Urban Canopy Parameterization Research Articles

WRF-Comfort: simulating microscale variability in outdoor heat stress at the city scale with a mesoscale model

Abstract. Urban overheating and its ongoing exacerbation due to global warming and urban development lead to increased exposure to urban heat and increased thermal discomfort and heat stress. To quantify thermal stress, specific indices have been proposed that depend on air temperature, mean radiant temperature (MRT), wind speed, and relative humidity. While temperature and humidity vary on scales of hundreds of meters, MRT and wind speed are strongly affected by individual buildings and trees and vary on the meter scale. Therefore, most numerical thermal comfort studies apply microscale models to limited spatial domains (commonly representing urban neighborhoods with building blocks) with resolutions on the order of 1 m and a few hours of simulation. This prevents the analysis of the impact of city-scale adaptation and/or mitigation strategies on thermal stress and comfort. To solve this problem, we develop a methodology to estimate thermal stress indicators and their subgrid variability in mesoscale models – here applied to the multilayer urban canopy parameterization BEP-BEM within the Weather Research and Forecasting (WRF) model. The new scheme (consisting of three main steps) can readily assess intra-neighborhood-scale heat stress distributions across whole cities and for timescales of minutes to years. The first key component of the approach is the estimation of MRT in several locations within streets for different street orientations. Second, mean wind speed and its subgrid variability are downscaled as a function of the local urban morphology based on relations derived from a set of microscale LES and RANS simulations across a wide range of realistic and idealized urban morphologies. Lastly, we compute the distributions of two thermal stress indices for each grid square, combining all the subgrid values of MRT, wind speed, air temperature, and absolute humidity. From these distributions, we quantify the high and low tails of the heat stress distribution in each grid square across the city, representing the thermal diversity experienced in street canyons. In this contribution, we present the core methodology as well as simulation results for Madrid (Spain), which illustrate strong differences between heat stress indices and common heat metrics like air or surface temperature both across the city and over the diurnal cycle.

Novel Geometric Parameters for Assessing Flow Over Realistic Versus Idealized Urban Arrays

AbstractUrban heterogeneity, such as the variation of street layouts, building shapes, and building heights, cannot be fully represented by density parameters commonly used in idealized urban environmental analyses. To address this shortcoming and better model flow fields over complex urban neighborhoods, we propose two novel descriptive geometric parameters, alignedness and building facet entropy, which quantify the connectivity of inter‐building spaces along the prevailing wind direction and the variation of building facet orientations, respectively. We then conducted large eddy simulations over 101 urban layouts, including realistic urban configurations with uniform building height as well as idealized building arrays with variable heights, and evaluated the resulting bulk flow properties. Urban canopy flow over realistic neighborhoods resembles staggered building arrays for low urban densities but becomes similar to aligned configurations beyond λp ∼ 0.25 where the realistic flow is less sensitive to changes in density. We further show that compared to traditional density parameters (such as plan and frontal area densities), the mean alignedness, a measure of connectivity of flow paths in street canyons, better predicts canopy‐averaged flow properties. Furthermore, for realistic urban flow, the dispersive momentum flux shows a clear increasing trend with building density, and a decreasing trend with alignedness, which is in contrast with idealized cases that exhibit no clear trend. This distinct behavior further highlights the necessity of evaluating flow over realistic urban layouts for flow parameterization. This study provides an improved method of describing urban layouts for flow characterization that can be applied in neighborhood‐scale urban canopy parameterization.

Parameterizing building effects on airflows within the urban canopy layer for high‐resolution models using a nudging approach

AbstractIn this study, a new multilayer urban canopy parameterization for high‐resolution (∼1 km) atmospheric models using the nudging approach to represent the impacts of urban canopies on airflow is presented. In our parameterization, a nudging term is added to the momentum equations and a source term to the turbulent kinetic energy equation to account for building effects. The challenge of this parameterization lies in defining appropriate values for the nudging coefficient and the weighting function used to reflect canopy effects. Values of both are derived and the parameterization developed is implemented and tested for idealized cases in the Mesoscale Transport and Stream model (METRAS). Comparison data are taken from obstacle‐resolving microscale model results. Results show that the parameterization using the nudging approach can simulate aerodynamic effects induced within the canopy by obstacles well, in terms of reduction of wind speeds and production of additional turbulent kinetic energy. Thus, models with existing nudging can use this approach as an efficient and effective method to parameterize dynamic urban canopy effects.

