Urban Heat Island Circulation Research Articles

Urban heat island circulations over the Beijing-Tianjin region under calm and fair conditions

The Beijing-Tianjin-Hebei region has experienced severe urban heat island (UHI) effects in recent decades. Using the Weather Research and Forecasting (WRF) model and the new World Urban Database and Access Portal Tools (WUDAPT) land-use dataset, the urban heat island circulations (UHICs) and other local atmospheric circulations over Beijing and Tianjin were studied during a severe UHI event in 2012. It was shown that in the daytime, various UHICs over Beijing and Tianjin developed separately under a southerly background wind. Two of these UHICs formed over Beijing due to multiple hot regions, involving a chain flow mechanism in which the outflows from one UHIC over nearby mountains were connected to the inflows to the other UHICs in a west-to-east direction. At night, more robust UHICs occurred and a chain flow mechanism again formed over the Beijing-Tianjin region, as the upper-level outflows from the UHIC over Tianjin were connected to the inflows of the UHIC over Beijing. Moreover, the rotary air movements formed by the flows over the mountain and the down-slope winds formed a convergent region in the northwest of Beijing. In particular, sensitivity experiments replacing either Beijing or Tianjin with cropland grids revealed that the presence of either of those urban areas modified the penetration of the sea-breeze circulation. The city shape was found to affect the UHIC over Beijing at night, as the convergent zones were distributed diagonally and the wind speeds were higher in the diagonal direction.

Dominance of Shortwave Radiative Heating in the Sea‐Land Breeze Amplitude and its Impacts on Atmospheric Visibility in Tokyo, Japan

AbstractThis study uses 21‐year observations to analyze the characteristics of sea‐land breeze (SLB) circulation and its potential influencing factors in the metropolis of Tokyo. Unlike in other megacities, the magnitude of the SLB in Tokyo does not show a decreasing trend over the years. It is found that the diurnal variation magnitude of the SLB fits an approximately sinusoidal function. The diurnal variation amplitude mainly depends on shortwave radiation that reaches the ground. Stronger radiation is often accompanied by SLB circulation with greater energy, which helps transport more air pollution in the urban area of Tokyo. Correspondingly, atmospheric visibility shows good correlation with in situ shortwave radiation. Generally, strong radiation heats the ground surface, which causes larger land‐sea temperature differences and thus greater SLB circulation amplitudes, which is favorable for pollution dispersion and better atmospheric visibility. The presence of less aerosols and the associated better atmospheric visibility weaken the umbrella effect on solar radiation, which allows more solar radiation to reach the ground. This positive feedback could further strengthen the connection between solar radiation, sea‐land breezes, and atmospheric visibility. Increasing solar radiation and atmospheric visibility even produces stronger SLB magnitudes at night. Moreover, it is noteworthy that urban heat island circulation plays a similar role to that of SLB circulation, which also makes the variation of in situ radiation and visibility synchronous.

Prediction of pollution dispersion under urban heat island circulation for different atmospheric stratification

The background atmospheric stratification with inversion layers in association with urban heat island phenomena affects the city ventilation and pollution transport. In this study, the effects of different stratification including ground inversion, isotherm, sub-adiabatic and super-adiabatic profiles with an inversion layer on pattern, intensity and extend of urban heat island induced circulation (UHIC) are investigated. The effect of the UHIC pattern on pollutant transport for two different scenarios are studied. The study is conducted using a modified CFD model that is adopted to include temperature background layers, compressibility and anelasticity of the atmosphere. The modified CFD model and results of simulation are validated against the available data in literature for UHIC pattern and pollution transport scenarios. The results indicate that background temperature profile controls the convergence and divergence of flow, and it affects the mixing height of flow inside of urban area, significantly. The flow speed at the urban border for inversion, isotherm, and sub-adiabatic stratification is 55%, 52%, and 40% less than super-adiabatic stratification, respectively. It is observed that the existence of inversion layer is essential for convergence of flow in case of super-adiabatic stratification. In case of ground inversion, pollutants that are released inside urban area are transferred to higher levels in rural area. The induced flow due to ground inversion transfers thepollutant in rural area into the urban dome. The pollutant concentration for inversion, isothermal and super-adiabatic profiles is 350%, 200%, and 150% more than sub-adiabatic stratification, respectively.

