Forcing mechanisms of a heavy precipitation event in the southeastern Adriatic area     

Branka Ivančan-Picek  Kristian Horvath  Nataša Strelec Mahović  Marjana Gajić-Čapka 《Natural Hazards》2014,72(2):1231-1252

The aim of this study was to identify the main mesoscale features and mechanisms responsible for the generation of an extreme precipitation event as a contribution to improving the modelling of processes that produce HPEs. The event occurred during the morning hours on 22 November 2010 over the Dubrovnik coast in Croatia and the hinterland mountain range of the southern Dinaric Alps and caused severe flash floods and landslides and consequent interruption of traffic and electricity supply as well as other infrastructural damage. The analysis is geographically focused on the southern portion of the eastern Adriatic region, which is prone to relatively frequent heavy precipitation events that occur mostly in autumn. This area is one of the rainiest in Europe with expected annual amounts of precipitation greater than 5,000 mm in the mountainous hinterland. The mechanisms responsible for the formation of convection were analysed using synop measurements, satellite data and numerical experiments performed with the WRF model, which was set up at the convection-permitting resolution in the innermost domain. Satellite data were used to identify the precipitation systems and to estimate the intensity of the precipitation during the period of interest. The development of the precipitation system was connected to a strong large-scale ascent over the southern Italy and southern Adriatic due to the advection of warm air and cyclonic vorticity advection, which increases with height. The numerical simulations highlighted the essential role of a southerly low-level jet stream in the transport of warm and moist air towards the affected area. The convergence of two branches of low-level marine air favoured convection triggered over the coast and sea. Furthermore, numerical sensitivity experiments suggested that the orography of the Dinaric Alps plays an essential role in the precipitation maximum over the mountainous hinterland, but also that the orography was not the crucial factor in the heavy precipitation near Dubrovnik. This study highlights the need for a dense network of observations, especially radar measurements, to validate the simulated mechanisms and improve numerical forecasts via data assimilation.  相似文献   

Cramer-Rao lower bounds for sonar broad-band modulation parameters     

Nielsen  R.O. 《Oceanic Engineering, IEEE Journal of》1999,24(3):285-290

A role of passive sonar signal processing is the detection and estimation of the parameters associated with amplitude modulated broad-band signals. An example of such signals is propeller noise. Discrete frequency lines occur at the rotational frequency of the propulsion shaft and at the blade frequency. This correspondence provides expressions for the Cramer-Rao lower bounds for the estimates of broad-band signal power, modulation level, modulation frequency, and modulation phase. It is shown that for low broad-band-signal-to-broad-band-noise ratios, the estimates of power and modulation level are uncoupled from the estimates of modulation frequency and phase  相似文献   

New 1982 — 1990 photometry of λ Andromedae and its 11-year cycle     

Douglas S. Hall  Gregory W. Henry  Dietmar Böhme  Peter A. Brooks  Sandy Chang  Ales Dolzan  George L. Fortier  Robert E. Fried  Russell M. Genet  Bruce S. Grim  James Hannon  Darrel B. Hoff  Kevin Krisciunas  Howard J. Landis  Howard P. Louth  Larry P. Lovell  Paul Nielsen  Bobby E. Powell  Harry D. Powell  Don Pray  Thomas R. Renner  Charles W. Rogers  Stephen Shervais  Douglas M. Slauson  Samuel Slote  Harold J. Stelzer  Arthur J. Stokes  Jack C. Troeger  Louis C. Turner  Norman F. Wasson  Kenneth W. Zeigler 《Journal of Astrophysics and Astronomy》1991,12(4):281-287

We present photoelectric photometry of λ And never before published, obtained between February 1982 and December 1990 at 29 different observatories. Then we combine it with all other photometry available to us (previously published, contained in the I.A.U. Commission 27 Archives, and obtained with the Vanderbilt 16-inch automatic telescope but not yet published), to yield a 14.8-year data base. Analysis reveals a long-term cycle in mean brightness, with a full range of 0^m.15 and a period of 11.4 ± 0.4 years. Because most of our new photometry was concentrated in the 1983-84 observing season, we analyze that one well-defined light curve with a two-spot model. Spot A keeps a 0^m.04 amplitude throughout four rotation cycles whereas the amplitude of spot B diminishes from 0^m.09 down almost to 0^m.03. The spot rotation periods were 55^d.9 ± 0^d.6 and 520^d.8 ± 1^d.0, respectively.  相似文献   

