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61.
Leif Kristensen Donald H. Lenschow David Gurarie Niels Otto Jensen 《Boundary-Layer Meteorology》2010,134(2):195-221
We have developed a simple, steady-state, one-dimensional second-order closure model to obtain continuous profiles of turbulent
fluxes and mean concentrations of non-conserved scalars in a convective boundary layer without shear. As a basic tool we first
set up a model for conserved species with standard parameterizations. This leads to formulations for profiles of the turbulent
diffusivity and the ratio of temperature-scalar covariance to the flux of the passive scalar. The model is then extended to
solving, in terms of profiles of mean concentrations and fluxes, the NO
x
–O3 triad problem. The chemical reactions involve one first-order reaction, the destruction of NO2 with decay time τ, and one second-order reaction, the destruction of NO and O3 with the reaction constant k. Since the fluxes of the sum concentrations of NO
x
= NO + NO2 and O3 + NO2 turn out to be constant throughout the boundary layer, the problem reduces to solving two differential equations for the
concentration and the flux of NO2. The boundary conditions are the three surface fluxes and the fluxes at the top of the boundary layer, the last obtained
from the entrainment velocity, and the concentration differences between the free troposphere and the top of the boundary
layer. The equations are solved in a dimensionless form by using 1/(kτ) as the concentration unit, the depth h of the boundary layer as the length unit, the convective velocity scale w
* as the velocity unit, and the surface temperature flux divided by w
* as the temperature unit. Special care has been devoted to the inclusion of the scalar–scalar covariance between the concentrations
of O3 and NO. Sample calculations show that the fluxes of the reactive species deviate significantly from those of non-reactive
species. Further, the diffusivities, defined by minus the flux divided by the concentration gradient may become negative for
reactive species in contrast to those of non-reactive species, which in the present model are never negative. 相似文献
62.
L. K. Kristensen 《Astronomische Nachrichten》1984,305(4):207-211
The mean diameter D = 147 ± 3 km of (106) Dione is derived from visual observations in Denmark, Germany and the Netherlands of the occultation of AGK3 + 25°0989 on 1983 January 19. Photoelectric magnitudes are: B(1, 2°.4) = 8m.51 ± 0m.06 and B – V = 0m.65 ± 0m.07. The visual albedo is pv = .064. Constraints on the axis of rotation are derived from the observed elliptical contour. 相似文献
63.
Gijs Allard Henstra Thomas Berg Kristensen Atle Rotevatn Robert L. Gawthorpe 《Basin Research》2019,31(6):1083-1097
Recent studies of natural, multiphase rifts suggest that the presence of pre‐existing faults may strongly influence fault growth during later rift phases. These findings compare well with predictions from recent scaled analogue experiments that simulate multiphase, non‐coaxial extension. However, in natural rifts we only get to see the final result of multiphase rifting. We therefore do not get the chance to compare the effects of the same rift phase with and without pre‐existing structural heterogeneity, as we may in the controlled environment of a laboratory experiment. Here, we present a case study from the Lofoten Margin that provides a unique opportunity to compare normal fault growth with and without pre‐existing structural heterogeneity. Using seismic reflection and wellbore data, we demonstrate that the Ribban Basin formed during Late Jurassic to Early Cretaceous rifting. We also show that the rift fault network of the Ribban Basin lacks a pre‐existing (Permian‐Triassic) structural grain that underlies the neighbouring North Træna Basin that also formed during the Late Jurassic to Early Cretaceous. Being able to compare adjacent basins with similar histories but contrasting underlying structure allows us to study how pre‐existing normal faults influence rift geometry. We demonstrate that in Lofoten, the absence of pre‐existing normal faults produced collinear fault zones. Conversely, where pre‐existing faults are present, normal fault zones develop strong “zigzag” plan‐view geometries. 相似文献
64.
Leif Kahl Kristensen 《Celestial Mechanics and Dynamical Astronomy》2009,105(4):275-287
Initial orbit determination by least squares of N observations is essentially a linear problem if the coordinates x
0 and x
1 at two standard epochs are used as elements. The orbit of a main belt object is approximated within the observational errors
by a third degree polynomial during a month. A 4-observation orbit is useful for the initial linking between two nights. Parallax
is treated rigorously and future simultaneous space and Earth based observations determine the critical distance directly.