High‐Resolution, Multilayer Modeling of Singapore's Urban Climate Incorporating Local Climate Zones

AbstractWe applied Weather Research and Forecasting (WRF) model's Multilayer Urban Canopy Model (MLUCM) to simulate the urban climate of Singapore during a hot period in April 2016. The high‐resolution local climate zone (LCZ) map was used as urban land use/land cover data in order to study the intraurban variability in different LCZ classes. The LCZ map for Singapore was developed by adopting the World Urban Database and Access Portal Tools (WUDAPT) methodology based on satellite remote sensing imageries and building height data. The coupled WRF/MLUCM model was validated using meteorological data from stations across Singapore. Higher index of agreement compared to observations and lower root‐mean‐squared error of 2‐m temperature, relative humidity, and global horizontal irradiance showed satisfactory model performance. A sensitivity analysis of initial and boundary conditions helped in determining the model configuration with the least error for quantifying the urban heat island (UHI) effect. The diurnal cycle and the spatial pattern of UHI were investigated, and it was found that the mean UHI intensity peaked in the early morning at 2.2 °C, reaching 3.6 °C in the Compact High Rise (LCZ1) areas. The anthropogenic heat due to indoor air conditioning was found to play a major role in all the processes studied, while the effect of the different land use types was most pronounced during nighttime and least visible near noon. The UHI circulation developed near the central catchment is found to prevent sea breezes from further propagating inland.

Effect of city shape on urban wind patterns and convective heat transfer in calm and stable background conditions

Extreme heatwaves and air pollution in urban areas often occur in calm and stable background conditions. In such situations, urban heat dome flows are induced by the urban heat island (UHI) effect, which has a strong influence on the urban thermal and wind environment. Cities have various shapes, but most studies of urban heat island circulation have focused on ideal circular shape and ignored the potential effects of shape. In this study, the effects of the various ideal urban shapes on the natural convective flow over urban areas (urban dome flow) are investigated experimentally. The urban dome flow over an equilateral polygonal city is characterised as convergent inflow along the interior angle bisector at the lower level, upward flow near the urban centre and divergent side outflow perpendicular to the side at the upper level. The characteristics of the urban dome flow over a rectangular urban area depend greatly upon the aspect ratio of the long side to the short side. As the aspect ratio increases, the divergent flows of the short sides disappear, and those of the two long sides dominate. Moreover, recirculation occurs between the divergent outflow perpendicular to the long edges and the convergent inflow near the short edges. Our results also show that the Nusselt (Nu) number (Eq. (1)), which quantifies the heat transfer efficiency, is significantly lower in a stable environment than in a neutral environment. A significant correlation between the shape and Nu was not found. It indicates that strong UHIs prefer stable background conditions.

Interacting urban heat island circulations as affected by weak background wind

This paper presents a numerical study of two interacting urban heat island circulations (UHICs) in the presence of weak background wind. The Weather Research and Forecasting (WRF) model was used with ideal settings without the Earth's rotation effect. UHICs were simulated over circular cities of the same size arranged in tandem or side by side with respect to the background wind direction, at different spacing distances. The study analyzed the flow structures and city ventilation arising from thermally driven UHICs and background wind. The arrangements of the cities have a significant modifying effect on the interaction between UHICs and winds. If side-by-side UHICs are sufficiently close (spacing distance less than half of the city diameter), they have a blockage effect similar to that of a single larger UHIC, and only one counter-rotating vortex pair structure exists around the cities. As the spacing distance increases, the city ventilation improves slightly. In the tandem arrangement, a chain flow is formed between UHICs at a spacing distance of between 0.5 and 1.5 times the city diameter. An upstream UHIC blocks the incoming wind flow to the downstream city, which results in a ventilation deficiency. Due to lateral entrainment, which partly compensates for the deficiency in the incoming flow, the ventilation of the downstream city is slightly improved by an increase in spacing distance.

