Antarctic Boundary Layer Research Articles

Number size distributions and concentrations of the continental summer aerosols in Queen Maud Land, Antarctica

Total particle number concentrations and particle size distributions in the diameter range 3–800 nm were measured in continental Antarctica during the austral summers 2000 and 2001. Observed total particle number concentrations varied between about 200 and 2000 cm−3, with on average higher concentrations in marine/coastal air masses than in continental air masses. Concentrations in excess of about 300–400 cm−3 were attributable to the presence of small (<20 nm) nucleation mode particles. Measured size distributions were fitted with two to four lognormal modes. All the spectra displayed an accumulation mode peaking at about 70–150 nm and an Aitken mode peaking at about 30–50 nm. The average mean diameter of the accumulation mode was much lower in continental air masses, whereas that of the Aitken mode was almost the same for the two air mass types. A nucleation mode was present in more than a half of the measured spectra, and occasionally two nucleation modes could be identified. Evidence on recent new particle formation with subsequent particle growth up to about 40 nm was observed in marine/coastal air masses only. Our measurements indicate that these recently formed particles contribute significantly to the overall particle budget of the Antarctic boundary layer, and they can be also a source of climatically important Aitken and accumulation mode particles.

The role of halogen species in the troposphere

While the role of reactive halogen species (e.g. Cl, Br) in the destruction of the stratospheric ozone layer is well known, their role in the troposphere was investigated only since their destructive effect on boundary layer ozone after polar sunrise became obvious. During these ‘Polar Tropospheric Ozone Hole’ events O 3 is completely destroyed in the lowest ≈ 1000 m of the atmosphere on areas of several million square kilometres. Up to now it was assumed that these events were confined to the polar regions during springtime. However, during the last few years significant amounts of BrO and Cl-atoms were also found outside the Arctic and Antarctic boundary layer. Recently even higher BrO mixing ratios (up to 176 ppt) were detected by optical absorption spectroscopy (DOAS) in the Dead Sea basin during summer. In addition, evidence is accumulating that BrO (at levels around 1–2 ppt) is also occurring in the free troposphere at all latitudes. In contrast to the stratosphere, where halogens are released from species, which are very long lived in the troposphere, likely sources of boundary layer Br and Cl are autocatalytic oxidation of sea salt halides (the ‘Bromine Explosion’), while precursors of free tropospheric BrO and coastal IO probably are short-lived organo-halogen species. At the levels suggested by the available measurements reactive halogen species have a profound effect on tropospheric chemistry: In the polar boundary layer during ‘halogen events’ ozone is usually completely lost within hours or days. In the free troposphere the effective O 3-losses due to halogens could be comparable to the known photochemical O 3 destruction. Further interesting consequences include the increase of OH levels and (at low NO X ) the decrease of the HO 2/OH ratio in the free troposphere.

On solitary wave mediated heat exchange in the Antarctic boundary layer

The adiabatic assumption that the vertical velocity can be estimated from the displacement of isotherms is examined for solitary wave events propagating in the stable boundary layer overlying an Antarctic ice shelf. Implications for the mathematical modelling of such waves when this assumption is invalid are discussed.

Forcing of the Wintertime Antarctic Boundary Layer Winds from the NCEP–NCAR Global Reanalysis

Antarctica is noted for strong and persistent winds in the lower atmosphere. The wind directions are controlled by the underlying ice terrain and are deflected in general 20 8‐508 to the left of the fall line. The Antarctic surface wind regime is thought to be the result of the dual influences of diabatic cooling of the terrain, responsible for the infamous katabatic winds, and the synoptic pressure gradient force in the free atmosphere. The relative importance of pressure gradients associated with katabatic and synoptic processes in forcing the wintertime Antarctic boundary layer winds is evaluated using output from the National Centers for Environmental Prediction‐ National Center for Atmospheric Research (NCEP‐NCAR) global reanalysis program for June, July, and August of 1997. Both katabatic and synoptic forces are found to be significant in shaping the near-surface winter winds over the Antarctic ice slopes. Analyses show that the synoptic force is influenced by the underlying ice terrain such that the net force over Antarctica is directed primarily downslope. This result reflects the adjustment of the large-scale ambient pressure gradient to the continental orography. The synoptic force over Antarctica thus differs significantly in both magnitude and direction from that found over the oceanic regions to the north. The adjustment of the synoptic force over the ice sheets enable even the nonwinter Antarctic winds to attain a high directional constancy and resemble a gravity-driven flow. This process also suggests that direction alone is insufficient in classifying Antarctic flows as katabatic.

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°S, 26.7°W) 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 5.5×105 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.

