It is well known that the urban environment changes local climate inside the city. This change of the local climate manifests itself mainly through di_erences in air temperature, where cities remain warmer than the rural environment during the night. This phenomenon is called the Urban Heat Island (UHI) e_ect, and is de_ned as di_erence in air temperature between the urban and rural environment. The UHI e_ect is found in many cities of di_erent sizes around the world, and ranges between 1 and 10oC during the night. The combination of the increasing urbanisation, global warming and the impact of increasing temperature on human health makes the urban heat island a topic that is gaining more and more attention. This thesis focusses on the urban micro-climate, which treats indivicual buildings and their direct surroundings. A numerical modelling approach is used in this thesis, such that the local urban climate can be investigated and perturbed in a systematic way. The developed 2D model, called URBSIM, combines computation of radiative transfer by a Monte-Carlo model, conduction of energy into the urban material and a Computational Fluid Dynamics (CFD) model to compute air ow and air temperature. With this model, it is shown that the main source of energy to the urban heat budget is due to radiative transfer. During the night, the long wave trapping e_ect (de_ned in this theses as radiation emitted by one surface and absorbed by an other) and absorbed long wave radiation emitted from the sky are of the same order of magnitude for a building height (H) over street width (W) ratio of H=W=0.5. With increasing building height, longwave trapping becomes the main source of energy to the urban energy budget. During the day time, absorbed shortwave radiation is the main source of energy, followed by the long wave trapping e_ect. The relative contribution of these radiative components is decreasing with increasing building height, vi Summary and the conductive heat ux becomes more important. The large impact of radiation sparked the question which high albedo adaptation measure (white surfaces) is best suited to reduce the Urban Heat Island e_ect. This thesis shows that there is a clear distinction between the atmospheric UHI (air temperature) and pedestrian heat stress. Lower air temperatures can be achieved by using high albedo materials, whereas thermal comfort at street level can be improved by using low albedo materials. By using a low albedo material, less radiation is reected back inside the canyon, thereby reducing the mean radiant temperature. The lowest pedestrian heat stress is found by using a vertical albedo gradient from high albedo at the bottom part to a low albedo at the top part of the wall for H=W=1.0. This study indicated that using a high albedo material can decrease the UHI e_ect, but increases pedestrian heat stress, which might not be the desired e_ect. The developed micro-scale model is also compared to a large-scale urban parametrisation scheme that is used in meso-scale models. In this parametrisation, a 2D geometry is used to compute the uxes of the 3D environment. Results indicate that radiative transfer is well captured in the parametrisation. Canyon wind speeds and the sensible heat ux showed much larger di_erences between the two models, which is most likely due to the 2D geometry that is used as a basis for the parametrisation. It is very likely that these parametrisations are adapted to better represent the 3D urban environment. The result of this thesis is an advanced numerical model that includes most processes relevant to the urban environment. Despite the fact that the model is limited to 2D cases, the studies presented in this thesis have aided the understanding of the elementary processes that control urban air temperature, the feedback processes and interactions between the di_erent mechanisms in the urban surface energy ... Read more

The authors regret to inform the readers that a programming error was found in the numerical code described and used in the original submission [1]. Longwave trapping was not correctly scaled with the local cell size and is underpredicted for deep canyons. Calculations have been repeated and this corrigendum provides an overview of the new results for a deep canyon. All new results will be published in a forthcoming Phd thesis [2] that will made publicly available through the TU Delft website (https://repository.tudelft.nl/). The authors would like to apologise for any inconvenience caused. ... Read more

This study investigated the surface temperature, air temperature and mean radiant temperature inside an idealized 2D street geometry during daytime. The goal was to unravel the relative impact of radiative transfer, heat conduction and ventilation to the urban heat budget. A building-resolving simulation model has been used, which represents these processes at a 1 m spatial resolution. Different combinations of the canyon height to width ratio (H/W) and physical mechanisms were investigated. Shortwave radiation is the main source of energy, and for small H/W can be higher at the canyon ground level compared to flat terrain due to multiple reflections. The longwave trapping effect has the second largest contribution and becomes relatively more important with increasing H/W ratio. The influence of the interior building temperature is small. Surface temperature and mean radiant temperature are closely related, since both are largely controlled by radiative properties. No straightforward relation was found between surface temperature and air temperature, since air temperature is dependent on the competing mechanisms of forced and natural convection. A small increase in air temperature inside the canyon was observed compared to the ambient temperature above roof level. The inclusion of all key physical processes in high detail resulted in large computational requirements. If multiple reflections by the building facades are small, the more traditional, yet much simpler view factor approach will strongly reduce the computational costs as compared to the Monte Carlo technique. The influence of using the view factors on the results must be investigated. ... Read more

This study investigates the effect of different high-albedo adaptation strategies on air temperature, mean radiant temperature and the Universal Temperature Climate Index (UTCI) for an idealized 2D street canyon. The used numerical model computes the heat transport in the canyon, and specifically takes into account the effect of multiple scattering of radiation. In general the mean radiant temperature has a much larger impact on the UTCI than the air temperature. Moreover, the mean radiant temperature exhibits strong spatial variations in the canyon due to its sensitivity to shading. The impact of albedo-differences on the UTCI is thus relatively small compared to the large shading effects. The best strategy to minimize the UTCI for the outdoor environment with building height to width ratio H/W = 0.5 is found to be a uniform albedo of 0.2. For H/W = 1.0, an albedo gradient from high at the bottom part to low at the top of the vertical walls showed the lowest UTCI. Although using high-albedo materials can mitigate the atmospheric urban heat island effect, it is very likely to increase pedestrian heat stress ... Read more

In the present study we numerically investigated the dispersion of photochemical reactive pollutants in complex urban areas by applying an integrated Computational Fluid Dynamics (CFD) and Computational Reaction Dynamics (CRD) approach. To model chemical reactions involved in smog generation, the Generic Reaction Set (GRS) approach is used. The GRS model was selected since it does not require detailed modeling of a large set of reactive components. Smog formation is modeled first in the case of an intensive traffic emission, subjected to low to moderate wind conditions in an idealized two-dimensional street canyon with a building aspect ratio (height/width) of one. It is found that Reactive Organic Components (ROC) play an important role in the chemistry of smog formation. In contrast to the NOx/O3 photochemical steady state model that predicts a depletion of the (ground level) ozone, the GRS model predicts generation of ozone. Secondly, the effect of direct sunlight and shadow within the street canyon on the chemical reaction dynamics is investigated for three characteristic solar angles (morning, midday and afternoon). Large differences of up to one order of magnitude are found in the ozone production for different solar angles. As a proof of concept for real urban areas, the integrated CFD/CRD approach is applied for a real scale (1 × 1 km2) complex urban area (a district of the city of Rotterdam, The Netherlands) with high traffic emissions. The predicted pollutant concentration levels give realistic values that correspond to moderate to heavy smog. It is concluded that the integrated CFD/CRD method with the GRS model of chemical reactions is both accurate and numerically robust, and can be used for modeling of smog formation in complex urban areas ... Read more