Urban heat island circulations of an idealized circular city as affected by background wind speed

The Weather Research and Forecasting (WRF) Model is used to study the interaction between urban heat island circulation (UHIC) and its background wind. Two distinct wind regimes are found depending on the ratio of the horizontal convective scale velocity to the background velocity, which is referred to as the buoyancy velocity ratio. When the buoyancy velocity ratio is above two, coherent asymmetric vortex rings are exhibited, similar to the UHIC structure under calm conditions. For buoyancy velocity ratios below two, upper return UHIC currents at the upstream disappear in the two-dimensional cases and the coherent asymmetric vortex rings are destroyed in the three-dimensional cases. In these low buoyancy velocity ratio cases, background wind inhibits UHIC development at the leading edge of the urban area and UHIC is transported downstream from the urban center. As a result, a trailing vortex forms instead of a vortex ring. In all simulated cases with a background wind speed, a counter-rotating vortex pair exists. This remarkable feature does not exist under calm conditions and is attributed to the corresponding background wind speed. Holding urban heat flux constant, as the background wind speed decreases, the reverse region expands.

An analytical model of an urban heat island circulation in calm conditions

We obtain an analytical model solution to an idealization of an urban heat island (UHI) circulation—the steady shallow convective flow of a viscous stably stratified fluid over a differentially heated lower boundary without system rotation (no Coriolis force) or background wind. The coupled linearized equations of motion and thermal energy are solved for flows in two-dimensional Cartesian (slab-symmetric) and axisymmetric geometries forced by laterally varying surface buoyancies considered as Gaussian and parabolic arch functions. The scaled problem is free of governing parameters, and the solutions are universal. In all cases the flow is dominated by an in-up-out circulation. The updraft is stronger in the axisymmetric case than in the slab-symmetric case, while the surrounding downdraft, and the inflow and outflow branches of the circulation are stronger in the slab-symmetric case. These differences are explained in terms of the response of the perturbation pressure to the thermal forcing in the different geometries. The scalings themselves provide insight into observations that daytime UHI circulations can be stronger than nighttime circulations despite the relative weakness of urban–rural temperature contrasts during the daytime.

Idealized large-eddy simulation study of interaction between urban heat island and sea breeze circulations

In this study, large-eddy simulations (LES) are conducted to investigate the interaction between the urban heat island (UHI) circulation and the sea breeze (SB) circulation. An idealized city, which has higher surface heat flux and lager roughness length than the rural surface, is located adjacent to or at a distance from the coastline. The heat flux over both rural and urban surfaces varies sinusoidally with time to mimic the diurnal variation of surface heating. Five urban cases (with differences in city size, location, surface heating strength, and surface roughness) and one non-urban case are simulated. The results show that the SB circulation is enhanced by stronger surface heating in urban area, which leads to a larger SB velocity, a deeper SB thickness, and a stronger ascending motion over the SB front. Moreover, the ascending motion over the SB front is significantly enhanced when it meets the updraft of the UHI circulation, and the maximum ascending motion occurs when the SB front enters the core of the updraft of the UHI circulation. Larger city size delays the occurrence of the maximum ascending motion. Higher surface drag contributes to the enhancement of the ascending motion of SB circulation by intensifying horizontal temperature gradient near the SB front. On the other hand, when the SB flow occupies the urban area, the boundary layer height rapidly decreases to a much lower level, and the UHI circulation is eliminated.

Effect of sea breeze propagation on the urban boundary layer of the metropolitan region of Sao Paulo, Brazil

