The purpose of this paper is to explore how a tropical cyclone forms from a pre-existing large-scale depression which has been observed and associated with cross-equatorial surges in the western North Pacific. Tropical cyclone Bilis(2000) was selected as the case to study.The research data used are from the results of the non-hydrostatic mesoscale model(MM5),which has successfully simulated the transformation of a pre-existing weak large-scale tropical depression into a strong tropical storm.The scale separation technique is used to separate the synoptic-scale and sub-synoptic-scale fields from the model output fields. The scale-separated fields show that the pre-existing synoptic-scale tropical depression and the subsynoptic scale tropical cyclone formed later were different scale systems from beginning to end.It is also shown that the pre-existing synoptic-scale tropical depression did not contract to become the tropical cyclone. A series of weak,sub-synoptic-scale low and high pressure systems appeared and disappeared in the synopticscale depression,with one of the low systems near the center of the synoptic-scale depression having deepened to become the tropical cyclone. The roles of the synoptic-scale flow and the sub-synoptic scale disturbances in the formation of the tropical cyclone are investigated by diagnoses of the scale-separated vertical vorticity equation.The results show that the early development of the sub-synoptic scale vortex was fundamentally dependent on the strengthening synoptic-scale environmental depression.The depression was strengthened by cross-equatorial surges,which increased the convergence of the synoptic-scale depression at low levels and triggered the formation of the tropical cyclone. 相似文献
Sea level extremes and their temporal variability have been explored based on the hourly measurements at Marseille tide gauge for the period 1885–2008. A careful quality check has first been applied to the observations to ensure consistency of the record by eliminating outliers and datum shifts. Yearly percentiles have been used to investigate long-term trends of extremes revealing that secular variations in extremes are linked to mean sea level changes. The associated decadal changes show discrepancies between mean sea level trend and extreme fluctuations, due to the influence of the atmospheric forcing. A local regression model based on the generalized Pareto distribution has been applied to derive trends in return levels. The 50-years return levels reach values between 80 and 120 cm. The most significant changes in return levels are characterized by an increase since the 1970s. 相似文献
A recently extended and spatially rich English Channel sea level dataset has been used to evaluate changes in extreme still water levels throughout the 20th century. Sea level records from 18 tide gauges have been rigorously checked for errors and split into mean sea level, tidal and non-tidal components. These components and the interaction between surge and tide have been analysed separately for significant trends before determining changes in extreme sea level. Mean sea level is rising at 0.8–2.3 mm/year, depending on location. There is a small increase (0.1–0.3 mm/year) in the annual mean high water of astronomical tidal origin, relative to mean sea level, and an increase (0.2–0.6 mm/year) in annual mean tidal range. There is considerable intra- and inter-decadal variability in surge intensity with the strongest intensity in the late 1950s. Storm surges show a statistically significant weak negative correlation to the winter North Atlantic Oscillation index throughout the Channel and a stronger significant positive correlation at the boundary with the southern North Sea. Tide–surge interactions increase eastward along the English Channel, but no significant long-term changes in the distribution of tide–surge interaction are evident. In conclusion, extreme sea levels increased at all of the 18 sites, but at rates not statistically different from that observed in mean sea level. 相似文献
Six large Late Miocene to Quaternary calderas, > 10 km in diameter, cluster together with several medium to small calderas and stratovolcanoes in a 60 × 30 km area of the Aizu volcanic field, southern NE Japan arc. These caldera volcanoes were built on a WNW–ESE trending highland coincident with a local uplifted swell since Late Miocene. The flare-up of felsic volcanism occurred synchronously along the NE Japan arc. Pyroclastic flow sheets from the calderas spread over the surrounding intra-arc basins and are interstratified with various sediments. Geochronological data indicates that the large-caldera eruptions have occurred six times since 8 Ma, at intervals of 1 to 2 million years. Late Miocene to Early Pliocene extra-caldera successions in the basin consist of nine sedimentary facies associations: (1) primary pyroclastics, (2) lahars, (3) gravelly fluvial channels, (4) sandy fluvial channels, (5) floodplains, (6) tidal flats, (7) delta fronts, (8) pro-delta slopes, and (9) pro-delta turbidites. The distribution of facies associations show westward prograding of volcaniclastic aprons, made up of braid delta, braidplain, pyroclastic flow sheet, and incised braided river deposits. The extra-caldera successions record: 1) an increase in felsic volcanism with an associated high rate of volcaniclastic sediment supply at about 10 Ma, prior to catastrophic caldera-forming eruptions; and 2) progradation of volcaniclastic aprons toward the back-arc side in response to the succeeding caldera-forming eruptions and sea-level changes, until about 3 Ma. 相似文献
Wind waves and elevated water levels together can cause flooding in low-lying coastal areas, where the water level may be
a combination of mean sea level, tides and surges generated by storm events. In areas with a wide continental shelf a travelling
external surge may combine with the locally generated surge and waves and there can be significant interaction between the
propagation of the tide and surge. Wave height at the coast is controlled largely by water depth. So the effect of tides and
surges on waves must also be considered, while waves contribute to the total water level by means of wave setup through radiation
stress. These processes are well understood and accurately predicted by models, assuming good bathymetry and wind forcing
is available. Other interactions between surges and waves include the processes of surface wind-stress and bottom friction
as well as depth and current refraction of waves by surge water levels and currents, and some of the details of these processes
are still not well understood. The recent coastal flooding in Myanmar (May 2008) in the Irrawaddy River Delta is an example
of the severity of such events, with a surge of over 3 m exacerbated by heavy precipitation. Here, we review the existing
capability for combined modelling of tides, surges and waves, their interactions and the development of coupled models. 相似文献
The evolution of the Somma-Vesuvius caldera has been reconstructed based on geomorphic observations, detailed stratigraphic
studies, and the distribution and facies variations of pyroclastic and epiclastic deposits produced by the past 20,000 years
of volcanic activity. The present caldera is a multicyclic, nested structure related to the emptying of large, shallow reservoirs
during Plinian eruptions. The caldera cuts a stratovolcano whose original summit was at 1600–1900 m elevation, approximately
500 m north of the present crater. Four caldera-forming events have been recognized, each occurring during major Plinian eruptions
(18,300 BP "Pomici di Base", 8000 BP "Mercato Pumice", 3400 BP "Avellino Pumice" and AD 79 "Pompeii Pumice"). The timing of
each caldera collapse is defined by peculiar "collapse-marking" deposits, characterized by large amounts of lithic clasts
from the outer margins of the magma chamber and its apophysis as well as from the shallow volcanic and sedimentary units.
