Certified coordinate selection for high-dimensional Bayesian inversion with Laplace prior

We consider high-dimensional Bayesian inverse problems with arbitrary likelihood and product-form Laplace prior for which we provide a certified approximation of the posterior in the Hellinger distance. The approximate posterior differs from the prior only in a small number of relevant coordinates that contribute the most to the update from the prior to the posterior. We propose and analyze a gradient-based diagnostic to identify these relevant coordinates. Although this diagnostic requires computing an expectation with respect to the posterior, we propose tractable methods for the classical case of a linear forward model with Gaussian likelihood. Our methods can be employed to estimate the diagnostic before solving the Bayesian inverse problem via, e.g., Markov chain Monte Carlo (MCMC) methods. After selecting the coordinates, the approximate posterior can be efficiently inferred since most of its coordinates are only informed by the prior. Moreover, specialized MCMC methods, such as the pseudo-marginal MCMC algorithm, can be used to obtain less correlated samples when sampling the exact posterior. We show the applicability of our method using a 1D signal deblurring problem and a high-dimensional 2D super-resolution problem

Certified coordinate selection for high-dimensional Bayesian inversion with Laplace prior

We consider high-dimensional Bayesian inverse problems with arbitrary likelihood and product-form Laplace prior for which we provide a certified approximation of the posterior density in the Hellinger distance. The approximate posterior density differs from the prior density only in a small number of relevant coordinates that contribute the most to the update from the prior to the posterior. We propose and analyze a gradient-based diagnostic to identify these relevant coordinates. Although this diagnostic requires computing an expectation with respect to the posterior, we propose tractable methods for the classical case of a linear forward model with Gaussian likelihood. Our methods can be employed to estimate the diagnostic before solving the Bayesian inverse problem via, e.g., Markov chain Monte Carlo (MCMC) methods. After selecting the coordinates, the approximate posterior density can be efficiently inferred since most of its coordinates are only informed by the prior. Moreover, specialized MCMC methods, such as the pseudo-marginal MCMC algorithm, can be used to obtain less correlated samples when sampling the exact posterior density. We show the applicability of our method using a 1D signal deblurring problem and a high-dimensional 2D super-resolution problem

First retrievals of carbon tetrafluoride (CF4) from ground-based FTIR measurements: production and analysis of the two-decadal time series above the Jungfraujoch

Carbon tetrafluoride (CF4 or PFC-14) is a potent greenhouse gas that is almost 7400 times more effective (100-yr horizon) than CO2 on a per molecule basis (IPCC, 2007). This high global warming potential, coming from its medium absorbance combined with a very long atmospheric lifetime (>50000 years; Ravishankara et al., 1993), makes CF4 a key species among the various greenhouse gases targeted by the Kyoto Protocol. In the Northern hemisphere, current atmospheric CF4 concentrations are close to 78 pptv, with a large fraction (around 35 pptv, Mühle et al., 2010) coming from natural processes like lithospheric emissions (Harnisch and Eisenhauer, 1998). In addition, CF4 has been used increasingly since the eighties in electronic and semiconductors industry. The primary aluminum production processes have also been clearly identified as an important anthropogenic source of CF4 emissions. The partitioning between these two main sources is however problematic, principally due to lacking or incomplete CF4 emission factors from inventories performed in industrial fields (e.g. International Aluminum Institute, 2009). Recent in situ ground level measurements of CF4 in the Northern hemisphere (Khalil et al., 2003; Mühle et al., 2010) or remotely from space (Rinsland et al., 2006) have indicated a significant slowdown in the increase rate of atmospheric CF4. This probably results from measures adopted by the aluminum industry aiming at the reduction of the frequency and duration of “anode effects” and therefore of related PFCs emissions (International Aluminum Institute, 2009). The present contribution reports on the long-term evolution (1990-2010) of the atmospheric carbon tetrafluoride total vertical abundance derived from ground-based Fourier transform infrared (FTIR) solar spectroscopy observations around 1285 cm-1 at the Jungfraujoch (46.5°N, 8.0°E, 3580m asl) and compares our findings with results available in the literature. To our knowledge, no equivalent time series (i.e. based on ground-based FTIR technique) has been published to date

Surveillance de l'atmosphère terrestre depuis la station du Jungfraujoch : une épopée liégeoise entamée voici plus de 65 ans !

C’est au début des années 1950 que des chercheurs de l’Université de Liège ont entrepris de caractériser l’atmosphère terrestre depuis la station scientifique du Jungfraujoch, dans les Alpes suisses, à une époque où les inquiétudes liées à l’évolution de la composition atmosphérique étaient inexistantes. Depuis, une spectrothèque infrarouge unique au monde a été soigneusement constituée. L’exploitation de ces observations a permis de constituer des séries temporelles multi-décennales indispensables à la caractérisation des changements subis par l’atmosphère et à l’identification de leurs causes. Après un bref historique rappelant les étapes qui ont jalonné la mise en place du programme observationnel liégeois et les découvertes qui ont justifié sa poursuite, nous passons en revue quelques résultats-clefs récemment acquis, en lien avec les Protocoles de Montréal et de Kyoto ou la surveillance de la qualité de l’air.It’s in the early 1950s that researchers from the University of Liège started to investigate the Earth’s atmosphere from the Jungfraujoch scientific station, in the Swiss Alps, at a time when concerns related to atmospheric composition changes were nonexistent. Since then, an infrared observational data base unique worldwide has been carefully collected. The exploitation of these observations has allowed constituting multi-decadal time series crucial for the characterization of the changes that affected our atmosphere and for the identification of their causes. In this paper, we first remind about the successive steps which led to establishing the observational program of the Liège team at the Jungfraujoch and we evoke important findings which justified its continuation. Then we present some recent results relevant to the Montreal and Kyoto Protocols, or related to the monitoring of air quality