Carbon cycling in a high-arctic marine ecosystem – Young Sound, NE Greenland     

Sren Rysgaard  Torkel Gissel Nielsen 《Progress in Oceanography》2006,71(2-4):426

Young Sound is a deep-sill fjord in NE Greenland (74°N). Sea ice usually begins to form in late September and gains a thickness of 1.5 m topped with 0–40 cm of snow before breaking up in mid-July the following year. Primary production starts in spring when sea ice algae begin to flourish at the ice–water interface. Most biomass accumulation occurs in the lower parts of the sea ice, but sea ice algae are observed throughout the sea ice matrix. However, sea ice algal primary production in the fjord is low and often contributes only a few percent of the annual phytoplankton production. Following the break-up of ice, the immediate increase in light penetration to the water column causes a steep increase in pelagic primary production. Usually, the bloom lasts until August–September when nutrients begin to limit production in surface waters and sea ice starts to form. The grazer community, dominated by copepods, soon takes advantage of the increased phytoplankton production, and on an annual basis their carbon demand (7–11 g C m⁻²) is similar to phytoplankton production (6–10 g C m⁻²). Furthermore, the carbon demand of pelagic bacteria amounts to 7–12 g C m⁻² yr⁻¹. Thus, the carbon demand of the heterotrophic plankton is approximately twice the estimated pelagic primary production, illustrating the importance of advected carbon from the Greenland Sea and from land in fuelling the ecosystem.In the shallow parts of the fjord (<40 m) benthic primary producers dominate primary production. As a minimum estimate, a total of 41 g C m⁻² yr⁻¹ is fixed by primary production, of which phytoplankton contributes 15%, sea ice algae <1%, benthic macrophytes 62% and benthic microphytes 22%. A high and diverse benthic infauna dominated by polychaetes and bivalves exists in these shallow-water sediments (<40 m), which are colonized by benthic primary producers and in direct contact with the pelagic phytoplankton bloom. The annual benthic mineralization is 32 g C m⁻² yr⁻¹ of which megafauna accounts for 17%. In deeper waters benthic mineralization is 40% lower than in shallow waters and megafauna, primarily brittle stars, accounts for 27% of the benthic mineralization. The carbon that escapes degradation is permanently accumulated in the sediment, and for the locality investigated a rate of 7 g C m⁻² yr⁻¹ was determined.A group of walruses (up to 50 adult males) feed in the area in shallow waters (<40 m) during the short, productive, ice-free period, and they have been shown to be able to consume <3% of the standing stock of bivalves (Hiatella arctica, Mya truncata and Serripes Groenlandicus), or half of the annual bivalve somatic production. Feeding at greater depths is negligible in comparison with their feeding in the bivalve-rich shallow waters.  相似文献   

High‐resolution reconstruction of a coastal barrier system: impact of Holocene sea‐level change          下载免费PDF全文

Mikkel Fruergaard  Thorbjørn J. Andersen  Lars H. Nielsen  Peter N. Johannessen  Troels Aagaard  Morten Pejrup 《Sedimentology》2015,62(3):928-969

This study presents a detailed reconstruction of the sedimentary effects of Holocene sea‐level rise on a modern coastal barrier system. Increasing concern over the evolution of coastal barrier systems due to future accelerated rates of sea‐level rise calls for a better understanding of coastal barrier response to sea‐level changes. The complex evolution and sequence stratigraphic framework of the investigated coastal barrier system is reconstructed using facies analysis, high‐resolution optically stimulated luminescence and radiocarbon dating. During the formation of the coastal barrier system starting 8 to 7 ka rapid relative sea‐level rise outpaced sediment accumulation. Not before rates of relative sea‐level rise had decreased to ca 2 mm yr^?1 did sediment accumulation outpace sea‐level rise. From ca 5·5 ka, rates of regionally averaged sediment accumulation increased to 4·3 mm yr^?1 and the back‐barrier basin was filled in. This increase in sediment accumulation resulted from retreat of the barrier island and probably also due to formation of a tidal inlet close to the study area. Continued transgression and shoreface retreat created a distinct hiatus and wave ravinement surface in the seaward part of the coastal barrier system before the barrier shoreline stabilized between 5·0 ka and 4·5 ka. Back‐barrier shoreline erosion due to sediment starvation in the back‐barrier basin was pronounced from 4·5 to 2·5 ka but, in the last 2·5 kyr, barrier sedimentation has kept up with and outpaced sea‐level. In the last 0·4 kyr the coastal barrier system has been prograding episodically. Sediment accumulation shows considerable variation, with periods of rapid sediment deposition and periods of non‐deposition or erosion resulting in a highly punctuated sediment record. The study demonstrates how core‐based facies interpretations supported by a high‐resolution chronology and a well‐documented sea‐level history allow identification of depositional environments, erosion surfaces and hiatuses within a very homogeneous stratigraphy, and allow a detailed temporal reconstruction of a coastal barrier system in relation to sea‐level rise and sediment supply.  相似文献   

Intermetallic compounds,copper and palladium alloys in Au–Pd ore of the Skaergaard pluton,Greenland     

