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.     

Frontal contraction processes leading to the formation of an intense narrow rainband

Summary Processes which played an important role in the sudden development of an intense narrow cold frontal rainband (NCFR) along the leading edge of a wintertime anafront are investigated. Quantitative analysis of the Regional Analysis and Forecast System (RAFS) initialization and Nested Grid Model (NGM) forecast fields, satellite-derived water vapor and ozone measurements, and conventional meteorological data are employed in this study.The NCFR developed upon merger of an arctic front with a nearly stationary cold front over the Appalachian mountains. Rapid intensification of an ageostrophic momentum surge associated with the formation of a sub-synoptic played a dominant role in forcing explosive frontogenesis during frontal merger. A deep tropopause fold and related mid-tropospheric subsidence feature within the entrance region of an upper-level jet crossed the mountains at the time of NCFR formation. Synthesis of the surface, NGM, and satellite data suggests that this subsidence resulted in the development of the sub-synoptic high pressure system, whose leading edge was marked by a 5mb pressure jump. Consequential amplifieation and scale contraction of the cross-front pressure gradient indicates that the scale of the merged frontal system contracted just prior to NCFR development.As precipitation developed, the pressure jump took on a microscale character and increased to 8mb amplitude. The subsequent intensity of this frontal rainband is explained by the interaction between vertical shear associated with a strong, pre-frontal, low-level jet and the gravity current-like structure at the leading edge of the merged cold front.With 22 Figures     

一次浙赣沿线梅雨锋暴雨过程诊断分析

利用日本GMS卫星数据TBB资料、多普勒雷达产品及NCEP逐6小时再分析资料对2013年6月27日20时—29日20时发生在江西省浙赣沿线附近的一次梅雨锋大暴雨过程进行诊断分析。结果表明:(1)中高纬双阻型和副热带高压位置偏南,是引发该暴雨过程的大尺度背景条件,梅雨锋上不断有中小尺度对流系统产生,这些中小尺度对流系统在受到大尺度强迫作用而增强并长时间维持,在导致暴雨区形成强降水;(2)此次过程是发生在不稳定流较弱、强迫流极强的环境下,暴雨区发生在低层西南急流出口区左侧、高层急流入口区,该区域上空具有低层正涡度、高层负涡度和低层负散度、高层正散度的藕合形势,这种形势非常有利于强垂直上升运动的维持,为这次梅雨锋暴雨过程提供了强大的动力机制。     

飑与冷锋过境的区别及其判定

飑是一种天气系统的活动，通常在冷锋前、切变线附近生成，多出现于夏季。飑是突然发作的强风，出现时风速的大小并无规定。但是飑出现时，风速达到大风标准，则要加记大风现象。     

黄淮地区一次冷锋暴雨天气过程的预报分析

利用常规天气图、地面加密资料、数值预报产品、卫星云图和多普勒天气雷达资料等,对2007年7月18-19日出现在黄淮地区的暴雨天气过程进行了综合分析.结果表明:暴雨是高空西风槽、冷锋和副热带高压边缘强盛的西南暖湿急流共同作用产生的;强降水产生在地面中尺度辐合线附近.在高层干冷、低层暖湿的大气不稳定条件下,地面冷空气的侵入触发了不稳定能量的释放,同时低层辐合、高空辐散的高低空配置,引发了强降水的产生.卫星云图和多普勒天气雷达资料在暴雨临近预报中起着重要作用.     

