P. S. Anderson

Measurements of NOx emissions from the Antarctic snowpack

It has been shown that NOx is produced photochemically within the snowpack of polar regions. If emitted to the atmosphere, this process could be a major source of NOx in remote snowcovered regions. We report here on measurements made at the German Antarctic station, Neumayer, during austral summer 1999, aimed at detecting and quantifying emissions of NOx from the surface snow. Gradients of NOx measured, and fluxes calculated using local meteorology measurements. On the 2 days of flux measurements, the derived fluxes showed continual release from the snow surface, varying between ∼0 and 3 × 108 molecs/cm²/s. When not subject to turbulence, the variation was coincident with the uv diurnal cycle, suggesting rapid release once photochemically produced. Scaling the diurnal average of Feb. 7th (1.3 × 108 molecs/cm²/s) suggests an annual emission over Antarctica of the order 0.0076TgN.

Profile measurements of blowing snow at Halley, Antarctica

Measurements of blowing snow particle concentration from the second Stable Antarctic Boundary Layer Experiment (STABLE 2) are presented. The measurements, made at Halley Station (75.6 degreesS, 26.7 degreesW) throughout the 1991 austral winter, are supplemented with profile measurements of wind speed, air temperature, and humidity. Threshold wind speeds for blowing snow are shown to be distinctly different for various episodes, often depending strongly on the the availability of loose snow. Blowing snow is measured to occur at Halley between 27 and 37% of the time during winter. Total winter (June, July, August) blowing snow sublimation at Halley is calculated to be around 4.7 mm water equivalent, 3.7% of total accumulation over the same period. Total winter blowing snow mass transport is calculated to be around 5.5 x 10(5) kg per metre width. Measured humidity profiles at Halley show that during winter blowing snow conditions, a layer of near-saturated air forms, causing total sublimation to be less than that for seasonal or limited fetch snow covers. The extent of sublimation is shown to be strongly dependent on wind speed, temperature and fetch.

Heat and water vapour fluxes and scalar roughness lengths over an Antarctic ice shelf

We present eddy-correlation measurements of heat and water vapour fluxes made during the Antarctic winter. The surface layer was stably stratified throughout the period of observation and sensible heat fluxes were always directed downwards. However, both upward and downward water vapour fluxes were observed. Their magnitude was generally small and the latent heat flux was not a significant fraction of the surface energy budget. The variation of heat and water vapour fluxes with stability is well described by Monin-Obukhov similarity theory but the scalar roughness lengths for heat and water vapour appear to be much larger than the momentum roughness length. Possible explanations of this effect are discussed.

The surface energy and mass balance at Halley, Antarctica, during winter

We present measurements of the components of the energy and mass balance of the snow surface at Halley Research Station, Antarctica. During the winter months, when insolation is small or zero, the surface energy balance is dominated by radiative cooling. This is mostly balanced by a downward transport of atmospheric sensible heat, with an upward conductive flux of heat through the snowpack making a secondary contribution. The average flux of atmospheric latent heat is downward but of negligible importance in the surface energy balance. During the winter, a significant imbalance is seen in the measured energy budget, with insufficient sensible and conductive heat fluxes to balance the radiative cooling. The wintertime surface mass balance is dominated by precipitation. Sublimation of blowing snow makes a small negative contribution to the budget and is observed to be highly dependent on wind speed. It is suggested that this may be an important mechanism for removing surface mass in some parts of Antarctica.

A method for rescaling humidity sensors at temperatures well below freezing

Abstract A method for extending the calibrated temperature rang of a solid-state capacitive humidity sensor is presented. This technique is applicable to relative humidity instruments that are based around solid-state sensors.

A role for newly forming sea ice in springtime polar tropospheric ozone loss? Observational evidence from Halley station, Antarctica

Since March 2003, measurements of surface ozone have been made at the British Antarctic Survey Clean Air Sector Laboratory (CASLab) at Halley station in coastal Antarctica. Detailed measurements of boundary layer meteorology, as well as standard meteorological parameters, are also measured at the CASLab. Combining these data allows us to probe the transport pathway of air masses during ozone depletion events (ODEs). ODEs were observed at Halley on several occasions during Antarctic spring 2003. On some occasions, extremely rapid loss of ozone was observed (loss of 16 ppbv in 1 min on one occasion), which was associated with regional-scale transport. For each such event during 2003, the air mass originated in the southern Weddell Sea, an area of vigorous sea-ice production. On other occasions the development of the event and its recovery were strongly associated with the build-up and decline of a stable boundary layer. In these cases, air masses had had recent contact with a nearby open water lead where sea-ice production is known to occur. The data presented here are entirely consistent with the idea that halogens responsible for ozone loss are derived during new sea-ice formation from an associated surface such as brine slush or frost flowers.

