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Andrew M. Coutts
,
Jason Beringer
, and
Nigel J. Tapper

Abstract

Variations in urban surface characteristics are known to alter the local climate through modification of land surface processes that influence the surface energy balance and boundary layer and lead to distinct urban climates. In Melbourne, Australia, urban densities are planned to increase under a new strategic urban plan. Using the eddy covariance technique, this study aimed to determine the impact of increasing housing density on the surface energy balance and to investigate the relationship to Melbourne’s local climate. Across four sites of increasing housing density and varying land surface characteristics (three urban and one rural), it was found that the partitioning of available energy was similar at all three urban sites. Bowen ratios were consistently greater than 1 throughout the year at the urban sites (often as high as 5) and were higher than the rural site (less than 1) because of reduced evapotranspiration. The greatest difference among sites was seen in urban heat storage, which was influenced by urban canopy complexity, albedo, and thermal admittance. Resulting daily surface temperatures were therefore different among the urban sites, yet differences in above-canopy daytime air temperatures were small because of similar energy partitioning and efficient mixing. However, greater nocturnal temperatures were observed with increasing density as a result of variations in heat storage release that are in part due to urban canyon morphology. Knowledge of the surface energy balance is imperative for urban planning schemes because there is a possibility for manipulation of land surface characteristics for improved urban climates.

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Stephanie J. Jacobs
,
Ailie J. E. Gallant
, and
Nigel J. Tapper

Abstract

The sensitivity of near-surface urban meteorological conditions to three different soil moisture initialization experiments under heat-wave conditions is investigated for the city of Melbourne, Australia. The Weather Research and Forecasting Model is used to simulate a domain over Melbourne and its surrounding rural areas. The experiments employ three suites of simulations. Two suites initialize the model with soil moisture from the top layer of the ERA-Interim soil moisture data with a 3-month and 24-h coupled spinup period, respectively. The third suite initializes the model with the arguably more realistic soil moistures from the Australian Water Availability Project (AWAP), which are an order of magnitude drier than the ERA-Interim data, again using a 24-h spinup period. The simulations employing the AWAP data are found to have smaller errors when compared with observations, with biases in urban maximum temperature reduced by 4.1°C and biases in the skin temperature reduced by 3.0°C relative to the biases of the 3-month-spinup experiment. Despite urban areas only having a small proportion of soil-covered surfaces, the results show that urban soils have a greater influence on urban near-surface temperatures at night, whereas rural soils have a greater influence on urban near-surface temperatures during the daytime.

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Roger K. Smith
,
Michael J. Redder
,
Nigel J. Tapper
, and
Douglas R. Christie

Abstract

This paper presents an observational study of the structure and behavior of cold fronts over central Australia during the late dry season, a time of year when the prefrontal convectively well-mixed layer is particularly deep. The study is based on the results of the Central Australian Fronts Experiments (CAFE) held in 1991. Three fronts were documented in unprecedented detail for the Australian region using a greatly enhanced surface-observing network and a boundary layer wind profiler, as well as serial upper-air soundings. Data on the surface energy balance were obtained also.

A common feature of the fronts observed during CAFE was that they were dry. shallow (∼1 km deep), and moved into a deep (∼4 km) convectively well-mixed boundary layer. One of them initiated major dust storms across central Australia. A prominent feature of the fronts was the marked diurnal variation of their surface signature as they moved through the network. Noteworthy was the tendency during the night for the initiation in the accompanying trough of undular borelike structures or other nonlinear wave disturbances that moved ahead of the main airmass change. One well-documented case illustrates the generation of a southerly morning glory bore wave in the southern Gulf of Carpentaria region. During the daytime, fronts over central Australia are difficult to locate from surface data and appear to decelerate and to weaken greatly. Some dynamical aspects of the observations are discussed.

