Diagnostics Of Hydraulic Jump And Gap Flow In Stratified Flows Over Topography

A criterion, allowing one to assess conditions likely togenerate gap flows and/or hydraulicjumps in stratified flows over a mountain ridge or a mountain pass,is derived. It is based on the one-dimensional reduced-gravity shallow-watertheory generalized to a three-dimensional orography with moderate streamwisevariations by introducing a variable effective flow cross-section. In this way,the hydraulic jump and gap flow are accommodated within the same model. Theresulting steady hyperbolic problem is shown to require the boundaryconditions to be expressed in terms of Riemann invariants. The latter yield the flow betweentwo given sites in a unique way. In particular, it is possible to relateunambiguously the existence of a hydraulic jump/gap flow and its energydiscontinuity to the boundary conditions. A simple method of flow interpolationand energy discontinuity calculation between two sites is presented.     

On the theory of the well-mixed layer containing a zero-order jump

Within the framework of a mixed layer (ML) containing a zero-order jump, the concept of ML is generalized for the case of horizontal non-homogeneity on the assumption that not only potential temperature, but also the wind does not change with height. It turns out that the components of the vertical turbulent stresses are quadratic functions of height.For such a well-mixed layer (WML), bounded below by uneven terrain with an adjacent surface layer, and above — by a stably stratified quasigeostrophic baroclinic atmosphere, a consistent system of equations with all terms independent of height, is obtained. This can be considered as a meteorological generalization of the known shallow-water equations.As an example of the use of these equations, an analytical solution of the large-scale one-dimensional steady-state problem concerning the development of the WML in a stable stratified barotropic air mass moving over a heated horizontal surface has been found.     

Numerical simulations of katabatic jumps in coats land,Antartica

A non-hydrostatic numerical model, the Regional Atmospheric Modeling System (RAMS), has been used to investigate the development of katabatic jumps in Coats Land, Antarctica. In the control run with a 5 m s^-1downslope directed initial wind, a katabatic jump develops near the foot of the idealized slope. The jump is manifested as a rapid deceleration of the downslope flow and a change from supercritical to subcritical flow, in a hydraulic sense, i.e., the Froude number (Fr) of the flow changes from Fr > 1 to Fr> 1. Results from sensitivity experiments show that an increase in the upstream flow rate strengthens the jump, while an increase in the downstream inversion-layer depth results in a retreat of the jump. Hydraulic theory and Bernoulli's theorem have been used to explain the surface pressure change across the jump. It is found that hydraulic theory always underestimates the surface pressure change, while Bernoulli's theorem provides a satisfactory estimation. An analysis of the downs balance for the katabatic jump indicates that the important forces are those related to the pressure gradient, advection and, to a lesser extent, the turbulent momentum divergence. The development of katabatic jumps can be divided into two phases. In phase I, the t gradient force is nearly balanced by advection, while in phase II, the pressure gradient force is counterbalanced by turbulent momentum divergence. The upslope pressure gradient force associated with a pool of cold air over the ice shelf facilitates the formation of the katabatic jump.     

天气预报分析型数据模型及生成

将原始数据转换为分析型数据,增强用户对海量数据的分析能力,是数据仓库技术最核心、最有价值的思想,也是数据仓库在气象领域应用的基础。该文针对天气预报领域数据空间性、瞬变性、物理性和多尺度性等特点,提出了五元组描述的天气预报分析型数据概念模型;总结了生成分析型数据的固定区域统计、划分区域统计、基本天气系统识别和天气学概念模型识别4种聚集变换,并对其关键技术进行了讨论。提出了基本天气系统自动识别的滤波-划分-测量算法,探讨了针对气象数据特点的模糊空间关系,定义了进行天气学概念模型识别的空间模糊产生式规则,并针对空间数据给出了定位条件等扩展。     