Case study of urban interactions with a synoptic scale cold front

Summary A two-day test period (10–11 March 1966) during which a slow moving synoptic cold front passed over New York City has been analyzed. As the front passed over the city it was destroyed near the surface by the urban roughness effect. The upper segment of the front passed over the city and rejoined the rest of the front downwind of the city. Evidence for the above phenomenon was seen in both the pibal wind and helicopter (temperature and SO₂) soundings.Distortion of the front forced polluted surface urban air upwards, as within the warm sector of an occluding synoptic frontal system. The outward spreading of this air aloft produced increasing concentrations over the city at heights above 200 m.With 7 Figures     

In situ observations of snow particle size distributions over a cold frontal rainband within an extratropical cyclone

Cloud microphysical properties of a mixed phase cloud generated by a typical extratropical cyclone in the Tongliao area, Inner Mongolia on 3 May 2014, are analyzed primarily using in situ flight observation data. This study is mainly focused on ice crystal concentration, supercooled cloud water content, and vertical distributions of fit parameters of snow particle size distributions (PSDs). The results showed several discrepancies of microphysical properties obtained during two penetrations. During penetration within precipitating cloud, the maximum ice particle concentration, liquid water content, and ice water content were increased by a factor of 2-3 compared with their counterpart obtained during penetration of a nonprecipitating cloud. The heavy rimed and irregular ice crystals obtained by 2D imagery probe as well as vertical distributions of fitting parameters within precipitating cloud show that the ice particles grow during falling via riming and aggregation process, whereas the lightly rimed and pristine ice particles as well as fitting parameters within non-precipitating cloud indicate the domination of sublimation process. During the two cloud penetrations, the PSDs were generally better represented by gamma distributions than the exponential form in terms of the determining coefficient (R²). The correlations between parameters of exponential /gamma form within two penetrations showed no obvious differences compared with previous studies.     

夏季华北冷锋暴雨个例分析

本文分析了1974年7月31日至8月1日华北地区的一次快速下滑冷锋暴雨过程,给出了此类冷锋的三维流场结构特征、暖区内中尺度雨带的分布及降雨团的活动特点。     

Airborne microphysical measurements and radar reflectivity observations near a cold frontal rainband observed during the FRONTS 87 experiment

Airborne microphysical measurements and ground-based radar-reflectivity observations, collected at the near of a narrow cold frontal rainband during the FRONTS 87 experiment, are used to infer some characteristics of the precipitation: size-distribution, liquid or ice-water content, mass-weighted fallspeed, radar-reflectivity and relationships between pairs of these quantities. In the ice phase region, two different methods are proposed to calculate the radar reflectivity from the aircraft data. The influence of the choice of the ice particle type on the calculated quantities is discussed from the results of the first method. The second method is new: it is based upon the use of a roughness parameter estimated from the analysis of the two-dimensional particle images of ice precipitation. Although this method is tentative, it gives conclusive results for the two different flight levels studied here.     