Thermally driven circulations over urban areas, such as sea, lake, and land breezes and mountain-valley and urban heat island (UHI) circulations, strongly determine local weather and air quality conditions over highly populated areas. This study investigates the influence of sea breeze (SB) propagation on the development of the urban boundary layer (UBL) in the Metropolitan Region of Sao Paulo (MRSP), Brazil, an urban area with complex topography. This investigation is undertaken observationally by analyzing surface and upper air observations conducted in 2013 during the MCITY BRAZIL Project field campaigns and is conducted numerically by performing a set of Weather Research and Forecasting (WRF) simulations. The observational analysis highlights two SB events in the MRSP, one in February (austral summer) and the other in August (winter). In both seasons, the passage of the SB front disrupts the convective growth of the UBL and establishes a thermal internal boundary layer, thereby reducing the UBL height. The cooling also momentarily disrupts the UHI circulation. The results indicate that topography enhances SB propagation because it helps the marine air to go up the MRSP coastal escarpment and to reach the plateau while forcing adiabatic cooling of this air, intensifying the thermal gradient between the southern part of the plateau and the urban area. The UHI further increases the thermal gradient (from 0.5 K to 2 K in summer and 1 to 2.5 K in winter), accelerating the SB propagation. Synoptic conditions determine whether the SB front reaches the urban area and also may displace the convergence center of the UHI circulation.

Theoretical Urban Heat Island Circulation in the Temperature Inversion Profile

The characteristics of urban heat island (UHI) circulation are analytically expressed as functions of the surface temperature in both temperature inversion and non-temperature inversion (NTI) profiles, in which the temperature declines with increasing altitudes. To identify how the inversion layer affects UHI circulation, two temperature profiles are specified to be nearly similar except within the temperature inversion layer. Theoretical calculations suggest that the UHI circulation in the temperature inversion case is weaker and lower than in the NTI case and that there is no significant difference between the two cases. When the inversion layer thickness is fixed, the relative size difference between the weakening inversion intensity and the strengthening temperature influence above the inversion lid controls the decrease or increase in UHI circulation.

Interaction of multiple urban heat island circulations under idealised settings

Urban heat island circulation (UHIC), commonly established under calm background conditions, is important for understanding the accumulation of pollutants and heat in a city. In a city cluster in which multiple cities exist in proximity, the resulting multiple UHICs can interact. As many city clusters continue to grow in size and in number, particularly in rapidly developing Asia, it is necessary to understand the interactions of multiple UHICs. In this study, the development of a single UHIC, interaction between two identical UHICs, and that of three different UHICs were investigated by water tank modelling experiments. UHIC is characterised as convergent inflow at lower levels, upward flow over the urban area and divergent outflow at upper levels. Stagnant zones were found between two adjacent cities due to competition between their inflows. If the vertical sizes (mixed heights) of two adjacent UHICs are different, the outflow of the smaller UHIC will be lower than that of the larger one and will join the inflow of the larger one; thus, the pollutants will be transported from the smaller to the larger UHIC. Because the outflow of the larger UHIC is higher than that of the smaller one, the outflow of the larger UHIC can shelter the smaller one and thus limit the vertical pollutant dispersion within the smaller one. These findings are useful to explain the haze accumulation phenomenon in major haze episodes such as those occurring in the Beijing-Tianjin-Hebei region in China.

Effects of anthropogenic heat due to air-conditioning systems on an extreme high temperature event in Hong Kong

Anthropogenic heat flux is the heat generated by human activities in the urban canopy layer, which is considered the main contributor to the urban heat island (UHI). The UHI can in turn increase the use and energy consumption of air-conditioning systems. In this study, two effective methods for water-cooling air-conditioning systems in non-domestic areas, including the direct cooling system and central piped cooling towers (CPCTs), are physically based, parameterized, and implemented in a weather research and forecasting model at the city scale of Hong Kong. An extreme high temperature event (June 23–28, 2016) in the urban areas was examined, and we assessed the effects on the surface thermal environment, the interaction of sea–land breeze circulation and urban heat island circulation, boundary layer dynamics, and a possible reduction of energy consumption. The results showed that both water-cooled air-conditioning systems could reduce the 2 m air temperature by around 0.5 °C–0.8 °C during the daytime, and around 1.5 °C around 7:00–8:00 pm when the planetary boundary layer (PBL) height was confined to a few hundred meters. The CPCT contributed around 80%–90% latent heat flux and significantly increased the water vapor mixing ratio in the atmosphere by around 0.29 g kg−1 on average. The implementation of the two alternative air-conditioning systems could modify the heat and momentum of turbulence, which inhibited the evolution of the PBL height (a reduction of 100–150 m), reduced the vertical mixing, presented lower horizontal wind speed and buoyant production of turbulent kinetic energy, and reduced the strength of sea breeze and UHI circulation, which in turn affected the removal of air pollutants. Moreover, the two alternative air-conditioning systems could significantly reduce the energy consumption by around 30% during extreme high temperature events. The results of this study suggest potential UHI mitigation strategies and can be extended to other megacities to enable them to be more resilient to UHI effects.

