11/13/2024 URBAN CLIMATOLOGY VII. Spatio-temporal variability of other meteorological elements in urban areas Table U2 Urban climate effects for a mid-latitude city with about 1 million inhabitants (values lor summer Lin less otherwise noted) Air humidity in urban climate Variable Change M agn i tude/eo m men is 1 uibnlcjicc intensity Greater 10-50% Wind -,pecJ Decreased 5-30% at 10 m in strong (low Increased In weak How with heat island Wind direction Altered 1-10 decrees UV radiation Much less 25-90% Solar radiation 1 .ess 1-25% Infrared input Greater 5^10% Visibility Reduced Evaporation Less About 50% Convective heat Mux Greater About 50% Heal storage dealer About 200% Air temperature Warmer ]-3°Cner 100 years: 1-3°C annual mean up In Hi' hourly mean H li nudity Drier S li in me r day I hue More moist Summer night, all day winter Cloud More haze In and downwind of city More cloud Especially in lee of city Fni; More or less Depends on aerosol and surroundings Precipitation Snow Less Some turns to ruin Total More.1 To the Ice of rather than in city Thunderstorms More 1 Atmospheric moisture in urban areas • Not many studies - atmospheric moisture is quite complex and variable • Importance 1) transition between the vapour, liquid and solid states has energetic implications due to an uptake and release of latent heat 2) As a suspended liquid or solid atmospheric humidity forms clouds and precipitation • In specific situations densely built up areas may be „arid" or „humid" due to impervious surface cover, relative lack of vegetation, UHI existence, local geography, water supply sources etc. • Existing studies are hardly comparable as they use different instrumentation as well as different air humidity measures Atmospheric moisture in urban areas Air humidity measures A hair tension dial hygrometer • Absolute humidity - (grams of water vapor per cubic meter volume of air) - it is a measure of the actual amount of water vapor (moisture) in the air, regardless of the air's temperature. The higher the amount of water vapor, the higher the absolute humidity. • Specific humidity (or moisture content) is the ratio of the mass of water vapor to the total mass of the air parcel. • Vapor pressure - the partial pressure of water vapor in the atmosphere • Relative humidity - the ratio of how much water vapour is in the air to how much water vapour the air could potentially contain at a given temperature. • RH varies with the temperature of the air: colder air can contain less vapour, and water will tend to condense out of the air more at lower temperatures. • Temperature-dependent and T-independent measures often provide contradictory results 11/13/2024 Spatial variability of air humidity and its relation to air temparature pr«tuit(fTfc] . . ..:....... • • Well developed UHI ^^^^^^ • Strong correspondence between T and RH fields • Vapor pressure spatial distribution follows city structure and density of buildings Spatial variability of air temperature (a), relative humidity (b) and vapor pressure (c) in Leicester (UK), calm, clear sky summer night (Oke et al. 2017) Spatial and temporal variability of air humidity in urban areas is the result of evapotranspiration, condensation and advection processes. There are several positive and negative feedbacks. • Higher temperature -> higher intensity of evapotranspiration (that is however low due to lack of vegetation) • No consumption of latent heat -> rising temperature • Fast runoff -> les intensity of evaporation 3 Humidity in urban areas • Atmospheric humidity is generally lower in cities during daytime (due to lower evapotranspiration compared to rural areas - there is smaller fraction of vegetation cover) ■ At night and in winter there is an urban moisture excess (UAAE) in mid- and high latitude cities. The reason is: i) additional water vapor from anthropogenic activities; ii) weak evapotranspiration in unstable atmosphere • In some situations (dry spells, arid climate) humidity can be higher in cities due to extensive irrigation compared to neighborhoods • Bad air quality mostly cause increase of fog frequency and intensity Urbanization-induced atmospheric moisture changes Urban dry island (UDI) - drier urban areas; first documented in 1950s in Europe Urban moisture island (UMI) - wetter urban areas compared to rural areas, in numerous European, North American, and Asian cities especially in clear summer nights The occurrence of