The role of climate change and urban development on compound dry-hot extremes across US cities

Compound dry-hot extreme (CDHE) events pose greater risks to the environment, society, and human health than their univariate counterparts. Here, we project decadal-length changes in the frequency and duration of CDHE events for major U.S. cities during the 21st century. Using the Weather Research and Forecasting (WRF) model coupled to an urban canopy parameterization, we find a considerable increase in the frequency and duration of future CDHE events across all U.S. major cities under the compound effect of high-intensity GHG- and urban development-induced warming. Our results indicate that while GHG-induced warming is the primary driver of the increased frequency and duration of CDHE events, urban development amplifies this effect and should not be neglected. Furthermore, We show that the highest frequency amplification of major CDHE events is expected for U.S. cities across the Great Plains South, Southwest, and the southern part of the Northwest National Climate Assessment regions.

Simulation of interaction between high-temperature process and heat emission from electricity system in summer

During the hot summer season, using electricity systems increases the local anthropogenic heat emission, further increasing the temperature. Regarding anthropogenic heat sources, electric energy consumption, heat generation, indoor and outdoor heat transfer, and exchange in buildings play a critical role in the change in the urban thermal environment. Therefore, the Weather Research and Forecasting (WRF) Model was applied in this study to investigate the heat generation from an indoor electricity system and its influence on the outdoor thermal environment. Through the building effect parameterization (BEP) of a multistorey urban canopy scheme, a building energy model (BEM) to increase the influence of indoor air conditioning on the electricity consumption system was proposed. In other words, the BEP+BEM urban canopy parameterization scheme was set. High temperatures and a summer heat wave were simulated as the background weather. The results show that using the BEP+BEM parameterization scheme of indoor and outdoor energy exchange in the WRF model can better simulate the air temperature near the surface layer on a sunny summer. During the day, the turning on the air conditioning and other electrical systems have no obvious effect on the air temperature near the surface layer in the city, whereas at night, the air temperature generally increases by 0.6 ℃, especially in densely populated areas, with a maximum temperature rise of approximately 1.2 ℃ from 22:00 to 23:00. When the indoor air conditioning target temperature is adjusted to 25–27 ℃, the total energy release of the air conditioning system is reduced by 12.66%, and the temperature drops the most from 13:00 to 16:00, with an average of approximately 1 ℃. Further, the denser the building is, the greater the temperature drop.

Simulating the meteorology during persistent Wintertime Thermal Inversions over urban areas. The case of Madrid

Persistent wintertime inversions over cities are the most critical conditions for air quality, and also one of the most challenging situations to simulate with a meteorological model. In this study, the ability of the meteorological Weather Research and Forecasting (WRF) model, coupled with the multilayer urban canopy parameterization BEP-BEM (Building Effect Parameterization - Building Energy Model), to simulate the evolution of the Planetary Boundary Layer (PBL) structure over the city of Madrid, Spain, during a multiday inversion episode, is assessed. The model results are evaluated against airport soundings, fourteen meteorological stations within and around the urban area, and remotely sensed surface temperatures. The study indicates that the PBL structure is determined by the interaction between the urban and rural heat and momentum fluxes, the topography, and the downward turbulent transport of heat. The best air temperature spatial distribution is obtained when the 6th order horizontal filter is applied only to wind and not to temperature, and when the soil moisture is reduced to 25% of the initial value provided by the global scale model. However, the comparison against satellite surface temperature data indicates that with such a dry soil the model underestimates the surface temperatures during the night and overestimates them during the day in rural areas. This compensating error points to a likely deficiency in the PBL and surface schemes during the persistent inversions. In urban areas, the simulations with the urban canopy parameterization tend to overestimate the nocturnal surface and air temperatures, while they reproduce wind speed correctly. Finally, a new methodology, based on a comparison of the differences between maximum and minimum temperatures for couples of stations, to assess the model's capability to reproduce the spatial variability of air temperature, is introduced, and the results show that the use of the multilayer urban canopy scheme and the adjustment of the soil moisture are both needed to improve the reproduction of such spatial distribution.