A Simple Daily Cycle Temperature Boundary Condition for Ground Surfaces in CFD Predictions of Urban Wind Flows

AbstractThe atmospheric boundary exhibits an obvious diurnal cycle. The daily cycle of climate variation has a significant effect on urban airflow, and an understanding of it is very important for city-scale environmental control. A new and simple daily cycle temperature boundary condition for simulations of urban airflows with computational fluid dynamics (CFD) is described herein. An analytical surface temperature formula was obtained after simplifying longwave radiation and sensible heat flux terms. The formula provides a reasonably good prediction of ground surface temperatures on sunny days without adding much complexity. The accuracy of the prediction of daily surface temperature variations in homogeneous soils was evaluated with a benchmark experiment and with weather station data. The new boundary condition was implemented in the commercial CFD software Fluent for a city-scale model. The implementation demonstrates the importance of including diurnal temperature profiles and atmospheric boundary conditions in CFD simulations of urban plumes in which an ideal urban heat island circulation occurs.

Horizontal extent of the urban heat dome flow

Urban heat dome flow, which is also referred to as urban heat island circulation, is important for urban ventilation and pollutant transport between adjacent cities when the background wind is weak or absent. A “dome-shaped” profile can form at the upper boundary of the urban heat island circulation. The horizontal extent of the heat dome is an important parameter for estimating the size of the area it influences. This study reviews the existing data on the horizontal extent of the urban heat dome flow, as determined by using either field measurements or numerical simulations. A simple energy balance model is applied to obtain the maximum horizontal extent of a single heat dome over the urban area, which is found to be approximately 1.5 to 3.5 times the diameter of the city’s urban area at night. A linearized model is also re-analysed to calculate the horizontal extent of the urban heat dome flow. This analysis supports the results from the energy balance model. During daytime, the horizontal extent of the urban heat dome flow is found to be about 2.0 to 3.3 times the urban area’s diameter, as influenced by the convective turbulent plumes in the rural area.

Impact of land surface heterogeneity on urban heat island circulation and sea‐land breeze circulation in Hong Kong

AbstractHong Kong is one of the most high‐rise and highly compact cities in the world. The urban land surface is highly heterogeneous, which creates low‐level convergence zones in urban areas, particularly the Kowloon Peninsula. The low‐level convergence zone is due to the combined effect of urban heat island circulation (UHIC) and sea‐land breeze circulation (SLBC) under weak northeasterly synoptic flow. To study the impacts of anthropogenic fluxes and built‐up areas on the local circulation, the Weather Research and Forecasting (WRF) mesoscale model is combined with the multilayer urban canopy building effect parameterization/building energy model (BEP/BEM) parameterization to produce a 3 day simulation of an air pollution episode in Hong Kong in September 2012. To better represent the city land surface features, building information is assimilated in the central part of the Kowloon Peninsula. The WRF‐BEP‐BEM model captures the 2 m temperature distribution and local wind rotation reasonably well but overestimates the 10 m wind speed with a mean bias error of 0.70 m/s. A dome‐shaped feature with a high level of moisture is captured in the convergence zones due to intensified UHIC and inflowing SLBC. The anthropogenic heat increases the air temperature by around 0.3°C up to 250 m, which in turn modifies the SLBC. A new drag coefficient based on λP, plan area per unit ground area, is tested. Besides the basic physical characteristics captured by the WRF‐BEP‐BEM model, the stagnation of wind in the lower level convergence zone is better captured by this approach than by the traditional constant value coefficient.

