Selection of Streets from a Network Using Self-Organizing Maps

We propose a novel approach to selection of important streets from a network, based on the technique of a self‐organizing map (SOM), an artificial neural network algorithm for data clustering and visualization. Using the SOM training process, the approach derives a set of neurons by considering multiple attributes including topological, geometric and semantic properties of streets. The set of neurons constitutes a SOM, with which each neuron corresponds to a set of streets with similar properties. Our approach creates an exploratory linkage between the SOM and a street network, thus providing a visual tool to cluster streets interactively. The approach is validated with a case study applied to the street network in Munich, Germany.     

Formation of turbid ice during autumn freeze-up in the Kara Sea

A one-dimensional (vertical) model is used to estimate the mass of ice-rafted sediment in turbid sea ice on the shallow Kara Sea shelf during autumn freeze-up. Sediment is entrained into the ice through aggregation with frazil ice crystals that are diffused downwards by wind-generated turbulence. Data from local meteorological stations are used to force the model, while water stratification and sediment concentrations from the area are used to initiate the model. Model results indicate a 0.2 m thick layer of slush ice created during 48 h with a mean wind of 6 m/s and an air temperature of −10°C. This ice contains ca. 20 mg/1 of sediment, or in total ca. 2% of the annual sediment discharge by nearby rivers. In shallow areas (<20 m depth) the process is very effective with winds of ca. 12 m/s, and the process can incorporate many years of sediment discharge. In the deeper areas (>20 m depth), the strong salinity stratification implies that winds above 18 m/s are needed for the process to be effective. For the rest of the winter months the same process may lead to additional sediment incorporated in a coastal polynya, but the freeze-up alone has the capacity to incorporate the total summer discharge of sediment into the surface ice. Calculated sediment concentrations in the surface ice cover are in the range 3 mg/1-19 g/1, in good agreement with available field data.     

Triassic lacustrine red-beds and palaeoclimate: The „Buntsandstein“ of Helgoland and the malmros Klint member of East Greenland

The playa-lake origin of the Lower Triassic Buntsandstein of Helgoland and the Upper Triassic Malmros Klint Member of East Greenland is suggested by: the continental setting; absence of marine fossils and occurrence of fresh-water trace fossils; evidence of shallow water, fluctuations in water level, frequent subaerial exposure and oxidizing depositional conditions; evidence of low-energy wave-action and absence of evidence for tidal influence. The Buntsandstein contains aeolian dunes and evaporites, but only rare fluvial sandstones and very rare fresh-water trace fossils. In contrast the Malmros Klint Member contains no aeolian sandstones and evaporites, but common fluvial sandstones and numerous fresh-water trace fossils. Wave ripple crest orientation suggests that both sequences were deposited during the influence of alternating NE and SE trade winds. The Buntsandstein appears to have been deposited in a central trade wind region with dominant winds from SE and short rainy seasons; the Malmros Klint Member was apparently deposited in a marginal trade wind region with dominant NE winds and longer rainy seasons.

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Zusammenfassung Der Buntsandstein auf Helgoland und der Malmros Klint Member (Obere Trias) in Ostgrönland werden als Playaseebildungen interpretiert auf Grund von: Vorkommen von Süßwasser-Lebenspuren und Mangel an marinen Fossilien; Beweise für Seichtwasser, Schwankungen in Wassertiefen, häufige Trockenlegung und oxydierende Ablagerungsverhältnisse; Beweise für schwachen Wellenschlag aber Mangel an Gezeiten; der stratigraphischen Einfügung in einer kontinentalen Schichtenfolge. Der Buntsandstein enthält Winddünen und Evaporite, aber nur wenige fluviatile Sandsteine und einzelne Süßwasser-Lebenspuren. Im Gegensatz dazu enthält der Malmros Klint Member keine äolische Sandsteine und keine Evaporite, sondern viele fluviatile Sandsteine und zahlreiche Süßwasser-Lebenspuren. Die Orientierung der Oszillationsrippelkämme beweist vermutlich, daß die beiden Schichtenfolgen während des Einflusses umlaufender Nordost- und Südostpassatwinde abgelagert wurden. Der Buntsandstein wurde wahrscheinlich in einer zentralen Passatwindregion mit vorherrschenden Südostwinden und kurzen Regenperioden abgelagert; der Malmros Klint Member wurde wahrscheinlich in einer Randpassatwindregion mit vorherrschenden Nordostwinden und längeren Regenperioden abgelagert.

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Résumé Le Grès bigarré (Trias inférieur) de Héligoland et le Malmros Klint Member (Trias supérieur) du Groenland oriental sont interprétés comme des formations lacustres éphémères à cause de: présence d'ichnofossiles d'eau douce et l'absence de fossiles marins; évidence d'eau peu profonde, variation de la profondeur d'eau fréquentes dessiccations et conditions sédimentaires oxydantes; évidence d'action calme des vagues, mais absence de marée; 1'emplacement stratigraphique dans une séquence continentale. Le Grès bigarré contient des dépôts éoliens et des évaporites, mais peu de grès fluviatiles et des ichnofossiles rares d'eau douce. Par contre le Malmos Klint Member ne contient ni de grès éoliens, ni d'évaporites, mais plusieurs grès fluviatiles et de nombreux ichnofossiles d'eau douce. L'orientation des crêtes ripplemarks semble prouver que les deux séquences ont été déposées sous l'influence des alizés du nord-est et du sud-est. Le Grès bigarré de Héligoland semble s'être déposé dans une région centrale des alizés avec des vents dominants du sud-est et de brèves saisons pluvieuses. Evidemment le Malmros Klint Member s'est déposé dans une région marginale des alizés avec des vents dominants du nord-est et des saisons pluvieuses plus longues.

