Very-Large-Scale Motions in the Atmospheric Boundary Layer Educed by Snapshot Proper Orthogonal Decomposition

Large-eddy simulations of the atmospheric boundary layer (ABL) under a wide range of stabilities are conducted to educe very-large-scale motions and then to study their dynamics and how they are influenced by buoyancy. Preliminary flow visualizations suggest that smaller-scale motions that resemble hairpins are embedded in much larger scale streamwise meandering rolls. Using simulations that represent more than 150 h of physical time, many snapshots in the \(xy\) -, \(yz\) - and \(xz\) -planes are then collected to perform snapshot proper orthogonal decomposition and further investigate the large structures. These analyses confirm that large streamwise rolls that share several features with the very-large-scale motions observed in laboratory studies arise as the dominant modes under most stabilities, but the effect of the surface kinematic buoyancy flux on the energy content of these dominant modes is very significant. The first two modes in the \(yz\) -plane in the neutral case contain up to 3 % of the total turbulent kinetic energy; they also have a vertical tilt angle in the \(yz\) -plane of about 0 to 30 \(^\circ \) due to the turning effect associated with the Coriolis force. Unstable cases also feature streamwise rolls, but in the convective ABL they are strengthened by rising plumes in between them, with two to four rolls spanning the whole domain in the first few modes; the Coriolis effect is much weaker in the unstable ABL. These rolls are no longer the dominant modes under stable conditions where the first mode is observed to contain sheet-like motions with high turbulent kinetic energy. Using these proper orthogonal decomposition modes, we are also able to extract the vertical velocity fields corresponding to individual modes and then to correlate them with the horizontal velocity or temperature fields to obtain the momentum and heat flux carried by individual modes. Structurally, the fluxes are explained by the topology of their corresponding modes. However, the fraction of the fluxes produced by the modes is invariably smaller than the fraction of energy they contain, particularly under stable conditions where the first modes are found to perform weak counter-gradient fluxes.     

The DEMETER Science Mission Centre

The DEMETER Scientific Mission Centre (SMC) has been developed and is operated by the Laboratoire de Physique et Chimie de l’Environnement (LPCE). The SMC commands the instruments of the scientific payload, collects and distributes DEMETER data and associated products to the DEMETER international community.The SMC has been designed to maximize scientific return and to reduce development and exploitation costs for the DEMETER project. This paper describes the SMC's data processing system, data server and methods of payload operation, and presents associated hardware and software architectures.     

Monin–Obukhov Similarity Functions for the Structure Parameters of Temperature and Humidity

Monin–Obukhov similarity functions for the structure parameters of temperature and humidity are needed to derive surface heat and water vapour fluxes from scintillometer measurements and it is often assumed that the two functions are identical in the atmospheric surface layer. Nevertheless, this assumption has not yet been verified experimentally. This study investigates the dissimilarity between the turbulent transport of sensible heat and water vapour, with a specific focus on the difference between the Monin–Obukhov similarity functions for the structure parameters. Using two datasets collected over homogeneous surfaces where the surface sources of sensible heat and water vapour are well correlated, we observe that under stable and very unstable conditions, the two functions are similar. This similarity however breaks down under weakly unstable conditions; in that regime, the absolute values of the correlations between temperature and humidity are also observed to be low, most likely due to large-scale eddies that transport unsteadiness, advection or entrainment effects from the outer layer. We analyze and demonstrate how this reduction in the correlation leads to dissimilarity between the turbulent transport of these two scalars and the corresponding Monin–Obukhov similarity functions for their structure parameters. A model to derive sensible and latent heat fluxes from structure parameters without measuring the friction velocity is tested and found to work very well under moderately to strongly unstable conditions (−z/L > 0.5). Finally, we discuss the modelling of the cross-structure parameter over wet surfaces, which is crucial for correcting water vapour effects on optical scintillometer measurements and also for obtaining surface sensible and latent heat fluxes from the two-wavelength scintillometry.     