Predicting Urban Surface Roughness Aerodynamic Parameters Using Random Forest

AbstractThe surface roughness aerodynamic parameters z0 (roughness length) and d (zero-plane displacement height) are vital to the accuracy of the Monin–Obukhov similarity theory. Deriving improved urban canopy parameterization (UCP) schemes within the conventional framework remains mathematically challenging. The current study explores the potential of a machine-learning (ML) algorithm, a random forest (RF), as a complement to the traditional UCP schemes. Using large-eddy simulation and ensemble sampling, in combination with nonlinear least squares regression of the logarithmic-layer wind profiles, a dataset of approximately 4.5 × 103 samples is established for the aerodynamic parameters and the morphometric statistics, enabling the training of the ML model. While the prediction for d is not as good as the UCP after Kanda et al., the performance for z0 is notable. The RF algorithm also categorizes z0 and d with an exceptional performance score: the overall bell-shaped distributions are well predicted, and the ±0.5σ category (i.e., the 38% percentile) is competently captured (37.8% for z0 and 36.5% for d). Among the morphometric features, the mean and maximum building heights (Have and Hmax, respectively) are found to be of predominant influence on the prediction of z0 and d. A perhaps counterintuitive result is the considerably less striking importance of the building-height variability. Possible reasons are discussed. The feature importance scores could be useful for identifying the contributing factors to the surface aerodynamic characteristics. The results may shed some light on the development of ML-based UCP for mesoscale modeling.

Precipitation response to climate change and urban development over the continental United States

Appropriately characterizing future changes in regional-scale precipitation requires assessment of the interactive effect owing to greenhouse gas-induced climate change and the physical growth of the built environment. Here we use a suite of medium resolution (20 km grid spacing) decadal scale simulations conducted with the Weather Research and Forecasting model coupled to an urban canopy parameterization to examine the interplay between end-of-century long-lived greenhouse gas (LLGHG) forcing and urban expansion on continental US (CONUS) precipitation. Our results show that projected changes in extreme precipitation are at least one order of magnitude greater than projected changes in mean precipitation; this finding is geographically consistent over the seven CONUS National Climate Assessment (NCA) regions and between the pair of dynamically downscaled global climate model (GCM) forcings. We show that dynamical downscaling of the Geophysical Fluid Dynamics Laboratory GCM leads to projected end-of-century changes in extreme precipitation that are consistently greater compared to dynamical downscaling of the Community Earth System Model GCM for all regions except the Southeast NCA region. Our results demonstrate that the physical growth of the built environment can either enhance or suppress extreme precipitation across CONUS metropolitan regions. Incorporation of LLGHGs indicates compensating effects between urban environments and greenhouse gases, shifting the probability spectrum toward broad enhancement of extreme precipitation across future CONUS metropolitan areas. Our results emphasize the need for development of management policies that address flooding challenges exacerbated by the twin forcing agents of urban- and greenhouse gas-induced climate change.

Sensitivity analysis of urban microclimatic conditions and building energy consumption on urban parameters by means of idealized numerical simulations

In the present work the mesoscale meteorological model Weather Research and Forecasting (WRF), coupled with a multilayer urban canopy parameterization and a building energy model (BEP + BEM), is used to analyze the sensitivity of both urban climatic conditions and building energy consumption on parameters characterizing urban materials and the building environment. In particular, simulations in an idealized urban area, with different urban geometry and under different climatic conditions, are compared to evaluate the relative importance of various parameters. Results highlight that the target indoor temperature has the strongest impact on both building energy consumption and external air temperature. Building energy consumption is also significantly affected by other internal building parameters, such as the ventilation rate and the heat produced by equipment and occupants. On the contrary, no significant influence, either on building energy consumption or on air temperature, is found from changes in wall albedo and emissivity.

Implementation of Observed Sky-View Factor in a Mesoscale Model for Sensitivity Studies of the Urban Meteorology

The sky view factor (SVF) is an important radiometric parameter for assessing the canopy energy budget of urban areas. There are several methods to determine the SVF observationally. The most common is taking a photo with a digital camera equipped with a fish-eye lens and then converting ratio of sky area to canopy area into SVF. However, most urban canopy models use this variable as derived from idealized canopy geometry. To evaluate the effect of inputting observed SVFs in numerical models, we evaluated a mesoscale model’s performance in reproducing surface wind and surface temperature when subjected to different ways of SVF prescription. The studied area was the Metropolitan Area of São Paulo (MASP) in Brazil. Observed SVFs were obtained for 37 sites scattered all over the MASP. Three simulations, A, B, and C, with different SVF and aspect-ratio prescriptions, were performed to analyze the effect of SVF on the urban canopy parameterization: Simulation A (standard) used the original formulation of the Town Energy Budget (TEB) model, computing the SVFs from the aspect-ratios; Simulation B used the observed SVFs, but keeps aspect-ratios as original; and Simulation C used the aspect-ratios computed from observed SVFs. The results show that in general inputting observed SVFs improves the model capability of reproducing temperature at surface level. The comparison of model outputs with data of regular meteorological stations shows that the inclusion of observed values of SVFs enhances model performance, reducing the RMSE index by up to 3 ∘C. In this case, the model is able to better reproduce the expected effects in the wind field, and consequently the temperature advection, of the urban boundary layer to a large urban area. The result of Simulation C shows that the surface wind and temperature intensity for all urban types is higher than those of Simulation A, because of the lower values of the aspect ratio. The urban type with high density of tall buildings increase up to 1 ms-1 in the wind speed, and approximately 1 ∘C in temperature, showing the importance of a better representation of the urban structure and the SVF database improvement.

