This study quantitatively analyses the spatial relationship between land surface cover and urban surface temperature distribution among different land use classes in metropolitan Melbourne. This relationship was explored through a detailed estimate of fractional land cover at 30 m grid resolution using LiDAR data along with land surface temperature (LST) and modeled heat fluxes using Landsat TM5 data. In this study spatially distributed energy fluxes specifically ET, in an urban area were modeled based on an energy balance approach incorporating the surface roughness parameters derived from the LiDAR Data. This study shows a strong and significant linear relationship between LST and the percentage of different types of land cover. The relationship is positive for built-up areas and negative for vegetated areas. Such relationship quantitively depicts that with increasing total vegetation cover the LST decreases; in contrast LST increases linearly with increased imperviousness.

Cities are experiencing significant warming and more frequent climate extremes, raising risks for over 90 Our recommendations span four key areas: city descriptive datasets, integrated observations, fit for purpose models, and a coordinated community of research and practice. Urgent action is needed to tailor models to Australia's unique urban landscapes and climates. This requires comprehensive, nationally consistent, high resolution datasets that capture the form, fabric, and function of contemporary and future cities. It also requires filling systematic gaps in integrated networks of urban climate observations for evaluation and benchmarking. At the same time, scientific understanding of key urban processes that influence weather and climate must advance, alongside improvements in their representation in physical models. This can be achieved through a national community of research and practice that codesigns and oversees an implementation plan, integrated with infrastructure such as ACCESS NRI and AURIN. Building this capability will enable us to answer critical questions about the interaction between cities and climate, protecting Australias urban populations and ensuring a resilient future.

The process of urbanisation has increased public health risks due to urban heat, risks that will be further exacerbated in future decades by climate change. However, the growing adoption of integrated water management (IWM) practices (coordinated stormwater management of water, land, and resources) provides an opportunity to support urban heat amelioration through water supply provision and irrigated and vegetated infrastructure that can provide cooling benefits. This study examines the thermal impacts of future implementations of IWM for nine Australian cities based on a review of Government policy documents in the present and over two future time frames (2030 and 2050) under different greenhouse gas emission scenarios (SSPs 1.2-6, 3.7-0 and 5.8-5). Statistical analysis of the future climate data using historical data shows that future warming is nuanced, with changes variable in both time and place, and with extremes becoming more pronounced in future. We have developed a unique approach to morph the future climate projections onto historical data (derived from the ERA5 Reanalysis product) for the 2010-2020 period. Additionally, we use locally appropriate Local Climate Zones (LCZs) for Australian cities, resulting from a holistic and global approach that is widely adopted by the urban climate modelling community. We developed scenarios for business-as-usual as well as implementation of moderate and high levels of IWM across each of the Australian LCZs and modelled them using TARGET (The Air temperature Response to Green infrastructure Evaluation Tool). Results generated at the LCZ level are aggregated to Australian statistical areas (SA4, the largest sub-city area) and city-wide levels. The thermal impacts associated with the various degrees of IWM were marked and geographically differentiated, depending on the climatic characteristics of the various cities. For the current climate, high IWM intervention provided reductions in annual mean daily maximum temperature ranging from -0.77°C in Darwin, up to -1.86°C in Perth. Generally, the drier southern cities of Sydney, Canberra, Albury, Melbourne, Adelaide, and Perth produced the greatest thermal response to implementation of IWM and the more tropical cities with higher rainfalls the least response. For some southern cities cooling was > -3.0°C at the time of maximum summer temperatures. Interestingly high levels of IWM in winter produced modest warming of minimum overnight temperatures, especially for the cooler southern cities. The cooling benefits of IWM were seen across all future climate scenarios and are a real opportunity to offset-projected temperature increases resulting from climate change.

