Dear Colleagues,

This Special Issue (SI) aims to draw attention on the inter-relationships between built environment, urban microclimate, building energy demand, and people’s health and wellbeing in cities characterized by a Mediterranean climate.

Several studies have shown that urban warming causes an increase in building cooling demand, especially in warm regions such as in the Mediterranean climate. Urban areas also increase the intensity of extreme events such as prolongued heat waves, with concerning impact for the health of vulnerable groups such as the elderly and children.

However, a peculiarity of the Mediterranean climate is its seasonal variability, and for this reason, buildings and urban spaces must be able to cope with both hot summer and cold winter periods to ensure comfortable living conditions and reduced energy demand on an annual basis. Considering this, the urban heat island (UHI) effect has contrasting impacts on people’s health and perceived comfort as well as buildings energy demand over the year.

Furthermore, while many studies have investigated the impact of the atmospheric UHI intensity, much less literature is available on the net impact of multiple urban microclimate modifications. For instance, the decrease in solar irradiance on facades and streets in urban canyons, the modification of wind speed, and the reduced sky view factors can be even more relevant than urban air temperature increase in determining thermal comfort in this climate region, characterized by mild temperatures and high solar irradiation. Urban canyon geometry also deeply affects daylight availability in buildings, and it can be responsible for a higher concentration of pollutants in the atmosphere, with negative consequences on air quality at the street level and indoors. All these phenomena are strongly interconnected and contribute to the environmental quality of indoor and outdoor spaces in urban areas.

This SI is open to studies investigating outdoor and indoor environmental quality (i.e., thermal and visual comfort and air quality) and urban building energy performance (i.e., cooling, heating, and lighting) in relation to the characteristics of the urban fabric and corresponding urban microclimate. Case studies and theoretical investigations as well as experimental and numerical studies are welcome, provided they deal with Mediterranean or similar climates. This climate type is characteristic of the Mediterranean basin but also present in several other regions, mainly within 30˚ to 45˚ latitude in both the north and the south hemispheres. Some similar climates are also found in the inter-tropical region at high altitude (from 2500 m above sea level).

Possible topics for this Special Issue include (but are not limited to):

• Urban heat island studies (mesoscale and local scale);
• Urban microclimate;
• Urban heat stress;
• Building energy performance in uban context;
• Tools and approaches for urban energy modeling (UBEM);
• Environmental quality in urban areas (indoor and outdoor);
• Urban heat island mitigation strategies;
• Urban and building adaptation stategies to climate change;
• Urban building passive cooling design;
• Urban building sustainable design and early-design strategies.

Dr. Agnese Salvati
Dr. Gianpiero Evola
Dr. Massimo Palme
Dr. Giacomo Chiesa
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All papers will be peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Sustainability is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 1900 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

• urban heat island
• urban microclimate
• building energy performance
• thermal comfort

Title: Development of an Urban Heat Mitigation Plan for the Greater Sacramento Valley, California
Authors: Haider Taha
Affiliation: Altostratus Inc., Martinez, United States
Abstract:

A detailed atmospheric modeling study, commissioned by the Sacramento Metropolitan Air Quality Management District with funding from the California Department of Transportation, was undertaken with the goal of informing the development of a heat-mitigation plan for the Capital region in California (Greater Sacramento Valley). Realistic levels of mitigation measures were characterized and ranked in terms of their effectiveness in producing urban cooling under current conditions and future climate and land use. The measures, in standalone fashion and in combinations, included (1) cool roofs; (2) cool pavements; (3) vegetation canopy and ground cover; (4) vehicles electrification / EV ownership; (5) solar photovoltaics; and (6) cool walls.

Based on the definition of time-varying upwind non-urban temperature reference points for each area in the region and the hourly temperatures at each model grid point per coincident wind direction, an urban heat-island index (UHII) and a temperature-weighted  UHII were computed for years 2013 – 2017 and for year 2050 (RCP 4.5 and 8.5). The UHII was calculated for all hours, specific hours, or ranges of hours and was used to prioritize areas for deployment of mitigation measures and as a yardstick to establish reasonably-attainable levels of heat mitigation for each urban area. The UHII for the period July 16 – 31, 2015, as an example, for all-hours averaged temperature equivalent (i.e., °C · hr hr^-1) ranged from 1.5 to 4.7 °C across various urban areas in the Greater Sacramento Valley.

