Modellrechnungen zu winderzeugten Bewegungen in einem Meer mit Sprungschicht

Zusammenfassung Nach Erörterung der benutzten vertikal integrierten Bewegungsgleichungen für ein Meer mit Sprungschicht sowie der Ansätze für Schubspannung und Reibung werden die Beziehungen zwischen Spiegel- und Sprungflächenlage und die Wirkung des Windschubs auf einen geschichteten Kanal diskutiert. Es folgt die Beschreibung der Windschubwirkung auf tropische Modellmeere mit Sprungschicht. Insbesondere wird das Ergebnis einer den Verhältnissen im Arabischen Meer bei beginnendem Sommermonsun nahekommenden Modellrechnung beschrieben, in der sich die Sprungschicht vor der afrikanisch-arabischen Küste um mehrere Dekameter hebt (upwelling), vor der indischen Küste ebenso stark absenkt. Parallel diesen Küsten treten zwischen dem 8. und 16. Tag Wellenkämme und-täler in der Sprungschicht und in der Oberfläche in genau gleicher Art auf, wie sie A. Defant [1950/1951] aus Beobachtungen vor der kalifornischen Küste im Zusammenhang mit auf- und ablandigem Windschub abgeleitet hat. Wir zeigen noch, daß diese Wellen nichts mit fortschreitenden zu tun haben können und leiten die dafür gebräuchliche Formel für die Fortpflanzungsgeschwindigkeit aus den vertikal integrierten Bewegungsgleichungen ab.Abschließend wird das Modell einer idealisierten Nordsee mit sommerlicher Sprungschicht beschrieben, in dem das Massen- und Geschwindigkeitsfeld durch ein vorüberziehendes Tiefdruckgebiet Änderungen erfährt. Dem Luftdruckminimum folgt eine Mulde mit zyklonaler Wasserzirkulation in der Oberschicht nach, welche noch geraume Zeit nach Abzug des Tiefs anhält. Die Sprungschicht hebt sich südlich der Bahn des Tiefs um einige Meter und entfaltet eine kräftige, südwärts fortschreitende Wellenbewegung an der östlichen Begrenzung. Auch in der Unterschicht herrscht eine zyklonale Wasserzirkulation vor; das untere Wirbelzentrum bleibt aber hinter dem der Oberschicht zurück und holt es erst nach Abzug der Zyklone ein. Beide Wirbelzentren liegen dann unter einer sich abschwächenden Mulde in der Wasseroberfläche.Unsere Modelle zeigen, daß es mit den heutigen Hilfsmitteln im Prinzip möglich ist, nicht nur die Einflüsse aktueller meteorologischer Faktoren auf die Wasseroberfläche, sondern auch ihre Wirkung auf die Massenverteilung innerhalb des Meeres zu ermitteln. Zur modellmäßigen Darstellung der allgemeinen Zirkulation des Meeres sind allerdings die hier nicht berücksichtigten Vermischungsvorgänge in Rechnung zu stellen.

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Numerical models on wind produced motions in a sea with a pycnocline

Summary After a treatment of the system of vertically integrated equations of motion concerning a sea with a pycnocline we discuss the stress and friction terms used here and the general relations connecting the variation of the surface with that of the pycnocline as well as the effect of wind stress in a double-layered canal. After that we regard the effects of wind stress and pressure of air on tropical model oceans with a pycnocline. Especially the result of a numerical computation of the behaviour of an idealized Arabian Sea in the beginning of the summer monsoon is described: The pycnocline is lifting by several decameters in front of the African and Arabian shores (upwelling) and is descending by the same number of decameters in front of the Indian coast. Between the eighth and sixteenth day there are contrary wave crests and troughs in the surface and in the pycnocline parallel to the African and Indian shores of exactly the same kind as those already derived from observations in front of the Californian coast by A. Defant [1950/1951] in connection with on-shore an off-shore winds.Concluding, we describe the model of an idealized North Sea with a summer thermocline where the fields of mass and velocity are affected by a passing low. Its centre is followed by a trough in the surface and a cyclonic circulation in the upper layer continuing even a long time after the low had passed. In the south of its path the thermocline ascents some meters and at the eastern shore it develops a vigorous wave motion propagating southward. A cyclonic circulation prevails also in the layer beneath the thermocline; its centre lags behind the curl centre of the upper layer but at last joins it after the low left the model sea. Both curl centres then come to lie underneath the gradually diminishing trough in the sea surface.As shown by our models, the modern aids make it possible, not only to compute the effects of the actual meteorological events on the sea surface but also their effects on the mass distribution within the sea. If, however, a model of the general circulation in the oceans is to be given, there should, at any rate, be taken into account the mixing processes of water masses neglected here.

