Computations of the buoyantly unstable Ekman layer are performed at low Reynolds number. The results are obtained by directly solving the three-dimensional time-dependen Navier-Stokes equations with the Boussinesq buoyancy approximation, resolving all relevant scales of motion (no turbulence closure is needed). The flow is capped by a stable temperature inversion and heated from below at a rate that produces an inversion-height to Obukhov-length ratio −zi/L* = 32. Temperature and velocity variance profiles are found to agree well with those from an earlier vigorously heated under-resolved computation at higher Reynolds number, and with experimental data of Deardorff and Willis (Boundary-Layer Meteorol., 32: 205–236, 1985). Significant helicity is found in the layer, and helical convection patterns of the scale of the inversion height are observed. 相似文献
The paleoclimatic variability at frequencies ranging from 10–4 cycle per year (cpy) to 10–5 cpy is investigated using a set of four deep-sea cores from the Atlantic, Pacific and Indian Oceans. Dominant features are the presence of orbital frequencies corresponding to mean periods of 117.7, 43.6, 24.9 and 19.3 kyr. These are statistically significant according to such advanced spectral tools as Blackman-Tukey, maximum entropy and the highly efficient Thomson technique. However, the main purpose of this paper is methodological, describing the statistical analyses of time series with modern methods in order to stress their relative power, advantages and disadvantages. The more advanced statistical methods confirm the coincidence of the dominant periods in the deep sea cores and those in the astronomical elements, including combination tones. Three frequency bands of high paleoclimatic variability centred at 15.4, 13 and 10.8 kyr are indeed also detected. These two last periods are very close to those predicted by the climatic non-linear model of Ghil and Le Treut and found by Pestiaux et al. and Yiou et al. 相似文献
PKKP signals from Novaya Zemlya recorded at LASA at distances around 60° show consistent anomalies in both slowness and azimuth. The observed anomaly suggests that the signal is a BC branch arrival, although the arrival time corresponds to the DF branch. The BC branch, however, does not extend back to this distance. The azimuth of approach is in the range 229–245°, instead of the expected 186°. These anomalies are associated only with PKKP; analysis of the core phases PKiKP and P′P′ (BC) from the same events show that they arrived at LASA with the appropriate slownesses and azimuths.The PKKP signals can be interpreted as “scattered” PKKP; the scattering occurs on underside reflection at the core-mantle boundary and is probably caused by topographic irregularities on the boundary itself. The calculated scattering region has a surface projection at about 60°S, 134°E, which is outside the diametral plane through source and receiver, and about 21° from the expected PKKP reflection point at 76°S, 95°E.Both the “direct” and “scattered” arms of the PKKP signal have a PK path close to that of the “C” end of the BC branch. The unexpectedly large amplitude of the arrival suggests that there may be a focusing of energy at C, which would indicate a change in velocity gradient just above the inner core boundary. The observations nevertheless require, on the scattering interpretation, lateral variations in the topography of the core-mantle boundary and a region of relatively large topography responsible for the anomalous PKKP observations. 相似文献
A similarity study of entrainment at the top of convectively driven mixed layer is presented. The similarity framework is used for a comparison between various parameterized models of mixed-layer growth rate and between closely related models for the ratio of heat fluxes at ground and inversion levels. These various models are also tested, in dimensionless form, against data from laboratory, field experiments and numerical higher-order-modeling of the convective layer. It is concluded that a rather accurate prediction of mixed-layer growth can be achieved with the simple constant flux ratio model, but that more refined studies of entrainment are required to account for the decrease of the heat flux ratio with increasing convective intensity.List of acronyms CRPE
Centre de Recherches en Physique de l'Environnement, Issy-les-Moulineaux, France
- EERM
Etablissement d'Etudes et de Recherches Météorologiques, Boulogne, France
- INRA
Institut National de la Recherche Agronomique, Versailles, France
- LAMP
Laboratoire Associé de Météorologie Physique, Clermont-Ferrand, France
- NCAR
National Center for Atmospheric Research, Boulder, U.S.A.
- NOAA
National Oceanic and Atmospheric Administration, Environmental Research Laboratories, Boulder, U.S.A. 相似文献
Properties of lunar crustal magnetization thus far deduced from Apollo subsatellite magnetometer data are reviewed using two of the most accurate presently available magnetic anomaly maps — one covering a portion of the lunar near side and the other a part of the far side.
