Sub-Cm Subsidence Measurements With The Wide-Angle Airborne Laser Ranging System

The Wide-Angle Airborne Laser Ranging System (WA-ALRS) is aninstrument developed at IGN with the aim of measuring landsubsidence with sub-cm accuracy. The instrument provides rangemeasurements that are used to estimate the co-ordinates of aground-based network of cube-corner retroreflectors (CCRs).Millimeter accuracy on relative radial co-ordinates has beendemonstrated in a ground-based experiment and centimeter accuracyhas been obtained in a recent airborne experiment. The mainlimitation in the latter experiment came from the low link budgetat longer distances. In this paper we analyze the performanceexpected with two different detector modules (avalanchephotodiodes with amplifiers). Laboratory measurements are used tocharacterize detector parameters and estimate ranging accuracy.Numerical simulations predict a 2–3 mm accuracy in the verticalco-ordinate with these detectors combined to a 2-km aircraftaltitude, a 20 laser beam divergence, a 250-m CCR separation and 10³ measurements per CCR.     

Benthic primary production during emersion: In situ measurements and potential primary production in the Seine Estuary (English Channel, France)

A two year survey of benthic primary production during periods of emersion was performed on two stations of an intertidal mudflat (a muddy-sand station and a muddy station) in the Seine Estuary (English Channel, France). The goals of this study were to investigate the seasonal variations of metabolism, to estimate daily potential primary production variation at the annual scale and to estimate the annual potential primary production of the mudflat. Primary production and respiration were estimated by in situ measurements of carbon dioxide fluxes. Chlorophyll a concentration exhibited a great variability on both locations. Gross community production ranged from ca. 0 to 77 mg C m(-2) h(-1) at the muddy-sand location and from ca. 0 to 122 mg C m(-2) h(-1) at the muddy location. Community respiration showed a seasonal trend following temperature variations (up to 28.51 mg C m(-2) h(-1) in the muddy-sand and up to 23.40 mg C m(-2) h(-1) in the mud). Daily potential primary production was calculated, according to seasonal variations of photosynthetic parameters calculated using three photosynthesis versus irradiance curves obtained for the muddy location. The annual gross community primary production was 135 g C m(-2) yr(-1), leading to a low autotrophic annual budget, considering an annual community respiration of 110 g C m(-2) yr(-1).     

The influence of stratification on heat and momentum turbulent transfer in antarctica

Data from the Antarctic winter at Halley Base have been used in order to evaluate qualitatively and quantitatively how the stratification in the low atmosphere (evaluated with the gradient Richardson number, Ri) influences the eddy transfers of heat and momentum. Vertical profiles of wind and temperature up to 32 m, and turbulent fluxes ( , and ) measured from three ultrasonic thermo-anemometers installed at 5, 17 and 32 m are employed to calculate Ri, the friction velocity (u _*) and the eddy diffusivities for heat (K _h ) and momentum (K _m ). The results show a big dependence of stability onK _m ,K _h andu _*, with a sharp decrease of these turbulent parameters with increasing stability. The ratio of eddy diffusivities (K _h /K _m ) is also analyzed and presents a decreasing tendency as Ri increases, reaching values even less than 1, i.e., there were situations where the turbulent transfer of momentum was greater than that of heat. Possible mechanisms of turbulent mixing are discussed.     

Large-Scale Temperature Changes across the Southern Andes: 20th-Century Variations in the Context of the Past 400 Years

