BIOASTY: Sustainable Urban Design for Climate Resilience ^†

Abstract

Climate change has led to the development of adaptation strategies at national, regional, and local levels. However, these interventions are often implemented without a strategic focus, either overlooking areas with the most urgent needs or failing to optimize efficiency at the local scale. BIOASTY aims to address this gap. The project’s primary objective was to assess thermal comfort in various urban environments using high-resolution modeling and propose targeted interventions to enhance thermal conditions. The final outcome was an operational system with comprehensive action protocols applicable to the city of Tripolis, Greece, strengthening urban resilience under different climate change scenarios.

1. Introduction

Mediterranean urban centers are increasingly susceptible to heat-related hazards due to both long-term climatic warming and the escalating severity, frequency and duration of extreme heat events [1]. An analysis of the Urban Audit database, encompassing 571 European cities [2], demonstrated that southern regions accounted for 69% of the observed increase in heatwave occurrences, highlighting the heightened thermal vulnerability of major cities such as Athens and Nicosia. Given rapid urbanization and the amplified effects of climate change, there is an urgent need for sustainable urban planning strategies that enhance climate resilience.

Bioclimatic urban design represents a scientifically sound approach that integrates biological and climatological principles to optimize urban microclimates and increase resilience. ENVI-met V5 Science, a high-resolution simulation tool, plays a pivotal role in this process by enabling the quantitative assessment of microclimatic variables [3,4]. This methodology seeks to improve outdoor thermal comfort, reduce energy demands, and mitigate Urban Heat Island effects through strategic manipulation of urban morphology, material properties, and vegetation. By analyzing the interplay between built environments, natural systems, and atmospheric dynamics—including solar exposure, airflow patterns, and thermal gradients—designers can develop urban configurations that diminish reliance on mechanical climate control.

This study explores the fundamentals of bioclimatic urban design and examines the use of ENVI-met in evaluating and improving the microclimate of Areos Square, the central plaza in Tripoli. The research aims to assess thermal comfort, pinpoint heat stress hotspots, and refine design interventions to mitigate unfavorable microclimatic conditions.

2. Materials and Methods

With the primary objective of optimizing urban microclimates, the methodology employed in this study evaluates heat stress in the central square of the city of Tripoli. Tripoli, located in the central Peloponnese region of Greece, has a Mediterranean climate (classified as Csa under the Köppen climate classification), but with continental influences due to its elevation (700 m ASL) and inland location. Because of its geography, Tripoli experiences larger temperature variations than coastal Mediterranean areas. It is considered one of the coldest major cities in Greece during the winter, while its summers are hot during the day but cooler at night.

A three-dimensional digital model of the study area was developed using the ENVI-met processor. Figure 1 displays the model, showcasing the spatial arrangement and physical characteristics of the study area.

For the chosen location, thermal comfort is assessed using the PET (physiologically equivalent temperature) index, a measure derived from the human body’s heat balance [5]. This indicator is calculated with the ENVI-met fluid mechanics model at the city level (1 × 1 m resolution). ENVI-met is a three-dimensional microscale climate model designed to simulate interactions among buildings, surface materials, vegetation, and atmospheric conditions.

The model setup incorporated the following key input parameters:

• Building Configurations: Detailed data were collected on the height, shape, and material properties of buildings in the study area, including architectural designs, construction materials, and spatial arrangements.
• Surface Materials: Information on the thermal and reflective properties of materials such as asphalt, concrete, grass, and vegetation was gathered to ensure accurate simulation of surface temperatures and heat exchange.
• Vegetation Coverage: The types, density and spatial distribution of vegetation—including trees, shrubs and lawns—were mapped. Vegetation plays a critical role in modifying microclimates through shading and evapotranspiration.
• Meteorological Data: Local weather conditions, including air temperature, humidity, wind speed and direction and solar radiation, were input into the model to reflect real atmospheric conditions.

ENVI-met modeling produces detailed, high-resolution outputs for microclimatic parameters. For boundary conditions, hourly measurements of air temperature and relative humidity recorded at a nearby meteorological station on the simulation day are used (i.e., manually defined diurnal profiles to fully control temperature and humidity in the model). Wind speed and direction are set according to the station’s mean daily values, measured at a height of 10 m [6]. The simulation outputs—detailed maps of temperature, humidity, wind flow and radiation—are validated against field measurements.