Three-dimensional modeling of synthetic cold fronts approaching the Alps

Summary A numerical model was used to study the behaviour of prototype cold fronts as they approach the Alps. Two fronts with different orientations relative to the Alpine range have been considered. One front approaches from west, a second one from northwest. The first front is connected with southwesterly large-scale air-flow producing pre-frontal foehn, whereas the second front is associated with westerly largescale flow leading to weak blocking north of the Alps.Model simulations with fully represented orography and parameterized water phase conversions have been compared with control runs where either the orography was cut off or the phase conversions were omitted. The results show a strong orographic influence in case of pre-frontal foehn which warms the pre-frontal air and increases the cross-frontal temperature contrast leading to an acceleration of the front along the northern Alpine rim. The latent heat effect was found to depend much on the position of precipitation relative to the surface front line. In case of pre-frontal foehn precipitation only falls behind the surface front line into the intruding cold air where it partly evaporates. In contrary, precipitation already appears ahead of the front in the case of blocking. Thus, the cooling effect of evaporating rain increases the cross-frontal temperature difference only in the first case causing an additional acceleration of the front.List of symbols C _pd specific heat capacity of dry air at constant pressure (C _pd =1004.71 J kg^–1 K^–1) - C _pv specific heat capacity of water vapour at constant pressure (C _pv =1845.96 J kg^–1 K^–1) - C _f propagation speed of a front - x, y horizontal grid spacing (cartesian system) - , horizontal grid spacing (geographic system) - t time step - E turbulent kinetic energy - f Coriolis parameter - g gravity acceleration (g=9.81 ms^–1) - h terrain elevation - H height of model lid (H=9000 m) - k Karman constant (k=0.4) - K _Mh horizontal exchange coefficient of momentum - K _Hh horizontal exchange coefficient of heat and moisture - K _Mz vertical exchange coefficient of momentum - K _Hz vertical exchange coefficient of heat and moisture - l mixing length - l _c specific condensation heat (l _c =2500.61 kJ kg^–1) - l _f specific freezing heat (l _f =333.56 kJ kg^–1) - l _s specific sublimation heat (l _s =2834.17 kJ kg^–1) - longitude - m ₁,m ₂,m ₃ metric coefficients - p pressure - Exner function - Pr Prandtl number - latitude - _M profile function - q _v specific humidity - q _c specific content of cloud droplets - q _i specific content of cloud ice particles - q _R specific content of rain drops - q _S specific content of snow - R _d gas constant of dry air (R _d =287.06 J kg^–1 K^–1) - R _v gas constant of water vapour (R _v =461.51 J kg^–1 K^–1) - r _E radius of earth (r _E =6371 km) - Ri _F flux Richardson number - density of dry air - t time - T temperature - _dia period of diastrophy - potential temperature - _v virtual potential temperature - _e equivalent potential temperature - U relative humidity - u, v, w cartesian wind components - u _F ,v _F front-normal and front-parallel wind components - x, y, z cartesian coordinates - w ^* transformed vertical wind component - W _R speed of falling rain - W _S speed of falling snow - z ^* transformed vertical coordinate Abbreviations GND (above) ground level - MSL (above) mean sea level With 12 Figures     

Fronts and the Alps: Findings from the front experiment 1987

Summary The German Front Experiment 1987 was an experimental effort to determine the influence of the European Alps on cold fronts using data collected by aircraft, radar, surface and upper-air systems between October and December 1987. Measurements were concentrated in the Alpine foreland south of Munich and in the Inn valley in Austria. It was a cooperative effort by groups from Austria, Germany and Switzerland. This study briefly recapitulates the conduct of the experiment and the intense observation periods which were carried through. Then it is outlined to what extent the scientific objectives have been achieved. The findings of the experiments are grouped into the following topics: orographically induced frontogenesis, the interaction of foehn and front, orographically trapped gravity currents, flow into valleys and the modification of precipitation by the Alps.     

台海地区一次冬季冷锋降水过程的数值模拟

利用WRF中尺度模式,结合FY-2E卫星云图和常规气象资料,对台海地区一次冬季冷锋降水过程进行了数值模拟研究.结果表明:1)微物理方案对台海地区冬季冷锋降水过程的模拟具有敏感性,Milbrandt双参数微物理方案能较好地再现云系层次结构、冰相降水过程及其云系的对流发展,24 h累积降水量模拟结果优于其他微物理方案.2)锋区的降水粒子(雨水、雪晶和霰)混合比大于锋后,锋区雨带集中在地面锋线的中段,锋后雨带偏向冷区的西南段.3)锋区附近云系受低空急流及台湾岛中部高山地形抬升共同作用,在迎风坡形成强降水中心,对应空中霰含量高值区.4)低空高相当位温、强辐合、正涡度和对流性不稳定与高空强辐散和负涡度的配置是本次冷锋云系维持与发展的重要原因.     