Climate change, rapid urbanisation, and ageing populations are reinforcing the need for urban heat mitigation techniques. Pavement watering is one such technique, where evaporative cooling is induced through wetting urban surfaces. The aim of this research is to assess the potential cooling benefits of pavement watering. To do this, a 10 x 10 m section of a car park was watered, and experiments were conducted at midday, the afternoon, and the evening across three days. Pavement watering was found to induce a mean cooling of up to 0.6 degrees C in air temperature and 2 degrees C in UTCI at 1.5 m. Benefits were related to prevailing conditions, with lower wind speeds associated with greater cooling. Surface temperature was also found to decrease by up to 9.0 degrees C, and the surface energy balance of the watered carpark was characteristic of a highly evaporative surface. However, there were limitations of the experiments; notably, the assumptions made to correct observations increased uncertainty, and the small scale of the experiment likely limited the observed cooling benefits. Despite this, pavement watering was shown to reduce air temperature and surface temperature, as well as improve thermal comfort, and thus may potentially be used in emergencies to provide cooling in urban areas.

Urban surface albedo is an essential biophysical variable in the surface energy balance across all scales, from micro-scale (materials) to the globe, changing with land covers and three-dimensional structures over urban areas. Urban albedos are dynamic over space and time but have not yet been quantified over global scales due to the lack of high-resolution albedo datasets. Here, we combined the direct estimation approach and Landsat surface reflectance product to generate a 30-m-resolution annual surface albedo dataset for 3037 large cities (area > 50 km(2)) worldwide for the period from 1986 to 2020, allowing spatial patterns and long-term temporal trends to be explored with possible causal drivers, and quantification of the surface radiative forcing from these albedo changes. Evaluation of this new albedo dataset using global urban flux tower-based measurements demonstrates its high accuracy with an overall bias and root-mean-square-error (RMSE) of 0.005 and 0.025, respectively. Analysis of the dataset reveals an overall decreasing trend of albedo during the 35-year evaluation period (1986-2020), which is robust accounting for uncertainties from training sample representativeness, Landsat data uncertainty, seasonal variation, and snow-cover contamination. Our results reveal that urban greening (measured by the positive Normalized Difference Vegetation Index (NDVI) trend) can well explain the total variances in the albedo trend for the 35-year period through two different pathways of tree planting and urban warming-enhanced vegetation growth. The decrease in urban albedo caused a warming effect indicated by positive surface radiative forcing, with a global city-level average surface radiative forcing of 2.76 W & sdot;m(- 2). These findings enhance our understanding of urbanization's impacts on albedo-related biophysical processes and can provide information to quantify urban surface radiation energy and design effective mitigation strategies to reduce urban warming.

Accurately predicting weather and climate in cities is critical for safeguarding human health and strengthening urban resilience. Multimodel evaluations can lead to model improvements; however, there have been no major intercomparisons of urban-focussed land surface models in over a decade. Here, in Phase 1 of the Urban-PLUMBER project, we evaluate the ability of 30 land surface models to simulate surface energy fluxes critical to atmospheric meteorological and air quality simulations. We establish minimum and upper performance expectations for participating models using simple information-limited models as benchmarks. Compared with the last major model intercomparison at the same site, we find broad improvement in the current cohort's predictions of short-wave radiation, sensible and latent heat fluxes, but little or no improvement in long-wave radiation and momentum fluxes. Models with a simple urban representation (e.g., 'slab' schemes) generally perform well, particularly when combined with sophisticated hydrological/vegetation models. Some mid-complexity models (e.g., 'canyon' schemes) also perform well, indicating efforts to integrate vegetation and hydrology processes have paid dividends. The most complex models that resolve three-dimensional interactions between buildings in general did not perform as well as other categories. However, these models also tended to have the simplest representations of hydrology and vegetation. Models without any urban representation (i.e., vegetation-only land surface models) performed poorly for latent heat fluxes, and reasonably for other energy fluxes at this suburban site. Our analysis identified widespread human errors in initial submissions that substantially affected model performances. Although significant efforts are applied to correct these errors, we conclude that human factors are likely to influence results in this (or any) model intercomparison, particularly where participating scientists have varying experience and first languages. These initial results are for one suburban site, and future phases of Urban-PLUMBER will evaluate models across 20 sites in different urban and regional climate zones.

