Three-dimensional modeling of synthetic cold fronts interacting with northern Alpine foehn

Summary The effect of the Alpine orography on prototype cold fronts approaching from the west is investigated by three-dimensional numerical model simulations. The numerical experiments cover a range of parameter constellations which govern the prefrontal environment of the front. Especially, the appearance and intensity of prefrontal northern Alpine foehn varies from case to case.The behaviour of a cold front north of the Alps depends much on the prefrontal condition it encounters. It is found that prefrontal foehn can either accelerate or retard the approaching front.An important feature is the pressure depression along the northern Alpine rim that results from the southerly foehn flow. In cases where this depression compensates the eastward directed pressure gradient associated with the largescale flow, the front tends to accelerate and the foehn breaks down as soon as the front passes. In contrast, the foehn prevents the front from a rapid eastward propagation if it is connected with a strong southerly wind component.No-foehn experiments are performed for comparison, where either the mountains are removed, or the static stability is set to neutral. Also shown are effects of different crossfrontal temperature contrasts.List of Symbols c _F propagation speed of a front - x, y horizontal grid spacing (cartesian system) - , horizontal grid spacing (geographic system) - t time step - z vertical grid spacing (cartesian system) - cross-frontal potential temperature difference - _i potential temperature step at an inversion - E turbulent kinetic energy - f Coriolis parameter - FGP frontogenesis parameter (see section 2.2) - g gravity acceleration (g=9.81 m s^–2) - vertical gradient of potential temperature - h terrain elevation (above MSL) - h _i height of an inversion (h _i =1000 m MSL) - H height of model lid (H=9000 m MSL) - K _M exchange coefficient of momentum - K _H exchange coefficient of heat and moisture - longitude - N Brunt-Väisäla-frequency - p pressure - Exner function (=T/) - latitude - q _v specific humidity - R _d gas constant of dry air (R _d =287.06 J kg^–1 K^–1) - density of dry air - t time - T temperature - potential temperature - TFP thermal front parameter (see section 2.2) - u, v, w cartesian wind components - u _g ,v _g geostrophic wind components - horizontal wind vector - x, y, z cartesian coordinates Abbreviations GND (above) ground level - MSL (above) mean sea level - UTC universal time coordinated With 20 Figures     

On the effect of a foehn on cold fronts in the vicinity of the Alps

The present note discusses physical mechanisms which may contribute to cold air channelling close to the Alps. This involves the modification of the prefrontal air by the warm foehn air and of the postfrontal air by blocking effects resulting in an increase in precipitation. Additionally the influence of a sloping surface in the vicinity of the orography is considered. The problems are discussed in term of a north-south-oriented cold front behaving as an atmospheric gravity current propagating along the east-west oriented Alps.     

The influence of the Alps on structure and behaviour of cold fronts over southern Germany

Summary In the paper results of investigations of the first two front events of the German Front Experiment 1987/88 are presented. In both cases a frontogenesis took place in the lower troposphere north of the Alps—in the first case directly at the cold front, in the second case ahead of it. The main reason for that was the incorporation of warm and dry air produced by foehn or advected along the Alps into the frontal zone of the approaching front. In the second case the cooling through evaporation of falling rain was an additional important factor for the increase of the temperature gradient. The computation of kinematic parameters yields as an interesting result the proof of areas with absolute vorticity near or below zero within the southwesterly current at the northwestern flank of the Alps.With 6 Figures     

Meso-scale surface wind changes associated with the passage of cold fronts along the eastern side of the Southern Alps,New Zealand

Summary The movement of cold fronts along the eastern side of the Southern Alps has been studied in detail during the Southerly Change Experiment (SOUCHEX). An enhanced network of surface wind monitoring stations was established in the Canterbury region of the South Island to allow detailed study of meso-scale wind fields during the passage of fronts. Fivesoutherly changes occurred during the experiment, one of which failed to produce a clear wind change over much of the area. The often erratic movement of these fronts along the eastern side of the mountains is illustrated by isochrone maps of the onset of the wind change. Other general characteristics of these events include their shallowness (1000–1500 m deep) and in most cases their abrupt onset. Detailed analysis of anemograph data collected during SOUCHEX indicates great variability in the surface wind field associated with passage of the fronts. Maps of the meso-scale wind field plotted for the first southerly change of the 14 January 1988 illustrate the complexity of the wind changes experienced over the eastern South Island in particular. The arrival of the cold fronts at individual sites provided features of air mass interaction which appear to relate to time of day, and regional and local site factors. Variations, in wind, temperature and relative humidity provide clear examples of both single and double air mass changes. However, nocturnal changes seem to be less distinct due to lower ambient temperatures and increased boundary layer stability.With 9 Figures     