Long-term trends of temperature variations across the southern Andes (37–55° S) are examined using a combination of instrumental and tree-ring records. A critical appraisal of surface air temperature from station records is presented for southern South America during the 20th century. For the interval 1930–1990, three major patterns in temperature trends are identified. Stations along the Pacific coast between 37 and 43° S are characterized by negative trends in mean annual temperature with a marked cooling period from 1950 to the mid-1970s. A clear warming trend is observed in the southern stations (south of 46°S), which intensifies at higher latitudes. No temperature trends are detected for the stations on the Atlantic coast north of 45° S. In contrast to higher latitudes in the Northern Hemisphere where annual changes in temperature are dominated by winter trends, both positive and negative trends in southern South America are due to mostly changes in summer (December to February) temperatures. Changes in the Pacific Decadal Oscillation (PDO) around 1976 are felt in summer temperatures at most stations in the Pacific domain, starting a period with increased temperature across the southern Andes and at higher latitudes.Tree-ring records from upper-treeline were used to reconstruct past temperature fluctuations for the two dominant patterns over the southern Andes. These reconstructions extend back to 1640 and are based on composite tree-ring chronologies that were processed to retain as much low-frequency variance as possible. The resulting reconstructions for the northern and southern sectors of the southern Andes explain 55% and 45% ofthe temperature variance over the interval 1930–1989, respectively. Cross-spectral analysis of actual and reconstructed temperatures over the common interval 1930–1989, indicates that most of the explained varianceis at periods >10 years in length. At periods >15 years, the squaredcoherency between actual and reconstructed temperatures ranges between 0.6 and 0.95 for both reconstructions. Consequently, these reconstructions are especially useful for studying multi-decennial temperature variations in the South American sector of the Southern Hemisphere over the past 360 years. As a result, it is possible to show that the temperatures during the 20thcentury have been anomalously warm across the southern Andes. The mean annual temperatures for the northern and southern sectors during the interval 1900–1990 are 0.53 °C and 0.86 °C above the1640–1899 means, respectively. These findings placed the current warming in a longer historical perspective, and add new support for the existence of unprecedented 20th century warming over much of the globe. The rate of temperature increase from 1850 to 1920 was the highest over the past 360 years, a common feature observed in several proxy records from higher latitudes in the Northern Hemisphere.Local temperature regimes are affected by changes in planetary circulation, with in turn are linked to global sea surface temperature (SST) anomalies. Therefore, we explored how temperature variations in the southern Andes since 1856 are related to large-scale SSTs on the South Pacific and South Atlantic Oceans. Spatial correlation patterns between the reconstructions and SSTs show that temperature variations in the northern sector of the southern Andes are strongly connected with SST anomalies in the tropical and subtropical Pacific. This spatial correlation pattern resembles the spatial signature of the PDO mode of SST variability over the South Pacific and is connected with the Pacific-South American (PSA) atmospheric pattern in the Southern Hemisphere. In contrast, temperature variations in the southern sector of the southern Andes are significantly correlated with SST anomalies over most of the South Atlantic, and in less degree, over the subtropical Pacific. This spatial correlation field regressed against SST resembles the `Global Warming' mode of SST variability, which in turn, is linked to the leading mode of circulation in the Southern Hemisphere. Certainly, part of the temperature signal present in the reconstructions can be expressed as a linear combination of four orthogonal modes of SST variability. Rotated empirical orthogonal function analysis, performed on SST across the South Pacific and South Atlantic Oceans, indicate that four discrete modes of SST variability explain a third, approximately, of total variance in temperature fluctuations across the southern Andes.     

Worth of secondary data compared to piezometric data for the probabilistic assessment of radionuclide migration

A common approach for the performance assessment of radionuclide migration from a nuclear waste repository is by means of Monte-Carlo techniques. Multiple realizations of the parameters controlling radionuclide transport are generated and each one of these realizations is used in a numerical model to provide a transport prediction. The statistical analysis of all transport predictions is then used in performance assessment. In order to reduce the uncertainty on the predictions is necessary to incorporate as much information as possible in the generation of the parameter fields. In this regard, this paper focuses in the impact that conditioning the transmissivity fields to geophysical data and/or piezometric head data has on convective transport predictions in a two-dimensional heterogeneous formation. The Walker Lake data based is used to produce a heterogeneous log-transmissivity field with distinct non-Gaussian characteristics and a secondary variable that represents some geophysical attribute. In addition, the piezometric head field resulting from the steady-state solution of the groundwater flow equation is computed. These three reference fields are sampled to mimic a sampling campaign. Then, a series of Monte-Carlo exercises using different combinations of sampled data shows the relative worth of secondary data with respect to piezometric head data for transport predictions. The analysis shows that secondary data allows to reproduce the main spatial patterns of the reference transmissivity field and improves the mass transport predictions with respect to the case in which only transmissivity data is used. However, a few piezometric head measurements could be equally effective for the characterization of transport predictions.     