Wind-borne redistribution of snow across an Antarctic ice rise

Redistribution of snow by the wind can drive spatial and temporal variations in snow accumulation that may affect the reconstruction of paleoclimate records from ice cores. In this paper we investigate how spatial variations in snow accumulation along a 13 km transect across Lyddan Ice Rise, Antarctica, are related to wind-borne snow redistribution. Lyddan Ice Rise is an approximately two-dimensional ridge which rises about 130 m above the surrounding ice shelves. Local slopes on its flanks never exceed 0.04. Despite this very smooth profile, there is a pronounced gradient in snow accumulation across the feature. Accumulation is highest on the ice shelf to the east ( climatologically upwind) of the ice rise and decreases moving westward, with the lowest accumulation seen to the west ( climatologically downwind) of the ice rise crest. Superimposed on this broad-scale gradient are large ( 20-30%), localized variations in accumulation on a scale of around 1 km that appear to be associated with local variations in surface slope of less than 0.01. The broad-scale accumulation gradient is consistent with estimates of wind-borne redistribution of snow made using wind speed observations from three automatic weather stations. The small-scale variability in accumulation is reproduced quite well using a snow transport model driven by surface winds obtained from an airflow model, providing that both the wind shear and static stability of the upwind flow are taken into account. We conclude that great care needs to be exercised in selecting ice core sites in order to avoid the possibility of blowing snow transport confounding climate reconstructions.

The seasonal cycle of sublimation at Halley, Antarctica

We have used micrometeorological data collected at Halley Research Station, Antarctica, to estimate monthly totals of snow sublimation. Direct sublimation from the snow surface is calculated using bulk-transfer formulae, while the sublimation of blowing snow is estimated using a model for suspended-particle number density and individual particle sublimation rates. During the winter months, sublimation losses are negligible, but between November and March sublimation removes around 25% of the snowfall. Surface sublimation and sublimation of blowing snow make roughly equal contributions to this total. Estimates of sublimation using micrometeorological data agree well with estimates made from daily snow-stake measurements.

A Climatological Study of Internal Gravity Waves in the Atmospheric Boundary Layer Overlying the Brunt Ice Shelf, Antarctica

Internal gravity waves are frequently observed in stably stratified regions of the atmospheric boundary layer. In order to determine the statistical influence of such waves on the dynamics of the boundary layer it is necessary to compile information concerning properties of the waves such as frequency of occurrence, propagation, and spectral characteristics. Gravity wave climatologies have been compiled from relatively few locations. In this paper a climatological study of gravity waves, in the period range 1‐20 min, propagating in the stably stratified atmospheric boundary layer overlying an Antarctic ice shelf is presented. An extensive set of boundary layer measurements were compiled throughout 1991. Surface pressure fluctuations were recorded from a spatial array of six sensitive microbarographs. Wind and temperature records from an instrumented mast were also available. A beam-steering technique has been used to determine wave parameters from the surface pressure data. The microbarographs detected the presence of internal gravity waves throughout the observational campaign. Rootmean-square pressure values were typically in the region 16‐40 mb, but a significant number of isolated events with amplitudes of up to 180 mb were also found. Wave properties have been studied in conjunction with the mean wind and temperature profiles in the boundary layer. It was found that most of the wave activity did not originate locally, but from shear layers aloft, or, more commonly, from the katabatic flow regime where the ice shelf joins the Antarctic continent.

Fine-scale structure observed in a stable atmospheric boundary layer by sodar and kite-borne tethersonde

Co-located high resolution profiles of acoustic backscatter,wind vector and potential temperature are presented, measured within the stable atmosphericboundary layer over an Antarctic ice shelf. Acoustic profiles from a monostaticacoustic radar (Sodar) indicate complex structure within the boundary layer, whilstwind and temperature profiles from a tethersonde show corresponding bands of differingstability. Internal waves and fossil convection are shown to invalidate attemptsto compare backscatter measurements with theoretical estimates based on local wind and temperature gradients, but during ideal conditions, a qualitative agreementis observed.