UMI can be attributed to the enhanced evapotranspiration and delayed dew/fall of urban vegetation at night due to UHI effect, as well as intensive anthropogenic moisture emissions. ,-'V sea-land breeze circulation (AA) urban rain island (F) valley circulation (AA) evapotran- 11 railing anthropogenk "Sp,i™"<"1 moisture emissions (Qt) T (£7> urban heat island circulation (A/I) mountain barrier effect {AA) í I frost (D) 4 Ail infiltration ( ) Sea (Rural) impeded ventilation quick surface runoff . . . r„ Lake and Mountain (Rural) and drainage (ff) Schematic of multi-faceted urban changes in focal and regional hydrometeorological cycles. Source: _https://iopscience.iop.ora/article/10.1088/1748-9326/acf7d7_ Urbanization-induced atmospheric moisture changes Detailed understanding on urban humidity changes still lacks exploration due to the following challenges: 1) the inconsistency of humidity metrics measured and analyzed in different studies (temperature dependent and independent metrics) 2) ensuring the richness and representativeness of observation data. 3) a comprehensive urban moisture modeling, due to the difficulty in accurate physical and mathematical representation of hydrometeorological cycles and water phase-changing processes 4) coupling and decoupling with UHI studies as the relationship between air temperature and humidity is complex and interdependent. Air humidity and rapid urbanization (a) Urban Expansion and Decreasing Relative Humidity {b) Urban Expansion and Increasing Vapor Pressure Deficit Logarithm of Urban Expansion Rale Logarithm of Urban Expansion Rate' Built-up area fraction 1112013 10 15 20 30 50 Rapid urban land expansion significantly accelerates the decrease in atmospheric moisture over the area, and urbanization contributes to approximately 50% of the increase of vapor pressure deficit and the decrease of atmospheric humidity in the urban areas of Guangdong Province, South China (Lin et al. 2020) Relative humidity in Brno Variability during a year Annual variations in daily means of relative humidity at three stations in Brno area in the period 1987-2010 Mean, min a max relative humidity in Brno area in individual seasons during days with radiation driven weather for urban stations (red) and rural stations (blue) Relative humidity in Brno Variability during a day m>:= summer 0:00 3:00 6:00 9:00 12:00 15:00 18:00 21.00 0:00 3:00 6:00 9:00 12:00 10:00 18:00 21:00 winter autumn 1 Mean daily variations of relative humidity at urban (M) and rural (P) stations in Brno area (a) and their differences P - M (b) in individual seasons 0:00 3:00 6:00 9:00 12:30 15:00 18:09 21:00 9:00 3:00 6:09 9:09 12:90 15:00 18:00 21:00 0:00 3:00 6:00 9:00 12:00 15:00 18:00 21:08 9:00 3:00 6:00 9:08 12:00 15:00 18:00 21:00 Urbanization-induced moisture changes summary • Mid-latitude cities predominantly exhibit moderate UMI and UDI effects, • Cities with low mean annual precipitatioi exhibit extreme UMI and UDI effects. ENVIRONMENTAL RESEARCH LETTERS TOPICAL REVIEW ■ OPEN ACCESS Urban moisture and dry islands: spatiotemporal variation patterns and mechanisms of urban air humidity changes across the globe Xinjie Huang1'2 and Jiyun Song6'1,3,4,5 Published 25 September 2023 ■ © 2023 The Authons). Published by IOP Publishing Ltd : ■: -:e re;ee-:- _e::eT. Vc _ - e ' c '■--■.e- ' : and distinct dry/wet seasons, however, • The diurnal cycle - more pronounced UMI effects at night, largely due to increased evapotranspiration and delayed dewfall linked with UHI. • On a seasonal scale, - UDI effects dominate in spring, while UMI effects peak in winter for mid-latitude cities and in summer for low-latitude cities. • Topography, morphology, and size significantly shape urban-rural humidity contrasts. • Coastal cities are subject to sea-breeze circulation, importing moisture from sea to land, whereas mountainous cities can accumulate humidity and precipitation due to geographical barriers and vertical airflow. • High-density urban areas generally experience heightened UMI effects due to restricted airflow and ventilation. • Larger cities with higher populations contribute to increased UMI effects, particularly in winter, due to stronger anthropogenic moisture sources. Wind field in urban climate Table U2 Urban climate effects for a micl-l.uilu<(v i il\ with aljuui 1 million inhabitants lvalues tor summer unless otherwise note' u,ban Bi™ Contribution of different urban neighborhoods