Geostatistical simulation of two-dimensional fields of raindrop size distributions at the meso-¿ scale

The large variability of the raindrop size distribution (DSD) in space and time must be taken into account to improve remote sensing of precipitation. The ability to simulate a large number of 2-D fields of DSDs sharing the same statistical properties provides a very useful simulation framework that nicely complements experimental approaches based on DSD ground measurements. These simulations can be used to investigate radar beam propagation through rain and to evaluate different radar retrieval techniques. The proposed approach uses geostatistical methods to provide structural analysis and stochastic simulation of DSD fields. First, the DSD is assumed to follow a Gamma distribution with three parameters. As a consequence, 2-D fields of DSDs can be described as a multivariate random function. The parameters are normalized using a Gaussian anamorphosis and simulated by taking advantage of fast Gaussian simulation algorithms. Variograms are used to characterize the spatial structure of the DSD fields. The generated fields have identical spatial structure and are consistent with the observations. Because intermittency cannot be simulated using this technique, the size of the simulation domain is limited to the meso-¿ scale (2-20 km). To assess the proposed approach, the method is applied to data collected during intense Mediterranean rainfall. Taylor's hypothesis is invoked to convert time series into 1-D range profiles. The anisotropy of the fields is derived from radar measurements. Simulated and measured reflectivity fields are in good agreement with respect to the mean, the standard deviation, and the spatial structure, demonstrating the promising potential of the proposed stochastic model of DSD field

Auswirkungen von Komposten und von Gärgut auf die Umwelt, Bodenfruchtbarkeit, sowie die Pflanzengesundheit: Ökologische Bewertung der organischen Substanz

Bisher war es für Kompost und Gärgut in Ökobilanzen üblich, die Nährstoffe als Substitute von Mineraldüngern und die Schwermetalle als Belastungen einzusetzen. Im Vergleich zur Verbrennung, wo die organische Substanz vollständig mineralisiert und energetisch genutzt wird, gibt es bei Kompost und Gärgut eine Lücke: wie soll die organische Substanz bewertet werden? Zur Bewertung der organischen Substanz in Ökobilanzen gibt es zwei grundlegend verschiedene Vorgehensweisen: 1. Einsetzen der einzelnen Effekte wie Erosionsreduktion, weniger Zugkraftbedarf, bessere Wasserhaltekapazität, weniger Krankheitsanfälligkeit etc. mit jeweils entsprechenden LCA-Modulen 2. Globales Einsetzen von Substituten, welche die Effekte möglichst gut abbilden. Gegen die erste Vorgehensweise sprechen die lückenhafte Datenbasis, der hohe Aufwand für die einzelnen Module und die Problematik der Interaktionen zwischen einzelnen Effekten. Das globale Einsetzen gibt ein rasterartiges Bild, das die physikalischen Effekte relativ gut abbildet, aber die biologischen Effekte (z.B. Krankheitsunterdrückung) nicht vollständig abdecken kann. Der Vorteil dieses Ansatzes besteht darin, dass man mit bestehenden Modulen arbeiten kann und diese gleichzeitig die Interaktionen nicht stören (die alternative organische Substanz wird nur einmal angewendet und erzeugt die Effekte). Aufgrund von Machbarkeitsüberlegungen gelangt die Studie zum Schluss, dass im Moment nur das globale Einsetzen von Substituten zum Erfolg führen kann. Als Substitut werden für die landwirtschaftliche Anwendung Stroh und für die gärtnerische Anwendung Torf gewählt. Es wird angenommen, dass zwei Drittel der Menge von Kompost und Gärgut in der Landwirtschaft eingesetzt wird. Das dritte Drittel, das im Gartenbau Anwendung findet, kann durch den dort üblichen Torf ersetzt werden. Die Mengen der Substituten werden aufgrund der Fähigkeit Humus zu reproduzieren berechnet. Ein Reifkompost kann fast viermal so viel Humus ersetzen wie frische organische Substanz in Stroh oder Gründüngung. Es wird auf viele Wissenslücken in diesem Bereich hingewiesen

Mass-based depth and velocity scales for gravity currents and related flows

Gravity driven flows on inclines can be caused by cold, saline or turbid inflows into water bodies. Another example are cold downslope winds, which are caused by cooling of the atmosphere at the lower boundary. In a well-known contribution, Ellison and Turner (ET) investigated such flows by making use of earlier work on free shear flows by Morton, Taylor and Turner (MTT). Their entrainment relation is compared here with a spread relation based on a diffusion model for jets by Prandtl. This diffusion approach is suitable for forced plumes on an incline, but only when the channel topography is uniform, and the flow remains supercritical. A second aspect considered here is that the structure of ET's entrainment relation, and their shallow water equations, agrees with the one for open channel flows, but their depth and velocity scales are those for free shear flows, and derived from the velocity field. Conversely, the depth of an open channel flow is the vertical extent of the excess mass of the liquid phase, and the average velocity is the (known) discharge divided by the depth. As an alternative to ET's parameterization, two sets of flow scales similar to those of open channel flows are outlined for gravity currents in unstratified environments. The common feature of the two sets is that the velocity scale is derived by dividing the buoyancy flux by the excess pressure at the bottom. The difference between them is the way the volume flux is accounted for, which—unlike in open channel flows—generally increases in the streamwise direction. The relations between the three sets of scales are established here for gravity currents by allowing for a constant co-flow in the upper layer. The actual ratios of the three width, velocity, and buoyancy scales are evaluated from available experimental data on gravity currents, and from field data on katabatic winds. A corresponding study for free shear flows is referred to. Finally, a comparison of mass-based scales with a number of other flow scales is carried out for available data on a two-layer flow over an obstacle. Mass-based flow scales can also be used for other types of flows, such as self-aerated flows on spillways, water jets in air, or bubble plume

Propagation of surge waves in channels with large-scale bank roughness

In open channels, a sudden rise in water elevation generates a positive surge. Positive surges are commonly observed in man-made channels (Bazin 1865, Treske 1994) and a natural occurrence is the tidal bore in macro-tidal estuaries (Tricker 1965, Chanson 2011a). The positive surge may propagate upstream or downstream (Fig. D1). It is a rapidly-varied flow and the flow properties immediately upstream and downstream of the front must satisfy the continuity and momentum principles (Rouse 1938, Liggett 1994). The authors investigated positive surge waves in a long channel with a range of sidewall configuration. Their configuration corresponded to a downstream surge configuration (Fig. D1, right). The contribution is a welcome addition to the literature on rapidly-varied unsteady open channel flows. In this discussion, it is shown that the effects of boundary friction were previously documented, and a recent investigation provided some insight into the energy dissipation induced by large-scale sidewall roughness