Urban Climates and Climate Change Masson, Valéry; Lemonsu, Aude; Hidalgo, Julia ...
Annual review of environment and resources,
10/2020, Letnik:
45, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Cities are particularly vulnerable to extreme weather episodes, which are expected to increase with climate change. Cities also influence their own local climate, for example, through the relative ...warming known as the urban heat island (UHI) effect. This review discusses urban climate features (even in complex terrain) and processes. We then present state-of-the-art methodologies on the generalization of a common urban neighborhood classification for UHI studies, as well as recent developments in observation systems and crowdsourcing approaches. We discuss new modeling paradigms pertinent to climate impact studies, with a focus on building energetics and urban vegetation. In combination with regional climate modeling, new methods benefit the variety of climate scenarios and models to provide pertinent information at urban scale. Finally, this article presents how recent research in urban climatology contributes to the global agenda on cities and climate change.
Abstract
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 to 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 °C–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.
Complex urban environments with diverse urban canyon structures and building materials mixed with human-modified green space substantially influence the directional variation in upwelling thermal ...radiance. The thermal anisotropy at a satellite pixel scale (e.g. 1km) is hard to quantify with current techniques through airborne observations and modeling. The various anisotropic features over urban and rural surfaces lead to an uncertainty regarding how this anisotropy of land surface temperature (LST) influences estimates of the surface urban heat island (SUHI). We quantify LST anisotropy over Chicago and New York using MODIS LST products during the warm months (May-September) for a 10-year period. The impacts of atmospheric attenuation and daily weather variability on LST are removed in order to isolate the anisotropic signal. We then calculate the anisotropy for different levels of urban land use intensity. The daytime maximum anisotropic effects can be up to 9K for the most urbanized areas, while the nighttime anisotropic effects are weaker but still discernible. Two anisotropic “hot spot” features are observed during daytime: one corresponds to the sensor-sun geometric effect with a higher proportion of sunlit surface components been seen a given view configuration, and the other occurs when sensor zenith angles are near nadir in the afternoon. Similar anisotropic features between the two cities confirm the broader applicability of our approach, while some unique features specific to each city emphasize that diverse urban surface properties and geographic settings can affect anisotropy. This study is the first of its kind to directly evaluate the influence of directional anisotropy on satellite-retrieved SUHI measurements, demonstrating that anisotropic effects can possibly modify the SUHI as measured with MODIS LST by 2.3K, which is about 25–50% of the total magnitude of the SUHI over Chicago and New York. Our findings provide a statistical basis for the quantification of satellite-retrieved LST anisotropy over heterogeneous urban environments globally.
•We quantify LST anisotropy using MODIS LST in May-Sept of 2003–2012.•A method is developed to isolate anisotropic effects on LST.•Two anisotropic “hot-spot” features are observed during daytime.•The daytime maximum anisotropic effects can be up to 9K in urban areas.•Anisotropic effects can possibly modify the SUHI as measured with MODIS LST by 2.3K.
With the expected increase in warmer conditions caused by climate change, heat-related illnesses are becoming a more pressing issue. One way that humans can protect themselves from this is to seek ...shade. The design of urban spaces can provide individuals with a variety of ways to obtain this shade. The objective of this study was to perform a detailed evaluation and comparison of three shading strategies that could be used in an urban environment: shade from a building, from a tree, and from an umbrella. This was done through using field measurements to calculate the impact of each strategy on a thermal comfort index (Comfort Formula (COMFA)) in two urban settings during sunny days of the summer of 2013 and 2014 in London, Canada. Building shade was found to be the most effective cooling strategy, followed by the tree strategy and the umbrella strategy. As expected, the main determinant of this ranking was a strategy’s ability to block incoming shortwave radiation. Further analysis indicated that changes in the convective loss of energy and in longwave radiation absorption had a smaller impact that caused variations in the strategy effectiveness between settings. This suggests that under non-sunny days, these rankings could change.
