A simplified hydrodynamic mesoscale model suitable for use over high plateau regions

Summary A simplified hydrodynamic primitive equation model in -coordinates is described. This model includes the main physical processes that influence development and motion of mesoscale vortices over plateau regions: elevated terrain effects, effects of large-scale and synoptic-scale systems on the mesoscale systems, sensible heating from the earth's surface, moisture cycle and condensation heating released by both large-scale stable precipitation and cumulus convective precipitation, evaporation and the feedback effect of precipitation on evaporation, friction in the planetary boundary layer, horizontal diffusion by subgrid scale eddies and vertical eddy flux of meteorological elements (momentum, temperature, moisture, etc.) due to turbulence, cumulus convection and other smaller-scale physical processes. The model incorporates observed data as much as possible and equations are simplified so that it may be run using either a high-speed computer or a desktop computer.Two developing vortices that originated over the Qinghai-Xizang (Tibet) Platau and moved eastward producing heavy precipitation over the lower elevations are used to test the model. It is shown that the model is capable of simulating the major features of the vortices including the precipitation distribution and may be used for studies of mesoscale and synoptic scale weather systems over plateau regions.

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Ein vereinfachtes für hochländer geeignetes hydrodynamisches mesoscale-modell

Zusammenfassung Es wird ein vereinfachtes hydrodynamisches Grundgleichungsmodell in -Koordinaten be schrieben. Dieses Modell beinhaltet die wichtigsten physikalischen Prozesse, die die Entwicklung und Bewegung von mesoskaligen Wirbeln über Hochplateaus beeinflussen: Effekte der Hochfläche, Effekte von großräumigen und synoptischen Systemen, die fühlbare Wärme von der Erdoberfläche, den Feuchtigkeitskreislauf und die Kondensationswärme, sowohl von großräumigen als auch von Konvektionsniederschlägen, die Evaporation und die Beeinflussung der Evaporation durch den Niederschlag, die Reibung in der planetaren Grenzschicht, die horizontale Diffusion durch Turbulenzen, die kleiner als die Gitterkonstante sind und den vertikalen Fluß meteorologischer Elemente (Impuls, Temperatur, Feuchte etc.) durch Turbulenzen, Konvektion und andere kleinräumige Prozesse. In das Modell sind die Beobachtungswerte weitestgehend integriert und die Gleichungen soweit vereinfacht, daß es sowohl auf einem Hochleistungs- als auch auf einem Mikrocomputer laufen kann.Zum Testen des Modells wurden zwei sich entwickelnde Wirbel verwendet, die vom Qinghai-Xizang-Plateu (Hochland von Tibet) ausgingen, sich ostwärts bewegten und dabei zu schweren Niederschlägen in den Tiefländern führten. Es wird gezeigt, daß das Modell die wichtigsten Eigenschaften der Wirbel und die Niederschlagsverteilung simulieren und für Untersuchungen von mesoskaligen und synoptischen Wettersystemen über Hochländern herangezogen werden kann.

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With 18 figures     

Damage risk assessment of breakwaters under tsunami attack

Turkey was struck by two major events on 17 August and 12 November 1999, named Izmit (M _w = 7.4) and Düzce (M _w = 7.2) earthquakes, respectively. Rubble mound breakwaters in Izmit Bay experienced little damage, as forecasted by the new risk assessment model in which tsunami occurrence risk was included in the damage estimations. In order to determine the occurrence probability of structural damage under design conditions, including the environmental loading parameters of tsunami and storm waves, tidal range and storm surge, the Conditional Expections Monte Carlo simulation was applied in the risk assessment model developed in this study for the Esenköy Fishery Harbour, Turkey. A tsunami was not the key design parameter when compared to storm waves for the main breakwater of the harbour, however, in places with great seismic activity, the tsunami risk should be important depending on the occurrence probability and magnitude of the tsunami.     

In situ U–Pb analysis of shocked zircon from the Charlevoix impact structure,Québec,Canada

Laser ablation inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) U–Pb geochronology of shocked zircon grains in a vesicular‐fluidal impact melt rock from the ≥54 km Charlevoix impact structure, Québec, Canada, suggests an Ordovician to Silurian age of 450 ± 20 Ma for the impact. This age is anchored by concordant U–Pb results of ~450 Ma for a U‐rich, cryptocrystalline zircon grain in the melt rock, interpreted as a recrystallized metamict zircon crystal; the U–Th–Pb system of the metamict grain was seemingly chronometrically reset by the Charlevoix impact, but withstood later tectonometamorphic events. The new zircon age for Charlevoix is in agreement with a stratigraphically constrained Late Ordovician maximum age of ~453 Ma and corroborates earlier suggestions that the impact occurred most likely in the Ordovician, and not ~100 Myr later, as indicated by previous K/Ar and ⁴⁰Ar/³⁹Ar geochronologic results. The latter may reflect postimpact thermal overprint of impactites during the Salinian (Late Silurian to Early Devonian) and/or Acadian (Late Devonian) orogenies. U–Pb geochronology of zircon crystals in anorthosite exposed in the central uplift of the impact structure yielded a Grenvillian crystallization age of 1062 ± 11 Ma. The preferred Ordovician age for the Charlevoix impact structure, which is partially overthrusted by the Appalachian front, suggests the impact occurred during a phase of Taconian tectonism and an episode of enhanced asteroid bombardment of the Earth. Our results, moreover, demonstrate that (recrystallized) metamict zircon grains may be of particular interest in impact geochronology.     

Small-scale heterogeneities below the Gräfenberg array, Germany from seismic wavefield fluctuations of Hindu Kush events

Small-scale elastic heterogeneities (<5  km) are found in the upper lithosphere underneath the Gräfenberg array, southeast Germany. The results are based on the analysis of broadband recordings of 17 intermediate-depth (201–272  km) events from the Hindu Kush region. The wavefront of the first P arrival and the following 40  s coda are separated into coherent and incoherent (scattered) parts in the frequency range from 0.05 to 5  Hz. The frequency-dependent intensities of the mean and fluctuation wavefields are used to describe the scattering characteristics of the lithosphere underneath the receivers. It is possible to discriminate a weak-fluctuation regime of the wavefield in the frequency range below approximately 1.5–2.5  Hz and a strong-fluctuation regime starting at 2.0–2.5  Hz and continuing to higher frequencies. In order to explain the observed wavefield fluctuations, an approach with seismic scattering at random media-type structures is proposed. The preferred model contains heterogeneities with 3–7 per cent perturbations in seismic velocity and correlation lengths of 0.6–4.8  km in the crust. This is compatible with models from active seismic experiments. Scattering in the lithospheric mantle is not required, but cannot be excluded at weak velocity contrasts (<3 per cent).