Mesoscale modelling of the summer climate response of Moscow metropolitan area to urban expansion

In this paper, the experience of applying a regional climate model to simulating the summer climate features of Moscow metropolitan area is examined. Also, an assessment is made of climate response to the implementation of a scenario of twofold city expansion. The model (COSMO-CLM) was adapted to the conditions of the region under investigation, supplemented by specific urban canopy parameterization and equipped with realistic parameters of urban surface. It was possible to successfully simulate the summer meteorological regime of Moscow region and, specifically, the temporal and spatial variability of the Moscow urban heat island (UHI). First results of the simulation for the city expansion show that the new urbanized areas on the periphery of Moscow have a heating impact on its central part, which leads to an increase of the UHI effect, on the average, of 10% (in the central area). In extreme heat events the temperature response to this scenario is much stronger, which may deteriorate human health and increase thermal stress. The simulation results also show that the city of Moscow is characterized by a positive anomaly of summer precipitation (in the city and its leeward side it increases by 10-30%). The scenario of urban expansion enhances this anomaly by 5-10% and increases its area.

A Study of the Oklahoma City Urban Heat Island Effect Using a WRF/Single-Layer Urban Canopy Model, a Joint Urban 2003 Field Campaign, and MODIS Satellite Observations

The urban heat island effect (UHI) for inner land regions was investigated using satellite data, ground observations, and simulations with an Single-Layer Urban Canopy Parameterization (SLUCP) coupled into the regional Weather Research Forecasting model (WRF, http://wrf-model.org/index.php). Specifically, using the satellite-observed surface skin temperatures (Tskin), the intensity of the UHI was first compared for two inland cities (Xi’an City, China, and Oklahoma City (OKC)), which have different city populations and building densities. The larger population density and larger building density in Xi’an lead to a stronger skin-level UHI by 2 °C. However, the ground observed 2 m surface air temperature (Tair) observations showed an urban cooling island effect (UCI) over the downtown region in OKC during the daytime of 19 July 2003, from a DOE field campaign (Joint Urban 2003). To understand this contrast between satellite-based Tskin and ground-based Tair, a sensitivity study using WRF/SLUCP was analyzed. The model reproduced a UCI in OKC. Furthermore, WRF/Noah/SLUCM simulations were also compared with the Joint Urban 2003 ground observations, including wind speeds, wind directions, and energy fluxes. Although the WRF/SLUCM model failed to simulate these variables accurately, it reproduced the diurnal variations of surface temperatures, wind speeds, wind directions, and energy fluxes reasonably well.

Impact of Lake Breezes on Summer Ozone Concentrations in the Salt Lake Valley

AbstractDuring the late afternoon of 18 June 2015, ozone concentrations in advance of a strong lake-breeze front arising from the Great Salt Lake in northern Utah were ~20 ppb lower than those in its wake. The lake-breeze progression and ozone concentrations in the valley were monitored by an enhanced observation network that included automated weather stations, a nearby Terminal Doppler Weather Radar, state air quality measurement sites, and mobile platforms, including a news helicopter. Southerly flow opposing the lake breeze increased convergent frontogenesis and delayed the onset of its passage through the Salt Lake valley. Ozone concentrations were exceptionally high aloft at the lake-breeze frontal boundary. The progression of this lake breeze was simulated using the Weather Research and Forecasting Model at 1-km horizontal grid spacing over northern Utah. The model was initialized using hourly analyses from the High Resolution Rapid Refresh model. Errors in the underlying surface initialization were improved by adjusting the areal extent and surface temperature of the lake to observed lake conditions. An urban canopy parameterization is also included. The opposing southerly flow was weaker in the simulation than that observed such that the simulated lake-breeze front occurred too early. Continuous passive tracers initialized within and ahead of the lake breeze highlight the dispersion and transport of pollutants arising from the lake-breeze front. Tracers within the lake breeze are confined near the surface while tracers in advance of the front are lofted over it.