Assessment of surface urban heat islands (SUHI) has been hampered by the lack of a consistent framework to permit consistent interpretation between cities. Local Climate Zones (LCZ) are a universal ...description of local scale landscape types based on expected variation at neighbourhood scale (≥1km2) in and around cities. In this study, we investigate the suitability of the LCZ scheme for SUHI studies based on 50 cities from across the globe. For comparability we use an annual temperature cycle model for MODIS land surface temperature (LST) at different overpass times and multi-year mean Landsat 8 LST. The SUHI analysis shows significant differences in the intra-urban estimate of SUHI for different built LCZ types. Substantial variability of SUHI within LCZ classes and between cities exists and SUHI patterns vary by time of day. Landsat derived estimates have very high correlations to those from MODIS at a similar time. The use of an LCZ approach combined with annual SUHI estimates provides a promising approach for a consistent and comprehensive SUHI analysis framework subject to further work to assess the spatial scale of matching LST and LCZ data, filter for topographic effects, and include the phenological status.
•A new approach for SUHI studies was presented and tested for a large number of cities•It allows consistent and comprehensive SUHI analysis based on LST time series and the LCZ scheme (WUDAPT L0 data)•· Significant LST differences between built and natural LCZs and within built types support the suitability of WUDAPT L0 data for SUHI analysis•The use of multi-year time series or ATC models is strongly recommended for further SUHI studies•There was also substantial variation in the SUHI patterns between cities and some questions remain to be solved
Surface temperature is a key variable in boundary-layer meteorology and is typically acquired by remote observation of emitted thermal radiation. However, the three-dimensional structure of cities ...complicates matters: uneven solar heating of urban facets produces an “effective anisotropy” of surface thermal emission at the neighbourhood scale. Remotely-sensed urban surface temperature varies with sensor view angle as a consequence. The authors combine a microscale urban surface temperature model with a thermal remote sensing model to predict the effective anisotropy of simplified neighbourhood configurations. The former model provides detailed surface temperature distributions for a range of “urban” forms, and the remote sensing model computes aggregate temperatures for multiple view angles. The combined model’s ability to reproduce observed anisotropy is evaluated against measurements from a neighbourhood in Vancouver, Canada. As in previous modeling studies, anisotropy is underestimated. Addition of moderate coverages of small (sub-facet scale) structure can account for much of the missing anisotropy. Subsequently, over 1900 sensitivity simulations are performed with the model combination, and the dependence of daytime effective thermal anisotropy on diurnal solar path (i.e., latitude and time of day) and blunt neighbourhood form is assessed. The range of effective anisotropy, as well as the maximum difference from nadir-observed brightness temperature, peak for moderate building-height-to-spacing ratios (H/W), and scale with canyon (between-building) area; dispersed high-rise urban forms generate maximum anisotropy. Maximum anisotropy increases with solar elevation and scales with shortwave irradiance. Moreover, it depends linearly on H/W for H/W < 1.25, with a slope that depends on maximum off-nadir sensor angle. Decreasing minimum brightness temperature is primarily responsible for this linear growth of maximum anisotropy. These results allow first order estimation of the minimum effective anisotropy magnitude of urban neighbourhoods as a function of building-height-to-spacing ratio, building plan area density, and shortwave irradiance. Finally, four “local climate zones” are simulated at two latitudes. Removal of neighbourhood street orientation regularity for these zones decreases maximum anisotropy by 3%–31%. Furthermore, thermal and radiative material properties are a weaker predictor of anisotropy than neighbourhood morphology. This study is the first systematic evaluation of effective anisotropy magnitude and causation for urban landscapes.