Urban-heat mitigation measures were ranked at 2-km and 500-m resolutions, using different modeling approaches for each scale. The 2-km modeling was intended for designing and deploying measures at the county and city levels (six counties in the region) whereas the 500-m modeling was carried out for specific projects within each city, such as roadway improvement, urban corridor greening, electrification of motor vehicles, installation of solar PV in specific areas, retrofitting particular buildings with cool roofs/walls, or other specific projects defined by the communities involved in this study.

The cooling efficacies of various measures were evaluated for different times of day, hours, or blocks of hours, so as to provide planning-relevant information on most effective strategies (since the efficacies of measures can differ from one block of hours to another). It was found that various measures could offset the localized UHII (in standalone fashion or combinations) and sometimes more than offset it. The mitigation measures could also inadvertently cause some warming outside of the modified areas, generally downwind of the urban land use, because of reduced mixing. However, the warming is small compared to the cooling effect both in magnitude (maximum of 0.3 °C compared to cooling of up to 4 °C locally) and in the spatial extents affected by the temperature changes (areas affected by warming were at most 5% of the areas affected by cooling).

In terms of thermal environmental conditions, it was found that urban-cooling measures can help decrease or offset exceedances in the National Weather Service Heat Index (HI) above several critical warning thresholds and reduce the number of heat-wave or excessive heat-event days. For example, measures that combine increased albedo and urban forests can reduce the exceedances above 106 °F HI (Danger) by between 50% and 100% and the exceedances above 91 °F HI (Extreme Caution) by between 18% and 36%.

An innovative approach (based on defining characteristic length scales) was applied to quantify UHII offsets from each mitigation measure in two situations: (1) a scenario where a community implements the measures and no other nearby communities take any action, and (2) a scenario where both the community and its upwind neighbors implement the measures. In this second situation, the community benefits from cooler air transported from upwind areas in addition to the local cooling resulting from implementation of its own heat-mitigation strategies. This is akin to “doubling” the local cooling effects and associated benefits.

The changes in local meteorology corresponding to conditions of future climate were quantified by applying a modified high-resolution urban-meteorology model in dynamically-downscaling the 2050 RCP 4.5 and RCP 8.5 fields from a climate model along with future urbanization and land-use change projections for each area. The changes in the future UHII relative to present were dominated mainly by two effects: (1) in areas that are currently urbanized, the main impacts on the temperature field and the UHII are those from local climate-change effects, whereas (2) in areas that will be urbanizing between now and 2050, the impacts on air temperature will result from both changes in land use (urbanization) and changes in climate. It was found that the effects of urbanization were of the same magnitude as that of the local climate-change effects and, thus, the urban-cooling measures have the potential to locally offset the effects of climate change.

Across the various urban areas in the region and the selected RCP scenarios, the all-hours UHII (°C · hr hr^-1) increased by between 0.24 and 0.80 °C, representing an increase of between 17% and 13% of their respective values in 2013 - 2017. Locally, temperatures could increase by up to ~3 °C because of climate change and up to ~5 °C because of both climate and urbanization changes. While temperatures in the region generally increase from current climate to future (e.g., to 2050 RCP 4.5 and then to 2050 RCP 8.5), the corresponding UHII also increases in this direction except for two urban areas where the UHII could be smaller in RCP 8.5 than in RCP 4.5 (although still larger than in current climate). This is a result of accelerated warming in the surrounding non-urban areas, hence, by definition, a smaller UHII at these two urban locations.

The modeling of future climates shows that except for a number of instances, the ranking (and ordering) of measures at each respective urban area remains mostly unchanged into the future. That is, the order of measures in terms of their effectiveness in mitigating urban heat in current climates and land use (at a given location) remains relatively the same under conditions of future climate and urbanization. While the ranking (order) can be relatively similar, the magnitudes of the cooling effects can differ. The effects of community-level heat-mitigation measures at local project scales (at the 500-m level) were also evaluated in the context of future climate and land use. The model results show that the effectiveness of the mitigation measures in 2050 is similar to their effectiveness in the current climate – this is because the increased extent of urbanization, while contributing to additional local warming, also results in an increase in technical potential (i.e., area available) for the deployment of mitigation measures, thus keeping the UHII offset levels similar to those in current climates or slightly larger in some cases.