An approach to retrieve information on the carbonyl fluoride (COF2) vertical distributions above Jungfraujoch by FTIR multi-spectrum multi-window fitting

We present an original multi-spectrum fitting procedure to retrieve volume mixing ratio (VMR) profiles of carbonyl fluoride (COF2) from ground-based high resolution Fourier transform infrared (FTIR) solar spectra. The multi-spectrum approach consists of simultaneously combining, during the retrievals, all spectra recorded consecutively during the same day and with the same resolution. Solar observations analyzed in this study with the SFIT-2 v3.91 fitting algorithm correspond to more than 2900 spectra recorded between January 2000 and December 2007 at high zenith angles, with a Fourier Transform Spectrometer operated at the high-altitude International Scientific Station of the Jungfraujoch (ISSJ, 46.5° N latitude, 8.0° E longitude, 3580 m altitude), Switzerland. The goal of the retrieval strategy described here is to provide information about the vertical distribution of carbonyl fluoride. The microwindows used are located in the ν4 or in the ν4 COF2 infrared (IR) absorption bands. Averaging kernel and eigenvector analysis indicates that our FTIR retrieval is sensitive to COF2 inversion between 17 and 30 km, with the major contribution to the retrieved information always coming from the measurement. Moreover, there was no significant bias between COF2 partial columns, total columns or VMR profiles retrieved from the two bands. For each wavenumber region, a complete error budget including all identified sources has been carefully established. In addition, comparisons of FTIR COF2 17–30 km partial columns with KASIMA and SLIMCAT 3-D CTMs are also presented. If we do not notice any significant bias between FTIR and SLIMCAT time series, KASIMA COF2 17–30 km partial columns are lower of around 25%, probably due to incorrect lower boundary conditions. For each times series, linear trend estimation for the 2000–2007 time period as well as a seasonal variation study are also performed and critically discussed. For FTIR and KASIMA time series, very low COF2 growth rates (0.4±0.2%/year and 0.3±0.2%/year, respectively) have been derived. However, the SLIMCAT data set gives a slight negative trend (−0.5±0.2%/year), probably ascribable to discontinuities in the meteorological data used by this model. We further demonstrate that all time series are able to reproduce the COF2 seasonal cycle, which main seasonal characteristics deduced from each data set agree quite well

Compact laser altimeter dedicated to drone ballistic carriers

peer reviewedOver the past few decades, the use of low cost Miniature Unmanned Aerial Vehicles (MUAV), as well as Micromechanical Flying Insects (MFI) in targets aerial surveillance and reconnaissance has experienced almost an exponentially growth not only in conventional military areas, but also in civil, industrial and aerospace applications. Despite obvious inherent advantages, such as a very attractive cost-efficiency ratio and a low risk to the drone operator safety, MUAV/MFI based technologies still present multiple challenges. The major drawback of conventional MUAV/MFI is that a relatively low on-board energy storage capacity and an inherent weakness of the MUAV/MFI propulsion drive systems. It results in a relatively low flight speed and an inability to fly in bad meteorological conditions, such as strong wind and could be critical for many applications, such as the forest fires or nuclear accident zones monitoring. In order to overcome these drawbacks, we propose to deliver MUAV/MFI to the designated target zone using ballistic carriers, such as mortar round or infantry man-portable unguided rocket. Accordingly, our research team intends to develop a prototype of MUAV/MFI ballistic carrier involving standard mortar rounds of 81 mm and 120 mm calibers. In this paper, we report the results of theoretical and experimental investigations on the MUAV/MFI ballistic carriers being developed by our research team and we address specifically the concept of compact laser altimeter dedicated to the ballistic carriers, as well as the first proof-of-concept experimental investigations, demonstrating its practical feasibility. One of the main challenges facing the implementation of the proposed concept of the mortar launched altimeter is to ensure its reliable operation after extremely high launch shock accelerations. This requirement determines the optical concept and materials selection and optimization, as well as the accuracy of alignment of the optical system. Recent mortar gun tests, performed in the frame of our experimental investigations, demonstrated that the key optical and electronic elements and sub-systems of the developed altimeter prototype can withstand launch accelerations of at least 5000g, as well as typical shock accelerations during parachute deployment phase. In additional, it should be noted that the fuze front surface area available for the placement of rangefinder optical lens assemblies is limited by military standards requirements, as well as by the air intake duct of the air drive turbine used as a power generator, that also poses some challenges for integrating the altimeter into mortar fuze head. The experimental investigations of the first altimeter prototype developed in compliance with these requirements, demonstrated the enabling range finding up to 1000 m, when operating at night time environments.ELECTROHO