Modification of fronts by the Alps: Simulations and numerical experimentation

Summary An isentropic limited area model is used to simulate and investigate the frontal passages of 3 May and 8 October 1987. It is demonstrated that a southward outbreak of air with high potential vorticity on 3 May 1987 affected the propagation of the front to the north of the Alps and the related formation of an orographic jet. Moreover, the outbreak plays a crucial role in the genesis of a lee cyclone. On the other hand, no such outbreak occurred on 8 October 1987 and it is shown that the propagation of the front near the ground was hardly affected by the flow at upper levels.With 10 Figures     

阵风锋、海风锋和冷锋等触发局地强对流风暴实例分析

利用CINRAD/SA雷达探测资料,结合地面实况和探空资料,对7次典型中尺度辐合线触发强对流风暴的特征进行了分析。结果表明：阵风锋、海风锋和冷锋等边界层辐合线在一定条件下雷达低层反射率因子产品上表现为清晰的窄带回波,某些辐合线在反射率因子产品上不能得到任何有用信息,但在低层径向速度上可识别出线性径向速度辐合;识别出窄带回波或清晰的径向辐合线约1 h后,是雷暴首次触发的主要时间段;对于干型强对流风暴产生的阵风锋,其右侧往往是雷暴触发的主要区域,导致风暴右向传播;湿型强对流风暴产生的阵风锋,激发雷暴的方向与雷暴平均移动方向基本相反,导致风暴呈后向传播特征;海风锋向内陆推进速度快的区域是雷暴触发的主要区域,后继雷暴具有两侧传播特征;单纯的线性低层径向速度辐合在合适的环境条件下触发强对流,主要特征是对流风暴移动缓慢,可造成局地灾害性强降雨天气。     

Boundary-layer effects on cold fronts at a coastline

The present note discusses one physical mechanism which may contribute to cold air channelling, manifest as a frontal bulge on a surface-analysis chart, in the coastal region of Victoria in southeast Australia. This involves the modification of boundary-layer air in both offshore (prefrontal) and onshore (postfrontal) flow, and the effect on cross-frontal thermal contrast. The problem is discussed in terms of a north-south-oriented cold front behaving as an atmospheric gravity current, propagating along an east-west-oriented coastline, in the presence of a prefrontal offshore stream.     

Orographically-forced cold fronts — Mean structure and motion

Shallow cold fronts with a north-west/south-east orientation sometimes experience blocking by the coastal mountain range of south-eastern Australia and greatly increase their propagation speed on the eastern (seaward) side of the range while they advance more slowly to the west of the range. The violent behaviour of some cold-frontal passages, or southerly busters, is found to be at least orographically initiated, but the phenomenon does not exhibit the characteristics of a coastally-trapped density current. The head of the front has the character of an evolving density current and its propagation is well predicted by density current theory over more than half of its lifetime. Nevertheless, it differs from steady laboratory-simulated examples in that a horizontal roll-vortex just behind the front is found to be accelerating relative to the rate of advection of cold air behind the front. This implies that its evolution will be governed by warm air entrainment, a fact which is confirmed by the observations. General agreement between extensive airborne observations over the ocean and measurements made at selected points along the coast indicates the usefulness of these basic studies for future improvement to forecasting in the near-coastal region.     

Development and propagation of precipitation cores on cold fronts

The structure and propagation of precipitation cores on a variety of cold fronts are described. The shapes of the cores, and their orientations with respect to the synoptic-scale front, are not always uniform either between fronts or within the same front. Interactions between cores depend on their proximity and relative strength. Large gaps between precipitation cores move along the cold front at a slower speed than the cores and affect the evolution of the precipitation cores. The spacing of precipitation cores, the height of the head of the density current associated with the cold front, and the strength of the cold front are positively correlated with precipitation strength. This suggests that interactions between cores are strongly influenced by precipitation.Current theories for precipitation cores on narrow cold-frontal rainbands cannot account for these observations. A new mechanism is proposed based on a positive feedback between the diabatic cooling produced by the melting and evaporation of precipitation, the head of the density current, and the strength of the front.     