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Modèles numériques pour le calcul des mouvements dûs au vent dans une mer ayant une couche de densité à discontinuité

Résumé Pour une mer ayant une couche de densité à discontinuité on discute le système des équations de mouvements, intégrées verticalement et on expose les termes du frottement tangentiel dû au vent et ceux de la friction interne employés dans ce travail. Puis on traite les relations générales qui existent entre les variations de la surface et celles de la couche de discontinuité et on décrit les effets du frottement tangentiel dû au vent dans un canal à deux couches. Aprés cela on étudie l'effet du frottement tangentiel dû au vent et la pression atmosphérique exercée sur des modèles de mers tropiques ayant des couches de discontinuité. On décrit surtout le résultat du calcul numérique du comportement d'une mer idéalisée arabique au commencement de la mousson estivale. Suivant ce calcul, la couche de discontinuité s'élève par plusieurs décamètres (upwelling) devant la côte afro-arabique et elle s'abaisse en même degré devant la côte indienne. Entre le 8^ième et le 16^ième jour il se présente, parallèlement à ces côtes, des crêtes et des creux de vagues opposées dans la couche de discontinuité et dans celle de la surface. Ces vagues sont de la même espèce que celles que A. Defant [1950/1951] a déjà dérivées des observations faites devant la côte de la Californie en présence du frottement tangentiel dû aux vents de terre et de mer. On montrera que ces vagues ne peuvent avoir aucun rapport aux vagues progressives et nous dérivons des équations verticalement intégrées de mouvement la formule en usage pour la vitesse de propagation.Ênfin, on décrit le modèle de la mer idéalisée du Nord, dans laquelle on suppose une couche estivale de discontinuité. Dans ce modèle le champ de masse et le champ de vitesse subissent de changements par le passage d'une aire de basse pression, dont le minimum atmosphérique est suivi d'une dépression qui de sa part, dans la couche supérieure de l'eau, est accompagnée d'une circulation cyclonique, qui se maintient même quelque temps après le passage de l'aire de basse pression. Au sud de sa trajectoire la couche de discontinuité s'élève par plusieurs mètres; à sa limite orientale elle développe de fortes vagues qui se propagagent vers le sud. Dans la couche qui se trouve au-dessous de la couche de discontinuité, une circulation cyclonique est également prédominante; son centre est en retard sur celui de la couche supérieure qu'il ne rattrapera qu'après le passage du cyclone. Les deux centres cycloniques se trouvent alors au-dessous de la dépression en voie de s'exténuer à la surface de l'eau.Les modèles montrent que les ressources modernes permettent, en principe, de calculer non seulement les effets des facteurs météorologiques actuels à la surface de l'eau, mais aussi leurs effets sur la distribution des masses dans l'océan. Si l'on veut, cependant, représenter la circulation générale en mer au moyen d'un modèle, il est nécessaire de tenir compte des processus de mélange non considérés dans cette étude.

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Hierzu Tafel 7 mit Abb. 7a-e, Tafel 8 mit Abb. 11a-d, Tafel 9 mit Abb. 12a-e

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Herrn Professor A. Defant zum 81. Geburtstag gewidmet.     

Extraterrestrial chromite distribution across the mid-Ordovician Puxi River section, central China: Evidence for a global major spike in flux of L-chondritic matter