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The length of closed geomorphic lines

This paper is based on the study of shore line length of 12 Swedish lakes on various maps ranging in scale from 1:10,000 1:1,000,000. The lakes differ in size, from Lake Munksjön, which has an area of 1.1 km², to Lake Vänern with an area of 5893 km², and also in shore line irregularity, ranging from the rather regular basins of Lake Munksjön and Lake Erken to the very irregular basin of Lake Mälaren. A “new ” method, the checkered transparent paper method (the CTP-method), was adopted to measure the shore length of certain lakes on various maps. Length determination by this method can be executed quickly and easily, and in a statistically definable way, giving comparable data from various types of map. A formula defining the functional relationship between scale, shore irregularity, shore length, and lake area has been derived: $$NF = F(K_{2} - K_{1})/[K_{2} - log(s + a)]$$ or $$1_{n} = 1(K_{2} - K_{1} )/[K_{2} - log(s + a)]$$ where NF = the normalized shore development (shore irregularity) at a scale of 1:1; F = the shore development as determined on a given map scale; s = the scale factor (10,000, 50,000 etc); a = 10⁵ ? log A, where 10⁵ = the area constant; A = the lake area in km²; K₁ = log(s + a) for s = 1, i.e. the reference scale; K₂ = log(s + a) for s = 6,000,000, where 6,000,000 is called the scale constant; 1 = the shore length as determined by the CTP-method on a given map; and 1_n = the normalized shore length at a scale of 1:1. The formula offers a high degree of accuracy and the length of any closed geomorphic line can be determined independently of map scale, under given practical limitations. The length value obtain is the normalized length, that is the best approximation of the real, natural length at a scale of 1:1.     

Plasma and Electromagnetic Simulations of Meteor Head Echo Radar Reflections

Recently, meteor head echo detections from high powered large aperture radars (HPLA) have brought new measurements to bear on the study of sporadic interplanetary meteors. These same observations have demonstrated an ability to observe smaller meteoroids without some of the geometrical restrictions of specular radar techniques. Yet incorporating data from various radar reflection types and from different radars into a single consistent model has proven challenging. We believe this arises due to poorly understood radio scattering characteristics of the meteor plasma, especially in light of recent work showing that plasma turbulence and instability greatly influences meteor trail properties at every stage of evolution. In order to overcome some of the unknown relationships between meteoroid characteristics (such as mass and velocity) and the resulting head echo radar cross-sections (RCS), we present our results on meteor plasma simulations of head echo plasmas using particle in cell (PIC) ions, which show that electric fields strongly influence early stage meteor plasma evolution, by accelerating ions away from the meteoroid body at speeds as large as several kilometers per second. We also present the results of finite difference time domain electromagnetic simulations (FDTD), which can calculate the radar cross-section of the simulated meteor plasma electron distributions. These simulations have shown that the radar cross-section depends in a complex manner on a number of parameters. In this paper we demonstrate that for a given head echo plasma the RCS as a function of radar frequency peaks at sqrt (2*peak plasma frequency) and then decays linearly on a dB scale with increasing radar frequency. We also demonstrate that for a fixed radar frequency, the RCS increases linearly on a dB scale with increasing head echo plasma frequency. These simulations and resulting characterization of the head echo radar cross-section will both help relate HPLA radar observations to meteoroid properties and aid in determining a particular radar facility’s ability to observe various meteoroid populations.     

The geoid-to-quasigeoid difference using an arbitrary gravity reduction model

Recently it was proved that the classical formula for computing the geoid to quasigeoid separation (GQS) by the Bouguer gravity anomaly needs a topographic correction. Here we generalize the modelling of the GQS not only to Bouguer types of anomalies, but also to arbitrary reductions of topographic gravity. Of particular interest for practical applications should be isostatic and Helmert types of reductions, which provide smaller and smoother components, more suitable for interpolation and calculation, than the Bouguer reduction.     

Modelling the tidal mixing fronts and seasonal stratification of the Northwest European Continental shelf

We investigate mixing processes under stratified conditions on the Northwest European Continental shelf using a numerical model (POLCOMS). Our results indicate that convection induced by vertical shearing of horizontal density gradients (‘shear-induced convection’) is a regularly occurring feature in the bottom and surface boundary layers in this open shelf-sea situation. Two types of turbulence models are investigated to study their capability for reproducing the observed location of tidal mixing fronts, and the physical processes occurring in seasonally stratified waters. The first model is a one-equation variant of the Mellor–Yamada model, whereas the second model combines a more recent second-momentum closure with a two-equation model. It is found that generally mean frontal positions (as estimated from ICES data) are predicted more accurately by the two-equation model. The one-equation model reproduces the mean frontal locations to 18.1 km (<3 grid spacings) and the two-equation model to 17.1 km; although in the Celtic Sea the accuracy is ∼33 and ∼12 km, respectively. Comparison with historical tide gauges, current metres, CTD stations, and thermistor chain data from the North Sea Project all show an improvement in accuracy when the two-equation model is used. This is particularly apparent in the model's ability to reproduce the spring–neap variability during stratification. We find that in the presence of shear-induced convection the routinely applied clipping of the turbulent length-scale, previously thought to be a minor ingredient in a turbulence model, has a dramatic effect on the results: if the length-scale clipping is not applied, substantial over-mixing is observed to occur. The causes and possible remedies of this effect are investigated. Overall our results demonstrate a sensitivity to the details of the turbulence model that is significantly greater than previously thought.