冷锋过境时的边界层阵风结构分析

本文对冷锋过境时强风条件下平均风速、阵风、阵风系数、阵风谱、水平和垂直自相关进行了计算和分析。证明在强风条件下各层最大风速可以通过平均风速乘以各层的大于1的阵风系数求得。并且指出,该阵风系数随高度呈负指数规律变化。 阵风谱和自相关函数的计算结果表明,所有大气湍流统计量都与高度有关。320米以下各层阵风谱的含能区均在20秒到20分钟周期范围内,呈现出多峰值现象,而且含能区的峰频随高度明显地向低频方向移动。湍流总动能随高度的分布不是单一性减小的,而且在30米和150米高度附近出现两个极大值。阵风的水平自相关和     

Orographical modification and large scale forcing of a cold front

Summary The development of a cold front influenced by orography and large scale forcing is examined with a two-dimensional meso-scale model. The model is based on the primitive equations and uses the hydrostatic and anelastic approximations. Gradients of the basic flow and temperature field in the third dimension are taken into account during the simulations. Low diffusive numerical schemes and radiation boundary conditions reduce the numerical errors to an acceptable minimum for a two day simulation and avoid reflections at the upper and lateral boundaries. Frontogenetical forcing is included in the simulations by specifying either a vertically sheared or horizontally convergent basic zonal flow field. Model runs with an idealized cold front were carried out over flat terrain and in the presence of a bell shaped mountain ridge.The simulations show a weakening of the cold front on the windward side of the mountain ridge and a strong reintensification on the leeward side relative to the control runs without topography. Analysis of frontogenesis terms demonstrates the importance of convergence in the ageostrophic circulation and of along-front temperature advection for the development of the cold front. The strong intensification of the cold front on the leeward side of the mountain ridge can only partly be explained by superposition with the mountain induced wave. It is mainly caused by ageostrophic deformation forcing in the strong downward flow of this wave.The results also show that the cold front passage over the mountain ridge is not a continuous process. The formation of a new frontal structure on the leeward side of the mountain ridge, well separated from the primary one, is observed while the initial cold front still exists in the upslope region. Generally nonlinear interactions between the mountain wave and the cold front are the important mechanisms to explain these phenomena.With 18 Figures     

台风与冷空气对重庆\

利用地面逐小时观测资料、NCEP/NCAR（1°×1°）再分析资料、FY-2F卫星加密观测（6 min分辨率）的云顶亮温（TBB）和常规观测资料,对1710号台风\     

湛江地区一次冷锋型海雾微物理特征

利用2010年3月31日—4月2日冷锋天气系统影响下湛江海雾综合观测资料,分析了海雾的微物理特征及海雾过程中气溶胶粒子谱的演变特征。结果表明,海雾的生消与风场密切相关,海雾生成和发展与较强的ESE气流相联系,而弱的NE气流则会促使海雾减弱或消散。湛江海雾的雾滴数浓度为100~102cm-3,液态含水量为0.001~0.232 g·m-3,雾滴平均半径小于10μm,雾滴峰值半径多位于1.4μm。海雾雾滴谱分布以单调递减谱为主,谱宽超过20μm,且雾发展过程中雾滴谱谱宽存在突然增宽和迅速减小的现象。海雾过程中雾滴数浓度的变化主要是由半径小于5μm的雾滴数密度变化引起的。海雾过程对气溶胶粒子的湿清除效果并不显著,雾过程中粒径小于0.1μm和大于4μm的气溶胶粒子数密度显著减少,但在雾消散后又迅速恢复到雾发生前的水平。     

Frontogenesis and cross frontal circulation in a strong summertime cold front

Summary The characteristics of a strong European summertime cold front are investigated using the wealth of synoptic observations, which were analysed by the dynamical initialization scheme of an operational meso-scale model. The study was also extended to forecasted fields.Cross-sections perpendicular to the front, covering a dormain of more than 2000 km in the horizontal and 10 km in the vertical direction, bring out the basic parameters (as potential temperature and the components of the geostrophic and ageostrophic wind vectors), the ageostrophic crossfrontal circulation and all cross-frontal frontogenesis terms.None of the many frontogenesis terms plays a dominant role, there is a concerted action of geostrophic and ageostrophic deformation and shear, of tilting and diabatic sources in producing a rather complex structure of total frontogenesis. The cross-sectional fields of all presented terms strongly underline the fact, that a front is not a line of discontinuity but a rather wide three-dimensional field phenomenon.With 12 Figures     

