CitySim: Urban Energy & Climate Simulator

• CitySim is a comprehensive urban simulation framework that integrates 3D radiosity modeling with urban canopy and building energy analysis.
• The framework employs a coupled model to compute detailed radiative fluxes and energy exchanges across complex urban morphologies.
• Its application in early-stage master planning demonstrates significant improvements in assessing energy demand and microclimate modulation.

CitySim is a physically integrated modeling framework and software environment developed to simulate urban canopy microclimate, building energy demand, and radiative energy exchange for three-dimensional urban designs. Designed for early-stage master planning and energy analysis, CitySim enables the assessment of how detailed urban morphology—beyond the limitations of simplified regular arrays or street canyons—impacts both indoor energy consumption and the local urban climate. It is employed as the radiosity (radiative transfer) engine within an urban modeling toolchain, coupling with analytical models for canopy thermodynamics and building energy flows. CitySim’s practical innovations and mathematical rigor position it as a reference tool in the field of urban energy and climate simulation.

1. Integration of Radiative Transfer in Urban Modeling

CitySim provides the core radiosity model within an integrated simulation of urban form. Unlike previous approaches that use simplified geometric assumptions, CitySim calculates the net radiative fluxes (both shortwave and longwave) for every individual surface in a 3D master plan, capturing the effects of shadows, mutual reflections, and surface orientations throughout complex, multi-building environments. Its radiosity solver is based on a simplified version proposed by Robinson (2005), designed to accelerate the computationally expensive calculation of form factors required for accurate surface-to-surface radiation exchange in urban settings. All geometry and material information are managed through an XML-based morphological description compatible with 3D urban design master plans.

This approach allows CitySim to faithfully represent the solar access and inter-building radiative interactions in not only idealized, but also realistically diverse urban forms, ensuring that urban microclimate effects are accurately propagated into energy demand calculations.

2. Coupled Model Components and Workflow

The CitySim ecosystem functions as part of an integrated suite, combining three tightly coupled modules:

1. Urban Canopy Model: Employs a multilayer (storey-by-storey) structure without the need for regular or canyon-form assumptions. It computes vertical profiles of urban air temperature, turbulent heat flux, and energy transfer using a parameterized eddy diffusivity.
2. Building Energy Model (BEM): Implements a two-node resistance-capacitance (RC) network for each building—one node representing interior air, the other for building thermal mass—calculating conduction, ventilation, anthropogenic heat, and incorporating CitySim-supplied radiative flux.
3. Radiosity Model via CitySim: Calculates the surface-level net radiative energy exchange for all walls, roofs, and other surfaces. These fluxes are then used to drive both the BEM and the canopy model, allowing bidirectional, time-stepped feedback between internal building loading and external microclimate.

Model interaction is mediated by an automated XQuery data exchange script, ensuring consistent and up-to-date information flow between modules at each timestep. All three subsystems use the same morphological parameters (areas, wall lengths, heights) to maintain strict coupling.

3. Impact of Urban Morphology: Archetypes and Findings

CitySim’s framework allows systematic comparison across archetypal urban morphologies while holding density and thermal parameters constant:

• Regular slabs: Parallel buildings with identical height; minimum surface area, least sun penetration.
• Convex slabs: Curved/courtyard-like slabs with greater sun exposure through geometric reflection.
• Even open block: Divides the urban block into multiple smaller, height-equivalent buildings, increasing total external surface area and varied solar access.
• Uneven open block: Varied building heights, causing intricate patterns of shade and exposure through vertical stratification.

Findings show that increased vertical solar access, especially to lower levels, can reduce net heating demand by up to a factor of two. Morphologies with a larger exterior surface-to-volume ratio and greater geometric complexity (e.g., uneven open block) both exploit increased insolation for lower building layers and introduce more uniform vertical distribution of energy loads. However, these benefits come at the cost of amplifying vertical air temperature gradients and can modify local microclimate, highlighting a fundamental trade-off faced by urban designers.

4. Mathematical Formulation

The core mathematical formulations in CitySim’s coupled system include:

• Canopy layer energy budget:

$$C_p \rho \frac{\partial \theta_c}{\partial z} = C_p \rho \frac{\partial}{\partial z} K_h \frac{\partial \theta_c}{\partial z} + H_w \frac{l_w}{S_c} + q_v \frac{S_f}{S_c}$$

where turbulent heat flux, radiative heating from walls, and ventilation all contribute to the vertical temperature gradient.

• Building energy model (two-node RC):

$$K_{int}(T_a - T_w) + K_{ext}(\theta_c^k - T_w) + K_r(\theta_c^{k+1}-T_w) + \Phi_w = C_w \frac{d T_w}{dt}$$

$$K_{int}(T_w - T_a) + K_{vent}(\theta_c^k - T_a) + Q_c + Q_a + \Phi_a = C_a \frac{d T_a}{dt}$$

Net radiative fluxes $\Phi_a, \Phi_w$ are supplied by CitySim for each model surface.

• Radiative model output linkage:

$$\Phi_a = \eta \Phi \qquad \Phi_w = \Phi (1 - \eta) K_{ext}/h_{conv}$$

• Eddy diffusivity for turbulent mixing:

$$K_h = \left( K_{max} \exp(1/2)/h_{max} \right) z \exp\left( -0.5 (z/h_{max})^2 \right)$$

These closed-form, parameterizable equations ensure computational tractability while capturing essential microclimate and radiative effects.

5. Practical Applications and Limitations

CitySim is suited for early-stage urban planning, where explicit window placement, zoning, and envelope details may be lacking. Its parameter-driven approach permits rapid scenario analysis and encourages urban designers to explore unconventional geometries (rather than defaulting to simplified energy indicators). The tight coupling of microclimate and energy models, with feedback loops, reveals subtle vertical and horizontal interactions otherwise inaccessible to uncoupled or oversimplified models.

However, principal limitations include:

• Reliance on prescribed input profiles for turbulence and upper atmospheric boundary conditions (which can be addressed at later stages by coupling with meso-scale models).
• Assumption of shoebox, abstracted geometries for buildings, meaning specific design features may not be fully resolved.
• Outputs, being comparative (rather than absolute), are best used for relative assessment between design alternatives during pre-construction phases.

6. Role in Urban Energy Modeling Ecosystem

Within the broader landscape of Urban Building Energy Modeling (UBEM) tools, CitySim is notable for its physics-based, reduced-order RC network approach, its sophisticated treatment of radiative coupling, and its capacity for large-scale simulation with relatively simple user input requirements. It supports extensive GIS and CityGML integration, offers graphical output and 3D visualization, and partially integrates mobility modeling with transport modules such as MATSim-T.

Recent review studies identify CitySim as a leading example for simulations requiring detailed 3D morphology, early-stage master plan evaluation, and the reconciliation of energy engineering with urban design language. Future directions highlighted for CitySim, in line with the overall UBEM research agenda, focus on improved data standardization, enhanced occupant and system modeling, integration with urban energy networks, advanced microclimate coupling, and support for large-scale calibration.

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Component	Methodology	Role of CitySim
Urban Canopy	Discrete multilayer energy equations	Supplies radiative input to canopy model
Building Energy	Two-node RC equivalent per storey	Receives radiative input from CitySim
Radiosity	Simplified radiosity (CitySim core)	Computes net radiative flux for all surfaces

CitySim’s integrated modeling approach empowers practitioners to quantitatively link urban design choices with both building energy performance and environmental microclimate, establishing it as a scientifically rigorous and practically useful tool for the exploration and optimization of sustainable urban masterplans.