Late Quaternary deglaciation and climate history of the Larsemann Hills (East Antarctica)

The Late Quaternary climate history of the Larsemann Hills has been reconstructed using siliceous microfossils (diatoms, chrysophytes and silicoflagellates) in sediment cores extracted from three isolation lakes. Results show that the western peninsula, Stornes, and offshore islands were ice‐covered between 30 000 yr BP and 13 500 cal. yr BP. From 13 500 cal. yr BP (shortly after the Antarctic Cold Reversal) the coastal lakes of the Larsemann Hills were deglaciated and biogenic sedimentation commenced. Between 13 500 and 11 500 cal. yr BP conditions were warmer and wetter than during the preceding glacial period, but still colder than today. From 11 500 to 9500 cal. yr BP there is evidence for wet and warm conditions, which probably is related to the early Holocene climate optimum, recorded in Antarctic ice cores. Between 9500 and 7400 cal. yr BP dry and cold conditions are inferred from high lake‐water salinities, and low water levels and an extended duration of nearshore sea‐ice. A second climate optimum occurred between 7400 and 5230 cal. yr BP when stratified, open water conditions during spring and summer characterised the marine coast of Prydz Bay. From 5230 until 2750 cal. yr BP sea‐ice duration in Prydz Bay increased, with conditions similar to the present day. A short return to stratified, open water conditions and a reduction in nearshore winter sea‐ice extent is evident between 2750 and 2200 cal. yr BP. Simultaneously, reconstructions of lake water depth and salinity suggests relatively humid and warm conditions on land between 3000 and 2000 cal. yr BP, which corresponds to a Holocene Hypsithermal reported elsewhere in Antarctica. Finally, dry conditions are recorded around 2000, between 760 and 690, and between 280 and 140 cal. yr BP. These data are consistent with ice‐core records from Antarctica and support the hypothesis that lacustrine and marine sediments on land can be used to evaluate the effect of long‐term climate change on the terrestrial environment. Copyright © 2004 John Wiley & Sons, Ltd.     

Signatures of Air–Wave Interactions Over a Large Lake

The air–water exchange of momentum and scalars (temperature and water vapour) is investigated using the Lake-Atmosphere Turbulent EXchange (LATEX) dataset. The wind waves and swell are found to affect the coupling between the water surface and the air differently. The surface-stress vector aligns with the wind velocity in the presence of wind waves, but a wide range of stress–wind misalignment angles is observed during swell. The momentum transport efficiency decreases when significant stress–wind misalignment is present, suggesting a strong influence of surface wave properties on surface drag. Based on this improved understanding of the role of wave–wind misalignment, a new relative wind speed for surface-layer similarity formulations is proposed and tested using the data. The new expression yields a value of the von Kármán constant (\(\kappa \)) of 0.38, compared to 0.36 when using the absolute wind speed, as well as reduced data fitting errors. Finally, the ratios of aerodynamic to scalar roughness lengths are computed and various existing models in the literature are tested using least-square fitting to the observed ratios. The tests are able to discriminate between the performance of various models; however, they also indicate that more investigations are required to understand the physics of scalar exchanges over waves.     

Exploring former subglacial Hodgson Lake,Antarctica. Paper II: palaeolimnology

Direct exploration of subglacial lakes buried deep under the Antarctic Ice Sheet has yet to be achieved. However, at retreating margins of the ice sheet, there are a number of locations where former subglacial lakes are emerging from under the ice but remain perennially ice covered. One of these lakes, Hodgson Lake (72°00.549′S, 068°27.708′W) has emerged from under more than 297–465 m of glacial ice during the last few thousand years. This paper presents data from a multidisciplinary investigation of the palaeolimnology of this lake through a study of a 3.8 m sediment core extracted at a depth of 93.4 m below the ice surface. The core was dated using a combination of radiocarbon, optically stimulated luminescence, and relative palaeomagnetic intensity dating incorporated into a chronological model. Stratigraphic analyses included magnetic susceptibility, clast provenance, organic content, carbonate composition, siliceous microfossils, isotope and biogeochemical markers. Based on the chronological model we provisionally assign a well-defined magnetic polarity reversal event at ca 165 cm in the lake sediments to the Mono Lake excursion (ca 30–34 ka), whilst OSL measurements suggest that material incorporated into the basal sediments might date to 93 ± 9 ka. Four stratigraphic zones (A–D) were identified in the sedimentological data. The chronological model suggests that zones A–C were deposited between Marine Isotope Stages 5–2 and zone A during Stage 1, the Holocene. The palaeolimnological record tracks changes in the subglacial depositional environment linked principally to changing glacier dynamics and mass transport and indirectly to climate change. The sediment composition in zones A–C consists of fine-grained sediments together with sands, gravels and small clasts. There is no evidence of overriding glaciers being in contact with the bed reworking the stratigraphy or removing this sediment. This suggests that the lake existed in a subglacial cavity beneath overriding LGM ice. In zone D there is a transition to finer grained sediments characteristic of lower energy delivery coupled with a minor increase in the organic content attributed either to increases in allochthonous organic material being delivered from the deglaciating catchment, a minor increase in within-lake production or to an analytical artefact associated with an increase in the clay fraction. Evidence of biological activity is sparse. Total organic carbon varies from 0.2 to 0.6%, and cannot be unequivocally linked to in situ biological activity as comparisons of δ¹³C and C/N values with local reference data suggest that much of it is derived from the incorporation of carbon in catchment soils and gravels and possibly old CO₂ in meteoric ice. We use the data from this study to provide guidelines for the study of deep continental subglacial lakes including establishing sediment geochronologies, determining the extent to which subglacial sediments might provide a record of glaciological and environmental change and a brief review of methods to use in the search for life.     