Normal mode theory of lee cyclogenesis and the initial value problem

Summary An initial value problem is proposed to study the influence of mountains upon atmospheric flows. The normal mode, quasigeostrophic theory of lee cyclogenesis is revisited; in order to model Alpine cyclogenesis, the flow, upon which the mountain acts, should represent a cyclone wave propagating eastward in a sheared zonal current. Two different models are considered in order to describe cyclone waves: an unstable Eady normal mode and a finite amplitude, neutral Rossby mode in the two layer model. It is shown that the topographic modifications proper of normal modes emerge very rapidly from the integration of the initial value problem. It is also shown that an enhanced deepening of the upper level trough can occur when finite amplitude effects are taken into account.With 8 Figures     

初夏至盛夏东亚副热带西风急流突变早晚与东亚环流异常的关系

利用1961-2004年NCEP／NCAR再分析逐候资料和全国160站月平均降水资料,分析了初夏至盛夏东亚副热带急流北跳和急流中心西移发生早晚对7月东亚大气环流和我国降水的影响。结果表明,急流北跳时间与7月长江中下游地区降水异常正相关,急流中心西移时间则与7月淮河流域降水异常正相关,与华北和河套地区降水异常负相关。急流北跳时间与南亚高压和西太平洋副热带高压南北位置异常及高纬贝加尔湖以东高压脊强度相关;而急流中心西移时间与南亚高压和西太平洋副热带高压的东西伸展及贝加尔湖以西高压脊强度相关,在急流中心西移偏晚年,南亚高压西缩,贝加尔湖西南侧高压脊增强,南下至华北和河套地区冷空气偏强,且西太平洋副热带高压东撤,冷暖空气在淮河流域交汇,使得华北和河套地区降水减少而淮河流域降水偏多;偏早年情况与偏晚年情况相反。     

Observational Study of Surface Wind along a Sloping Surface over Mountainous Terrain during Winter

The 2018 Winter Olympic and Paralympic Games will be held in Pyeongchang, Korea, during February and March. We examined the near surface winds and wind gusts along the sloping surface at two outdoor venues in Pyeongchang during February and March using surface wind data. The outdoor venues are located in a complex, mountainous terrain, and hence the near-surface winds form intricate patterns due to the interplay between large-scale and locally forced winds. During February and March, the dominant wind at the ridge level is westerly; however, a significant wind direction change is observed along the sloping surface at the venues. The winds on the sloping surface are also influenced by thermal forcing,showing increased upslope flow during daytime. When neutral air flows over the hill, the windward and leeward flows show a significantly different behavior. A higher correlation of the wind speed between upper-and lower-level stations is shown in the windward region compared with the leeward region. The strong synoptic wind, small width of the ridge, and steep leeward ridge slope angle provide favorable conditions for flow separation at the leeward foot of the ridge. The gust factor increases with decreasing surface elevation and is larger during daytime than nighttime. A significantly large gust factor is also observed in the leeward region.     

Post-Onset Behaviour of the Pure Katabatic Flow

An analytic improvement of the classic Prandtl model for pure katabatic flows is obtained. The one-dimensional unsteady problem employs gradually-varying eddy diffusivity K(z) fixed in time. A new solution is found for thefourth-order governing equation that couples the momentum and heat transfer in an approximate but still systematic way. The solution for wind and temperature perturbations is a generalization of the Prandtl solution allowing for: (1) Local andcumulative K(z) effects, (2) gradual evolution from the initial, discontinuous- towardthe steady-state profiles and (3) the given two-dimensional background potential temperature gradient, the surface slope and its potential temperature deficit atthe surface. The solution that is based on a relaxation equation compares adequatelywith its numerical counterpart soon after the estimated flow onset. It is a product ofthe steady-state solution with a spatio-temporal transfer function. The results can beuseful for data analyses, especially for the scale estimations of inclined stable boundarylayers and for surface flux calculations.     