As input data, measurements from the nearest weather stations are used, as well as downscaling of the ERA5 data of the European Copernicus service on a local scale of 1 × 1 km with the WRF atmospheric model [7]. The results of the simulations are verified through measurements of high spatial and temporal resolution (i) with an electric bicycle equipped with weather sensors and (ii) with a drone equipped with a thermal camera. Subsequently, the simulations for the optimization of bioclimatic parameters are carried out for the current situation. For the optimization, alternative types of vegetation, building materials and bioclimatic–architectural design were examined through the digital modification of the input data of the model. The final assessment of the improvement scenarios results in the proposal of specific measures, taking into account additional environmental, social and techno-economic parameters. This approach allows users (e.g., regions, municipalities, public and private bodies) to be provided with concrete solutions to adapt to national and European policies on the sustainability of cities and their resilience to climate change.

3. Results

3.1. Assessment of the Current Situation

To assess thermal conditions, a simulation of human thermal sensation was conducted for 19 July 2023—a representative summer day—using ENVI-met.

The model results for PET, as presented in Figure 2, show considerably lower PET values near green areas.

3.2. Optimized Bioclimatic Conditions

As seen in Figure 2, Areos Square presents a significant thermal load during the afternoon hours. The square is fully landscaped, so the proposed interventions, in consultation with the Mayor of Tripoli, should be targeted.

Figure 3 depicts a two-dimensional presentation of Areos Square as it is currently and with the scenario for optimized bioclimatic conditions. More specifically, changing the covering materials of the square has not been proposed due to its recent remodeling. The proposed redesign of Areos Square focuses on improving thermal conditions and aesthetics by introducing two 106.5 m lines of cherry trees flanking a central fountain (93 m long × 1.5 m wide). This intervention seeks to encourage public use at all hours, alleviating thermal discomfort and providing a lush, inviting retreat.

For the selection of the optimal scenario, different planting arrangements were examined in order to investigate the optimal location of the tree trunks and to achieve better conditions of thermal comfort. The criteria considered for the final selection are described below:

Architectural criteria:

Since the square is fully formed, the arrangement of the proposed plantings was made taking into account the installation of the existing layout.

Environmental criteria:

In any case, we chose not to use building materials to avoid additional interventions in the environment, while the presence of greenery increased.

Social criteria:

The ability to utilize the space by the residents and the creation of places for gathering, playing and resting have a positive impact on the local community.

Techno-economic criteria:

The proposed solutions do not differ in terms of ease of implementation and cost since they all concern the planting of tree rows.

In Figure 4 the spatiotemporal mapping of the Physiological Equivalent Temperature (PET) simulations at 15:00 for the study area of Areos Square in the Municipality of Tripoli is presented. It is observed that, with the application of these simple interventions, the PET index decreases, especially in an area with tree planting, where the PET decreases by 3–5 °C.

4. Conclusions

In this work, the effect of planting solutions and additional water surfaces on thermal comfort in public spaces, squares and parks was examined. For the various scenarios, the choices of the relevant municipal authorities were taken into account, and solutions with significant and high-cost interventions were avoided. Interestingly, the proposed solutions are simple to implement and do not cost much. Alternative scenarios were examined regarding the layout and the choice of location. The urban planning optimization simulations for the areas under study showed that

• The increase in green spaces for heat absorption, the provision of shading and the promotion of evaporative transpiration lead to a reduction in the PET index by mitigating the intensity of the Urban Heat Island phenomenon.
• The application of shading structures, tree lines on the sides of the streets and tree planting in public places (stadiums, parks, playgrounds) offers protection from direct solar radiation, taking into account the meridian arc of the sun by optimizing human thermal comfort.
• The integration of aquatic features (fountains, streams, lakes) into the public urban space works effectively for absorbing heat from the built urban environment, dissipating excess heat through evaporation and creating refreshing microclimates.
• In addition, the model’s optimization features allow for fine-tuning of the above scenarios to achieve better results.

By iterating parameters and variables, urban planning can help develop mitigation strategies in specific urban contexts, taking into account factors such as local climate, population density, and infrastructure constraints.

The optimization of mitigation scenarios for the Urban Heat Island is a proactive approach to enhancing urban resilience and sustainability and contributes significantly to climate change adaptation.