The N-observation method is a great simplification of the classical 3-observation orbit followed by a differential correction by
N observations. 相似文献
65.
66.
Time series tapering for short data samples 总被引:1,自引:0,他引:1
We explore the effect of applying tapered windows on atmospheric data to eliminate overestimation inherent in spectra computed from short time series. Some windows are more effective than others in correcting this distortion. The Hamming window gave the best results with experimental data. The Hanning window, which performed almost as well with the same data set, was used to demonstrate the effectiveness of the window in a numerical simulation of the process. We recommend that either of these windows be applied to short data samples prior to spectral analysis. 相似文献
67.
We propose a simple gust definition based on the theory of excursions by Rice (1944 and 1945). We discuss the relation to the distribution of extreme events and demonstrate theoretically and experimentally that the most probable extreme event is very close to being identical to the gust according to our definition. We demonstrate how it is possible to predict the gust on the basis of the measured mean wind and variance rather than rely on actually measured extreme excursions. Our gust definition also allows us to predict the average duration of a gust.Supported by the Catalonian Research Council (CIRIT), Spain. 相似文献
68.
Jørgen Primdahl Lone Søderkvist Kristensen 《Geografisk tidskrift / udgivet af Bestyrelsen for Det Kongelige danske geografiske selskab》2013,113(2):107-116
Geografisk Tidsskrift-Danish Journal of Geography 111(2):107–116, 2011 Rural landscapes are mainly maintained and changed through farmers' decisions and practices. As a landscape manager the farmer has many different roles of which three roles related to the farm as a whole is investigated in this paper: producer, owner and citizen. Although most farmers take landscape decisions in all these three roles production based decisions are assumed to be more important for full time farmers than for hobby farmers who have their main income outside the farm and who may consider their farm more as a living place than as a production place. Based on a large survey carried out in Hvorslev, Eastern Jutland, Denmark in 2008 farmers' landscape practices are analysed in relation to their background, occupational status and view of their farms as a production place versus a living place. Altogether 377 farmers were interviewed and some comparisons are made to a similar survey in the same area in 1996. It was found that a significant proportion of farmers are hobby farmers who mainly see the farm as a living place and who to a large degree have different landscape practices than full time farmers have. The findings indicate that more research is needed to fully understand the reasons for and implications of the differences in landscape management practices. 相似文献
69.
Ida Malene Berstad Hans Petter Sejrup Dorthe Klitgaard‐Kristensen Haflidi Haflidason 《第四纪科学杂志》2003,18(7):591-602
Core P1‐003MC was retrieved from 851 m water depth on the southern Norwegian continental margin, close to the boundary between the Norwegian Current (NC) and the underlying cold Norwegian Sea Deep Water. The core chronology was established by using 210Pb measurements and 14C dates, suggesting a sampling resolution of between 2 and 9 yr. Sea‐surface temperature (SST) variations in the NC are reconstructed from stable oxygen isotope measurements in two planktonic Foraminifera species, Neogloboquadrina pachyderma (d.) and Globigerina bulloides. The high temporal resolution of the SST proxy records allows direct comparison with instrumental ocean temperature measurements from Ocean Weather Ship (OWS) Mike in the Norwegian Sea and an air temperature record from the coastal island Ona, western Norway. The comparison of the instrumental and the proxy SST data suggests that N. pachyderma (d.) calcify during summer, whereas G. bulloides calcify during spring. The δ18O records of both species suggest that the past 70 yr have been the warmest throughout the past 600 yr. The spring and summer proxy temperature data suggest differences in the duration of the cold period of the Little Ice Age. The spring temperature was 1–3°C colder throughout most of the period between ca. AD 1400 and 1700, and the summer temperature was 1–2°C colder throughout most of the period between ca. AD 1400 and 1920. Fluctuations in the depth of the lower boundary of the NC have been investigated by examining grain size data and benthic foraminiferal assemblages. The data show that the transition depth of the lower boundary of the NC was deeper between ca. AD 1400 and 1650 than after ca. AD 1750 until present. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献