中国东部高空颠簸时空分布特征及其与热带中东太平洋海温的关系

本文利用1979～2014年NCEP-DOE日平均再分析资料和中国区域2375份航空器空中颠簸报告资料,研究中国东部区域高空颠簸的时空分布特征及其与热带中东太平洋海温异常（简称“海温异常”;空间范围:5°S～5°N,120°～170°W）的关系以及产生这种关系的可能原因。结果表明:中国东部地区高空颠簸与东亚副热带西风急流之间存在显著时空相关关系,其原因是高空纬向风引起的垂直风切变是构成高空颠簸时空分布的主导因素。中国东部夏季高空颠簸与海温异常存在正相关关系;冬季呈现南北两个正负相关区:以30°N为界,北部区域存在显著的负相关,南部区存在显著的正相关,在30°N急流轴附近区域无显著相关关系。海温异常影响中国高空颠簸时空分布的可能原因是海温变化引起对流层高层温度出现异常,进而影响温度的经向梯度,导致东亚副热带西风急流强度和位置出现异常（夏季,急流轴南侧出现西风异常;冬季,急流轴北侧出现东风异常,南侧出现西风异常）。高空纬向风的变化导致纬向风的垂直梯度和经向梯度出现异常,最终影响高空颠簸的时空分布特征。对流层高层温度的异常变化可能是由与热带海温异常相关的平流层水汽变化所引起。     

南海台风边界层湍流小波分析

利用小波变换（WT）对香港天文台飞机观测台风“妮妲”（1604）资料进行分析,研究在不稳定、不均匀的台风边界层中湍流涡旋的垂直传输作用。在0.1～5 Hz惯性子区内横风和顺风分量功率谱密度能较好符合-5/3幂律。小波分析显示：横风的小波功率谱峰值集中在1 km之下,顺风分量的小波功率谱峰值集中在1～6km之间;眼区动量通量的主要贡献尺度为2.3 km,眼区外主要贡献尺度在1～2 km,中低层为较小尺度（<1.0km）;湍流功能(TKE)的生成尺度主要集中在4 km之下。这项研究定量描述了南海北部台风边界层各个区域湍流结构的差异特征,讨论了对台风边界层通量参数化的可能影响。     

Wave Flow Simulations Over Arctic Leads

We investigate the flow over Arctic leads using a mesoscale numerical model, typical of both summer and winter, under idealised conditions. We find that Arctic leads may be the source of standing atmospheric internal gravity waves during both seasons. The summertime wave may be compared with the wave generated by a small ridge, though with the phase reversed. The mechanism for exciting the wave is found to be the internal boundary layer developing due to horizontal variations in surface temperature and roughness length. During the more exploratory wintertime simulations, with substantial temperature difference between the lead and the ice surface, we find that secondary circulations and intermittent wave-breaking may occur. The effects of the lead appear far downstream.     

A PCSWMM/GIS-based water balance model for the Reesor Creek watershed

This paper presents the results of a study of a watershed experiencing the pressures of land-use change resulting from urban development. The study was undertaken to facilitate an understanding of the water balance of the watershed by developing and implementing watershed procedures that are to be addressed in a watershed plan. There were three components to the research: firstly, observation of the effects of spatially distributed rainfall measurements and their effect on modelling were assessed. Secondly, the model was then calibrated by observing how differing techniques can discretize both the landscape (e.g. land-use and soil type) and incoming precipitation. Finally, a modelling methodology was developed to integrate a Geographic Information System and a hydrologic model (e.g. Storm Water Management Model) in a water balance analysis on a watershed basis. Results show that, under certain conditions, kriging spatially distributed rainfall values can help predict rainfall at ungauged (virtual) sites. Discretization of a watershed was found to affect the differences between measured and generated runoff volumes; however, this can be refined with calibration. It was seen that a strong correlation between measured and predicted rainfall values did not always guarantee a strong relationship between measured and generated runoff Recommendations include the use of a longer time series of rainfall, streamflow and predicted rainfall to observe temporal variations, and the need to assess the differences in modelled rainfall values generated by various surface interpolation methods (e.g. Inverse Distance Weighting and other kriging options) currently available in GIS packages.     

Joint PDF Modelling of Turbulent Flow and Dispersion in an Urban Street Canyon

The joint probability density function (PDF) of turbulent velocity and concentration of a passive scalar in an urban street canyon is computed using a newly developed particle-in-cell Monte Carlo method. Compared to moment closures, the PDF methodology provides the full one-point one-time PDF of the underlying fields containing all higher moments and correlations. The small-scale mixing of the scalar released from a concentrated source at the street level is modelled by the interaction by exchange with the conditional mean (IECM) model, with a micro-mixing time scale designed for geometrically complex settings. The boundary layer along no-slip walls (building sides and tops) is fully resolved using an elliptic relaxation technique, which captures the high anisotropy and inhomogeneity of the Reynolds stress tensor in these regions. A less computationally intensive technique based on wall functions to represent the boundary layers and its effect on the solution are also explored. The calculated statistics are compared to experimental data and large-eddy simulation. The present work can be considered as the first example of computation of the full joint PDF of velocity and a transported passive scalar in an urban setting. The methodology proves successful in providing high level statistical information on the turbulence and pollutant concentration fields in complex urban scenarios.     

