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971.
972.
大连地区辐射雾与平流雾边界层温度场及风场对比 总被引:3,自引:1,他引:2
选取2008年3月和6月出现在大连及其沿海地区的辐射雾和平流雾过程,采用GTS1型数字式探空仪探测资料、能见度仪自动观测资料和常规观测资料,对其边界层温度场及风场结构特征进行了对比分析。结果表明,两种性质不同的大雾(辐射雾和平流雾)具有共同特征:都产生于纬向环流背景,中层东南暖湿气流为大雾的形成提供了充沛的水汽和热力条件。但两者存在明显的差异:底层东北风平流降温是辐射雾生成的重要条件,却造成了平流雾的消亡,底层东南风是平流雾生成的条件,却对应于辐射雾的消亡;辐射雾逆温厚而强,平流雾则为弱的逆温或无逆温;辐射雾生消演变对应于大气层结由稳定发展为不稳定,平流雾则对应于由不稳定趋于稳定。 相似文献
973.
The present study solves a two-layer atmospheric wave equation model with a lower atmosphere concave wind profile and cold-air outbreak over sea, while simultaneously proving that such a wind shear may cause neutral boundary layer roll vortices in the presence of disturbing sources upstream. Without thermal effects, the wind shear-induced waves have band structures at the top of the boundary layer that are similar to cloud street patterns observed over sea. This study proves that dynamic and thermal effects can act independently to initiate the roll vortices in the lower atmosphere. At the same time, a quantitative comparison shows that dynamic effects play a large role in the formation of roll vortices in the initial stage of cold-air outbreak and will be surpassed by thermal effects soon after surface heating commences. 相似文献
974.
A Parameterization of Dry Thermals and Shallow Cumuli for Mesoscale Numerical Weather Prediction 总被引:3,自引:1,他引:2
Julien Pergaud Valéry Masson Sylvie Malardel Fleur Couvreux 《Boundary-Layer Meteorology》2009,132(1):83-106
For numerical weather prediction models and models resolving deep convection, shallow convective ascents are subgrid processes
that are not parameterized by classical local turbulent schemes. The mass flux formulation of convective mixing is now largely
accepted as an efficient approach for parameterizing the contribution of larger plumes in convective dry and cloudy boundary
layers. We propose a new formulation of the EDMF scheme (for Eddy Diffusivity\Mass Flux) based on a single updraft that improves
the representation of dry thermals and shallow convective clouds and conserves a correct representation of stratocumulus in
mesoscale models. The definition of entrainment and detrainment in the dry part of the updraft is original, and is specified
as proportional to the ratio of buoyancy to vertical velocity. In the cloudy part of the updraft, the classical buoyancy sorting
approach is chosen. The main closure of the scheme is based on the mass flux near the surface, which is proportional to the
sub-cloud layer convective velocity scale w
*. The link with the prognostic grid-scale cloud content and cloud cover and the projection on the non- conservative variables
is processed by the cloud scheme. The validation of this new formulation using large-eddy simulations focused on showing the
robustness of the scheme to represent three different boundary layer regimes. For dry convective cases, this parameterization
enables a correct representation of the countergradient zone where the mass flux part represents the top entrainment (IHOP
case). It can also handle the diurnal cycle of boundary-layer cumulus clouds (EUROCS\ARM) and conserve a realistic evolution
of stratocumulus (EUROCS\FIRE). 相似文献
975.
Parameterization of Sheared Convective Entrainment in the First-Order Jump Model: Evaluation Through Large-Eddy Simulation 总被引:2,自引:0,他引:2
In this note, two different approaches are used to estimate the entrainment-flux to surface-flux ratio for a sheared convective
boundary layer (CBL); both are derived under the framework of the first-order jump model (FOM). That suggested by Sun and
Wang (SW approach) has the advantage that there is no empirical constant included, though the dynamics are described in an
implicit manner. The second, which was proposed by Kim et al. and Pino et al. (KP approach), explicitly characterizes the
dynamics of the sheared entrainment, but uncertainties are induced through the empirical constants. Their performances in
parameterizing the CBL growth rate are compared and discussed, and a new value of the parameter A
3 in the KP approach is suggested. Large-eddy simulation (LES) data are employed to test both approaches: simulations are conducted
for the CBL growing under varying conditions of surface roughness, free-atmospheric stratification, and wind shear, and data
used when the turbulence is in steady state. The predicted entrainment rates in each case are tested against the LES data.
