T.J. Lyons

Simulation of Surface Urban Heat Islands under ‘IDEAL’ Conditions at Night Part 1: Theory and Tests against Field Data

Observations show that the urban heat island in the atmospheric layer below roof level is most strongly developed during calm, cloudless conditions at night. This paper outlines two versions of a numerical model to describe the cooling of rural and street canyon surfaces under these conditions using surface thermal and radiative properties and the radiative geometry of the canyons. One version uses a full system of differential equations and the other the simpler force-restore approach. The two approaches are shown to be in general agreement and the output of the simpler model is shown to give a faithful representation of cooling of rural and urban surfaces, and therefore heat islands, when compared with field observations.

Monoterpene and isoprene emissions from 15 Eucalyptus species in Australia

Monoterpene and isoprene emission rates of 15 Eucalyptus species were measured using an air exchange chamber technique and GC-FID analysis. The normalised monoterpene emission rates (leaf temperature 30°C) of these Eucalyptus species ranged from zero for E. forrestiana to 5.4±2.2 μg g−1 h−1 (or 871±33 μg m−2 h−1) for E. globulus. The dominant monoterpene compounds emitted from these Eucalyptus species were α-pinene, 1,8-cineole, β-pinene and limonene. The normalised isoprene emission rates (leaf temperature 30°C, PAR flux 1000 μmol m−2 s−1) ranged from 5.3±1.6 (E. botryoides) to 69±34 μg g−1 h−1 (E. globulus) or from 0.74 (E. cladocalyx) to 9.5 mg m−2 h−1 (E. rudis). Based on monoterpene emission rate data from four Eucalyptus species (E. globulus, E. robusta, E. rudis, and E. sargentii), there were clear exponential relationships between leaf temperature and monoterpene emission rate for these Eucalyptus species. The mean exponential value (β value) of the four Eucalyptus species was 0.103±0.012 K−1.

Land-Atmosphere Interaction in a Semiarid Region: The Bunny Fence Experiment

Southwestern Australia, with a semiarid Mediterranean climate, has been extensively cleared of native vegetation for winter-growing agricultural species. The resultant reduction in evapotranspiration has increased land salinisation. Through detailed meteorological and vegetation measurements over both agricultural and native vegetation, the bunny fence experiment is addressing the impact on the climate of replacing native perennial vegetation with winter-growing annual species. Such measurements will give a better understanding of the interaction between the land surface and the atmosphere and are important for improved parameterization of the boundary layer in climate models.

Landscape-scale assessment of tree crown dieback following extreme drought and heat in a Mediterranean eucalypt forest ecosystem

Mediterranean regions are under increasing pressure from global climate changes. Many have experienced more frequent extreme weather events such as droughts and heatwaves, which have severe implications for the persistence of forest ecosystems. This study reports on a landscape-scale assessment investigating potential associated factors of crown dieback in dominant tree species following an extreme dry and hot year/summer of 2010/11 in the Northern Jarrah Forest of Western Australia. Analyses focussed on the influence of (i) geology, (ii) topography, (iii) climate, and (iv) fire history. The results showed that trees on specific soils were more likely to show canopy dieback. Generally, trees on rocky soils with low water holding capacity were found to be affected more frequently. Other explanatory factors identified that dieback occurred (i) on sites that were close to rock outcrops, (ii) in areas that received a slightly higher amount of annual rainfall compared to the surrounding landscape, (iii) on sites at high elevations and (vi) on steep slopes, and (v) in areas that were generally slightly warmer than their surroundings. These results expand our understanding of how landscape-scale factors contribute to the effects of an extreme drought and heating event in Mediterranean forest ecosystems, and give indications of where changes are likely to occur within the landscape in the future. The analogues with other Mediterranean climate regions make the results of this study transferable and a starting point for further investigations.

Effects of land use in Southwest Australia: 1. Observations of cumulus cloudiness and energy fluxes