N. S. Rudashevsky  V. N. Rudashevsky  T. F. D. Nielsen 《Geology of Ore Deposits》2015,57(8):674-690

Copper–palladium intermetallic compounds and alloys (2314 grains) from the Au–Pd ore of the Skaergaard layered gabbroic pluton have been studied. Skaergaardite PdCu, nielsenite PdCu₃, (Cu,Pd)_β, (Cu,Pd)_α, (Pd,Cu,Au,Pt) alloys, and native palladium have been identified as a result of 1680 microprobe analyses. The average compositions and various chemical varieties of these minerals are characterized, as well as vertical and lateral zoning in distribution of noble metals. The primary Pd–Cu alloys were formed within a wide temperature interval broadly synchronously with cooling and crystallization of host gabbro and in close association with separation of Fe–Cu sulfide liquid. In the course of crystallization of residual gabbroic melt enriched in iron, noble and heavy metals and saturated with the supercritical aqueous fluid, PGE and Au are selectively concentrated in the Fe–Cu sulfide phase as Pd–Cu and Cu–Au alloys.  相似文献   

First results of micropulsation activity observed by sabre     

J.A. Waldock  T.B. Jones  E. Nielsen  D.J. Southwood 《Planetary and Space Science》1983,31(5):573-578

Between September 1981 and March 1982 many Pc5 micropulsation events have been observed by the Wick component of the new SABRE auroral radar. A feature of these observations is that in addition to the characteristic poleward drifting regions of radar backscatter, equatorward moving regions are frequently observed. A consistent diurnal pattern in the direction of drift emerges from a statistical analysis of the data.  相似文献   

X-ray, Lyα and Hα emission from simulated disk galaxies     

Jesper Sommer-Larsen  Sune Toft  Jesper Rasmussen  Kristian Pedersen  Martin Götz  Laura Portinari 《Astrophysics and Space Science》2003,284(2):693-696

The X-ray properties of the haloes of disk galaxies formed in fully cosmological, hydro/gravity simulations are discussed. The results are found to be consistent with observational X-ray detections and upper limits. Disk galaxy haloes are predicted to be about an order of magnitude brighter at z ∼ 1 than at z=0. The Lyα and Hα surface brightness of an edge-on, Milky Way like model galaxy has been determined. The emission is found to be quite extended, with a scale height of about 600 pc, neglecting dust corrections. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

Joint two-dimensional observations of ground magnetic and ionospheric electric fields associated with auroral zone currents: Current systems associated with local auroral break-ups     

W. Baumjohann  R.J. Pelunen  H.J. Opgenoorth  E. Nielsen 《Planetary and Space Science》1981,29(4):431-447

On 15 February, 1977, ground magnetic, ionospheric electric and auroral signatures of a multiple onset substorm were observed simultaneously by the Scandinavian Magnetometer Array (SMA), the Scandinavian Twin Auroral Radar Experiment (STARE) and the Finnish all-sky camera chain. Between 21:00 and 21:30 U.T., i.e. around local magnetic midnight, three consecutive local auroral break-ups were observed over Scandinavia. Each of these break-ups was preceded by a clear fading of the aurora and magnetic fields (while the electric fields remained unaffected), and occurred slightly south of the Harang discontinuity in the region of north-westward-directed electric fields. They were associated with a sudden change in direction of the electric field from north-west to south-west and the appearance of a westward equivalent current in the localized active region (about 1200 × 300 km²). These observations matched the features to be expected during the generation of a Cowling channel by a strong increase of the ionospheric conductivities due to precipitating auroral electrons. Numerical model calculations, based on the observations during the initial brightening and peak development of the second, most conspicuous break-up, show that the field-aligned currents at the northern and southern border of the active region are indeed very weak. However, highly localized and intense upward field-aligned currents at the western edge of the active region and more widespread and less intense downward currents in the eastern half preserve current continuity of the westward Cowling current and complete the substorm current wedge.  相似文献   

Coherent integration time limit of a mobile receiver for indoor GNSS applications     

Ali Broumandan  John Nielsen  Gérard Lachapelle 《GPS Solutions》2012,16(2):157-167

There is an emerging requirement for processing global navigation satellite system (GNSS) signals indoor where the signal is very weak and subjected to spatial fading. Typically, longer coherent integration intervals provide the additional processing gain required for the detection and processing of such weak signals. However, the arbitrary physical motion of the handset imputed by the user limits the effectiveness of longer coherent integration intervals due to the spatial decorrelation of the multipath-faded GNSS signal. In this paper, limits of coherent integration due to spatial decorrelation are derived and corroborated with experimental verification. A general result is that the processing gain resulting from direct coherent integration saturates after the antenna has moved through a certain distance, which for typical indoor propagation, is about half a carrier wavelength. However, a refined Doppler search coupled with a prolonged coherent integration interval extends this limit, which is effectively a manifestation of selective diversity.  相似文献