Arctic amplification (AA) – referring to the enhancement of near-surface air temperature change over the Arctic relative to lower latitudes – is a prominent feature of climate change with important impacts on human and natural systems. In this review, we synthesize current understanding of the underlying physical mechanisms that can give rise to AA. These mechanisms include both local feedbacks and changes in poleward energy transport. Temperature and sea ice-related feedbacks are especially important for AA, since they are significantly more positive over the Arctic than at lower latitudes. Changes in energy transport by the atmosphere and ocean can also contribute to AA. These energy transport changes are tightly coupled with local feedbacks, and thus their respective contributions to AA should not be considered in isolation. It is here emphasized that the feedbacks and energy transport changes that give rise to AA are sensitively dependent on the state of the climate system itself. This implies that changes in the climate state will lead to changes in the strength of AA, with implications for past and future climate change.

Arctic amplification of climate change: a review of underlying mechanisms

Infrastructure-based heat reduction strategies can help cities adapt to high temperatures, but simulations of their cooling potential yield widely varying predictions. We systematically review 146 studies from 1987-2017 that conduct physically based numerical modelling of urban air temperature reduction resulting from green-blue infrastructure and reflective materials. Studies are grouped into two modelling scales: neighbourhood scale, building-resolving (i.e., microscale); and city scale, neighbourhood-resolving (i.e., mesoscale). Street tree cooling has primarily been assessed at the microscale, whereas mesoscale modelling has favoured reflective roof treatments, which are attributed to model physics limitations at each scale. We develop 25 criteria to assess contextualization and reliability of each study based on metadata reporting and methodological quality, respectively. Studies have shortcomings with respect to neighbourhood characterization, reporting areal coverages of heat mitigation implementations, evaluation of base case simulations, and evaluation of modelled physical processes relevant to heat reduction. To aid comparison among studies, we introduce two metrics: the Albedo Cooling Effectiveness (ACE), and the Vegetation Cooling Effectiveness (VCE). A sub-sample of 47 higher quality studies suggests that high reflectivity coatings or materials offer ≈0.2-0.6 °C cooling per 0.10 neighbourhood albedo increase, and that trees yield ≈0.3 °C cooling per 0.10 canopy cover increase, for afternoon clear-sky summer conditions. VCE of low vegetation and green roofs varies more strongly between studies. Both ACE and VCE exhibit a striking dependence on model choice and model scale, particularly for albedo and roof-level implementations, suggesting that much of the variation of cooling magnitudes between studies may be attributed to model physics representation. We conclude that evaluation of the base case simulation is not a sufficient prerequisite for accurate simulation of heat mitigation strategy cooling. We identify a three-phase framework for assessment of the suitability of a numerical model for a heat mitigation experiment, which emphasizes assessment of urban canopy layer mixing and of the physical processes associated with the heat reduction implementation. Based on our findings, we include recommendations for optimal design and communication of urban heat mitigation simulation studies.

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Cooling hot cities: a systematic and critical review of the numerical modelling literature

Processes in the climate system that can either amplify or dampen the climate response to an external perturbation are referred to as climate feedbacks. Climate sensitivity estimates depend critically on radiative feedbacks associated with water vapor, lapse rate, clouds, snow, and sea ice, and global estimates of these feedbacks differ among general circulation models. By reviewing recent observational, numerical, and theoretical studies, this paper shows that there has been progress since the Third Assessment Report of the Intergovernmental Panel on Climate Change in (i) the understanding of the physical mechanisms involved in these feedbacks, (ii) the interpretation of intermodel differences in global estimates of these feedbacks, and (iii) the development of methodologies of evaluation of these feedbacks ( or of some components) using observations. This suggests that continuing developments in climate feedback research will progressively help make it possible to constrain the GCMs' range of climate feedbacks and climate sensitivity through an ensemble of diagnostics based on physical understanding and observations.

