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461.
Boundary-layer measurements made from the Swedish icebreaker Oden during the Arctic Ocean Experiment 2001 (AOE-2001) are analysed. They refer mainly to ice drift in the central Arctic during
the period 2–21 August 2001. On board Oden a remote sensing array with a wind profiler, cloud radar and a scanning microwave radiometer, and a regular weather station
operated continuously; soundings were also released during research stations. Turbulence and profile measurements on an 18-m
mast were deployed on the ice, along with two sodar systems, a microbarograph array and a tethered sounding system. Surface
flux and meteorological stations were also deployed on nearby ice floes. There is a clear diurnal cycle in radiation and also
in wind speed, cloud base and visibility. It is absent in temperature and humidity, probably due to the very strong control
by melting/ freezing ice and snow. In the advection of warm air, latent heat of melting maintains the surface temperature
at 0 °C, while with a negative energy balance the latent heat of freezing of the salty ocean water acts to maintain the surface
temperature > −2 °C. The constant presence of water at the surface maintains a relative humidity close to 100%, and this is
also often facilitated by an increasing specific humidity through the capping inversion, making entrainment a moisture source.
This ensures cloudy conditions, with low cloud and fog prevailing most of the time. Intrusions of warm and moist air from
beyond the ice edge are frequent, but the local Arctic boundary layer remains at a relatively constant temperature, and is
shallow and well mixed with strong capping inversions. Power spectra of surface-layer wind speed sometimes show large variance
at low frequency. A scanning radiometer provides a monitoring of the vertical thermal structure with a spatial and temporal
resolution not seen before in the Arctic. There are often two inversions, an elevated main inversion and a weak surface inversion,
and occasionally additional inversions occur. Enhanced entrainment across the main inversion appears to occur during frontal
passages. Variance of the scanning radiometer temperatures occurs in large pulses rather than varying smoothly, and the height
to the maximum variance appears to be a reasonable proxy for the boundary-layer depth. 相似文献
462.
Ulrike?WackerEmail author K.?V.?Jayaraman?Potty Christof?Lüpkes J?rg?Hartmann Matthias?Raschendorfer 《Boundary-Layer Meteorology》2005,117(2):301-336
The mesoscale weather prediction model ’Lokal-Modell’ (LM) of the Deutscher Wetterdienst is applied to the situation of an
Arctic cold air outbreak in the Fram Strait region in April 1998. Observations are available from a flight along 50E carried out during the ARTIST campaign. Initial and time-dependent boundary data for the simulation are taken from a larger
scale operational model system.
Using the standard configuration of LM, the simulation reproduced the propagation of cold air and the characteristic structure
of the atmospheric boundary layer (ABL) in fair agreement with the observations. However, a detailed comparison revealed three
basic problems. Firstly, there is evidence that the available data on sea-ice conditions were insufficient approximations
to the true state for several reasons. A modification of the sea-ice data towards observations revealed that parts of the
discrepancies were due to the original sea-ice data. Secondly, a control run with the model in its standard configuration
shows an insufficient warming of the ABL downstream of the ice edge due to underestimation of surface heat fluxes. A simple
modification of the approach for the scalar roughness length resulted in the strongest benefit, while comparative studies
showed only a slight sensitivity to different types of parametrisation of turbulent mixing or the inclusion of an additional
moist convection parametrisation. Thirdly, in all the simulations the deepening of the convective ABL downstream of the ice
edge is weaker than observed. This may be partly due to the thermal stratification above the ABL in the analysis data, which
is more stable than observed; but it may also be a hint to the fact that processes near the inversion are insufficiently parametrised
in mesoscale models with resolutions as used in LM. The simulated cloud layer in the convective ABL is similar to that observed
with respect to condensate content, a sharply defined cloud top, a diffuse lower bound, and continuous light precipitation. 相似文献
463.