A total of 20 urban neighbourhood-scale eddy covariance flux tower datasets are made openly available after being harmonized to create a 50 site–year collection with broad diversity in climate and urban surface characteristics. Variables needed as inputs for land surface models (incoming radiation, temperature, humidity, air pressure, wind and precipitation) are quality controlled, gap-filled and prepended with 10 years of reanalysis-derived local data, enabling an extended spin up to equilibrate models with local climate conditions. For both gap filling and spin up, ERA5 reanalysis meteorological data are bias corrected using tower-based observations, accounting for diurnal, seasonal and local urban effects not modelled in ERA5. The bias correction methods developed perform well compared to methods used in other datasets (e.g. WFDE5 or FLUXNET2015). Other variables (turbulent and upwelling radiation fluxes) are harmonized and quality controlled without gap filling. Site description metadata include local land cover fractions (buildings, roads, trees, grass etc.), building height and morphology, aerodynamic roughness estimates, population density and satellite imagery. This open collection can help extend our understanding of urban environmental processes through observational synthesis studies or in the evaluation of land surface environmental models in a wide range of urban settings. These data can be accessed from https://doi.org/10.5281/zenodo.7104984 (Lipson et al., 2022).

An increase in energy demands and positive public acceptance of clean energy resources have contributed to a growing need for using solar energy in cities. Solar photovoltaic (PV) deployment relies on suitable locations with high solar energy potential. In the urban context, building rooftops are often considered one of the most available locations for solar PV installation. This work demonstrates a new geospatial-method for spatiotemporal modeling and mapping solar energy potential based on a high-resolution (0.2 m) digital surface model (DSM) and solar radiation dataset. The proposed method identifies building rooftops with a high solar energy potential by using the Solar Analyst (SA) model. The results show that 93.5% of the rooftop area has high solar energy potential in the study area. The annual averaged sum of solar irradiation values is estimated to be 1.36 MWh/m 2 . In addition, the study showed that sloped rooftops facing to the north received up to 30% more incoming solar radiation than other rooftops with different geometry and orientation. The results are validated using recorded energy output data from four existing solar PV systems in the study area. The return on the initial investment of PV systems installation is estimated to be from four to five years.

Interconnected systems are what keep cities functioning. But climate change can undo one part, and unravel the whole system. Extreme heat takes more lives in Australia than any other natural hazard and, because of the high level of urbanisation, most of those deaths are in cities. But our settlements and the many devices that are […]

Backyards play important roles for individual households because they provide a private and safe green space for social and environmental interactions, relaxation, gardening and children's ac-tivities. The use of backyards is highly dependent on their thermal conditions. Turf is a common surface type in backyards but unirrigated turf can be as warm as pavement, bringing thermal discomfort and discouraging people from using them. Under certain conditions, turf irrigation provides an opportunity to reduce thermal stress by increasing evapotranspiration. This study aims to measure the impacts of turf irrigation on microclimate in a backyard environment in the warm season in Melbourne, Australia. The experiment consisted of four 6 m x 6 m turf-covered plots. Daily irrigation was applied at four amounts: 0, 2, 4 and 7 mm for six weeks. In Week 6, the 4-mm irrigation reduced daytime soil temperature, turf surface temperature, air temperature and universal thermal climate index by 1.7, 2.3, 0.6 and 0.4 degrees C, respectively. All daytime impacts were significant (p < 0.05, t-test). Irrigation has the potential to significantly improve the thermal conditions of backyards in combination with the use of tree shade.

A Correction to this paper has been published: https://doi.org/10.1038/s41597-021-00851-9.