is blended ML wind flow integrate both the roughness and the thermal effect Airflow in ML may strongly affect convection processes and precipitation Wind flow usually leads to convergence (slowing due to roughness), and uplift Low pressure zone over warmer city initiates UHI circulation (a) (b) Divergence Urban dome- {Wvo^jřTj ^ří^uow) Outflow Divergence Outflow^ ^ Oke etal., 2017, Urban Climates © Cambridge University Press 2017 Combined roughness and thermal effects (a) Strong flow, weak UHI (h) Moderate flow, moderate UHI Plan view - Decelerate —H h— Accelerate —H : — * v _^ Divergence Coni-ergence p | „_)___ I * t * ' * * *; t t t i_t mi ,^iT..i. ij— "__■-■ :~ ■■- -__* Oke etal., 2017, Urban Climates © Cambridge University Press 2017 ENVIRONMENTAL RESEARCH LETTERS LETTER • OPEN ACCESS Introducing the urban wind island effect A M Droste1 ©, G J Steeneveld1 and A A M Holtslag1 Published 31 August 2018- e 2018 The Authorfs}. Published by IOP Publishing Ltd :T./i-c-:-^-_5 _ett^'5. Vcii.i~t 13. \lu~iber9 Free|atmnsphere Citation A M Droste et al 2018 Environ. Res. Lett. 13 094007 1 DOI 1O.108S/1748-9326/aad8ef Geostrophic wimT^] ^ 1U.AV J Urban BL ___ au.AV U,V ' ' 1 Rural BL J . , 7 . ft/ • For certain atmospheric conditions the boundary-layer mean wind speed in a city can surprisingly be higher than its rural counterpart, despite the higher roughness of cities. • This urban wind island effect (UWI) prevails in the afternoon, and appears to be caused by a combination of differences in ABL growth, surface roughness and the ageostrophic wind, between city and countryside. • The UWI phenomenon challenges the commonly held perception that urban wind is usually reduced due to drag processes. Wind velocity in Brno 0 2 4 6 8 10 12 14 16 18 20 22 Daily course of mean wind speed at urban (blue) and rural (red) stations in the 2005-2011 period 5 i 2011 Variability of mean annual wind speed at rural (blue, 1961-2011) and urban (red, 1987-2011) stations The ratio between maximum daily wind speed and mean daily wind speed may be used as a simple measure of intensity of turbulence. The ratio is clearly higher at the urban station. Intensity of turbulence Daily variation of tneon maximum wind speed at rural and urban stations in the period 2005-2011 10 12 14 16 18 20 22 Daily variation of the intensity of turbulence defined as a ratio of maximum wind speed (Fmax) and mean wind speed (Fprum) at urban and rural stations, period 2005-2011 rural 1.4 2.1 10 12 14 16 Intensity of turbulence is much higher at urban station The maximum and minimum in the daily course of the turbulence occur earlier in the city compared to countryside Wind field modification směr větru [% rychlost větru [m.s~*] 0,7 6.0 0 2 Modelled mean annual wind velocity in Brno area and wind roses for selected points at 10 m above terrain (model WAsP, data from Turany station, period 1961-1998) Localities: 1 - Brněnská přehrada 2 - oblast Staré dálnice 3 - Troubsko 4 - ul. Hroznová 5 - ul. Šumavská 6 - údolí Svitavy 7 - Tuřany, letiště. Wind field modification Modification of wind speed near obstacles calculated using WAsP model - an example for for high-rise buildings (60 m) at Šumavská str. a) study area; b) mean wind speed near buildings for NW wind direction; c) relative reduction of mean wind speed near buildings for NW wind direction Wind field modification ■ o,09 [m.s"1] 16,07 H ■ l,45 [m.s-1] 16,07J 1 ^m ij Modification of mean wind speed (a) and wind direction (b) near high-rise buildings at Šumavská str. calculated using WAsP model for wind speed 8.0 m.s-1 and for wind direction 260° (reference values for Tuřany airport station) Final remarks and questions Transition between water vapor, liquid water and solid states has energetic implications due to consumption or release of latent heat Extensive irrigation may significantly modify moisture conditions especially in an arid climate. Wind field may be significant factor reducing thermal heat load and the urban heat island intensity. Intentional wind field modifications in scale of city may contribute significantly to mitigation strategies (see lecture 10) 1. How does the vegetation in urban areas influence humidity? 2. What is a typical variability of humidity in urban area during a day and during a year? 3. What are main factors modifying wind field in urban areas 4. What can be the most important negative effects of wind field modification in urban areas?