Four months of eddy correlation data collected over a grass field and a nearby sage brush community are analyzed to examine
the adjustment of the boundary-layer structure as it flows from the heated brush to the snow-covered grass. The grass site
includes a 34-m tower with seven levels of eddy correlation data. The midday heat flux over the snow-covered grass and bare
ground surfaces is often downward particularly with melting conditions, while the corresponding heat flux over the brush is
almost always upward. For most of these cases, a stable internal boundary layer over the snow is well defined in terms of
vertical profiles of the buoyancy flux over the snow-covered grass. The stable internal boundary layer is generally embedded
within a deeper layer of flux divergence corresponding to increasing upward heat flux with height above the internal boundary
layer. With thin snow cover, the surface heat flux over the grass is weak upward due to heating of grass protruding above
the snow so that the flow adjusts to a decrease of the upward surface heat flux in the downwind direction. This common case
of an adjusting boundary layer contrasts with the formation of an internal boundary layer due to a change of sign of the surface
heat in flux the downwind direction. The adjustment of the boundary layer to the decrease of the surface heat flux leads to
vertical divergence of the upward heat flux in contrast to the usual heated boundary layer over homogeneous surfaces. The
consequences of the cooling due to the vertical divergence of the heat flux are discussed in terms of the heat budget of the
adjusting and internal boundary layers. 相似文献
464.
P. Martano D. Cava G. Mastrantonio S. Argentini A. Viola 《Boundary-Layer Meteorology》2005,115(1):85-103
Nocturnal convection, originating in a well-mixed marine cloud-topped boundary layer, advected onshore, was observed using a Doppler sodar on the Tyrrhenian coast in Italy. The horizontal and vertical dimensions of the downdrafts were evaluated. The oscillation frequency triggered by the downdrafts at the inversion layer, derived from the harmonic analysis of the sodar measured vertical velocity (w), is compared with the Brunt-Vaisala frequency, obtained from the rawinsonde temperature profile. A similarity function for the 2w vertical profile was used to fit the sodar experimental data and to retrieve the depth of the mixing layer and the sensible heat flux at the top of the cloud layer. The results are in agreement with the convection layer depth observed in the sodar echoes facsimile record, and with the energy budget evaluated at the top of the cloud layer using the rawinsonde profiles. 相似文献
465.
Magma underplating and Hannuoba present crust-mantle transitional zone composition: Xenolith petrological and geochemical evidence 总被引:7,自引:0,他引:7
FAN Qicheng ZHANG Hongfu SUI Jianli ZHAI Mingguo SUN Qian & LI Ni . Institute of Geology Geophysics Chinese Academy of Sciences Beijing China . Institute of Geology China Seismological Bureau Beijing China 《中国科学D辑(英文版)》2005,48(8):1089-1105
Studies on the deep-seated xenoliths from global volcanoes reveal that the present petrological crust-mantle boundary between the lower crust and the upper mantle actually is a transitional layer from mainly mafic granulites to ultramafic spinel lher-zolites[1,2], i.e. a transitional zone distinctive from the seismological Moho[3]. Oceanic lithosphere crust- mantle transitional zone can be established from the study on the exposed ophiolites. However, as for the continental lithosphere, since … 相似文献
466.
467.
The Pliocene-Pleistocene boundary, defined using pollen analysis, has been recorded in sections at the Belchatów outcrop, Kleszczów Graben, central Poland. The boundary is located at the top of the green clay subunit of the Tertiary Clayey-sandy unit. A pollen flora of Reuverian C type, dominated by pine, 10–20% of ‘Reuverian elements’ and scarce thermophilous Tertiary trees, was found directly below this subunit. Cold stage floras of Praetiglian type were, in turn, recorded in the ?ekińsko Formation, lying directly on the green clays. The Pliocene—Pleistocene transition at Belchatów demonstrates the characteristic features of this boundary in western Europe, that is, the disappearance of Tertiary thermophilous tree pollen taxa, followed by a cooling to open-forest or forest-steppe conditions. Both pollen analysis and geological data suggest continuous sedimentation at least from the Upper Pliocene to the Prae-Tiglian, with no change of sediment sources and no marked hiatuses. The fluvial deposits of the Praetiglian ?ekińsko Formation, although Pleistocene in age, mark a final period in the Tertiary evolution of the Kleszczów Graben. Later, there is a hiatus to the deposition of the first glacigenic sediments in the region possibly of Elsterian age. 相似文献
468.