How well do we understand and evaluate climate change feedback processes

Urban environments lie at the confluence of social, cultural, and economic activities and have unique biophysical characteristics due to continued infrastructure development that generally replaces natural landscapes with built-up structures. The vast majority of studies on urban perturbation of local weather and climate have been centered on the urban heat island (UHI) effect, referring to the higher temperature in cities compared to their natural surroundings. Besides the UHI effect and heat waves, urbanization also impacts atmospheric moisture, wind, boundary layer structure, cloud formation, dispersion of air pollutants, precipitation, and storms. In this review article, we first introduce the datasets and methods used in studying urban areas and their impacts through both observation and modeling and then summarize the scientific insights on the impact of urbanization on various aspects of regional climate and extreme weather based on more than 500 studies. We also highlight the major research gaps and challenges in our understanding of the impacts of urbanization and provide our perspective and recommendations for future research priorities and directions.城市环境位于社会、文化和经济活动的结合点,城市下垫面具有独特的生物物理特征。在城市化过程中持续的基础设施建设导致自然景观被建筑物替代。在过去很长时间,绝大部分城市影响天气气候的研究和城市热岛效应(城市及上空温度高于周边地区)有关。除了城市热岛效应,城市化也影响大气湿度、风、边界层结构、云的形成、污染物扩散、降水和暴雨。在这篇综述文章中,我们阅览了超过五百篇文献,从观测和模拟两个方面,首先介绍了用于城市化及影响研究的数据资料和方法,总结了城市化影响区域气候和极端天气的各个领域的科学要点。我们也例举了在理解城市化影响方面目前存在的主要问题和挑战,提出了相应的未来研究重点和方向。. 

/pdf/urbanization-impact-on-regional-climate-and-extreme-weather-1v8dakqv.pdf

Urbanization Impact on Regional Climate and Extreme Weather: Current Understanding, Uncertainties, and Future Research Directions

Urban heat island (UHI) effect poses great challenges to urban environmental quality, building energy consumption, and public health. Over decades, research efforts have been made to develop and evaluate different UHI mitigation strategies. Governments, organizations, and commercial companies have also been working to develop building codes, standards, and green rating systems to improve energy efficiency and urban sustainability. However, a lack of synergies between the research community, governments, organizations, and commercial companies may create knowledge gaps that can become a considerable obstacle to the implementation and effectiveness of UHI mitigation strategies. This study aims to synthetically review recent research advances, building codes and green rating systems, and urban implementation projects related to the use of cool pavements for UHI mitigation. In particular, we review efforts on reflective pavements, permeable pavements, and other innovative pavement strategies. Based on this synthetic review, we identify six gaps in existing cool pavement research and five gaps related to the implementation of cool pavements in building codes, standards, and municipal projects. We further provide recommendations and suggestions for each gap. Further efforts are needed to bridge these gaps, which will be critical to proactively coping with the negative impacts of the UHI effect and promoting more sustainable urban energy use.

Cool pavements for urban heat island mitigation: A synthetic review

Cities worldwide are piloting the use of solar reflective coatings on roads to mitigate the Urban Heat Island (UHI). Solar reflective pavement has been praised as simple, low-cost solution with a demonstrated ability to reduce surface temperature and, potentially, energy use. Less well understood is the effect of solar reflective coatings on radiant heat, which influences human thermal exposure and comfort. We present the first biometeorological observations of solar reflective coating to investigate its thermal performance from a pedestrian perspective. Hourly transects were conducted in two Los Angeles neighborhoods with MaRTy, a mobile platform that measures air, surface, and mean radiant temperature. Transects were performed on July 30, 2019, a typical summer day with low wind speeds and maximum air temperature of 31°C. The surface temperature of the coated asphalt concrete was 4°C to 6°C lower than that of the regular asphalt concrete, but coated surfaces reflected 118 Wm-2 more shortwave radiation on average and up to 168 Wm-2 more at noon. In the evening, MaRTy observed 20 to 30 Wm-2 of added reflected shortwave radiation on sidewalks next to the reflective pavement. Mean radiant temperature over reflective pavement was 4°C higher during midday. Although air temperature was reduced by 0.5°C in the afternoon, after-sunset cooling was negligible. Findings illustrate the benefits and disadvantages of reflective pavement with respect to various thermal performance metrics. Cities should weigh the tradeoffs of UHI mitigation, thermal exposure, implementation and maintenance costs, lifecycle, and other competing priorities in the context of space use.

https://iopscience.iop.org/article/10.1088/1748-9326/ab87d4/pdf

Solar reflective pavements-A policy panacea to heat mitigation?