A modeling analysis of urban canopy parameterization representing the vegetation effects in the megacity of São Paulo

The representation of the influence of vegetation on meteorological variables inside the urban canopy is a modeling challenge. The interaction of vegetation canopy parameterization for urban regions in mesoscale modeling was implemented and analyzed in this work. The BRAMS model equipped with the TEB was used to simulate a calm wind and clear sky period in the Metropolitan Area of São Paulo. The impact of vegetated areas on cities was estimated by calculating the surface fluxes due to vegetation and artificial materials, weighing individual contributions by using each component fraction inside a model grid. An evaluation of the vegetation impact on atmospheric conditions was made by comparing simulated temperature and specific humidity with surface observations, atmospheric sounding values and land-surface temperature product retrieved from MODIS/TERRA. The inclusion of vegetation clearly improves the simulated air temperature, the vertical structure of the urban boundary layer and the calculation of surface fluxes. Although some improvements in the model are still necessary, it is clear that the model is very sensitive to the presence of the vegetation and continuous efforts should be made to better understand the related process and to provide better tools for urban planning and other important activities in urban areas.

Large-scale urbanization effects on eastern Asian summer monsoon circulation and climate

Impacts of large-scale urbanization over eastern China on East Asian summer monsoon circulation and climate are investigated by comparing three 25-year climate simulations with and without incorporating modified land cover maps reflecting two different idealized large-scale urbanization scenarios. The global atmospheric general circulation model CAM4.0 that includes an urban canopy parameterization scheme is employed in this study. The large-scale urbanization over eastern China leads to a significant warming over most of the expanded urban areas, characterized by an increase of 3 K for surface skin temperature, 2.25 K for surface air temperature, significant warming of both daily minimum and daily maximum air temperatures, and 0.4 K for the averaged urban–rural temperature difference. The urbanization is also accompanied by an increase in surface sensible heat flux, a decrease of the net surface shortwave and long-wave radiation, and an enhanced surface thermal heating to the atmosphere in most Eastern Asia areas. It is noted that the responses of the East Asian summer monsoon circulation exhibits an evident month-to-month variation. Across eastern China, the summer monsoon in early summer is strengthened by the large-scale urbanization, but weakened (intensified) over southern (northern) part of East Asia in late summer. Meanwhile, early summer precipitation is intensified in northern and northeastern China and suppressed in south of ~35°N, but late summer precipitation is evidently suppressed over northeast China, the Korean Peninsula and Japan with enhancements in southern China, the South China Sea, and the oceanic region south and southeast of the Taiwan Island. This study highlights the evidently distinct month-to-month responses of the monsoon system to the large-scale urbanization, which might be attributed to different basic states, internal feedbacks (cloud, rainfall) as well as a dynamic adjustment of the atmosphere. Further investigation is required to understand the dynamic mechanisms by which a large-scale urbanization in China affects eastern Asian climate and summer monsoon circulation, especially possible internal feedbacks relevant to the sub-seasonal changes of the monsoon system.

Simulations of a Heat-Wave Event in New York City Using a Multilayer Urban Parameterization

AbstractThe Weather Research and Forecasting mesoscale model coupled to a multilayer urban canopy parameterization was used to evaluate the evolution of a 3-day heat wave in New York City, New York, during the summer of 2010. Results from three simulations with different degrees of urban modeling complexity and one with an absence of urban surfaces are compared with observations. To improve the city morphology representation, building information was assimilated and the land cover land-use classification was modified. The thermal and drag effects of buildings represented in the multilayer urban canopy model improve simulations over urban regions, giving better estimates of the surface temperature and wind speed. The accuracy of the simulation is further assessed against more simplified urban parameterizations models. The nighttime excessive cooling shown by the Building Energy Parameterization is compensated for when the Building Energy Model is activated. The turbulent kinetic energy is vertically distributed when using the multilayer scheme with a maximum at the average building height, whereas turbulence production is confined to a few meters above the surface when using the simplified scheme. Evidence for the existence of horizontal roll vortices is presented, and the impact that the horizontal resolution and the time step value have on their formation is assessed.

