Microclimatic modelling in assessing the impact of urban geometry on urban thermal environment
Introduction
Urban microclimate is a complex consequence of different parameters which involves innumerable natural and urban processes. The natural parameters like air temperature and humidity, vapour pressure, wind speed, solar radiation, soil temperature and humidity are very sensitive to any 3-dimensional changes in the urban settings. Due to the diverse processes involved, causing different microclimates, one of the most feasible ways to predict their impacts is through the use of numerical methods (Arnfield, 2003, Bruse, 1999). Numerical modelling and computer simulation techniques are therefore playing increasingly important roles in present day thermal comfort and building performance studies in the urban context. Furthermore, the constraints associated with the in-situ measurements make numerical modelling more convenient for the researchers, especially in terms of comparing theoretical models with different combinations of parameters.
The integration of microclimatic simulation in this study enables the limitations of direct monitoring to be overcome. Direct monitoring and field measurements only allow observation of a few points at a time. It is difficult to measure climatic parameters in a large number of canyons at the same time as it will require multiple sets of equipment and measurement tools which are expensive and involve the risk of theft. Therefore, along with field measurements, ENVI-met V4 (Bruse, 2015), a numerical microclimatic tool with high temporal and spatial resolution was used in this study as an important tool for measuring microclimatic dynamics.
The primary interest of using numerical modelling in this study lies in checking its responsiveness in identifying the diversity in urban forms. Studies in the mid-latitude and tropical climate cities have shown that even though deep urban canyons can improve daytime microclimate, they could generate a nocturnal urban heat-island effect (Qaid & Ossen, 2014). Due to the inconsistency between day and night situations, uniform, homogeneous canyons are not climatically ideal for a tropical, hot-humid climate. A field measurement by Sharmin, Steemers, and Matzarakis (2015) in the tropical megacity Dhaka reveal, varying traditional urban forms are cooler in comparison to more regular formal residential areas.
Therefore, the study examines the proficiency of a numerical modelling tool ENVI-met V4 in distinguishing uniform versus variable urban geometry conditions as identified during the field measurements. It aims to quantify the microclimatic differences measured between the actual case-study areas with variable and uniform geometry, mainly in terms of air temperature and mean radiant temperature and, subsequently, to compare the differences with the modelled variations. The objective is full-filled in a series of analysis: firstly, by examining ENVI-met’s ability in reproducing microclimatic conditions as observed in field-study conditions with the use of site-specific measured data as input for simulation models. Secondly, by examining ENVI-met’s responsiveness to the diversity in urban geometry by using the same input data for each case-study site. In this case, it is assumed that whatever difference is occurring between the microclimate of different sites, is due to the variation in their urban geometry. It attempts to answer the research questions:
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To what extent ENVI-met modelling can reproduce the microclimatic conditions for the existing case-study areas by using field measurements as boundary conditions?
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Is ENVI-met able to recognise the diversity in urban form when all case-study areas are modelled using identical boundary condition?
ENVI-met is an advanced simulation system that recreates the microclimatic dynamics of the outdoor environment by addressing the interaction between climatic parameters, vegetation, surfaces, soil and the built environment (Bruse & Fleer, 1998). The new features in ENVI-met V4 include the simple forcing of air temperature and humidity in 2 m levels which needs input data, such as the initial temperature of the atmosphere, specific humidity at the model top and maximum and minimum values over a 24 h cycle. The forcing also has the option to input the values on an hourly basis which are collected either from weather stations or directly from on-site measurements. This study uses simple forcing together with the hourly forcing (for air temperature and humidity) options to perform simulation of the case-study areas.
ENVI-met has been extensively used in urban design and thermal comfort studies for its ability to reproduce microclimatic conditions within the urban canopy layer (UCL) (Ali-Toudert & Mayer, 2007; Krüger, Minella, & Rasia, 2011; Ng et al., 2012). Although there are several microclimatic tools such as RayMan (Matzarakis, Rutz, & Mayer, 2010), SOLWEIG (Lindberg, Holmer, & Thorsson, 2008) and Townscope (Teller & Azar, 2001), ENVI-met is particularly popular for its high temporal and spatial resolution, its advanced 3D interface and modelling techniques and its ability to adjust air temperature and relative humidity. The latest version (ENVI-met V4) considers the heat capacity of the building materials (Huttner, 2012, Yang et al., 2013), a unique feature that other microclimatic simulation tools are yet to accomplish. ENVI-met is based on the fundamental laws of fluid dynamics and thermodynamics, while other models such as RayMan and SOLWEIG are 3D radiation models. It is thus a rare example of a model which can be used to explore the relationships between urban form and the urban microclimate.
The study has been carried out in three steps, named as Step 1–Step 3. Firstly, in Step 1, field measurements of microclimatic dynamics are discussed. Secondly, actual case-study areas are modelled in ENVI-met V4 (Step 2) and a comparison of microclimatic conditions between the modelled sites is carried out. Actual microclimatic measurements from the field-study are used as the model boundary conditions. Thirdly, in Step 3, the same case-study areas are modelled using boundary conditions from the worst-case scenario, obtained from the EPW (EnergyPlus Weather) data for Dhaka. In this step, all sites have the same boundary conditions in order to understand how they respond to the differences in urban form. Finally, microclimatic deviations among the case-study areas reported from modelling in the second and third steps are evaluated against the actual differences reported in the field measurements. The results particularly identify the limitations of the microclimatic simulations in terms of predicting air temperature and mean radiant temperature.
Section snippets
Uniform versus variable urban geometry
The geometry of urban canyons has been proven to play a key role in determining the heat island effect and affecting thermal comfort in streets. By definition the urban canyon is a basic geometric unit estimated by a two-dimensional cross-section of buildings (Oke, 1988). The urban geometry parameters used in this study are H/W ratio and sky view factor (SVF). H/W ratio is a key urban geometry parameter affecting the incoming and outgoing solar radiation, radiation flux and wind flow in an
Introduction to microclimatic simulation tool: ENVI-met
ENVI-met is a CFD microclimatic model to simulate the interactions between building, pavement and natural surfaces in a virtual environment by reproducing the major atmospheric processes (Bruse, 1999). This involves a sequence of mathematical calculations established by the laws of fluid dynamics and thermodynamics which govern the atmospheric motions. It is a non-hydrostatic, RANS5 model with a typical horizontal resolution
Analysis of results of Step 1
Box plot analysis and related statistics of autumn 2014 and summer 2015 data are presented in Fig. 4, Fig. 5 respectively. In Fig. 4, both EW and NS oriented streets of the formal area FRA2 have a higher air temperature compared to the traditional area TRA1. The average (median) air temperatures of the formal sites FRA2EW and FRA2NS are 3.3 °C and 2.5 °C higher than the corresponding traditional sites TRA1EW and TRA1NS respectively. Among the EW oriented canyons, the site FRA1EW has the lowest
Conclusion
Recent studies have shown that the geometry and the aspect ratio of urban canyons play a crucial role in moderating the microclimate at the street level (Shashua-Bar et al., 2004). This study has examined the shortcomings of modelling techniques in responding sufficiently to the urban geometry characteristics, such as H/W ratio and its standard deviation, SVF and its standard deviation. It has addressed a specific argument: whether microclimatic modelling techniques can address the impact of
Acknowledgement
This paper is drawn from research funded by the Schlumberger Foundation at the University of Cambridge, Department of Architecture.
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