Russell S. Ladkin

Imaging of firn isochrones across an Antarctic ice rise and implications for patterns of snow accumulation rate

It has been an underlying assumption in many studies that near-surface layers imaged by ground-penetrating radar (GPR) can be interpreted as depositional markers or isochrones. It has been shown that GPR layers can be approximately reproduced from the measured electrical properties of ice, but these material layers are generally narrower and more closely spaced than can be resolved by typical GPR systems operating in the range 50-400 MHz. Thus GPR layers should be interpreted as interference patterns produced from closely spaced and potentially discontinuous material layers, and should not be assumed to be interpretable as precise markers of isochrones. We present 100 MHz GPR data from Lyddan Ice Rise, Antarctica, in which near-surface (< 50 m deep) layers are clearly imaged. The growth of the undulations in these layers with depth is approximately linear, implying that, rather than resulting from a pattern of vertical strain rate, they do correspond to some pattern of snowfall variation. Furthermore, comparison of the GPR layers with snow-stake measurements suggests that around 80% of the rms variability in mean annual accumulation is present in the GPR layers. The observations suggest that, at least in this case, the GPR layers do approximate isochrones, and that patterns of snow accumulation over Lyddan Ice Rise are dominated by extremely persistent spatial variations with only a small residual spatial variability. If this condition is shown to be widely applicable it may reduce the period required for measurements of surface elevation change to be taken as significant indications of mass imbalance.

Observations of cloud and precipitation particles on the Avery Plateau, Antarctic Peninsula

Surface-based observations were taken of cloud and precipitation particles on the Avery Plateau (66°50.34′S 65°29.58′W), Antarctic Peninsula from 25 November to 13 December 1995. This paper considers cloud parameters on three days during this period when the cloud base reached ground level and snow was falling. It was found that on all three days more ice crystals were present in the cloud than would be expected from simple theoretical considerations. The rate of snowfall decreased as the number of ice crystals increased, the large number of ice crystals present effectively suppressing the formation of large precipitation-sized crystals. The source of the ice nuclei that allowed the formation of the large number of crystals is not known for certain but is thought to be the snow surface, possibly in the form of very fine ice crystals blown from the surface during blowing/drifting snow episodes.

An operational, real-time cloud detection scheme for use in the Antarctic based on AVHRR data

A description is given of an automatic cloud detection scheme that has been developed for year-round, routine use on full (1.1 km) horizontal-resolution, five-channel Advanced Very High Resolution Radiometer (AVHRR) satellite imagery of the Antarctic continent and the adjacent ocean areas. The scheme is based on multiple independent tests to determine whether a pixel is cloudy or cloud-free. Tests used include the use of various channel thresholds, inter-channel differences and assessment of the spatial coherence of infrared data. During that part of the year when there is solar radiation, the channel 3 (3.7 m) data are the most valuable as clouds composed of supercooled water droplets can be detected via their high reflectivity against the dark ice surface. The scheme is successful at detecting most types of cloud, but some problems still remain. During the daytime, low-level, optically thin cloud can be difficult to detect, while during the winter, thick, featureless cloud with a temperature that is similar to the surface is difficult to identify. Assessment of the early climatological fields suggests that the scheme gives a realistic distribution of cloud over the southern part of the Antarctic Peninsula and resolves the greater amount of cloud that is present in King George VI Sound and over the Southern Weddell Sea coastal polynya, and the lower cloud fraction down the spine of the Peninsula.

An autonomous Doppler sodar wind profiling system

Abstract An autonomous Doppler sodar wind profiling system has been designed, built, tested, and then deployed for 2 years at a remote site in Coats Land, Antarctica. The system is designed around a commercially available phased-array sodar (a Scintec flat-array sodar, FAS64) and powered from five modular power system units. Each power unit comprises two batteries, two photovoltaic solar panels, and two vertical axis wind generators, plus charging control and isolation circuitry. The sodar’s main processing unit is located at the antenna, but is controlled from a manned research station 50 km distant, in real time, by a line-of-sight UHF radio link. Data from an integral automatic weather station (AWS) are also transmitted over the radio link, allowing meteorologically informed decisions on whether or not to operate the Doppler sodar. Over the 2-yr experiment dozens of sounding episodes, lasting from a few hours to a few days, were obtained. Successful soundings were obtained in temperatures down to −33°C...