条件不稳定大气中二维小尺度双脊地形上空对流及降水特征

在条件不稳定大气条件下,二维小尺度双脊钟形地形上空对流触发、传播和降水分布特征主要决定于地形上游基流强度、双脊地形配置形式、地形高度及其山谷宽度。双脊地形在沿基流方向上有两种配置:高脊地形位于上游和低脊地形位于上游。对于高脊地形位于上游的双脊地形,上游高地形将起主导作用,山地上空对流及降水特征与单脊地形类似。对于低脊地形位于上游的双脊地形,上游低地形可明显地改变下游高地形的前方来流,同时,下游高地形也能够对上游低地形背风侧流动产生影响,从而导致出现地形上空复杂的对流传播、降水分布特征。对于低脊地形位于上游的双脊地形,其山谷宽度主要决定了双脊地形与单脊地形之间在对流、降水分布等的差异;当山谷宽度较小时,双脊地形可以近似为一个包络地形,此时地形上空的对流、降水特征与单脊地形类似;当山谷宽度较大时,双脊上空流动相互影响较小,此时双脊地形可以分成两个单脊地形;当山谷宽度在一定范围内,其上空的对流及其降水分布与单脊地形有明显差异。对于低脊地形位于上游、中等山谷宽度的双脊地形上空降水主要呈现4种类型:(1)山谷与低脊迎风坡降水;(2)高脊迎风坡降水;(3)低脊山峰与高脊迎风坡降水;(4)低脊背风侧、双脊山峰准静止降水。     

Orographic and stability effects on day-time,valley-side slope flows

The effects of orography and initial stability upon the magnitude and configuration of daytime, valley-side slope flows were investigated. A three-dimensional, time-dependent, non-hydrostatic numerical model provided simulations over a range of idealised valley forms for a range of vertical stabilities. The model's short-wave radiation scheme was improved and the runs were for a virtually dry atmosphere. Airflow over the valley is influenced by two distinct stability regimes, separated by a sharp threshold value of 0.37°C km^–1. At lower stabilities, flow is strong and predominantly downward. Above the threshold, uplift occurs for all stabilities, decreasing in magnitude with increasing stability. Cross-valley flow increases in the stability range 0.06°C–0.6°C/100 m and decreases at higher stabilities. For a given stability above the threshold value, vertical velocities are directly related to slope angle. Horizontal velocities increase with slope at low angles but there is a suggestion that they decrease with increasing slope angle at high angles. The effect of valley half-width is much smaller than that of slope; greater valley width leads to a weaker cross-valley circulation. Conditions for the development of valley-slope flow configuration in harmony with the underlying orography are derived. A quantitative relationship between the magnitude of the average flow and the average slope and the initial stability is presented.     

Nonlinear chemical couplings in the tropospheric NO x -HO x gas phase chemistry

A bifurcation phenomenon with relevance to atmospheric chemistry is discussed. The gasphase reactions in the troposphere exhibit two types of temporal evolution which are controlled by the strength of the source,Q, of nitric oxide, NO, via the nonlinear chemical coupling between the hydrogen oxides and nitrogen oxides chemistry. IfQ remains below a threshold value, all short-lived species, including NO, approach steady-state concentrations, while above the threshold bifurcation to another state with increasing (nonstationary) NO concentrations accompanied by a depletion of the OH and HO₂ abundances takes place.     

基于DKPCA的气象卫星遥测故障检测方法

针对气象卫星遥测数据在阈值范围内变化发生异常,简单的阈值法无法检测出来的问题,本文提出基于动态核主元分析法(DKPCA)对气象卫星遥测数据进行故障检测。首先采集卫星正常运行的遥测数据,建立DKPCA模型求得统计量的控制限,然后对当前的遥测数据进行检测,判断是否有故障发生。该方法通过初始数据的动态化表示解决了观测值间存在序列相关性的问题,引入的核主元分析法能够将复杂的遥测数据的非线性问题转化为线性问题。通过在轨运行的气象卫星FY3C的实际数据进行效果验证,结果表明利用正常状况下的观测数据,可以实时检测卫星遥测数据出现的异常,有效避免故障漏报。     

