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731.
Boundary-layer depth and entrainment zone characterization with a boundary-layer profiler 总被引:13,自引:0,他引:13
A technique for determining the height of the convective atmospheric boundary layer (CBL) with a 915 MHz boundary-layer profiler is discussed. The results are compared with CBL heights determined from radiosonde measurements. The profiler provides continuous CBL height measurements with very good time resolution (30 minutes or less), allowing for detailed understanding of the growth and fluctuations of the CBL. In addition, the profiler provides information about the degree of definition of the CBL top and the thickness of the entrainment zone. The measurements discussed were taken during the Rural Oxidants in the Southern Environment II (ROSE II) experiment. 相似文献
732.
733.
Stable carbon isotope variations in northwest Europe during the last glacial–interglacial transition
Chris S. M. Turney David J. Beerling Douglas D. Harkness J. John Lowe E. Marian Scott 《第四纪科学杂志》1997,12(4):339-344
Stable carbon isotope data that span part of the last glacial–interglacial transition (ca. 14-9 ka 14C BP; ca. 15–11 ka cal. BP), and which derive from organ-specific plant macrofossils recovered from two lake sediment profiles in the UK and one in Norway, are compared. The recorded temporal variations show similar trends, which, over a millennial time-scale appear to parallel the main drift in δ18O as determined for the GRIP ice-core. It is postulated that some smaller scale variations in the δ13C profiles may reflect the shorter term oscillations in δ18O values evident in the GRIP record, although this is less certain. Overall, however, the results suggest that stable carbon isotope measurements based on organ-specific terrestrial plant macrofossils may provide (i) a means for establishing correlations between terrestrial successions and (ii) additional paleoenvironmental information, as the apparent ‘shadowing’ of the GRIP record indicates a common forcing mechanism for both Greenland δ18O and northwest European δ13C variations. From the evidence available we suggest that the recorded δ13C variations reflect fluctuations in air temperature and/or changes in water vapour pressure in the atmosphere. © 1997 John Wiley & Sons, Ltd. 相似文献
734.
735.
736.
A laboratory experiment was performed to investigate mixing across a density interface which separates two turbulent fluid layers and coexists with a stabilizing buoyancy flux. It was found that the buoyancy flux (q0) across the interface and through the turbulent layers (of depth D) becomes steady and constant in magnitude in the vertical direction, only when
, where u is the horizontal r.m.s. velocity at the base of the mixed layers. The results suggest that mixing across the density interface is controlled by a dynamically important buoyancy gradient induced in the turbulent layers and that parameters such as the bulk Richardson number,
, where Δb is the interfacial buoyancy jump, are of secondary importance. Measurements are used to infer the mixing mechanism at the interface, the mixing efficiency of stratified fluids and the entrainment law. Some geophysical applications of the results are also discussed. 相似文献
737.
Abstract Laboratory tests were conducted on compacted marine sediments to study the effect of salt concentration of permeating fluid on its permeability characteristics. Deep sea sediment samples were collected from water depths varying from 3700 to 4500 m off Mauritius coast. Liquid limit and plasticity index varied widely from 45 to 75 and 10 to 35, respectively. Permeability was found at different void ratios with distilled water and 0.2, 0.4, and 0.8 N NaCl solutions as permeating fluid. It was found that permeability increases with an increase in salt concentration for a given void ratio. This is explained by diffused double layer theory. Also, the rate of increase in permeability decreases with increase in salt concentration. The effect of salt concentration seemed to be less at higher void ratios. 相似文献
738.
739.
C. P. Goff L. van Driel-Gesztelyi P. Démoulin J. L. Culhane S. A. Matthews L. K. Harra C. H. Mandrini K. L. Klein H. Kurokawa 《Solar physics》2007,240(2):283-299
A series of flares (GOES class M, M and C) and a CME were observed in close succession on 20 January 2004 in NOAA 10540. Radio
observations, which took the form of types II, III and N bursts, were associated with these events. We use the combined observations
from TRACE, EIT, Hα images from Kwasan, MDI magnetograms and GOES to understand the complex development of this event. Contrary
to a standard interpretation, we conclude that the first two impulsive flares are part of the CME launch process while the
following long-duration event flare represents simply the recovery phase. Observations show that the flare ribbons not only
separate but also shift along the magnetic inversion line so that magnetic reconnection progresses stepwise to neighboring
flux tubes. We conclude that “tether cutting” reconnection in the sheared arcade progressively transforms it to a twisted
flux tube, which becomes unstable, leading to a CME. We interpret the third flare, a long-duration event, as a combination
of the classical two-ribbon flare with the relaxation process following forced reconnection between the expanding CME structure
and neighboring magnetic fields.
Electronic Supplementary Material The online version of this article () contains supplementary material, which is available to authorized users. 相似文献
740.
S. J. Watkins A. S. Bhattal H. M. J. Boffin N. Francis & A. P. Whitworth 《Monthly notices of the Royal Astronomical Society》1998,300(4):1205-1213
It is expected that an average protostar will undergo at least one impulsive interaction with a neighbouring protostar whilst a large fraction of its mass is still in a massive, extended disc. Such interactions must have a significant impact upon the evolution of the protostars and their discs. We have carried out a series of simulations of coplanar encounters between two stars, each possessing a massive circumstellar disc, using an SPH code that models gravitational, hydrodynamic and viscous forces. We find that during a coplanar encounter, disc material is swept up into a shock layer between the two interacting stars, and the layer then fragments to produce new protostellar condensations. The truncated remains of the discs may subsequently fragment; and the outer regions of the discs may be thrown off to form circumbinary disc-like structures around the stars. Thus coplanar disc–disc encounters lead efficiently to the formation of multiple star systems and small- N clusters, including substellar objects. 相似文献