The results show that the SW approach describes the evolution of the sheared CBL quite well, and the KP approach also reproduces
the growth of the CBL reasonably, so long as the value of A
3 is modified to 0.6. 相似文献
976.
We estimated the turbulent kinetic energy (TKE) dissipation rate for thirty-two 1-h intervals of unstable stratification covering the stability range 0.12 ≤ −z/L ≤ 43 (z/L is the ratio of instrument height to the Obukhov length), by fitting Kolmogorov’s inertial subrange spectrum to streamwise
spectra observed over a desert flat. Estimated values are compatible with the existence of local equilibrium, in that the
TKE dissipation rate approximately equalled the sum of shear and buoyant production rates. Only in the neutral limit was the
turbulent transport term in the TKE budget measured to be small. 相似文献
977.
J. M. Edwards 《Boundary-Layer Meteorology》2009,132(2):349-350
The structure of the radiatively dominated stable boundary layer is analysed using idealized calculations at high vertical
and spectral resolution. The temperature profile of a nocturnal radiative boundary layer, developing after the evening transition,
is found to be well described in terms of radiative cooling to the surface, although radiative exchanges within the atmosphere
become increasingly important with time. The treatment of non-black surfaces is discussed in some detail and it is shown that
the effect of reducing the surface emissivity is to decrease rather than to increase the radiative cooling rate in the surface
layer. It is also argued that an accurate assessment of the impact of non-black surfaces requires careful attention to the
spectral and directional characteristics of the surface emissivity. A polar nocturnal boundary layer, developing above snow-covered
ground, is simulated and found to reach a slowly evolving state characterized by a strong radiative divergence near the surface
that is comparable to observed values. Radiative boundary layers are characterized by large temperature gradients near the
surface.
An erratum to this article can be found at 相似文献
978.
Ralph C. Foster 《Boundary-Layer Meteorology》2009,131(3):321-344
We present similarity solutions for the mean boundary-layer profiles under an axisymmetric vortex that is in gradient wind
balance; the similarity model includes the nonlinear momentum advection and curvature terms. These solutions are a generalization
of the Ekman layer mean flow, which is the canonical boundary-layer basic state under a uniform, geostrophically-balanced
flow. Near-surface properties such as inflow angle, surface wind factor, diffusive transport of kinetic energy into the surface
layer and dissipational heating are calculated and shown to be sensitive to the choice of turbulence parameterization. 相似文献
979.
In this paper, by using the sounding data collected in LOPEX05, we have analyzed the vertical atmospheric structure and boundary layer characteristics of temperature and humidity in the late summer over the east Gansu loess plateau. The results show that the bottom of the stratosphere is at about 16 500 m and varies between 14 000 m and 18 000 m above the ground. The center of the westerly jet is located between 8300 m and 14 300 m above the ground and its direction moves between 260° and 305°. There is an ... 相似文献
980.
复杂地形风场的精细数值模拟 总被引:1,自引:0,他引:1
风能是一种重要气候资源,随着我国风电规模的迅速增大,发展风能资源评估系统和风功率预测系统已成为一项重要的研究内容。国内外对复杂地形风场结构的数值模拟有大量研究,随着计算机能力增强,以往用于空气动力学精细流场计算的计算流体力学(Computational Fluid Dynamics,CFD)模式越来越多地在气象领域得到应用,人们开始研究用中尺度预报模式和CFD模式结合进行复杂地形风场的数值模拟。本文的耦合模式系统采用中尺度气象模式(WRF),通过嵌套网格到内层尺度(一般是几公里),然后通过耦合CFD模式Fluent软件获得高分辨率(水平30~100 m,垂直150 m高度以下10 m)的风速分布资料,得到精细化的风场信息。通过对鄱阳湖北部区域和云南杨梅山复杂地形的风场模拟,提供了风能评估和预报的一种可行的方法。 相似文献