The sensitivity of landscape evolution models to spatial and temporal rainfall resolution

© Author(s) 2016. Climate is one of the main drivers for landscape evolution models (LEMs), yet its representation is often basic with values averaged over long time periods and frequently lumped to the same value for the whole basin. Clearly, this hides the heterogeneity of precipitation - but what impact does this averaging have on erosion and deposition, topography, and the final shape of LEM landscapes? This paper presents results from the first systematic investigation into how the spatial and temporal resolution of precipitation affects LEM simulations of sediment yields and patterns of erosion and deposition. This is carried out by assessing the sensitivity of the CAESAR-Lisflood LEM to different spatial and temporal precipitation resolutions - as well as how this interacts with different-size drainage basins over short and long timescales. A range of simulations were carried out, varying rainfall from 0.25 h × 5 km to 24 h × Lump resolution over three different-sized basins for 30-year durations. Results showed that there was a sensitivity to temporal and spatial resolution, with the finest leading to & gt; 100 % increases in basin sediment yields. To look at how these interactions manifested over longer timescales, several simulations were carried out to model a 1000-year period. These showed a systematic bias towards greater erosion in uplands and deposition in valley floors with the finest spatial- and temporal-resolution data. Further tests showed that this effect was due solely to the data resolution, not orographic factors. Additional research indicated that these differences in sediment yield could be accounted for by adding a compensation factor to the model sediment transport law. However, this resulted in notable differences in the topographies generated, especially in third-order and higher streams. The implications of these findings are that uncalibrated past and present LEMs using lumped and time-averaged climate inputs may be under-predicting basin sediment yields as well as introducing spatial biases through under-predicting erosion in first-order streams but over-predicting erosion in second- and third-order streams and valley floor areas. Calibrated LEMs may give correct sediment yields, but patterns of erosion and deposition will be different and the calibration may not be correct for changing climates. This may have significant impacts on the modelled basin profile and shape from long-timescale simulations

Multi-scale approaches for high-speed imaging and analysis of large neural populations

Progress in modern neuroscience critically depends on our ability to observe the activity of large neuronal populations with cellular spatial and high temporal resolution. However, two bottlenecks constrain efforts towards fast imaging of large populations. First, the resulting large video data is challenging to analyze. Second, there is an explicit tradeoff between imaging speed, signal-to-noise, and field of view: with current recording technology we cannot image very large neuronal populations with simultaneously high spatial and temporal resolution. Here we describe multi-scale approaches for alleviating both of these bottlenecks. First, we show that spatial and temporal decimation techniques based on simple local averaging provide order-of-magnitude speedups in spatiotemporally demixing calcium video data into estimates of single-cell neural activity. Second, once the shapes of individual neurons have been identified at fine scale (e.g., after an initial phase of conventional imaging with standard temporal and spatial resolution), we find that the spatial/temporal resolution tradeoff shifts dramatically: after demixing we can accurately recover denoised fluorescence traces and deconvolved neural activity of each individual neuron from coarse scale data that has been spatially decimated by an order of magnitude. This offers a cheap method for compressing this large video data, and also implies that it is possible to either speed up imaging significantly, or to “zoom out” by a corresponding factor to image order-of-magnitude larger neuronal populations with minimal loss in accuracy or temporal resolution

High-order harmonic spectroscopy for molecular imaging of polyatomic molecules

High-order harmonic generation is a powerful and sensitive tool for probing atomic and molecular structures, combining in the same measurement an unprecedented attosecond temporal resolution with a high spatial resolution, of the order of the angstrom. Imaging of the outermost molecular orbital by high-order harmonic generation has been limited for a long time to very simple molecules, like nitrogen. Recently we demonstrated a technique that overcame several of the issues that have prevented the extension of molecular orbital tomography to more complex species, showing that molecular imaging can be applied to a triatomic molecule like carbon dioxide. Here we report on the application of such technique to nitrous oxide (N2O) and acetylene (C2H2). This result represents a first step towards the imaging of fragile compounds, a category which includes most of the fundamental biological molecules

Towards attosecond 4D imaging of atomic-scale dynamics by single-electron diffraction