Geological constraints on glacio-isostatic adjustment models of relative sea-level change during deglaciation of Prince Gustav Channel,Antarctic Peninsula

The recent disintegration of Antarctic Peninsula ice shelves, and the associated accelerated discharge and retreat of continental glaciers, has highlighted the necessity of quantifying the current rate of Antarctic ice mass loss and the regional contributions to future sea-level rise. Observations of present day ice mass change need to be corrected for ongoing glacial isostatic adjustment, a process which must be constrained by geological data. However, there are relatively little geological data on the geometry, volume and melt history of the Antarctic Peninsula Ice Sheet (APIS) after Termination 1, and during the Holocene so the glacial isostatic correction remains poorly constrained. To address this we provide field constraints on the timing and rate of APIS deglaciation, and changes in relative sea-level (RSL) for the north-eastern Antarctic Peninsula based on geomorphological evidence of former marine limits, and radiocarbon-dated marine-freshwater transitions from a series of isolation basins at different altitudes on Beak Island. Relative sea-level fell from a maximum of c. 15 m above present at c. 8000 cal yr BP, at a rate of 3.91 mm yr^?1 declining to c. 2.11 mm yr^?1 between c. 6900–2900 cal yr BP, 1.63 mm yr^?1 between c. 2900–1800 cal yr BP, and finally to 0.29 mm yr^?1 during the last c. 1800 years. The new Beak Island RSL curve improves the spatial coverage of RSL data in the Antarctic. It is in broad agreement with some glacio-isostatic adjustment models applied to this location, and with work undertaken elsewhere on the Antarctic Peninsula. These geological and RSL constraints from Beak Island imply significant thinning of the north-eastern APIS by the early Holocene. Further, they provide key data for the glacial isostatic correction required by satellite-derived gravity measurements of contemporary ice mass loss, which can be used to better assess the future contribution of the APIS to rising sea-levels.     

Late Holocene changes in cyanobacterial community structure in maritime Antarctic lakes

Despite the dominance of cyanobacteria in polar freshwater aquatic ecosystems, little is known about their past biodiversity and response to climate and environmental changes. We explored the use of light microscopy of microfossils, high performance liquid chromatography of the fossil pigment composition and denaturing gradient gel electrophoresis of fossil 16S rRNA genes to study past and present-day differences in cyanobacterial community structure in response to climate changes in two adjacent maritime Antarctic lakes with contrasting depths (4 and 26 m) and light climates. Light microscopy was of limited use because of degradation of cell structures. Fossil cyanobacterial pigment concentrations were below the detection limits of our method in several sediment samples in the deep lake, but abundant and diverse in the sediment core from the shallow pond, probably as a consequence of increased light availability and/or a more diverse and abundant benthic cyanobacterial flora. Total carotenoid and chlorophyll concentrations were highest in both lakes between ca. 2,950 and 1,800 cal yr BP, which coincides with the late Holocene climate optimum recognised elsewhere in maritime Antarctica. Cyanobacterial molecular diversity was higher in the top few centimeters of the sediments in both lakes. In deeper sediments, the taxonomic turnover of cyanobacteria appeared to be relatively small in response to past climate anomalies in both lakes, underscoring the broad tolerance of cyanobacteria to environmental variability. This, however, may in part be explained by the low taxonomic resolution obtained with the relatively conserved 16S rRNA gene and/or the preferential preservation of particular taxa. Our results highlight the potential of fossil DNA in lake sediments to study colonization and succession dynamics of lacustrine cyanobacteria and warrant further investigation of the factors that affect preservation of cyanobacterial DNA.     

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