of a surface radiation inversion
lofting
If turbulent eddies are smaller than the pollutant cloud / plume
Will diffuse it
if turbulent eddies are larger than the pollutant cloud / plume
Will transport it
