Stuart Bradley

The PLATO Dome A Site-Testing Observatory : instrumentation and first results

The PLATeau Observatory (PLATO) is an automated self-powered astrophysical observatory that was deployed to Dome A, the highest point on the Antarctic plateau, in 2008 January. PLATO consists of a suite of site-testing instruments designed to quantify the benefits of the Dome A site for astronomy, and science instruments designed to take advantage of the unique observing conditions. Instruments include CSTAR, an array of optical telescopes for transient astronomy; Gattini, an instrument to measure the optical sky brightness and cloud cover statistics; DASLE, an experiment to measure the statistics of the meteorological conditions within the near-surface layer; Pre-HEAT, a submillimeter tipping radiometer measuring the atmospheric transmission and water vapor content and performing spectral line imaging of the Galactic plane; and Snodar, an acoustic radar designed to measure turbulence within the near-surface layer. PLATO has run completely unattended and collected data throughout the winter 2008 season. Here we present a detailed description of the PLATO instrument suite and preliminary results obtained from the first season of operation.

The PLATO Antarctic site testing observatory

Over a decade of site testing in Antarctica has shown that both South Pole and Dome C are exceptional sites for astronomy, with certain atmospheric conditions superior to those at existing mid-latitude sites. However, the highest point on the Antarctic plateau, Dome A, is expected to experience colder atmospheric temperatures, lower wind speeds, and a turbulent boundary layer that is confined closer to the ground. The Polar Research Institute of China, who were the first to visit the Dome A site in January 2005, plan to establish a permanently manned station there within the next decade. As part of this process they conducted a second expedition to Dome A, arriving via overland traverse in January 2008. This traverse involved the delivery and installation of the PLATeau Observatory (PLATO). PLATO is an automated self-powered astrophysical site testing observatory, developed by the University of New South Wales. A number of international institutions have contributed site testing instruments measuring turbulence, optical sky background, and sub-millimetre transparency. In addition, a set of science instruments are providing wide-field high time resolution optical photometry and terahertz imaging of the Galaxy. We present here an overview of the PLATO system design and instrumentation suite.

Wind speed errors for LIDARs and SODARs in complex terrain

All commercial LIDARs and SODARs are monostatic and hence sample distributed volumes to construct wind vector components. We use an analytic potential flow model to estimate errors arising for a range of LIDAR and SODAR configurations on hills and escarpments. Wind speed errors peak at a height relevant to wind turbines and can be typically 20%.

Rainfall redistribution over low hills due to flow perturbation

Abstract A simple potential flow model is developed for airflow over low hills. The model readily allows incorporation of simple rain microphysics so that drop trajectories can be traced in the region of a hill complex. Rainfall intensity variations are found in the horizontal and in the vertical, and a straightforward modification to the model allows for time dependence to be studied. The model is shown to serve as a useful diagnostic tool to describe rain drop redistribution due to perturbed airflow. In particular, increased rainfall is predicted in the lee of hill peaks and depleted rainfall is predicted on ridges. The redistribution effects dominate at low levels, in contrast to predictions based on seeder–feeder enhancement. Comparison with field measurements shows good qualitative agreement over low hills and, in some cases, close quantitative agreement. Redistribution of rain appears to be a significant process that should be considered in rainfall interpolation and prediction schemes involving hilly terrain.

Model for optical forward scattering by nonspherical raindrops

We describe a numerical model for the interaction of light with large raindrops using realistic nonspherical drop shapes. We apply geometrical optics and a Monte Carlo technique to perform ray traces through the drops. We solve the problem of diffraction independently by approximating the drops with area-equivalent ellipsoids. Scattering patterns are obtained for different polarizations of the incident light. They exhibit varying degrees of asymmetry and depolarization that can be linked to the distortion and thus the size of the drops. The model is extended to give a simplified long-path integration.

A Combined Microphone and Camera Calibration Technique With Application to Acoustic Imaging

We present a calibration technique for an acoustic imaging microphone array, combined with a digital camera. Computer vision and acoustic time of arrival data are used to obtain microphone coordinates in the camera reference frame. Our new method allows acoustic maps to be plotted onto the camera images without the need for additional camera alignment or calibration. Microphones and cameras may be placed in an ad-hoc arrangement and, after calibration, the coordinates of the microphones are known in the reference frame of a camera in the array. No prior knowledge of microphone positions, inter-microphone spacings, or air temperature is required. This technique is applied to a spherical microphone array and a mean difference of 3 mm was obtained between the coordinates obtained with this calibration technique and those measured using a precision mechanical method.

SODAR measurements of wing vortex strength and position

Abstract A method is developed for robust real-time visualization of aircraft vortex spatial and temporal development based on measurement data from a line array of sodars. The method relies on using a potential-flow vortex model, with spatial averaging according to the along-beam and transverse spatial resolution of the sodar. The model comprises the wing vortex pair, together with two image vortices below ground such that there is no flow through the ground surface. An analytic solution for the temporal–spatial evolution of this four-vortex system is obtained as an aid to establishing relevant scales and performance criteria for any sodar. Field results from an array of four sodars are used on an individual profile basis (every 2 s of real time) to fit the model parameters of vortex circulation, position, and spacing. This method gives vortex trajectories and strength as a function of real time without dependence on assumptions regarding interactions with the atmosphere. Estimates of parameter uncertain...

A Rain Gauge for the Measurement of Finescale Temporal Variations

Abstract A rain gauge is described that quantizes rainwater collected by a funnel into equal-sized drops. Using a funnel of 150-mm diameter, the quantization corresponds to 1/160 mm of rainfall, enabling the measurement of low rainfall rates and the attainment of a fine temporal resolution on the order of 15 s without unduly large sampling errors. Two drop-producing units are compared and an operational rain gauge design is presented. Field comparisons with conventional rain gauges are made, showing excellent correlations for daily rain totals, and intercomparisons between clusters of dropper gauges are also given. Examples of highly resolved rainfall events are shown demonstrating the ability to measure low rainfall accumulations and also coherent high intensity events of short duration, which are not detectable with conventional rain gauges.

An Improved High-Resolution Raingage

Abstract Evaluation of an earlier raingage design based on counting drops formed on the tip of a small-diameter stainless-steel tube shows a defect due to resonant oscillation of the water column in the dropper unit. The defect causes nonlinearity in the drop rate-flow rate relationship, precluding useful integration of the gage output. An improved design is presented which eliminates this defect. Laboratory tests on the new design show linear performance over two orders of magnitude of rainfall intensity with time resolution of better than 5 s and accuracy limited by sampling errors. The new gage is also shown to perform well in field testing.

Underestimation of Monostatic Sodar Measurements in Complex Terrain

Recent investigations in complex terrain have found that remote sensing instrumentation commonly finds mean wind-speed differences when compared to cup anemometery. In many cases the difference is found to be an underestimation and varies from 2 to 9% depending on topology. We describe these differences in a theoretical sense for a five-beam sodar. An investigation is conducted on a New Zealand ridge with a five-beam sodar and three computational models, consisting of a potential flow model and two computational fluid dynamical simulations, OpenFOAM and the industry standard software WindSim. All models predict the difference to within 0.1–2.5%. A comparative assessment is made and it is found that, given the computing overheads, the potential flow model provides a good compromise in the prediction of mean wind-speed difference.