临近寒潮相互影响关系及高层位涡的预报作用

利用ERA-Interm 1°×1°逐6 h再分析资料,对2017年1—2月发生的两次寒潮事件(简称过程A和过程B)的天气环流形势和位势涡度进行对比分析。结果表明:(1)过程A爆发后的冷空气促进过程B的形成;(2)等熵面上的位势涡度可用来追踪并判断寒潮的变化特征;(3)通过350~200 hPa处的位涡值特征可提前6 h预测寒潮开始爆发的时间和强度;(4)上游和下游系统对判断寒潮爆发的特征同样有着重要作用。     

Modifications of the structure of cold fronts over the foreland and in a mountain valley

Summary The investigations during the German Frontal Experiment 1987 show the modification of structure and movement of cold fronts in the vicinity of the Alps and in an Alpine valley. The two cold fronts that moved nearly parallel to the mountains were modified at the mountain barrier by the channelling effect. In those cases the postfrontal air flowed into the Inn valley from the valley's exit up-valley. The upper and lower parts of the fronts became detached, and the structure of the lower part changes on its way from the foreland to the mountain barrier.In contrast the fourth cold front, which moved from north-west to south-east, crossed the mountains without appreciable deformation although it suffered some degree of blocking and retardation.With 7 Figures     

Potential and equivalent-potential temperature patterns at cold fronts with pre-frontal foehn

Summary The separation of potential temperature and equivalent-potential temperature patterns at cold fronts with pre-frontal foehn is explained by means of conceptual considerations and discussed with reference to observations and numerical simulations. It is shown that foehn increases the potential temperature north of the Alps but does not necessarily affect the equivalent-potential temperature. As a consequence potential temperature and equivalent-potential temperature may form different structures and generate zones of large gradients at different locations even if no diabatic heat conversions are present.With 5 Figures     

Three-dimensional air pollutant modeling in the lower atmosphere

A steady-state, three-dimensional turbulent diffusion equation describing the concentration distribution of an air pollutant from an elevated point source in the lower atmosphere is solved analytically. The same formulation can be used to obtain solutions from line, area or other kinds of sources. The solution is developed for the cases in which the velocity, vertical and lateral diffusivities are given by the power law. The model preserves the beauty of analytical solution without sacrificing much on the accuracy of approximating the velocity and eddy diffusivities. Methods of evaluating the parameters, which are required for the model applications, are discussed. Results indicate that the ratio of the plume width to the plume length increases with decreasing stability and with increasing source height. These consequences are in response to the variations of the size of eddies in the vertical direction.     

Numerical studies on cold fronts Part II: Orographic effects on gravity flows

Summary A two-dimensional nonhydrostatic numerical model was used to investigate the behaviour of a cold air gravity current, moving along complex terrain. It is found, that the model with a high horizontal and vertical resolution and with a closure scheme, using the turbulent kinetic energy, is suitable to simulate currents, which have the main features of those found in laboratory experiments.Simulations are presented for different orographic structures (mountain and valley), for varying thermal stratification of the environmental atmosphere (neutral, stable and stable with an elevated inversion) and for different heights of the cold air reservoir.The major effect of a hill on the advance of a gravity current is a reduction of the front speed upstream as well as (even stronger) downstream of the obstacle, where the amount of this decrease depends on thermal stratification. Near surface blocking of the air flow on the windward side occurs for all simulations. However, for small depths of the oncoming cold air, the current cannot surmount the hill and remains on the lee side.With 11 Figures     

Lee cyclogenesis resulting from the combined outbreak of cold air and potential vorticity against the Alps

Summary A conceptual model of lee cyclogenesis is given which combines the commonly observed predominant features inherent in Alpine lee cyclogenesis, to be: blocking of cold air due to the Alps and progression of a potential vorticity maximum over the Alps. Numerical simulations of observed cases of Alpine lee cyclogenesis are carried out whose flow development turn out to bear close resemblance to this model. Therefore far-reaching conclusions are drawn.With 14 Figures     