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Stephanie J. Jacobs
,
Ailie J. E. Gallant
,
Nigel J. Tapper
, and
Dan Li

Abstract

The ability of cool roofs and vegetation to reduce urban temperatures and improve human thermal stress during heat wave conditions is investigated for the city of Melbourne, Australia. The Weather Research and Forecasting Model coupled to the Princeton Urban Canopy Model is employed to simulate 11 scenarios of cool roof uptake across the city, increased vegetation cover across the city, and a combination of these strategies. Cool roofs reduce urban temperatures during the day, and, if they are installed across enough rooftops, their cooling effect extends to the night. In contrast, increasing vegetation coverage reduces nighttime temperatures but results in minimal cooling during the hottest part of the day. The combination of cool roofs and increased vegetation scenarios creates the largest reduction in temperature throughout the heat wave, although the relationship between the combination scenarios is nonsynergistic. This means that the cooling occurring from the combination of both strategies is either larger or smaller than if the cooling from individual strategies were to be added together. The drier, lower-density western suburbs of Melbourne showed a greater cooling response to increased vegetation without enhancing human thermal stress due to the corresponding increase in humidity. The leafy medium-density eastern suburbs of Melbourne showed a greater cooling response to the installation of cool roofs. These results highlight that the optimal urban cooling strategies can be different across a single urban center.

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Jason Beringer
,
Jorg Hacker
,
Lindsay B. Hutley
,
Ray Leuning
,
Stefan K. Arndt
,
Reza Amiri
,
Lutz Bannehr
,
Lucas A. Cernusak
,
Samantha Grover
,
Carol Hensley
,
Darren Hocking
,
Peter Isaac
,
Hizbullah Jamali
,
Kasturi Kanniah
,
Stephen Livesley
,
Bruno Neininger
,
Kyaw Tha Paw U
,
William Sea
,
Dennis Straten
,
Nigel Tapper
,
Richard Weinmann
,
Stephen Wood
, and
Steve Zegelin

Savannas are highly significant global ecosystems that consist of a mix of trees and grasses and that are highly spatially varied in their physical structure, species composition, and physiological function (i.e., leaf area and function, stem density, albedo, and roughness). Variability in ecosystem characteristics alters biophysical and biogeochemical processes that can affect regional to global circulation patterns, which are not well characterized by land surface models. We initiated a multidisciplinary field campaign called Savanna Patterns of Energy and Carbon Integrated across the Landscape (SPECIAL) during the dry season in Australian savannas to understand the spatial patterns and processes of land surface–atmosphere exchanges (radiation, heat, moisture, CO2, and other trace gasses). We utilized a combination of multiscale measurements including fixed flux towers, aircraft-based flux transects, aircraft boundary layer budgets, and satellite remote sensing to quantify the spatial variability across a continental-scale rainfall gradient (transect). We found that the structure of vegetation changed along the transect in response to declining average rainfall. Tree basal area decreased from 9.6 m2 ha−1 in the coastal woodland savanna (annual rainfall 1,714 mm yr−1) to 0 m2 ha−1 at the grassland site (annual rainfall 535 mm yr−1), with dry-season green leaf area index (LAI) ranging from 1.04 to 0, respectively. Leaf-level measurements showed that photosynthetic properties were similar along the transect. Flux tower measurements showed that latent heat fluxes (LEs) decreased from north to south with resultant changes in the Bowen ratios (H/LE) from a minimum of 1.7 to a maximum of 15.8, respectively. Gross primary productivity, net carbon dioxide exchange, and LE showed similar declines along the transect and were well correlated with canopy LAI, and fluxes were more closely coupled to structure than floristic change.

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Wouter Dorigo
,
Stephan Dietrich
,
Filipe Aires
,
Luca Brocca
,
Sarah Carter
,
Jean-François Cretaux
,
David Dunkerley
,
Hiroyuki Enomoto
,
René Forsberg
,
Andreas Güntner
,
Michaela I. Hegglin
,
Rainer Hollmann
,
Dale F. Hurst
,
Johnny A. Johannessen
,
Christian Kummerow
,
Tong Lee
,
Kari Luojus
,
Ulrich Looser
,
Diego G. Miralles
,
Victor Pellet
,
Thomas Recknagel
,
Claudia Ruz Vargas
,
Udo Schneider
,
Philippe Schoeneich
,
Marc Schröder
,
Nigel Tapper
,
Valery Vuglinsky
,
Wolfgang Wagner
,
Lisan Yu
,
Luca Zappa
,
Michael Zemp
, and
Valentin Aich

ABSTRACT

Life 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.

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