Gap flow measurements during the Mesoscale Alpine Programme

Summary The lowest pass through the Alpine crest, the Brenner Pass, was heavily instrumented with ground-based and air-borne in-situ and remote sensors during the Special Observation Period (SOP) of the Mesoscale Alpine Programme (MAP) in the fall of 1999 to study gap flow. The main objectives were to study the combined effects of changes of terrain height and changes of width in altering the flow characteristics, to investigate the coupling of the gap flow to the flow aloft, and to provide high-density measurements in the along- and cross-gap directions.Gap flows occurred during one third of the 70-day SOP, a frequency above the long term average. Gap flows took place with and without accompanying cross-barrier flow and with and without a capping inversion. A case study demonstrates the hydraulic jump-like features that occurred in gap flow on 30 October 1999 and illustrates the types of data available for further analyses.     

Geostrophic adjustment and frontogenesis in a continuously stratified fluid

The structure of geostrophic flows that are formed from smooth initial unbalanced data with zero or uniform potential vorticity has been investigated in the work. Using methods of the theory of complex-variable functions, a class of exact analytical solutions describing these flows has been constructed. It has been shown that if the amplitude of a smooth initial disturbance exceeds the critical one, the final state will contain discontinuities of frontal type. A closed system of equations for discontinuity surfaces determination in the final states has been formulated. This system includes the Margules formula as a condition at the jump. It has been established that geometric configurations of discontinuity surfaces have an universal character, i.e. they do not depend on the details of initial distributions. To describe the nonstationary wave-processes of adjustment, a numerical hydrostatic model and a simplified analytical model based on the linearized dynamic equations in the Lagrange variables have been proposed. It has been shown that the establishment of discontinuious geostrophic flows has a clearly defined wave character with the alternations of smooth and multivalued phases. At the initial rather short interval, these phases are repeated with the inertial period, i.e. a nonstationary oscillatory front is formed.     

伊朗高压东伸对西藏高原汛期降水的影响

利用NCEP/NCAR再分析资料和西藏高原气象观测站逐日降水资料对1980—2011年5—9月967个伊朗高压东伸影响高原个例进行了分析,将西藏高原降水分布类型分为大雨型、中雨型、小雨型和无雨型四类分布,分别有158、516、165和128 d;当伊朗高压脊线偏北(南)时,500 hPa的南北气流辐合偏强(弱),200 hPa辐散偏强(弱),高原上的降水偏大(小);伊朗高压东伸除直接影响西藏高原的环流外,当其脊线偏北(南),索马里越赤道急流强度偏强(弱),导致孟加拉湾水汽输送多(少),高原降水偏强(弱)。     

An objective classification system of air mass types for Szeged, Hungary, with special interest in air pollution levels

Summary This paper determines the characteristic air mass types over the Carpathian Basin for the winter (December, January, and February) and summer (June, July and August) months dependant on levels of the main air pollutants. Based on the ECMWF data set, daily sea-level pressure fields analysed at 00 UTC were prepared for each air mass type (cluster) in order to relate sea-level pressure patterns with the level of air pollutants in Szeged. The data comprise daily values of twelve meteorological and eight pollutant parameters for the period 1997–2001. Objective definition of the characteristic air mass types is achieved by using the methods of Factor Analysis and Cluster Analysis. According to the results, during the winter months five air mass types (clusters) were detected based on higher concentrations of primary pollutants that occur with high irradiance and low wind speed. This is the case when an anticyclone is found over the Carpathian Basin and over the region south of Hungary, influencing the weather of the country. Low levels of pollutants occur when zonal currents exert influence over Hungary. During the summer months anticyclones and anticyclone ridge situations are found over the Carpathian Basin. (During the prevalence of anticyclone ridge situations, the Carpathian Basin is found at the edge of a high pressure centre.) As a result of high irradiance and very low NO levels, secondary pollutants are highly enriched.     

