The effects of spatial averaging on airfoil probe measurements of oceanic velocity microstructure

The effects of spatial averaging on the measurement of velocity power spectra by an airfoil probe have been studied. The probe is a miniature airfoil of revolution with a piezo-ceramic sensor that outputs a voltage proportional to the time-varying cross-stream velocity component. The spatial transfer function of the probe was measured in grid turbulence set up in a water tunnel by comparing the power spectra obtained with the airfoil probe to those measured using high spatial resolution laser Doppler anemometry (LDA). The LDA system was designed and constructed specifically for this purpose. The LDA was shown to be capable of measuring low intensity turbulence, giving results consistent with similarity theory and previous constant temperature anemometry measurements. The transfer function exhibited unexpected behavior at low wavenumbers, namely, an initial positive slope and peak away from the origin. A three-dimensional model is proposed and used to explain the experimental transfer function and to derive a large-scale transfer function suitable for correcting shear probe measurements of oceanic rates of dissipation of turbulent kinetic energy. Although in general the transfer function depends on the form of the energy spectrum, it is shown that for oceanic spectra the transfer function is independent of scaling in the wavenumber domain and hence independent of the rate of dissipation. For the two probe designs studied, the large-scale transfer functions have a 1/2-power response at a scale of about 1.5cm and approximately 50% of the total dissipation is recovered at the highest dissipation value considered, 10⁻¹ cm²/s³, (which is larger than the dissipation rates encountered in most of the ocean). The dissipation measurements can be corrected to an accuracy of 5%.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat

The effects of a moratorium on land-clearing in the Douglas-Daly region, Northern Territory, Australia

Land-clearing represents the first step in agricultural development and signals a shift in landscape function towards provisioning ecosystem services, in particular food production. In the process, other types of ecosystem services are often unintentionally lost as illustrated by the associated decline in biodiversity, increased soil erosion and emission of greenhouse gases. In 2003, the Northern Territory state government in Australia promulgated a moratorium on the clearing of native vegetation on freehold land in the Douglas-Daly river catchment, an area experiencing increasing pressure from agricultural development. The moratorium was intended to limit the rate and extent of land-clearing for a period of time so that informed policy could be concurrently developed to guide future land-clearing and minimise negative impacts. Under the moratorium, land-clearing required a permit and had to conform to broad guidelines; clearing was confined to freehold land, was prohibited in close proximity to wetlands, rivers and rainforest to safeguard water quality, and there were prescribed limits on percentages cleared by property, vegetation type, sub-catchment, and the whole catchment. Remotely sensed data (1977–2011) were used to explore the effectiveness of the moratorium. The analysis shows that, during moratorium years (2002–2009), clearing rates accelerated rather than slowed in the moratorium area and was mostly (81%) conducted without the required permits. The extent of land cleared after the moratorium was declared, and the fallow nature of some of this land a decade later, suggests that much of the land-clearing may have been completed in anticipation of stricter future controls. The moratorium failed because it was not formally legislated and was too broadly defined. Consequently, the non-binding nature of the land-clearing guidelines, and the absence of systematic monitoring of land cover change or penalties for clearing land without a permit, led to uninformed and uncontrolled clearing. This paper demonstrates that effective policy is only as good as its level of implementation