A New Convective Velocity Scale for Studying Diurnal Urban Heat Island Circulation

AbstractUrban heat island circulation establishes an urban dome under stable stratification and no background wind conditions. Small-scale water models have been a very useful tool in the exploration of the mechanisms by which urban domes and their associated wind flows are formed. Data are available from a number of water-tank heat island models. Data from field measurements, computational fluid dynamics, and small-scale water-tank experiments are compared in this paper. The small-scale water-tank experiments were found to produce relatively low radial velocities, such as the radial horizontal velocity. Different relevant velocity scales developed in the literature were reviewed. The influence of the Prandtl number on convective flows was analyzed. The analysis resulted in a new convective velocity scale that is a function of the Prandtl number, and the new scale was found to work well. This new development is expected to render small-scale models more useful in urban wind studies. The new convective velocity scale may be extended to water-modeling studies of other buoyancy-driven airflows.

Modeling of urban heat island and its impacts on thermal circulations in the Beijing–Tianjin–Hebei region, China

Through regulating the land–atmosphere energy balance, urbanization plays an important role in modifying local circulations and cross-border transport of air pollutants. The Beijing–Tianjin–Hebei (BTH) metropolitan area in northern China is frequently influenced by complex atmospheric thermal circulations due to its special topography and geographic position. In this study, the Weather Research and Forecasting (WRF) model combined with remote sensing is used to explore the urbanization impacts on local circulations in the BTH region. The urban heat island (UHI) effect generated around Beijing and Tianjin shows complex interactions with local thermal circulations. Due to the combined effects of UHI and topography, the UHI circulation around Beijing and valley breeze at the southern slopes of Yan Mountain are coupled together to reinforce each other. At the coastal cities, the increased land/sea temperature gradient considerably accelerates the sea breeze along Bohai Bay and moves the sea breeze front further inland to reach as far as Beijing. This study may lay a foundation for the better understanding of air pollutant dispersion on complex terrain.

Predicting urban heat island circulation using CFD

Most existing microscale computational fluid dynamics (CFD) models and mesoscale meteorological models cannot consider multi-scale urban wind flows, as neither can completely take into account the mesoscale and microscale physics, and their interactions. Here we suggest a CFD model which has good potential for development of the meso-micro scale models for predicting and designing multi-scale urban airflows. The principal idea is to use CFD numerical methods to solve a set of governing equations with appropriate boundary conditions that can govern both major mesoscale and microscale flow characteristics. Based on the coordinate transformation method proposed by Kristóf, Rácz and Balogh (2009), we derived a similar set of governing equations with some improvements and used an existing porous turbulence model for modeling the urban canopy layer. The approach was then successfully implemented using a commercial CFD package (Fluent) for studying urban heat island circulation (UHIC), which is considered to be one of the most difficult problems in CFD. Our predicted mean quantities agree well with existing data in the literature obtained from large eddy simulations, mesoscale models, and laboratory experiments. Our predicted results also reveal the effects of the different heat fluxes and urban height of a city on UHIC characteristics.

Impacts of thermal circulations induced by urbanization on ozone formation in the Pearl River Delta region, China

Abstract Thermal circulations induced by urbanization could exert important effects on regional ozone (O 3 ) formation through regulating the chemical transformations and transport of O 3 and its precursors. In this study, the Weather Research and Forecasting/Chemistry (WRF/Chem) model combined with remote sensing are used to investigate the impacts of urbanization-induced circulations on O 3 formation in the Pearl River Delta (PRD) region, China. The urban heat island (UHI) effect in PRD significantly enhances turbulent mixing and modifies local circulations, i.e., initiates the UHI circulation and strengthens the sea breeze, which in turn cause a detectable decrease of daytime O 3 concentration (−1.3 ppb) and an increase of O 3 (+5.2 ppb) around the nocturnal rush-hours. The suppressed O 3 titration destruction due to NOx dilution into the deeper urban boundary layer (200–400 m) is the main reason for elevated nocturnal O 3 levels. In the daytime, however, the upward transport of O 3 precursors weakens near-surface O 3 photochemical production and conversely enhances upper-level O 3 generation. Furthermore, the surface UHI convergence flow and intensified sea breeze act to effectively trap O 3 at the suburban and coastal regions.