Title: Application of Urban Scale Energy Modelling and Multi-Objective Optimization Techniques for Building Energy Renovation at District Scale
Authors: Andrea Gasparella; Fahad Haneef; Giovanni Pernigotto; Jérôme Henri Kämpf
Affiliation: 1.Andrea Gasparella, Fahad Haneef and Giovanni Pernigotto Free University of Bozen-Bolzano, Bozen-Bolzano, Italy 2. Jérôme Henri Kämpf Institut Dalle Molle D'intelligence Artificielle Perceptive, Martigny, Switzerland
Abstract: Nearly-Zero Energy Buildings are now a standard for new constructions. However, the real challenge for a decarbonized economy relies in the renovation of the existing building stock, selecting energy efficiency measures considering also building life cycle environmental impacts and economic sustainability. Even if the literature is full of examples of multi-objective optimization coupled with building energy simulation to identify the best energy efficiency measures, the adoption of such approaches for district and urban scale simulation is still limited. In this research, a hybrid approach based on building envelope archetypes and simplified physical modeling is applied to CitySim, to simulate a residential district built in the 90s in Bolzano, Italy. Different sets of renovation measures for the building envelope and three objectives – i.e., energy efficiency, economic performance and environmental sustainability, are compared. Despite energy savings from 30 % to 52 %, energy efficiency measures applied just to the building envelope are found insufficient to meet the carbon neutrality goals without interventions on the system, in particular considering a mechanical ventilation with heat recovery. Furthermore, public subsidization has revealed necessary, since none of the proposed measures is able to pay back the initial investment for this case-study.

Title: Applications of Models and Tools for Mesoscale and Microscale Thermal Analysis in Mid-Latitudes Climate Regions – A Review
Authors: Gabriele Lobaccaro (corresponding)1, Koen De Ridder 2, Juan Angel Acero 3, Hans Hooyberghs 4, Dirk Lauwaet 4, Bino Maiheu 4, Richa Sharma 5
Affiliation: 1. Norwegian University of Science and Technology 2. Flemish Institute for Technological Research 3. Singapore-​ETH Centre 4. Vlaamse Instelling voor Technologisch Onderzoek 5. ITS Engineering College
Abstract: The analysis of local climate conditions became more important to test typical artificial urban boundaries and related climate hazards. The multitude of finishing materials and sheltering objects especially within built structures, such as street canyons produce a very distinct pattern of different climate conditions mostly during the daytime, in which the combination of high temperatures and intense solar radiation strongly perturb the environment by increasing the thermal heat stress at pedestrian level. Therefore, it is becoming common practice to inform urban planners and decision-makers about the attractiveness and effectiveness of new urban spaces by using modelling and simulation tools. Their use enables multiple design and planning alternatives to be quantitatively and qualitatively tested, predicted, and compared, to understand the relationships between the micro-climatic environment, the subjective thermal assessment, and the social behavior. In the early design phases, this process works as urban planning instrument to develop more sustainable and livable open public spaces. This paper presents a review of selected environmental numerical tools and models and their applications to predict human thermal stress at pedestrian level at mesoscale (e.g. Satellite thermal image, UrbClim, Enviro-HIRLAM) and microscale (e.g. Mobile measurements, ENVI-met, RayMan, UrbClim HR) in case study cities located in mid-latitudes climate regions.

Title: Nature-Based Solutions: Thermal Comfort Improvement and Psychological Wellbeing.A Case Study in Genoa, Italy.
Authors: Francesca Mosca1, Giulia Dotti Sani2, Andrea Giachetta1, Katia Perini1
Affiliation: 1 Università degli Studi di Genova, Dipartimento Architettura e Design (Italy) 2Università degli Studi di Milano, Dipartimento di Scienze Sociali e Politiche
Abstract: Urban heat island (UHI) is among the most critical issues caused by human activities and high built density. UHI has severe impacts on urban and natural environment and on citizen health and wellbeing. The research presented aims at evaluating the effects of nature-based solutions (NBS) in improving the livability of a district in the city of Genoa, heavily cemented and highly critical regarding the heat island phenomenon; the study focuses on both microclimatic benefits for urban heat island mitigation and psychological and perceptual aspects. A preliminary analysis of the district through CFD simulations (Envi-met software) allowed selecting the most suitable areas for a system of punctual interventions of urban regeneration using nature-based solutions. For each area identified, we simulated the effects of different design scenarios in terms of microclimate mitigation and thermal comfort improvement. In order to evaluate perceptive benefits of the most performing design scenarios, we set up a web-based survey which was filled by a convenience sample of Genoa residents. The preferred design outcomes, in terms of aesthetic satisfaction, perception of improved conditions of physical and psychological well-being, are the freer and more natural environment. The study shows that nature based solutions can improve the overall conditions of dense urban areas: microclimate performance and psycho¬logical effects should be both considered in the design process in order to improve citizens wellbeing.