The directional variation in upwelling thermal radiance (referred to as "thermal anisotropy") is one of the important issues in retrieving representative urban land surface temperature (LST) from ...remote sensing data. Yet, there is a gap between urban thermal anisotropy assessed from different observational and modeling approaches at different time scales. Thermal anisotropy at the satellite observing scale (~1 km) is usually derived from multiangular thermal observations that are a result of temporal aggregation over long time series. How well this type of anisotropy estimate represents the instantaneous thermal anisotropy (e.g., from airborne and some modeling techniques) at the satellite scale remains unclear. To close this knowledge gap, we comprehensively compare Moderate-Resolution Imaging Spectroradiometer (MODIS)-derived anisotropy and quasi-simultaneous airborne observations in urban areas and assess the factors that control anisotropic features of LST at seasonal and diurnal scales. Furthermore, we use model simulations to separately assess the impacts of 1) weather variability on anisotropy and 2) solar angle variation on zenithal variation in anisotropy, both of which have been largely simplified in the MODIS-derived approach. The instantaneous model-derived or airborne-measured anisotropy in early morning (~10:00) and late afternoon (~14:00) is closer to the seasonally aggregated MODIS-derived anisotropy from Terra-Day (with a median overpass time of 11:00) and Aqua-Day (13:00), respectively, whereas it has a good representation during winter. The investigation of the diurnal and seasonal characteristic of MODIS-derived anisotropy is a critical step needed to understand its performance for global urban thermal anisotropy profiles.
Urbanization modifies surface energy and water budgets, and has significant impacts on local and regional hydroclimate. In recent decades, a number of urban canopy models have been developed and ...implemented into the Weather Research and Forecasting (WRF) model to capture urban land-surface processes. Most of these models are inadequate due to the lack of realistic representation of urban hydrological processes. Here, we implement physically-based parametrizations of urban hydrological processes into the single layer urban canopy model in the WRF model. The new single-layer urban canopy model features the integration of, (1) anthropogenic latent heat, (2) urban irrigation, (3) evaporation from paved surfaces, and (4) the urban oasis effect. The new WRF–urban modelling system is evaluated against field measurements for four different cities; results show that the model performance is substantially improved as compared to the current schemes, especially for latent heat flux. In particular, to evaluate the performance of green roofs as an urban heat island mitigation strategy, we integrate in the urban canopy model a multilayer green roof system, enabled by the physical urban hydrological schemes. Simulations show that green roofs are capable of reducing surface temperature and sensible heat flux as well as enhancing building energy efficiency.
The anisotropy of directional radiative surface temperature measurements over urban building surfaces is expected to have strong temporal variation. However, a model with the capability to simulate ...the temporal variation of urban thermal anisotropy (UTA) and with inversion abilities has not yet been developed. In this paper, the relationship between the temporal variation of UTA and temperature contrasts among surface components is investigated. Based on a system of previously developed geometric models for simulation of thermal anisotropy in simplified urban neighborhoods of different densities, a system of advanced geometric models, GUTA-T, is proposed that incorporates the temporal variability of UTA through the dependence of temperature contrasts on solar zenith angle θs under clear skies. A 3-D microscale urban surface temperature model and a sensor view model are combined to generate a synthetic UTA dataset for a variety of simplified urban geometries and with which to evaluate GUTA-T. The results show that parameter inversions are sensitive to sun-surface-sensor geometries. The seasonal and hourly behavior of anisotropy is simulated by GUTA-T with mean absolute differences of 1.06 °C, 1.25 °C, 1.69 °C and 1.04 °C for the view zenith and azimuth angles (θv, φv) of (40°, 90°), (60°, 270°), (60°, 180°) and (40°, 130°), respectively. Evaluation using diurnal measurements over an urban scale model shows that the root mean square difference between simulated anisotropy and the reference anisotropy generated from observed directional temperatures and a high-accuracy computer model is 2.02 °C for θs ≤ 60°. If the proposed model is to be calibrated with satellite data, a second directional observation from the opposite side of the surface target area can improve model performance significantly. Calibrations using samples including only small (e.g., θs ≤ 30°) or large θs (e.g., θs ≥ 50°) tend to decrease the anisotropy time-series simulation accuracy. Due to its inversion abilities, the GUTA-T model has the potential to simulate and correct anisotropy in time-series thermal infrared remote sensing data, thereby improving the analysis either of spatial variation within an image, or of temporal variation of angular urban surface temperature.
•The GUTA-T system of models is proposed to assess urban thermal anisotropy (UTA).•Temperature differences between surface components drive temporal variation of UTA.•GUTA-T successfully simulates the seasonal and hourly behavior of anisotropy.•GUTA-T is able to simulate diurnal anisotropy measured over an urban scale model.
PDF