一次冷锋过境时引发多种天气现象的成因分析

利用观测数据、雷达产品、ERA5再分析资料对2021年4月15日河北保定一次天气过程进行综合分析,结果表明：(1)天气背景高层干冷,低层有暖脊,整层大气较为干燥。冷锋过境引起地面风力加大,锋面触发干对流后,冷池出流进一步加大了地面风速,湍流加强并出现扬沙。(2)对流云内存在水平弱风区,其径向速度近于0 m/s,弱风区后侧的雷达回波强度呈增加趋势。雷达低仰角0径向速度区的上方有水凝物粒子迅速集中、生长,生长区内径向速度为5～10 m/s,降雹区域上空各仰角均为0径向速度。(3)下击暴流和动量下传引发了干对流大风,降水粒子的蒸发、拖曳作用进一步加剧了气流下沉,其中蒸发作用的贡献更显著。(4)干对流消亡后,锋面东移,锋后西北风减弱,扬沙天气结束,较小的近地层风通量有利于沙尘粒子缓慢下降,地面能见度好转但PM₁₀浓度依然较高。(5)夜间冷空气下沉,上游输送的沙尘粒子随之沉降,造成了浮尘天气。     

持续性冷涡暖锋暴雨成因及特征分析

利用常规观测资料、自动气象站降水量以及NCEP FNL再分析资料,对2019年8月6—8日一次持续性东北冷涡暴雨过程成因及特征进行诊断分析。结果表明:暖锋稳定维持在同一区域且不断锋生造成持续性暴雨。强降水出现在850 hPa锋区南侧,呈东西带状分布,暴雨与最大锋生区相对应。大气中层为弱对流不稳定,有利于强降水的维持。高层正位涡大值区向下层扩展,促使中低层涡度增加,在暖锋前形成正涡柱结构,在地面锋区上诱发出气旋性环流,有低压新生。锋区低层的强辐合区位于迎风坡,锋面辐合抬升和地形强迫抬升的共同作用,使低层强辐合区持续3 d维持在同一区域。冷涡东移减弱阶段,台风携带大量暖湿空气北上促使锋区北抬,强降水维持。冷涡新生和维持阶段,T_(850-500)≥25℃,K≥35℃,且有一定的对流有效位能,对暴雨的出现有较好的指示意义。暴雨区东边界的水汽输入最为关键,占到整个水汽输入总量的一半,尽管南边界的总水汽输入量不大,但集中出现在第一个暴雨日,是6日暴雨过程主要水汽贡献者。     

Boundary-layer structure near the cold front of a marine cyclone during “ERICA”

The boundary layer in the warm sector of a moderately deepening winter cyclone during the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA) is studied near the cold front. Data from the National Center for Atmospheric Research Electra research aircraft are used to examine mean and turbulence quantities. The aircraft data and supplemental data from ships, drifting buoys and moored buoys reveal an equivalent-barotropic pressure field. The area is found to be dominated by gradients in temperature and in turbulent fluxes, with changes occurring over 100 km horizontally being comparable to changes over 350 m vertically. The horizontal components of the gradients are found to be a maximum in a direction perpendicular to the front. Cross-sections perpendicular to the front are used to illustrate boundary-layer structure. Profiles of wind speed, stress, wind direction and stress direction are estimated from an Ekman model that is modified to take into account the equivalent-barotropic pressure field. Comparison of profiles from the model to the aircraft-measured data show reasonable agreement far from the front (100 km) when the model uses a constant eddy viscosity of approximately 6 kg m^–1 s^–1. Near the front there is less agreement with the model. Profiles of turbulent fluxes of momentum, heat and latent heat are divergent, with along-wind momentum flux negative and decreasing upward, cross-wind momentum flux positive and increasing upward, and heat flux and latent heat flux small, positive and decreasing upward. Far from the front, the turbulent kinetic energy budget shows that dissipation balances shear production. However, near-front behavior has an imbalance at low altitude, with shear production appearing as a TKE sink.