G. D. Hess 《Boundary-Layer Meteorology》2004,110(3):319-355
The presence of a low-level, capping inversion layer will affect the height and structure of the planetary boundary layer (PBL). Results from models of varying levels of sophistication, including analytical, turbulent kinetic energy (TKE), second-order closure (SOC), large-eddy simulation (LES) and direct numerical simulation (DNS) models, are used to investigate this influence for the neutral, barotropic PBL. Predicted and observed profiles of stress and geostrophic departure components, and integral measures, such as the parameters of Rossby-number similarity theory, are compared for the KONTUR, Marine Stratocumulus, JASIN, Leipzig, Pre-Wangara and Upavon field experiments.Analytical models of the equilibrium value of inversion height zi, which depend on the surface friction velocity u*, and both the Coriolis parameter f and the free-flow Brunt-Väisälä frequency N, are found to give reasonable estimates of the PBL height. They also indicate that only the KONTUR and Marine Stratocumulus experiments were strongly influenced by N. More quantitative comparisons would require larger, more comprehensive datasets. The effects of the presence of a capping inversion on the profile structure were found to be insignificant for h* = |f|zi/u* > 0.15.The simple analytical model performed quite well over all values of h*; it predicted the profiles of the longitudinal stress component (in the direction of the surface stress) better than the lateral component. The more advanced models performed well for small values of h* (for flow over the sea), but systematically underestimated the cross-isobaric angle for flow over land. These models predicted the profiles of the lateral stress component better than the longitudinal component. The profiles of the analytical model agreed with those of the advanced models when the constant eddy viscosity of the outer layer was increased.Agreement with DNS was achieved by increasing the eddyviscosity of the analytical model by a factor of 5.Zilitinkevich and Esau(2002, Boundary-Layer Meteorology, 104, 371–379)suggest that the neutral, barotropic values of A and B of Rossby-numbersimilarity theory are not universal constants, but depend on the ratio N/|f|. The dependence for A and B is calculated using the analytical model and TKE models. Over the sea (h* 0.1; N/|f| 100, where we have used the Zilitinkevich-Esau relation to convert between h* and N/|f|) there is agreement between the model predictions and observations; however over land where the equilibrium boundary-layer height is greater (h* 0.35; N/|f| 10) the inconsistency between the advanced model predictions (TKE, SOC, LES, and DNS) and observations, as noted previously by Hess and Garratt, still exists. We attribute this disagreement to violations of the strict assumptions of steady, horizontally homogeneous, neutral, barotropic conditions implicit in the observations. At small values of zi and a strongly stable background stratification (h* 0.04; N/|f| 1000) both the TKE and analytical models predict that A and B depend significantly on h*, however observations are unavailable to confirm these predictions. Zilitinkevich and Esau call this case the `long-lived near-neutral PBL', and state that it is found in cold weather at high latitudes. 相似文献
469.
A new algebraic turbulent length scale model is developed, based on previous one-equation turbulence modelling experience in atmospheric flow and dispersion calculations. The model is applied to the neutral Ekman layer, as well as to fully-developed pipe and channel flows. For the pipe and channel flows examined the present model results can be considered as nearly equivalent to the results obtained using the standard k– model. For the neutral Ekman layer, the model predicts satisfactorily the near-neutral Cabauw friction velocities and a dependence of the drag coefficient versus Rossby number very close to that derived from published (G. N. Coleman) direct numerical simulations. The model underestimates the Cabauw cross-isobaric angles, but to a less degree than the cross-isobar angle versus Rossby dependence derived from the Coleman simulation. Finally, for the Cabauw data, with a geostrophic wind magnitude of 10 ms–1, the model predicts an eddy diffusivity distribution in good agreement with semi-empirical distributions used in current operational practice. 相似文献
470.
Using a Love number formalism, the elastic deformations of the mantle and the mass redistribution gravitational potential within the Earth induced by the fluid pressure acting at the core–mantle boundary (CMB) are computed. This pressure field changes at a decadal time scale and may be estimated from observations of the surface magnetic field and its secular variation. First, using a spherical harmonic expansion, the poloidal and toroidal part of the fluid velocity field at the CMB for the last 40 years is computed, under the hypothesis of tangential geostrophy. Then the associated geostrophic pressure, whose order of magnitude is about 1000 Pa, is computed. The surface topography induced by this pressure field is computed using Love numbers, and is a few millimetres. The mass redistribution gravitational potential induced by these deformations and, in particular, the zonal components of the related surface gravitational potential perturbation (J2, J3 and J4 coefficients), are calculated. Overall perturbations for the J2 coefficient of about 10–10, for J3 of about 10–11 and for J4 are found of about 0.3×10–11. Finally, these theoretical results are compared with recent observations of the decadal variation of J2 from satellite laser ranging. Results concerning J2 can be described as follows: first, they are one order of magnitude too small to explain the observed decadal variation of J2 and, second, they show a significant linear trend over the last 40 years, whose rate of decrease amounts to 7% of the observed value. 相似文献