The 2019/20 Black Summer bushfire disaster in southeast Australia was unprecedented: the extensive area of forest burnt, the radiative power of the fires, and the extraordinary number of fires that developed into extreme pyroconvective events were all unmatched in the historical record. Australia’s hottest and driest year on record, 2019, was characterised by exceptionally dry fuel loads that primed the landscape to burn when exposed to dangerous fire weather and ignition. The combination of climate variability and long-term climate trends generated the climate extremes experienced in 2019, and the compounding effects of two or more modes of climate variability in their fire-promoting phases (as occurred in 2019) has historically increased the chances of large forest fires occurring in southeast Australia. Palaeoclimate evidence also demonstrates that fire-promoting phases of tropical Pacific and Indian ocean variability are now unusually frequent compared with natural variability in pre-industrial times. Indicators of forest fire danger in southeast Australia have already emerged outside of the range of historical experience, suggesting that projections made more than a decade ago that increases in climate-driven fire risk would be detectable by 2020, have indeed eventuated. The multiple climate change contributors to fire risk in southeast Australia, as well as the observed non-linear escalation of fire extent and intensity, raise the likelihood that fire events may continue to rapidly intensify in the future. Improving local and national adaptation measures while also pursuing ambitious global climate change mitigation efforts would provide the best strategy for limiting further increases in fire risk in southeast Australia. Multiple climate contributors to fire risk in southeast Australia have led to an increase in fire extent and intensity over the past decades that will likely continue into the future, suggests a synthesis of climate variability, long-term trends and palaeoclimatic evidence.

This global study, which has been coordinated by the World Meteorological Organization Global Atmospheric Watch (WMO/GAW) programme, aims to understand the behaviour of key air pollutant species during the COVID-19 pandemic period of exceptionally low emissions across the globe. We investigated the effects of the differences in both emissions and regional and local meteorology in 2020 compared with the period 2015–2019. By adopting a globally consistent approach, this comprehensive observational analysis focuses on changes in air quality in and around cities across the globe for the following air pollutants PM2.5, PM10, PMC (coarse fraction of PM), NO2, SO2, NOx, CO, O3 and the total gaseous oxidant (OX = NO2 + O3) during the pre-lockdown, partial lockdown, full lockdown and two relaxation periods spanning from January to September 2020. The analysis is based on in situ ground-based air quality observations at over 540 traffic, background and rural stations, from 63 cities and covering 25 countries over seven geographical regions of the world. Anomalies in the air pollutant concentrations (increases or decreases during 2020 periods compared to equivalent 2015–2019 periods) were calculated and the possible effects of meteorological conditions were analysed by computing anomalies from ERA5 reanalyses and local observations for these periods. We observed a positive correlation between the reductions in NO2 and NOx concentrations and peoples’ mobility for most cities. A correlation between PMC and mobility changes was also seen for some Asian and South American cities. A clear signal was not observed for other pollutants, suggesting that sources besides vehicular emissions also substantially contributed to the change in air quality. As a global and regional overview of the changes in ambient concentrations of key air quality species, we observed decreases of up to about 70% in mean NO2 and between 30% and 40% in mean PM2.5 concentrations over 2020 full lockdown compared to the same period in 2015–2019. However, PM2.5 exhibited complex signals, even within the same region, with increases in some Spanish cities, attributed mainly to the long-range transport of African dust and/or biomass burning (corroborated with the analysis of NO2/CO ratio). Some Chinese cities showed similar increases in PM2.5 during the lockdown periods, but in this case, it was likely due to secondary PM formation. Changes in O3 concentrations were highly heterogeneous, with no overall change or small increases (as in the case of Europe), and positive anomalies of 25% and 30% in East Asia and South America, respectively, with Colombia showing the largest positive anomaly of ~70%. The SO2 anomalies were negative for 2020 compared to 2015–2019 (between ~25 to 60%) for all regions. For CO, negative anomalies were observed for all regions with the largest decrease for South America of up to ~40%. The NO2/CO ratio indicated that specific sites (such as those in Spanish cities) were affected by biomass burning plumes, which outweighed the NO2 decrease due to the general reduction in mobility (ratio of ~60%). Analysis of the total oxidant (OX = NO2 + O3) showed that primary NO2 emissions at urban locations were greater than the O3 production, whereas at background sites, OX was mostly driven by the regional contributions rather than local NO2 and O3 concentrations. The present study clearly highlights the importance of meteorology and episodic contributions (e.g., from dust, domestic, agricultural biomass burning and crop fertilizing) when analysing air quality in and around cities even during large emissions reductions. There is still the need to better understand how the chemical responses of secondary pollutants to emission change under complex meteorological conditions, along with climate change and socio-economic drivers may affect future air quality. The implications for regional and global policies are also significant, as our study clearly indicates that PM2.5 concentrations would not likely meet the World Health Organization guidelines in many parts of the world, despite the drastic reductions in mobility. Consequently, revisions of air quality regulation (e.g., the Gothenburg Protocol) with more ambitious targets that are specific to the different regions of the world may well be required.