Previous studies of mid-Ordovician limestone in Sweden have shown that over a stratigraphic interval representing a few million years there is a two orders-of-magnitude enrichment in fossil L-chondritic meteorites (Ø = 1-21 cm) and sediment-dispersed extraterrestrial chromite (EC) grains (>63 μm). This has been interpreted as a dramatic increase in the flux of L-chondritic matter to Earth following the breakup of the L-chondrite parent body, which based on Ar-Ar gas retention ages (470 ± 6 Ma) of recently fallen meteorites occurred at about this time. Here we show that the general trend in the distribution of sediment-dispersed EC grains can be reproduced in the Puxi River section in central China. A total of 288 kg of limestone was searched for chrome spinels. In samples spanning the lower 8 m of the section, representing the Paroistodus originalis and Lenodus antivariabilis conodont zones, a total of 110 kg of limestone yielded only one EC grain. Similarly to the Swedish sections, EC grains begin to be common in the overlying L. variabilis Zone and remain common throughout the upper 9 m of the section, representing the L. variabilis, Yangtzeplacognathus crassus and L. pseudoplanus zones. In this part of the section 178 kg of limestone yielded 290 EC grains, with an average chemical composition very similar to chromite from recent L chondrites. In most of the beds over this interval one finds 1-4 EC grains per kilogram rock, a clear two orders-of-magnitude enrichment relative to the lower part of the section. Small bed-by-bed variations in the EC content over the upper interval most likely reflect small variations in sedimentation rates. The Puxi River section contains only very rare terrestrial chrome spinel grains, which can be distinguished already by their rounded, abraded appearance compared to the angular, pristine extraterrestrial spinels. In the mid-Ordovician, based on paleoplate reconstructions, the Puxi River site was positioned at mid-latitudes on the southern hemisphere a couple of thousand kilometers east of the Swedish sites. The prominent enrichment of EC grains over the same stratigraphic interval in China and Sweden is supporting evidence for a dramatic increase in the flux of L-chondritic matter to Earth shortly after the disruption of the L-chondrite parent body in the asteroid belt.     

Re-assessment of the paleoclimate implications of the Shell Bar in the Qaidam Basin,China

The Shell Bar in the Qaidam Basin, China, is a prominent geological feature composed of millions of densely packed Corbicula shells. Since the mid 1980s, it has been regarded as evidence for existence of a large lake during Marine Isotope Stage (MIS) 3 in the presently hyper-arid Qaidam Basin. Early studies suggested the bivalve shells accumulated at the shore of a large lake, whereas more recent work led to the conclusion that the Shell Bar was formed within a deeper water body. Based on our re-assessment of sediments and fossils from the Shell Bar, investigation of exposed fluvio-lacustrine sections upstream of the Shell Bar and study of nearby modern streams, we infer that the Shell Bar represents a stream deposit. Corbicula is a typical stream-dweller around the world. Preservation of Corbicula shells of different sizes, as well as occurrence of many articulated shells, provide evidence against post-mortem transport and accumulation along a lake shore. Additionally, the SE-NW alignment of the Shell Bar is similar to modern intermittent stream beds in its vicinity and corresponds to the present-day slope towards the basin centre further NW, and furthermore, the predominantly sandy sediments also indicate that the Shell Bar was formed in a stream. Abundant ostracod shells in the Shell Bar sediments originated from stream-dwelling species that are abundant in modern streams in the vicinity of the Shell Bar, or in part from fluvio-lacustrine sediments exposed upstream of the Shell Bar, as a result of erosion and re-deposition. Deflation of alluvial fine-grained sediments in the Shell Bar region and protection of the stream deposits by the large and thick-walled Corbicula shells reversed the former channel relief and yielded the modern exposure, which is a prominent morphological feature. Occurrence of Corbicula shells in the Qaidam Basin indicates climate was apparently warmer than present during the formation of the Shell Bar because Corbicula does not live at similar or higher altitudes in the region today. Because the Shell Bar is no longer considered a deposit formed within a lake, its presence does not indicate paleoclimate conditions wetter than today.     

GNSS satellite transmit power and its impact on orbit determination

Antenna thrust is a small acceleration acting on Global Navigation Satellite System satellites caused by the transmission of radio navigation signals. Knowledge about the transmit power and the mass of the satellites is required for the computation of this effect. The actual transmit power can be obtained from measurements with a high-gain antenna and knowledge about the properties of the transmit and receive antennas as well as losses along the propagation path. Transmit power measurements for different types of GPS, GLONASS, Galileo, and BeiDou-2 satellites were taken with a 30-m dish antenna of the German Aerospace Center (DLR) located at its ground station in Weilheim. For GPS, total L-band transmit power levels of 50–240 W were obtained, 20–135 W for GLONASS, 95–265 W for Galileo, and 130–185 W for BeiDou-2. The transmit power differs usually only slightly for individual spacecraft within one satellite block. An exception are the GLONASS-M satellites where six subgroups with different transmit power levels could be identified. Considering the antenna thrust in precise orbit determination of GNSS satellites decreases the orbital radius by 1–27 mm depending on the transmit power, the satellite mass, and the orbital period.     