OH photochemistry and methane sulfonic acid formation in the coastal Antarctic boundary layer

Studies of dimethylsulfide (DMS) oxidation chemistry were conducted at Palmer Station on Anvers Island, Antarctica, during the austral summer of 1993/1994. Part of the study involved gas phase measurements of OH, methane sulfonic acid (MSA), and H2SO4 using a chemical ionization mass spectrometer, as well as measurements of the NO, CO, and O3 concentrations. Mean 24 hour concentrations from February 16–23 of OH, MSA, and H2SO4 were 1.1 × 105, 9.5 × 105, and 1.61 × 106 molecules cm−3, respectively. Model calculations of OH compared well with observed levels (e.g., within 30%). The modeling results suggest that the dominant source of OH is from the reaction of O(1D) with H2O, where O(1D) is the product of O3 photolysis. Because of the clean atmospheric environment and predicted low nonmethyl hydrocarbon levels in Antarctica, the dominant OH sink was found to be reaction with CO and CH4. Particulate levels of MSA were higher than could be attributed to condensation of boundary layer (BL) gas phase MSA on to the aerosol surface. Alternate mechanisms for generating MSA in the particle phase were speculated to involve either in‐cloud oxidation of dimethylsulfoxide or OH oxidation of DMS in the atmospheric buffer layer above the boundary layer followed by condensation of gas phase MSA on aerosols and transport back to the BL [Davis et al., this issue].

A numerical study of the diurnal variation of the Adelie Land katabatic wind regime

Katabatic winds occur with great frequency over the Adelie Land sector of Antarctica. Data collected by automatic weather stations (AWSs) along a string of stations from the coast of Adelie Land to the high interior plateau show a marked diurnal cycle of the wind and temperature regimes of the lower atmosphere during the short austral summer period. Numerical simulations of the katabatic wind regime have been conducted for a variety of solar forcing and synoptic conditions assuming a clear sky environment. Results suggest that the katabatic wind is a robust feature of the Antarctic boundary layer. Significant disruption of the low‐level drainage features occurs only during the midsummer months owing to the solar heating of the ice slopes and from synoptic activity along the Adelie Land coastline. During midsummer, modeled ground temperatures undergo a diurnal cycle with an amplitude of 15°C near the coast, in good agreement with the AWS observations. The katabatic wind decreases considerably during the middle of the day; wind speeds at coastal sites become reduced by more than one half of the early morning values. The numerical simulations depict a rapid summer to winter transition of katabatic wind conditions over the 1‐month period from mid‐February to mid‐March. The surface energy budget becomes negative for the entire 24‐hour period after mid‐February, corresponding to the rapid onset of the wintertime katabatic wind regime.

Complexity of the wind field as observed by a sodar system and by automatic weather stations on the Nansen Ice Sheet, Antarctica, during summer 1988-1989: two case studies

In the Antarctic boundary layer, some phenomena appear to be particularly enhanced with respect to those at midlatitudes. Thermal stratifications are present almost all year and can substain drainage flows of different types. Several episodes of flows with different potential temperature and consequently with different buoyancy were observed on the Nansen Ice Sheet by a sodar. Two case studies are presented in which a comparison with the data of the automatic weather stations located on the Reeves and the Priestley glaciers as well on the Nansen Ice Sheet have been used. As a main result, we saw that during the summer time, the flow from the Reeves glacier may superimpose the one from the Priestley, in contrast with the current opinion based on the automatic weather station measurements averaged over the entire year. Also, an episode of barrier-katabatic flow interaction is shown in which the first is negatively buoyant in comparison with the second. Since the wind profile frequently showed a great variability with height, the importance of using remote sensing sensors in studies that realistically want to model the dynamics of the Antarctic boundary layer has been emphasized. DOI: 10.1034/j.1600-0889.1992.00017.x

Sodar observations of the antarctic boundary layer in a deglaciated area: Preliminary results

A single-axis sodar system, with off-line Doppler analysis, has been operated in the month of January 1987 at the Italian Antarctic base near Terra Nova Bay. The instrument was installed in complex terrain at a distance of approximately 1 km from the shore-line. During the campaign the area around the station, for a width of about (2÷3) km from the shore, was completely deglaciated and exposed to periods of direct solar irradiation. The measurements were carried out almost continuously with the antenna pointing to the zenith or at zenith angles of 20° and 30° in the NE direction. From a preliminary analysis of the data, the main phenomena observed were as follows:a) Very intense convection during periods of insolation with vertical velocities up to 4 ms−1.b) Evidence of layering and Kelvin-Helmholtz instabilities.c) Strong subsidence.