Many physical and chemical processes which define our daily life take place on atomic scales in space and time. Time-resolved electron diffraction is an excellent tool for investigation of atomic-scale structural dynamics (4D imaging) due to the short de Broglie wavelength of fast electrons. This requires electron pulses with durations on the order of femtoseconds or below. Challenges arise from Coulomb repulsion and dispersion of non-relativistic electron wave packets in vacuum, which currently limits the temporal resolution of diffraction experiments to some hundreds of femtoseconds. In order to eventually advance the temporal resolution of electron diffraction into the few-femtosecond range or below, four new concepts are investigated and combined in this work: First, Coulomb repulsion is avoided by using only a single electron per pulse, which does not repel itself but interferes with itself when being diffracted from atoms. Secondly, dispersion control for electron pulses is implemented with time-dependent electric fields at microwave frequencies, compressing the duration of single-electron pulses at the expense of simultaneous energy broadening. Thirdly, a microwave signal used for electron pulse compression is derived from an ultrashort laser pulse train. Optical enhancement allows a temporal synchronization between the microwave field and the laser pulses with a precision below one femtosecond. Fourthly, a cross-correlation between laser and electron pulses is measured in this work with the purpose of determining the possible temporal resolution of diffraction experiments employing compressed single-electron pulses. This novel characterization method uses the principles of a streak camera with optical fields and potentially offers attosecond temporal resolution. These four concepts show a clear path towards improving the temporal resolution of electron diffraction into the few-femtosecond domain or below, which opens the possibility of observing electron densities in motion. In this work, a compressed electron pulse's duration of 28±5 fs full width at half maximum (12±2 fs standard deviation) at a de Broglie wavelength of 0.08 Å is achieved. Currently, this constitutes the shortest electron pulses suitable for diffraction, about sixfold shorter than in previous work. Ultrafast electron diffraction now meets the requirements for investigating the fastest primary processes in molecules and solids with atomic resolution in space and time

CO emission and variable CH and CH+ absorption towards HD34078: evidence for a nascent bow shock ?

The runaway star HD34078, initially selected to investigate small scale structure in a foreground diffuse cloud has been shown to be surrounded by highly excited H2. We first search for an association between the foreground cloud and HD34078. Second, we extend previous investigations of temporal absorption line variations (CH, CH+, H2) in order to better characterize them. We have mapped the CO(2-1) emission at 12 arcsec resolution around HD34078's position, using the 30 m IRAM antenna. The follow-up of CH and CH+ absorption lines has been extended over 5 more years. In parallel, CH absorption towards the reddened star Zeta Per have been monitored to check the homogeneity of our measurements. Three more FUSE spectra have been obtained to search for N(H2) variations. CO observations show a pronounced maximum near HD34078's position, clearly indicating that the star and diffuse cloud are associated. The optical spectra confirm the reality of strong, rapid and correlated CH and CH+ fluctuations. On the other hand, N(H2, J=0) has varied by less than 5 % over 4 years. We also discard N(CH) variations towards Zeta Per at scales less than 20 AU. Observational constraints from this work and from 24 micron dust emission appear to be consistent with H2 excitation but inconsistent with steady-state bow shock models and rather suggest that the shell of compressed gas surrounding HD34078, is seen at an early stage of the interaction. The CH and CH+ time variations as well as their large abundances are likely due to chemical structure in the shocked gas layer located at the stellar wind/ambient cloud interface. Finally, the lack of variations for both N(H2, J=0) towards HD34078 and N(CH) towards Zeta Per suggests that quiescent molecular gas is not subject to pronounced small-scale structure.Comment: 19 pages, 15 figures, accepted for publication in A&

Fast Back-Projection for Non-Line of Sight Reconstruction

Recent works have demonstrated non-line of sight (NLOS) reconstruction by using the time-resolved signal frommultiply scattered light. These works combine ultrafast imaging systems with computation, which back-projects the recorded space-time signal to build a probabilistic map of the hidden geometry. Unfortunately, this computation is slow, becoming a bottleneck as the imaging technology improves. In this work, we propose a new back-projection technique for NLOS reconstruction, which is up to a thousand times faster than previous work, with almost no quality loss. We base on the observation that the hidden geometry probability map can be built as the intersection of the three-bounce space-time manifolds defined by the light illuminating the hidden geometry and the visible point receiving the scattered light from such hidden geometry. This allows us to pose the reconstruction of the hidden geometry as the voxelization of these space-time manifolds, which has lower theoretic complexity and is easily implementable in the GPU. We demonstrate the efficiency and quality of our technique compared against previous methods in both captured and synthetic dat

δ Orionis: Further temporal variability and evidence for small-scale structure in the interstellar medium

We report here the detection of both spatial and temporal variations in interstellar absorption in the line of sight to δ Orionis. First, we present new high-resolution (R≈110 000) observations of the interstellar D lines of Na i towards both δ Ori A and C. Comparison of these spectra highlights variations in absorption between the two stars, indicative of small-scale spatial structure in the interstellar medium in this direction over distances of less than ≈15 000 au (the projected separation of the two stars). Components with the largest Na i column densities and lowest velocity dispersions are, in general, found to be subject to the greatest differences; in fact the narrowest component detected is only observed in one of the sightlines. This effect has also been reported by Meyer & Blades. Secondly, we present new ultra-high-resolution (R≈900 000) Na i D1 observations and high-resolution (R≈110 000) Ca ii H & K observations of δ Ori A which, through ultra-high-resolution work conducted between 1994 and 2000, has been shown to exhibit a time-variable interstellar Na i absorption component. These new observations, while revealing the further reduction in intensity of the time-variable Na i absorption, indicate constant Ca ii absorption over the same period. This results in a dramatic reduction in the Na°/Ca+ abundance ratio, perhaps indicating the line of sight to be gradually probing a less-dense outer region of an absorbing filament