Cities increasingly recognize the importance of shade to reduce heat stress and adopt urban forestry plans with ambitious canopy goals. Yet, the implementation of tree and shade plans often faces maintenance, water use, and infrastructure challenges. Understanding the performance of natural and non-natural shade is critical to support active shade management in the built environment. We conducted hourly transects in Tempe, Arizona with the mobile human-biometeorological station MaRTy on hot summer days to quantify the efficacy of various shade types. We sampled sun-exposed reference locations and shade types grouped by urban form, lightweight/engineered shade, and tree species over multiple ground surfaces. We investigated shade performance during the day, at peak incoming solar, peak air temperature, and after sunset using three thermal metrics: the difference between a shaded and sun-exposed location in air temperature (ΔTa), surface temperature (ΔTs), and mean radiant temperature (ΔTMRT). ΔTa did not vary significantly between shade groups, but ΔTMRT spanned a 50°C range across observations. At daytime, shade from urban form most effectively reduced Ts and TMRT, followed by trees and lightweight structures. Shade from urban form performed differently with changing orientation. Tree shade performance varied widely; native and palm trees were least effective, while non-native trees were most effective. All shade types exhibited heat retention (positive ΔTMRT) after sunset. Based on the observations, we developed characteristic shade performance curves that will inform the City of Tempe’s design guidelines towards using “the right shade in the right place” and form the basis for the development of microclimate zones (MCSz).

50 Grades of Shade

Climate feedbacks have been found to strongly impact the observed amplified Arctic warming. However, Arctic amplification is modeled with a wide spread which partly arises from intermodel differenc...

Climate models disagree on the sign of total radiative feedback in the Arctic

ABSTRACT Regional land surface temperature (LST) maps derived from remote sensing data are most available to cities to assess and respond to heat. Yet, LST only captures one dimension of urban climate. This study investigates the extent to which remote sensing derived estimates of LST are a proxy for multiple climate variables at hyper-local scales (<10s of meters). We compare remotely sensed estimates of LST (RS-LST) to field and simulated LST, MRT, and air temperature (AT), in a neighborhood in Tucson, Arizona, USA. We find that LST, MRT, and ST follow different diurnal trends masked by RS-LST. We also find that three-dimensional urban design is a better predictor of MRT than two-dimensional land cover and albedo – a known determinant of RS-LST. Shade is a better predictor of both simulated LST and MRT than RS-LST. We conclude that RS-LST is not adequate for guiding heat mitigation at hyper-local scales in cities.

https://www.tandfonline.com/doi/pdf/10.1080/1747423X.2021.2015003?needAccess=true

More than surface temperature: mitigating thermal exposure in hyper-local land system

Urban overheating is an increasing threat to people, infrastructure, and the environment. Common heat mitigation strategies, such as green infrastructure, confront space limitations in current car-centric cities. In 2020, the City of Phoenix, Arizona, piloted a "cool pavement" program using a solar reflective pavement seal on 58 km of residential streets. Comprehensive micrometeorological observations are used to evaluate the cooling potential of the reflective pavement based on three heat exposure metrics-surface, air, and mean radiant temperatures-across three residential reflective pavement-treated and untreated neighborhoods. In addition, the solar reflectivity of reflective pavement is observed over 7 months across eight residential neighborhoods. Results are synthesized with the literature to provide context-based reflective pavement implementation guidelines to mitigate urban overheating where common strategies cannot be applied. The three most important contextual factors to consider for effective implementation include urban location, background climate type, and heat exposure metric of interest. 

https://www.nature.com/articles/s41467-023-36972-5.pdf

Florian Arwed Schneider

Papers

Solar reflective pavements-A policy panacea to heat mitigation?

50 Grades of Shade

Climate models disagree on the sign of total radiative feedback in the Arctic

More than surface temperature: mitigating thermal exposure in hyper-local land system

Evidence-based guidance on reflective pavement for urban heat mitigation in Arizona