The largest single anomaly found within the region of coverage on the near-side map correlates exactly with a conspicuous, light-colored marking in western Oceanus Procellarum called Reiner Gamma. This feature is interpreted as an unusual deposit of ejecta from secondary craters of the large nearby primary impact crater Cavalerius. An age for Cavalerius (and, by implication, for Reiner Gamma) of 3.2 ± 0.2 × 109 y is estimated. The main (30 × 60 km) Reiner Gamma deposit is nearly uniformly magnetized in a single direction, with a minimum mean magnetization intensity of 7 × 10−2 G cm3/g (assuming a density of 3 g/cm3), or about 700 times the stable magnetization component of the most magnetic returned samples. Additional medium-amplitude anomalies exist over the Fra Mauro Formation (Imbrium basin ejecta emplaced 3.9 × 109 y ago) where it has not been flooded by mare basalt flows, but are nearly absent over the maria and over the craters Copernicus, Kepler, and Reiner and their encircling ejecta mantles.
The mean altitude of the far-side anomaly gap is much higher than that of the near-side map and the surface geology is more complex, so individual anomaly sources have not yet been identified. However, it is clear that a concentration of especially strong sources exists in the vicinity of the craters Van de Graaff and Aitken. Numerical modeling of the associated fields reveals that the source locations do not correspond with the larger primary impact craters of the region and, by analogy with Reiner Gamma, may be less conspicuous secondary crater ejecta deposits. The reason for a special concentration of strong sources in the Van de Graaff-Aitken region is unknown, but may be indirectly related to the existence of strongly modified crustal terrain which also occurs in the same region. The inferred directions of magnetization for the several sources of the largest anomalies are highly inclined with respect to one another, but are generally depleted in the north-south direction. The north-south depletion of magnetization intensity appears to continue across the far-side within the region of coverage.
The mechanism of magnetization and the origin of the magnetizing field remain unresolved, but the uniformity with which the Reiner Gamma deposit is apparently magnetized, and the north-south depletion of magnetization intensity across a substantial portion of the far side, seem to require the existence of an ambient field, perhaps of global or larger extent. The very different inferred directions of magnetization possessed by nearly adjacent sources of the Van de Graaff-Aitken anomalies, and the depletion in their north-south component of magnetization, do not favor an internally generated dipolar field oriented parallel to the present spin axis. A variably oriented interplanetary magnetizing field that was intrinsically strong or locally amplified by unknown surface processes is least inconsistent with the data. 相似文献
The MECO hydrodynamic model (MECO Technical Report No. OMR-118/120, CSIRO Marine Research, 1998) was adapted for the Torres Strait–Gulf of Papua region at 0.05° resolution. Validation of the hydrodynamic model was carried out against observed current meter data and calculated tidal sea levels. Dispersal pathways of sediments derived from the Fly River, and from a resuspension event along the northern Great Barrier Reef were investigated using an Eulerian approach. Sediment input into Torres Strait is found to be greater during the Trade season by approximately 10%. Wave data were also obtained, and together with hydrodynamic model output, sediment mobility due to currents, waves and wave–current interactions was considered for both the Trade and Monsoon seasons. Sediment mobility in the Gulf of Papua is dominated by wave motion, whereas Torres Strait is a mixed environment of waves and tidal currents. 相似文献
The crossover adjustment plays a central role in the processing of satellite altimeter measurements. The usual procedure
is to form sea surface height differences at crossover points, solve for the radial orbit error (with due attention to the
singular nature of the estimation problem) and then to construct altimetric sea-level maps using the mean sea surface heights
at the crossover points. Our approach is very different, to make direct use of measurements at crossover points without differencing
and to estimate simultaneously orbit parameters, mean sea surface height and sea surface height variability in a single, unified
adjustment. The technique is suited for repeat data over an area small enough that adjoining passes may be considered to be
parallel and to permit the solution of a set of linear equations of dimension equal to the number of crossover points. The
size of the numerical problem is almost independent of the number of repeat cycles of the altimeter mission. Explicit recognition
is given to the rank defect of the least-squares estimation problem; we show that, for an orbit model with r parameters, the rank defect of the local crossover problem is exactly r2. The defect may be overcome by choosing an appropriate set of constraints – either giving a best fit of mean sea surface
heights to a reference surface, or minimising orbit parameters, or a minimum norm solution in which both mean sea surface
heights and orbit parameters are minimised. There is no need to choose a reference pass, all passes are treated equally and
data gaps are easily accommodated. Numerical results are presented for the south-western Indian Ocean, based on the first
2 years of altimeter data from the Geosat Exact Repeat Mission.
Received: 31 May 1996 / Accepted: 19 April 1997 相似文献
Multiple-point simulation, as opposed to simulation one point at a time, operates at the pattern level using a priori structural information. To reduce the dimensionality of the space of patterns we propose a multi-point filtersim algorithm that classifies structural patterns using selected filter statistics. The pattern filter statistics are specific linear combinations of pattern pixel values that represent directional mean, gradient, and curvature properties. Simulation proceeds by sampling from pattern classes selected by conditioning data. 相似文献