共查询到20条相似文献，搜索用时 541 毫秒
1.
A generalized frontogenesis function and its application 总被引：3，自引：0，他引：3
With the definition of generalized potential temperature, a new generalized frontogenesis function, which is expressed as the Lagrangian change rate of the magnitude of the horizontal generalized potential temperature gradient, is derived. Such a frontogenesis function is more appropriate for a real moist atmosphere because it can reflect frontogenesis processes, in which the atmosphere in a frontal zone is typically characterized by neither completely dry nor uniform saturation. Furthermore, by derivation, the expression of generalized frontogenesis function includes both temperature and humidity gradients, which is different from and superior to the traditional frontogenesis function in moist processes, which also uses equivalent potential temperature. Diagnostic studies of real cases are performed and show that the generalized frontogenesis function in non- uniformly saturated moist atmosphere indeed provides a useful tool for frontogenesis, compared to using the traditional frontogenesis function. The new frontogenesis function can be used in situations involving either a strong temperature or moisture gradient and is closely correlated with precipitation. 相似文献
2.
3.
Climatological Characteristics of Frontogenesis and Related Circulations over East China in June and July 总被引：1，自引：0，他引：1 
下载免费PDF全文
下载免费PDF全文 Based on daily precipitation data from 212 stations in East China and NCEP/NCAR daily global final analysis data in June and July from 2000 to 2010, the climatological characteristics of frontogenesis and related circulations have been analyzed. The results demonstrate that frontogenesis function distributes nonuniformly in East China. The different terms of the kinematic frontogenesis show different intensities and distributions of frontogenesis. The strongest frontogenesis integrated from different kinematic frontogenesis terms is observed in the Jianghuai area. Frontogenesis events are separated into four types as per the different shear and convergence types of horizontal wind along the strong frontogenesis band at 850 hPa. The four types include the warm shear type, cold shear type (with two subtypes), west wind convergence type, and east wind convergence type. The events in different frontogenesis types occur with different frequencies over the past decade. The warm shear type occurs most frequently. Different types of frontogenesis have distinctive horizontal and vertical structures. Strong frontogenesis is featured with 340-K contour of θse parallel to the frontal zone in the vertical. Varied large-scale circulation patterns manifested in shifted location and strength of cyclone or anticyclone, wind convergence, as well as vertical circulation structure are associated with different types of frontogenesis. Moreover, a strong positive correlation between frontogenesis and precipitation is found, i.e., the stronger the frontogenesis, the more precipitation there is. Daily precipitation related to the warm shear type frontogenesis is the largest, and often occurs inside the frontal zone with the same orientation with the strong frontogenesis belt. The second largest daily precipitation occurs with the cold shear type frontogenesis, and related rainfall is usually observed to the south of the frontal zone, with the rain belt extending northeastward. Precipitation related to the west wind convergence type frontogenesis is the third largest, occurring mostly in the south of the frontal zone. 相似文献
4.
华南局地锋生及对流系统发展的模拟分析研究 总被引：5，自引：1，他引：4
2009年3月28日一条东西向的锋面出现在华南25°N附近。伴随着锋面的活动,对流降水回波午后开始在广西的梧州附近发展,并在随后几小时向东移动,组织发展成为中尺度对流系统(MCS),为广东中西部及珠三角地区带来了雷暴、暴雨、冰雹等灾害天气。应用地面自动站、雷达回波、卫星云图以及NCEP-FNL再分析资料和WRF模式的模拟结果,对锋面暴雨形成的天气特征进行了诊断分析,考察了中尺度对流系统的发展演变过程及其与局地锋生的相互关系。结果表明,锋面对流系统形成发展于一个东西向水平尺度约200km的地面中尺度辐合线附近,对流起始发展于具有较大对流有效位能(CAPE)和较小对流抑制位能(CIN)的区域。伴随着对流的发展,锋面强度增强。锋生函数的计算发现,非绝热项和倾斜项分别在由对流引起的次级环流的上升运动支和下沉运动支起锋生作用,是引发中尺度锋生的主要影响因子。而相对来说,水平辐合项和形变项的作用却比较小。这与大尺度的锋生过程不同,中尺度锋生更主要的是由热力直接触发的非地转环流所驱动。涡度场的演变分析还发现,沿着850hPa锋区大的正涡度区与500hPa的强上升运动区对应良好,对流系统发展与中尺度锋生之间存在着类似于第二条件不稳定机制的相互作用,对流增强了锋生过程,锋面则对中尺度对流系统的发展起组织作用。中尺度锋生对对流组织发展的作用作为此类灾害天气形成的原因值得关注。 相似文献
5.