Further Studies Of Atmospheric Turbulence In Layers Near The Surface: Scaling The Tke Budget Above The Roughness Sublayer

The second of two experimental studies of the TKE budget conducted on sites of different roughness is described, and results are compared. The first took place within a shallow layer above a small field of mostly bare, cultivated soil; the second was carried out above a roughness sublayer of significant depth on an extensive plain of tall dry grass. Budget terms observed in the second study were scaled with a modified u which compensated for effects of an unusually large stress gradient and ensured that the _m functions would be collinear. By showing that the modification becomes negligible in smaller gradients, it is demonstrated that in normal conditions, budgets observed above significant roughness sublayers should be normalized by scaling in terms of the unreduced Reynolds stress at the sublayer's upper surface. This procedure is shown to be consistent with the expectation that TKE budgets in layers near the surface all scale in fundamentally the same way.Other findings include: (1) the fact that most _m functions previously reported are not quite collinear is attributed to a type of overspeeding known to affect three-cup anemometers; (2) revised _m functions, collinear and largely free of the effects of overspeeding, are determined from a well-established characteristic of the linear _m relation for the stable case; (3) data that define collinear _m functions can also be represented with single hyperbolic curves; (4) dissipation is found to be 10 to 15% too small to balance total TKE production in unstable and neutral conditions and to decrease with increasing z/L in thestable regime; and (5) new relations for based on the observed behaviour of the dissipation deficit provide an improved closure for the set of equations that express the budget terms as functions of _m and z/L.     

Intermittent Bursting of Turbulence in a Stable Boundary Layer with Low-level Jet

The atmospheric stable boundary layer (SBL) with a low-level jet is simulated experimentally using a thermally stratified wind tunnel. The turbulence structure and flow characteristics are investigated by simultaneous measurements of velocity and temperature fluctuations and by flow visualization. Attention is focused on the effect of strong wind shear due to a low-level jet on stratified boundary layers with strong stability. Occasional bursting of turbulence in the lower portion of the boundary layer can be found in the SBL with strong stability. This bursting originates aloft away from the surface and transports fluid with relatively low velocity and temperature upward and fluid with relatively high velocity and temperature downward. Furthermore, the relationship between the occurrence of turbulence bursting and the local gradient Richardson number (Ri) is investigated. The Ri becomes larger than the critical Ri, Ri_cr = 0.25, in quiescent periods. On the other hand, the Ri number becomes smaller than Ri_cr during bursting events.     

“2007.7.6”飞机颠簸事件数值模拟与成因分析

利用WRF模式模拟了2007年7月6日发生在菲律宾南部海域上空的一次民航飞机颠簸事件.结果表明,综合分析WRF模式计算得到的Ri和EI等预报指数,能够确定飞机颠簸发生的区域、高度和强度.飞机遭遇颠簸的区域是位于涡旋云系外围的螺旋云带的上空,飞机在飞越由重力波形成的螺旋云带时,在强烈的垂直上升和下沉气流的转换区域产生了急速的上抛和下降运动.     

Numerical study of flow and gas diffusion in the near-wake behind an isolated building

To assist validation of the experimental data of urban pollution dispersion, the effect of an isolated building on the flow and gaseous diffusion in the wake region have been investigated numerically in the neutrally stratified rough-walled turbulent boundary layer. Numerical studies were carried out using Computational Fluid Dynamics (CFD) models. The CFD models used for the simulation were based on the steady-state Reynolds-Average Navier-Stoke equations (RANS) with κ-ε turbulence models; standard κ-ε and RNG κ-ε models. Inlet conditions and boundary conditions were specified numerically to the best information available for each fluid modeling simulation. A gas pollutant was emitted from a point source within the recirculation cavity behind the building model. The accuracy of these simulations was examined by comparing the predicted results with wind tunnel experimental data. It was confirmed that simulation using the model accurately reproduces the velocity and concentration diffusion fields with a fine-mish resolution in the near wake region. Results indicated that there is a good agreement between the numerical simulation and the wind tunnel experiment for both wind flow and concentration diffusion. The results of this work can help to improve the understanding of mechanisms of and simulation of pollutant transport in an urban environment.     

Large-Eddy Simulation of Flows over Random Urban-like Obstacles

Further to our previous large-eddy simulation (LES) of flow over a staggered array of uniform cubes, a simulation of flow over random urban-like obstacles is presented. To gain a deeper insight into the effects of randomness in the obstacle topology, the current results, e.g. spatially-averaged mean velocity, Reynolds stresses, turbulence kinetic energy and dispersive stresses, are compared with our previous LES data and direct numerical simulation data of flow over uniform cubes. Significantly different features in the turbulence statistics are observed within and immediately above the canopy, although there are some similarities in the spatially-averaged statistics. It is also found that the relatively high pressures on the tallest buildings generate contributions to the total surface drag that are far in excess of their proportionate frontal area within the array. Details of the turbulence characteristics (like the stress anisotropy) are compared with those in regular roughness arrays and attempts to find some generality in the turbulence statistics within the canopy region are discussed.