In proximal sites the deposits consist of coarse breccias resulting from emplacement of either dense pyroclastic flows (Pomici
di Base and Pompeii eruptions) or fall layers (Avellino eruption). During each caldera collapse, the destabilization of the
shallow magmatic system induced decompression of hydrothermal–magmatic and hydrothermal fluids hosted in the wall rocks. This
process, and the magma–ground water interaction triggered by the fracturing of the thick Mesozoic carbonate basement hosting
the aquifer system, strongly enhanced the explosivity of the eruptions.
Received: 24 November 1997 / Accepted: 23 March 1999 相似文献
Analogue experiments in part I led to the conclusion that pyroclastic flows depositing very high-grade ignimbrite move as
dilute suspension currents. In the thermo–fluid–dynamical model developed, the degree of cooling of expanded turbulent pyroclastic
flows dynamically evolves in response to entrainment of air and mass loss to sedimentation. Initial conditions of the currents
are derived from column-collapse modeling for magmas with an initial H2O content of 1–3 wt.% erupting through circular vents and caldera ring-fissures. The flows spread either longitudinally or
radially from source up to a runout distance that increases with higher mass flux but decreases with higher gas content, temperature,
bottom slope and coarser initial grain size. Progressive dilution by entrainment and sedimentation causes pyroclastic currents
to transform into buoyant ash plumes at the runout distance. The ash plumes reach stratospheric heights and distribute 30–80%
of the erupted material as widespread co-ignimbrite ash. Pyroclastic suspension currents with initial mass fluxes of 107-1012 kg/s can spread for tens of kilometers with only limited cooling, although they move as supercritical, strongly entraining
currents for the eruption conditions considered here. With increasing eruption mass flux, cooling during passage through the
fountain diminishes while cooling during flow transport increases. The net effect is that eruption temperature exerts the
prime control on emplacement temperature. Pyroclastic suspension currents can form welded ignimbrite across their entire extent
if eruption temperature is To>1.3.Tmw, the minimum welding temperature. High eruption rates, a large fraction of fine ash, and a ring-fissure vent favor the formation
of extensive high-grade ignimbrite. For very hot eruptions producing sticky, partially molten pyroclasts, analysis of particle
aggregation systematics shows that factors favoring longer runout also favor more efficient aggregation, which reduces runout.
As a result, very high-grade ignimbrites cannot spread more than a few tens of kilometers from their source. In cooler pyroclastic
currents, particles do not aggregate, and the sedimentation process may involve re-entrainment of particles, which potentially
leads to more extensive cooling and longer runout; such effects, however, are only significant when net erosion of substrate
occurs. Model results can be employed to estimate mass flux and duration of ignimbrite eruptions from measured ignimbrite
masses and aspect ratios. The model also provides an alternative explanation of the observed decrease in H/Lratios with ignimbrite
mass.
Received: 10 May 1998 / Accepted: 21 October 1998 相似文献
Fast-moving landslides are one of the most significant dangers deriving from slope instabilities. Landslides involving large volumes can develop in rock or debris avalanches with extreme mobility and enormous destructiveness. Nevertheless, a relevant number of casualties and damages derive from small, fast-moving landslides with flow-like behaviour.
The Las Colinas landslide occurred at Santa Tecla (El Salvador, Central America) during a strong earthquake. It slid off the northern flank of the Bálsamo ridge, and resulted in almost 500 casualties and can be considered one of the most destructive landslides ever known. Earthquake shaking was amplified by the rock mass and the steep ridge topography.
We collected original geological, geomorphological and geophysical data in the Cordillera del Bálsamo area. The involved materials, ranging from lapilli to tuff layers of different strength, have been mapped and characterized.
Slope stability analyses have been performed both under static and dynamic conditions through limit equilibrium and finite element methods.
Hazard zonation for this type of landslides requires the forecast of the movement velocity and final deposition area. We used a fully two-dimensional FEM model to simulate landslide spreading downslope. The developed code allows the use of different constitutive models and yield rules with the possibility to model and study internal deformation of the landslide mass, as well material entrainment and deposition. 相似文献