Sensitivity of an atmospheric general circulation model to prescribed SST changes: feedback effects associated with the simulation of cloud optical properties

This paper was presented at the International Conference on Modelling of Global Climate Change and Variability, held in Hamburg 11–15 September 1989 under the auspices of the Meteorological Institute of the University of Hamburg and the Max Planck Institute for Meteorology. Guest Editor for these papers is Dr. L. Dumenil     

Changes in European greenhouse gas and air pollutant emissions 1960–2010: decomposition of determining factors

This paper analyses factors that contributed to the evolution of SO₂, NO_x and CO₂ emissions in Europe from 1960 to 2010. Historical energy balances, along with population and economic growth data, are used to quantify the impacts of major determinants of changing emission levels, including energy intensity, conversion efficiency, fuel mix, and pollution control. Time series of emission levels are compared for countries in Western and Eastern Europe, throwing light on differences in the importance of particular emission-driving forces. Three quarters of the decline in SO₂ emissions in Western Europe resulted from a combination of reduced energy intensity and improved fuel mix, while dedicated end-of-pipe abatement measures played a dominant role in the reduction of NO_x emissions. The increase in atmospheric emissions in Eastern Europe through the mid-1990s was associated with the growth of energy-intensive industries, which off-setted the positive impact of better fuel quality and changes in fuel mix. A continuous decrease in energy intensity and higher conversion efficiencies have been the main factors responsible for the moderate rate of growth of European CO₂ emissions.     

Digital thermal monitoring of the Amazon forest: an intercomparison of satellite and reanalysis products

Remote sensing and climate digital products have become increasingly available in recent years. Access to these products has favored a variety of Digital Earth studies, such as the analysis of the impact of global warming over different biomes. The study of the Amazon forest response to drought has recently received particular attention from the scientific community due to the occurrence of extreme droughts and anomalous warming over the last decade. This paper focuses on the differences observed between surface thermal anomalies obtained from remote sensing moderate resolution imaging spectroradiometer (MODIS) and climatic (ERA-Interim) monthly products over the Amazon forest. With a few exceptions, results show that the spatial pattern of standardized anomalies is similar for both products. In terms of absolute anomalies, the differences between the two products show a bias near to zero with a standard deviation of around 0.2?K, although the differences can be up to 1?K over particular regions and months. Despite this general agreement, the proper filtering of MODIS daily values in order to construct a new monthly product showed a dramatic reduction in the number of reliable pixels during the rainy season, while for the dry season this reduction is only seen in Northern Amazonia.     

A multi-technique approach for characterizing the SVN49 signal anomaly, part 1: receiver tracking and IQ constellation

A characterization of the signal anomaly of SVN49 is presented. A mathematical model is developed to relate the observed multipath to the internal signal reflection. The analyses provided are based on measurements, which have been collected during a dedicated tracking campaign with a 30-m dish antenna. Data on the L1 and L2 frequency have been collected with four different receivers. In addition, IQ samples have been recorded directly with a spectrum analyzer. The multipath combination of the receiver measurements on L1 and L2 is analyzed to demonstrate the effect of the signal reflections on different correlator spacing. The capability to suppress the signal reflection with receiver multipath mitigation methods is demonstrated. Finally, preliminary estimates of the attenuation, delay, and phase shift over elevation are obtained from an IQ sample analysis.     

Wide-angle airborne laser range data analysis for relative height determination of ground-based benchmarks

 A wide-angle airborne laser ranging system has been developed for the determination of relative heights of ground-based benchmarks in regional-scale networks (typically 100 laser reflectors spread over 100 km²). A first prototype demonstrated a 1–2 mm accuracy in radial distance measurement in a ground-based experiment in 1995. The first aircraft experiment was conducted in 1998, over a small area (1 km²) equipped with a network of 64 benchmarks. The instrument was modified before that experiment, in order to minimize echo superimposition due to the high density of benchmarks. New data processing algorithms have been developed, for the deconvolution of strongly overlapped echoes and a high a priori uncertainty in the aircraft flight path, and for the estimation of benchmark coordinates. A special methodology has been developed for the parameterization of these algorithms and of outlier detection tests. From a total of 2×10⁴ pseudo-range measurements, that have been acquired from two flights composed of 30 legs each, only 3×10³ remain after outlier detection. A positioning accuracy of 1.5 cm in the vertical coordinate (2.1 cm in the difference between the two flights) has been achieved. It is shown that the errors are normally distributed, with a nearly zero mean, and are consistent with the a posteriori uncertainty. It is also shown that the accuracy is limited mainly by the sensitivity of the photodetector used for this experiment (due to reduced response time). Another limiting factor is the effect of aircraft attitude changes during the measurements, which produces additional uncertainties in absolute distance measurements. It is planned to test new photodetectors with high internal gains. These should provide, in future experiments with smaller benchmark density, an improvement in signal-to-noise ratio of a factor of 5–10, leading to sub-centimeter vertical positioning accuracy. Received: 19 June 2001 / Accepted: 3 January 2002