Sub-Recent Sexual Populations of Limnocythere inopinata Recorded for the First Time from > 3500 m Altitude on the Tibetan Plateau

<正>The central and northwestern Tibetan Plateau region,also called"Qiangtang Plateau"(30°27'25"-35°39'13"N,83o41'14"-95°10'46"E),is the highest and largest arcticalpine area of the Earth covering approximately 600,000km~2 with altitude ranging from 4600 to 5100 m a.s.l.(Song et al.,2012,Fig.1).Ostracod species of     

C‐band SAR backscatter characteristics of Arctic sea and land ice during winter

Abstract

Synthetic Aperture Radar (SAR) data has become an important tool for studies of polar regions, due to high spatial resolution even during the polar night and under cloudy skies. We have studied the temporal variation of sea and land ice backscatter of twenty‐four SAR images from the European Remote Sensing satellite (ERS‐1) covering an area in Lady Ann Strait and Jones Sound, Nunavut, from January to March 1992. The presence of fast ice in Jones Sound and glaciers and ice caps on the surrounding islands provides an ideal setting for temporal backscatter studies of ice surfaces. Sample regions for eight different ice types were selected and the temporal backscatter variation was studied. The observed backscatter values for each ice type characterize the radar signatures of the ice surfaces. This time series of twenty‐four SAR images over a 3‐month period provides new insights into the degree of temporal variability of each surface. Ice caps exhibit the highest backscatter value of ‐3.9 dB with high temporal variability. Valley glacier ice backscatter values decrease with decreasing altitude, and are temporally the most stable, with standard deviations of 0.08–0.10 dB over the 90‐day period.

First‐year ice and lead ice show a negative trend in backscatter values in time and a positive correlation of up to 0.59 with air temperature over the 90‐day period. For first‐year ice and lead ice, episodes of large temperature fluctuations (±12°C) are associated with rapid changes in backscatter values (±2 dB). We attribute the backscatter increase to a temperature‐induced increase in brine volume at the base of the snow pack. Multi‐year ice, conglomerate ice and shore ice are relatively stable over the 3‐month period, with a backscatter variation of only a few dBs. An observed lag time of up to three days between backscatter increase/decrease and air temperature can be attributed to the insulation effect of the snow cover over sea ice. The net range of the backscatter values observed on the most temporally stable surface, valley glacier ice, of about 0.30 dB indicates that the ERS‐1 SAR instrument exceeds the 1 dB calibration accuracy specified for the Alaska SAR Facility processor for the three winter months.     

Karst spring responses examined by process-based modeling

Ground water in karst terrains is highly vulnerable to contamination due to the rapid transport of contaminants through the highly conductive conduit system. For contamination risk assessment purposes, information about hydraulic and geometric characteristics of the conduits and their hydraulic interaction with the fissured porous rock is an important prerequisite. The relationship between aquifer characteristics and short-term responses to recharge events of both spring discharge and physicochemical parameters of the discharged water was examined using a process-based flow and transport model. In the respective software, a pipe-network model, representing fast conduit flow, is coupled to MODFLOW, which simulates flow in the fissured porous rock. This hybrid flow model was extended to include modules simulating heat and reactive solute transport in conduits. The application of this modeling tool demonstrates that variations of physicochemical parameters, such as solute concentration and water temperature, depend to a large extent on the intensity and duration of recharge events and provide information about the structure and geometry of the conduit system as well as about the interaction between conduits and fissured porous rock. Moreover, the responses of solute concentration and temperature of spring discharge appear to reflect different processes, thus complementing each other in the aquifer characterization.     

Spatial and temporal changes in dry-snow line altitude on the Antarctic Peninsula

Drastic changes were detected in glacial systems of the Antarctic Peninsula in the last decades. The observed phenomena comprise the disintegration of ice shelves, acceleration and thinning of glaciers, and retreat of glacier fronts. However, due to the lack of consistent systematic observations in particular of the higher parts of the glacial systems, it is difficult to predict further responses of the Antarctic Peninsula glaciers to climatic change. The present paper analyses spatial and temporal variations of changes in the dry-snow line altitude on the Antarctic Peninsula as extracted from a time series (1992–2005) of ERS-1/2 SAR and Envisat ASAR data. Upward changes in dry-snow line altitude were observed in general, and are attributed to extreme high-temperature events impacting the central plateaus of the Antarctic Peninsula and the increasing duration of warming periods. A mean decrease in dry-snow line altitude was detected on the west side of the peninsula and is identified as a response to recorded increase in precipitation and accumulation. These results validate the capability of SAR data for deriving superficial parameters of glaciers to be used as indicators of climatic changes in high-latitude regions where operational restrictions limit conventional meteorological observations.