ABSTRACTLife on Earth vitally depends on the availability of water. Human pressure on freshwater resources is increasing, as is human exposure to weather-related extremes (droughts, storms, floods) caused by climate change. Understanding these changes is pivotal for developing mitigation and adaptation strategies. The Global Climate Observing System (GCOS) defines a suite of essential climate variables (ECVs), many related to the water cycle, required to systematically monitor Earth’s climate system. Since long-term observations of these ECVs are derived from different observation techniques, platforms, instruments, and retrieval algorithms, they often lack the accuracy, completeness, and resolution, to consistently characterize water cycle variability at multiple spatial and temporal scales. Here, we review the capability of ground-based and remotely sensed observations of water cycle ECVs to consistently observe the hydrological cycle. We evaluate the relevant land, atmosphere, and ocean water storages and the fluxes between them, including anthropogenic water use. Particularly, we assess how well they close on multiple temporal and spatial scales. On this basis, we discuss gaps in observation systems and formulate guidelines for future water cycle observation strategies. We conclude that, while long-term water cycle monitoring has greatly advanced in the past, many observational gaps still need to be overcome to close the water budget and enable a comprehensive and consistent assessment across scales. Trends in water cycle components can only be observed with great uncertainty, mainly due to insufficient length and homogeneity. An advanced closure of the water cycle requires improved model–data synthesis capabilities, particularly at regional to local scales.

Increasing urban green spaces and canopy cover requires careful planning of irrigation strategies, especially in arid and semiarid areas. This study investigates how vegetation cover and irrigation affect the water balance and vegetation productivity of a small urban reserve in the Melbourne metropolitan area, Australia. Using a mechanistic ecohydrological model, a series of numerical experiments were carried out for the period 1999–2018, which included a prolonged drought. Results indicated that irrigation played an essential role in helping both trees and grass productivity by increasing soil moisture and vegetation water access during the drought. With 10% tree cover, grass benefitted more than trees by increasing irrigation, and trees coped well with drought even without additional water. However, trees strongly relied on irrigation to maintain productivity when tree cover increased, highlighting the need for a sustainable balance between increasing urban greening and water conservation. Differences in soil properties and rooting strategies were also found to strongly modify the need for irrigation and the competition for water. These results provide quantitative insights on how increasing tree cover and vegetation diversity may impact irrigation requirements, highlighting the key role of mechanistic numerical models to support urban planners in the evaluation and design of urban green spaces.