On the appearance of a cold front with a narrow rainband in the vicinity of the Alps

Summary First intensive observations of a narrow cold frontal rainband in southern Germany are described. The observations by Doppler radar and surface observations are in good agreement with others made in Great Britain and at the west coast of the United States. There are no principal differences between the observations close to the sea and those during the passage of the front across the Alpine foreland. The observed coexistence of short and elongated precipitation cells is explained by the local modification of convergence and vorticity. When approaching the Alps the precipitation patterns increased in size due to the upslope winds, and the pre-frontal low-level jet is reorganized completely in a manner such that the cross-frontal circulation was enhanced while the pre-frontal low-level jet disappeared.With 20 Figures     

Diagnosis and prediction of middle level cloud development associated with cold fronts over southern Australia

Summary Broad scale vertical velocities have been used to predict the development over southern Australia of cloud associated with cold fronts. The vertical velocities were determined by solving explicity the quasi-geostrophic omega equation expressed in terms ofQ-vectors. The frontal cloud band was located within the are where up-motion was diagnosed, with the deepest cloud in the vicinity of the strongest upward velocities, and the rear of the band coinciding closely with the change from up- to down-motion. On those occasions when a major cloud mass evolved the region of up-motion extended to low latitudes as a clearly defined band. This band was indicative free a well developed warm conveyor belt associated with the frontal system. Areas of diagnosed down-motion were free of deep, organised cloud masses. Stronger upward motion did not produce an immediate increase in the cloudiness; typically there was a delay of 12 to 24 h before the corresponding cloud development became apparent.With 10 Figures     

Numerical studies on cold fronts Part I: Gravity flows in a neutral and stratified atmosphere

Summary We have used a two dimensional version of a nonhydrostatic mesoscale model to simulate atmospheric gravity currents for different thermal stratification. The horizontal and vertical grid increments are chosen such, that the major features of a current like head and elevated nose are resolvable.When the density current propagates into a neutral stratified environment it was found, that frontspeed agrees well with an empirical formula. Also characteristic length scales like depth of the head or height of the following cold air body agree well with observations found in water tank experiments.When a stable atmosphere is adopted, the front moves faster and the generated gravity waves have a significant influence on the atmospheric variables ahead of the current. This results especially in a pressure increase before the front arrives, an effect, which was found in observations, too.Finally, it is shown, that an elevated inversion, embedded in a stable layer, intensifies the vertical velocities and therefore the mesoscale heat flux, which results in a stronger entrainment. For this case a remarkable decrease of front speed is simulated with time.With 7 Figures     

Mesoscale ageostrophic circulations of two fronts observed during the fronts87 experiment: Diagnosis and interpretation

Summary This paper deals with the first results concerning mesoscale ageostrophic circulations as they were presented during the International Meeting on Fronts and Orography, held in Burghausen, Germany, on June 1991. The aim is to test the importance of some specific small scale and mesoscale processes on the observed ageostrophic circulations. For this, various available theoretical models including these processes are used, in order to put in evidence their effect or to diagnose their presence or not in the observed circulations. These secondary circulations have been observed in the vicinity of two surface cold fronts documented during the European experiment FRONTS87 and they are quantitatively evaluated and compared with the theoretical ones. The diagnoses are performed by numerically solving two versions of the two-dimensional Sawyer-Eliassen equation for the ageostrophic streamfunction: a form based upon the Geostrophic Momentum approximation of along-front geostrophy and a Primitive Equation form that allows the existence of a along-front ageostrophic component. A simple parameterization of the Ekman pumping effect in the secondary circulations is added, through a boundary condition in the Sawyer-Eliassen equation resolution.This application of the Sawyer-Eliassen equation to measured data from a network of two or three sounding stations allows the identification of the significant frontogenetic terms and the testing of the assumptions underlying this diagnostic tool. Also, the impact of various parameterizations is considered. For instance, the introduction of a boundary layer parameterization scheme permits the reproduction of low-level updrafts observed in reality ahead of the frontal zone and predicted by numerical models found in literature.With 18 Figures