An idealized study of city structure, urban climate, energy consumption, and air quality

The interactions between the structure of a city and the atmosphere have an impact on thermal comfort, air quality and building energy consumption for space heating and cooling. A mesoscale model, with a multilayer urban canopy parameterization, coupled with a simple building energy model is used to investigate such interactions by simulating 22 idealized cities in 3D with the same total population, but different population densities and vegetation fraction in the urban areas. Simulations are performed for summer and winter periods at mid latitude and for a hot dry climate. Results indicate that compact cities, with buildings with low surface-to-volume ratios, minimize the building energy consumption for space heating/cooling, but maximize the outdoor heat stress. For air quality, the optimum is for cities with intermediate population densities. The inclusion of vegetation is most of the time positive, and never detrimental, in this climate.

Simulation of flow and turbulence in the Phoenix area using a modified urbanized mesoscale model

ABSTRACTThe urban canopy parameterization (UCP) of Dupont et al. implemented in the MM5v3.7 mesoscale meteorological modelling system (MM5‐UCP‐Basic) is evaluated against the non‐urbanized model (MM5‐NoUCP) using measurements taken in urban Phoenix during two field studies. In general, MM5‐UCP‐Basic improved the predictions of typical meteorological parameters. Nevertheless, significant discrepancies still exist between observations and the predictions of MM5‐UCP‐Basic, and new parameterizations and land‐use classes are introduced to improve the model performance. Anthropogenic heat flux from buildings and roadways is also included. The land‐use classes in the improved model (MM5‐UCP‐MOD) represent roadways and rivers, in addition to five classes of buildings identified in MM5‐UCP‐Basic. New parameterizations include those for momentum roughness, velocity decay during evening transition, and heat and momentum diffusivities for the nocturnal period that account for different heat and momentum transfer rates under stable atmospheric conditions. Five nested grid domains are used for simulations, with the highest resolution (1 km) implemented in the two UCP versions. In general, substantial improvements in the prediction of wind speed, temperature (especially during the night time), and momentum flux as well as a smaller improvement in the heat flux are noted, pointing to possible further enhancement to model performance by including improved physics in the model.

Forecasting the New York City Urban Heat Island and Sea Breeze during Extreme Heat Events

Abstract Two extreme heat events impacting the New York City (NYC), New York, metropolitan region during 7–10 June and 21–24 July 2011 are examined in detail using a combination of models and observations. The U.S. Navy's Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) produces real-time forecasts across the region on a 1-km resolution grid and employs an urban canopy parameterization to account for the influence of the city on the atmosphere. Forecasts from the National Weather Service's 12-km resolution North American Mesoscale (NAM) implementation of the Weather Research and Forecasting (WRF) model are also examined. The accuracy of the forecasts is evaluated using a land- and coastline-based observation network. Observed temperatures reached 39°C or more at central urban sites over several days and remained high overnight due to urban heat island (UHI) effects, with a typical nighttime urban–rural temperature difference of 4°–5°C. Examining model performance broadly over both heat events and 27 sites, COAMPS has temperature RMS errors averaging 1.9°C, while NAM has RMSEs of 2.5°C. COAMPS high-resolution wind and temperature predictions captured key features of the observations. For example, during the early summer June heat event, the Long Island south shore coastline experienced a more pronounced sea breeze than was observed for the July heat wave.

On the representation of urban heterogeneities in mesoscale models

The size and arrangement of the obstacles and the presence of a source of heat (anthropogenic heat flux) are distinctive characteristics of an urban area. These two elements, together with the specific applications oriented to improve citizen’s comfort, determine the way urban heterogeneities are represented in mesoscale models. In this contribution two examples are presented. In the first a microscale fluid dynamics model is used to investigate the role of organized motions (dispersive fluxes) of a passive tracer emitted at the surface in a staggered and in an aligned array of cubes. The impact of the dispersive flux, that can reach 90 % of the total flux in the staggered array, is then assessed in a column model. The second example deals with the representation of anthropogenic heat fluxes and the estimation of thermal comfort by means of an urban canopy parameterization with a simple building energy model, implemented in a mesoscale model. The simulation of a typical summer day over the city of Madrid (Spain) shows that the anthropogenic heat fluxes have the largest impact on the air temperature in the evening-night, and that the presence of the city prolongs to the late evening the period of thermal discomfort, compared with the rural areas surrounding the city. The paper is concluded by pointing out that future work must be devoted to deep on the relationship between the real morphology of a city and the simplified morphology adopted in the urban canopy parameterizations.