Reducing night-time air temperature by irrigating urban greenery can ameliorate heat stress in cities. Past studies often used modelling approaches to investigate the irrigation cooling effect during heatwaves. However, few observational studies have compared the differences in micro-scale irrigation cooling effect between heatwave and non-heatwave conditions. This study examines whether nocturnal irrigation cooling effects intensify during heatwaves, as there have been conflicting results in the literature regarding this issue. During the Australian summer of December 2013 to March 2014, this study was conducted in the Royal Botanic Gardens Victoria's Melbourne Gardens (mixture of Australian and exotic vegetation) and in the Australian Gardens of its Cran-bourne Gardens (specialises in Australian vegetation). We used the excess heat factor (EHF) to identify one severe heatwave (EHF = 34.9) and one extreme heatwave (EHF = 155.6) in our study period. Using meteorological observations, irrigation data and one-way ANOVA, we present the differences in cooling associated with irrigation between different types of landscape in the two gardens. During non-heatwave conditions, there was 0.5 degrees C - 1 degrees C of cooling associated with overnight irrigation of 20-25 mm. During heatwave conditions, several lawn areas recorded greater cooling (2 degrees C - 4 degrees C) associated with irrigation of similar amounts, but this phenomenon was not observed in areas dominated by hard surfaces. The magnitude and timing of the irrigation cooling effects also differed in various garden sites, with residual cooling effects observed lasting for several hours in certain sites following nocturnal irrigation events. Our results suggest that some significant cooling at the micro-scale is apparent from irrigated landscape in Melbourne, which has a temperate oceanic climate with occasional heat waves. Moreover, the irrigation cooling effect in the Melbourne Gardens intensifies during heatwaves. Our findings could inform landscape succession planning to prepare for a hotter future climate.

Urban parks are important within cities and towns for mitigating high air temperatures and improving human thermal comfort, especially during hot summer conditions. We demonstrate this benefit through an analysis of the climatic interactions between a relatively small (1.5 Ha), inner-city park and its surrounding urban environment in Melbourne, Australia, during the hot Austral summer of 2013-14. On average, the park was cooler than its surrounding built-up area at all times and in all weather conditions. The park's mean maximum cooling reached 1.0 degrees C during peak daytime heating (15:00), with the magnitude of difference between the park and its surroundings varying from 0.5-3 degrees C. During the day, the magnitude of park cooling was greater in sunny conditions and under higher wind speeds in the park (relative to the surrounding streets). However at night, park cooling was less variable and dominantly influenced by the characteristics of the site such as Sky View Factor (SVF), vegetation distribution and irrigation. The results showed a downwind propagation of the cooling effect of the park that extended up to one half a park width away from the park. Tree shading and evapotranspiration in the park could reduce the level of heat stress from strong in the nearby streets, to comfortable within the park.

Groundwater can have a critical role in sustaining the functioning of natural ecosystems during droughts, especially in dry and seasonally dry climates. However, the response to droughts of ecosystems embedded in urban areas is not well known. This study investigates how different scenarios of groundwater availability control the water balance and vegetation productivity of two urban reserves hosting native vegetation in the Melbourne metropolitan area, Australia. Using a mechanistic ecohydrological model supported by field observations, long‐term simulations were run to explore the impact of groundwater flow on water, carbon, and energy fluxes under present climatic conditions, including the Millennium Drought (2001–2009), and in response to perturbations in key environmental variables (air temperature, atmospheric CO 2 concentrations, and rainfall). It was found that the presence of a water table and its capillary fringe within the root depths supports ecosystem transpiration and vegetation productivity. The effects of declining groundwater were found to be more severe in predominantly sandy soils because of the lower water holding capacity, identifying that the water status of vegetation differs significantly depending on soil type. Differences in rooting strategies and groundwater availability also had a pivotal role in helping plants soften the impacts of increased air temperature ( T a ) and make use of higher atmospheric CO 2 concentrations. Increased T a strongly affected evapotranspiration, enhancing the competition for water between different vegetation types. These results provide quantitative insights of how vegetation responds to groundwater depletion and climate variability, highlighting the essential role of groundwater resources in urban ecosystems characterized by seasonally dry climates.