Flow Over Hills Covered by a Plant Canopy: Extension to Generalised Two-Dimensional Topography

An understanding of how topography, including that covered by a plant canopy, influences the flow and turbulence is important in many meteorological and micrometeorological applications. We revisit the framework of Finnigan and Belcher (Q J Roy Meteorol Soc 130:1–29, 2004) for neutral flow over sinusoidal two-dimensional topography covered by a canopy and present a minor modification that enables a formal match between the forms for the flow within and above the canopy, including within the deep canopy. The modification then allows the easy extension of the framework to generalised two-dimensional topography. The revised framework affirms the conclusions of Finnigan and Belcher (op.cit.); above the canopy the flow perturbations are maximised near to where the perturbation pressure is a minimum, whereas deep within the canopy the flow perturbations are maximised when the streamwise gradient of the perturbation pressure is largest. However, the extension to general topography strongly illustrates that it is the pressure perturbation, not the hill shape directly, that controls the magnitude and location of the topographic impacts on the flow. For isolated ridges with a plateau, the flow perturbations above the canopy maximise, not at the ridge crest, but where the hill curvature is largest. Multiple recirculation regions can also exist within the canopy, as determined by the streamwise gradient of the pressure perturbation. These results indicate that knowledge of the terrain over a larger area than hither-to-thought is required to assess whether the use of micrometeorological techniques is appropriate at individual sites.     

A laboratory model of a convectively driven ocean

Many large-scale flows in the ocean are driven by an imposed horizontal density gradient and the resulting circulation is strongly influenced by the Earth's rotation. Some of the essential features of such flows have been incorporated into a laboratory model by differentially heating and cooling the vertical end walls of a shallow rectangular cavity rotating about a vertical axis. Buoyancy driven boundary currents produced by the heating and cooling were unstable and the resulting eddy structures eventually dominated the system. A broad mean flow, perpendicular to the side walls, developed in the central region of the cavity. The resultant steady-state flow had a barotropic component consisting of two large-scale gyres of cyclonic and anticyclonic senses. In this paper, the evolution of the flow towards its final steady state is described. Measurements of the time-scales for the establishment of stratification in the cavity, point to the existence of two dynamical density adjustment modes.     

积温效应在电力日峰谷负荷中的应用及检验

从电力气象服务需求出发,利用2001—2010年5—9月河北省南电网逐日电力日峰负荷、日谷负荷与对应时间的气象资料,探讨晴热天气和闷热天气对电力日峰负荷、日谷负荷的影响特征。分析发现持续3 d以上的闷热天气相对晴热天气使电力日峰负荷、日谷负荷增长更显著;日最高气温32℃是引起河北省南电网日峰负荷增长的初始气温敏感点,35℃为强气温敏感点,38℃为极强气温敏感点,日最低气温25℃为引起日谷负荷增加的敏感气温临界点;建立了引入积温热累积效应的日峰负荷、日谷负荷多元回归气象预测模型,经2011—2013年应用检验,日峰负荷、日谷负荷预测平均相对误差分别为4.8%和3.5%,提高了预测准确率,对电力调度具有参考价值。     

全国综合气象信息共享系统的设计与实现

为满足对海量气象数据管理和服务的需求,国家气象信息中心设计开发了全国综合气象信息共享系统 (CIMISS)。该文描述了系统基本设计思路、功能结构、基础平台体系结构、信息流程,阐述了系统为用户提供的基础数据使用环境。系统由数据收集与分发系统、数据加工处理系统、数据存储管理系统、数据共享服务系统、业务监控系统5个业务子系统组成,承担数据收集、加工处理、存储管理、共享服务和业务监控任务。系统设计和开发采用了一系列现代信息技术,包括基于消息和文件共享的平台内信息交换、气象数据标准化分类、数据处理作业调度和算法的动态扩展、元数据的设计和应用、公共配置信息管理、全流程业务监视和调度控制、面向服务的多维度数据存储策略、全局数据访问视图和统一访问接口设计等。该系统为我国国家级和省级气象业务提供了统一规范的气象数据使用环境。