The Southwest Australian region has large homogeneous tracts of differing vegetation types separated by a sharp transition called the vermin or bunny fence which runs for almost 750 km. Seasonal winter agriculture is found to the west of the fence, whereas to the east native perennial vegetation grows. Geostationary Meteorological Satellite-5 imagery are used to construct monthly cumulus cloud frequency of occurrence maps for the region 0800 to 1500 LT in hourly increments for 1999 and 2000. Moderate Resolution Imaging Spectroradiometer (MODIS) imagery are used to retrieve regional values of surface temperature, albedo, Normalized Difference Vegetation Index, fractional soil moisture availability, sensible and latent heat fluxes. High spatial resolution Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) imagery are used to retrieve detailed values along the fence. MODIS imagery also is utilized to retrieve cloud optical thickness, droplet sizes, and liquid water paths. This study shows that higher soil moisture availability is found over agricultural areas during winter (September) and over native vegetation areas during summer (December). Latent heat fluxes are higher over native vegetation than over agricultural areas during summer, while sensible heat fluxes are lower. Cumulus clouds occur with higher frequency and have higher optical thicknesses, cloud liquid water contents, and effective radii over agricultural areas during the winter and over native perennial vegetation during the dry summer. This is due to higher latent heat fluxes and available energy over agriculture during winter and over native vegetation during summer. We conclude that land use differences result in differences in available soil moisture and surface energy fluxes, which in turn lead to the observed preferential enhancement of cumulus cloudiness and cumulus cloud properties.

UNDERSTANDING AND PREDICTING PRIVATE MOTORISED URBAN MOBILITY

Dimensional analysis is used to derive a simple model of private motorised mobility for any urban area based on detailed land use and travel pattern data from a large international sample of cities. This highlights the dependence of vehicle kilometres of travel on urban population and area and shows that despite time, social and cultural differences, urban areas behave in a systematic way. Private motorised mobility, although arising from local decisions, is in the mean determined by the structure of the urban environment. As private motorised mobility is based on vehicle kilometres of travel, which in turn is a surrogate for urban transportation emissions, urban air quality is directly linked to urban structure. This has implications for further research in urban air pollution and supports arguments in favour of the compact city.

An international urban air pollution model for the transportation sector

Analysis of cities across the globe illustrates a strong relationship between vehicle kilometres of travel (VKT) and urbanised land area. As VKT can be used as a surrogate for vehicular emissions, this provides a method of estimating urban vehicular pollution. These emission rates are incorporated with simple meteorological forcing into a box model to provide a generalised urban air pollution model for transportation within any city and validated against published data. The results suggest direct air pollution benefits from minimising the outward growth of cities, and add weight to urban policies that emphasise compact city planning principles.

Climate and landscape drivers of tree decline in a Mediterranean ecoregion

Climate change and anthropogenic land use are increasingly affecting the resilience of natural ecosystems. In Mediterranean ecoregions, forests and woodlands have shown progressive declines in health. This study focuses on the decline of an endemic woodland tree species, Eucalyptus wandoo (wandoo), occurring in the biodiversity hotspot of southwest Western Australia. We determined the change in health of wandoo stands between 2002 and 2008 across its geographic and climatic range, and associated this change in health with non-biotic variables focusing on: (1) fragment metrics; (2) topography; (3) soil characteristics; and (4) climate. Only fragment metrics and climate variables were found to be significantly related to the observed change in health. Stands that were small with high perimeter/area ratios were found to be most sensitive to health declines. Recent increases in autumn temperatures and decreases in annual rainfall were negatively affecting health of wandoo most prominently in the low rainfall zone of its climatic range. Together, these results suggest the onset of range contraction for this ecologically important species, which is likely to be exacerbated by projected future changes in climate. Our results emphasize the importance of establishing monitoring programs to identify changes in health and decline trends early to inform management strategies, particularly in the sensitive Mediterranean ecoregions.

Recirculation of coastal urban air pollution under a synoptic scale thermal trough in Perth, Western Australia

Air pollutant recirculation is a common feature of coastal cities as a result of the diurnal variation of the land/sea breeze circulation. On the western coast of Australia, a thermally induced synoptic scale trough interacts with this local circulation to enhance the sea breeze flow, allowing pollutant to penetrate further inland. As well a low wind speed zone formed offshore near the trough axis leads to increased ozone concentrations and the rotation of the trough axis during its onshore movement spreads these higher concentrations of air pollutant over a larger area.

Estimation of Regional Evapotranspiration through Remote Sensing

Abstract Models used for the remote estimation of evapotranspiration are evaluated using aircraft observations over two distinct vegetation regimes in southwestern Australia. Single-source models using an empirically determined excess resistance term performed better than a two-source model, which does not require such a parameterization. The mean absolute difference between measured and estimated values of the sensible heat flux is below 17 W m−2 in comparison with approximately 40 W m−2 for evapotranspiration. Estimates of evapotranspiration depend on the closure of the surface energy balance and incorporate all residual errors in this closure. All models performed better over the agricultural vegetation than over the native vegetation.