A new frontogenesis function is developed and analyzed on the basis of a local change rate of the absolute horizontal gradient of the resultant deformation. Different from the traditional frontogenesis function, the newly defined deformation frontogenesis is derived from the viewpoint of dynamics rather than thermodynamics. Thus, it is more intuitive for the study of frontogenesis because the compaction of isolines of both temperature and moisture can be directly induced by the change of a flow field. This new frontogenesis function is particularly useful for studying the mei-yu front in China because mei-yu rainbands typically consist of a much stronger moisture gradient than temperature gradient, and involve large deformation flow. An analysis of real mei-yu frontal rainfall events indicates that the deformation frontogenesis function works remarkably well, producing a clearer mei-yu front than the traditional frontogenesis function based on a measure of the potential temperature gradient. More importantly, the deformation frontogenesis shows close correlation with the subsequent(6 h later) precipitation pattern and covers the rainband well, bearing significance for the prognosis or even prediction of future precipitation. 相似文献
6.
本文用WKBJ方法讨论了叠加在Hoskins-Bretherton［1］]锋生模型背景上的小扰动发展问题。考虑了大尺度变形场、锋区斜压气流及锋生环流对扰动发展的作用，发现锋生过程有利于与锋平行的中小尺度扰动发展，而锋消则有利于沿锋传播的横波型扰动发展。 相似文献
7.
本文选取2018年7月31日（简称“7.31”暴雨）和2016年8月8日（简称“8.8”暴雨）两次东天山哈密地区强降水天气过程,利用NCEP/NCAR的FNL资料（0.25°×0.25°）、中国地面卫星雷达三源融合逐小时降水产品、新疆地区常规观测资料、FY-2G卫星产品,通过对暴雨期间锋生函数计算诊断,证实了两次强降水过程中尺度对流系统触发因子差异,取得如下主要结果:（1）“7.31”暴雨期间,500 hPa西太平洋副热带高压位置异常偏北,700 hPa暖舌沿副高南侧偏东急流向西北伸展,低层增暖增湿,暴雨区上空形成不稳定大气层结,多个中尺度对流系统在700 hPa低空急流前生成,向东北方向移动和发展。“8.8”暴雨期间,500 hPa西太平洋副热带高压位置异常偏西,对流云团在对流层低层西南急流前生成向东北方向移动。（2）对流层低层暴雨区暖锋锋生是“7.31”暴雨中尺度对流云团的触发因子,云团初生阶段对流触发主要是锋生水平散度项和由垂直运动发展引起的倾斜项决定,成熟阶段暖锋锋生主要由锋生形变项和倾斜项所致。低空东南急流的维持加强利于锋面次级环流发展,是造成中尺度对流系统长时间维持的主要原因。（3）“8.8”暴雨对流云团由对流层低层弱冷锋触发。对流云团发展初始阶段,对流层低层冷锋锋生主要由水平辐散项决定;对流云团成熟阶段,对流层低层冷锋锋生主要由倾斜项决定。低层切变线长时间维持和加强利于低层冷锋进一步锋生,是造成中尺度对流系统长时间维持的主要原因。 相似文献
8.
Summary The problem of along-stream ageostrophic frontogenesis is studied by employing a numerical model at meso-alpha and meso-beta scales in simulations of the downstream circulations over the Front Range of the Rocky Mountains. Three-dimensional real data simulations at these two scales of motion are used to diagnose the transition from semigeostrophic cross-stream frontogenesis accompanying a propagating baroclinic upper-level jet streak to midtropospheric along-stream ageostrophic frontogenesis. This along-stream ageostrophic frontogenesis results from the perturbation of the jet streak by the Rocky Mountain range. The case study represents an example of internal wave dynamics which are forced by the drag of the Rocky Mountains on a strong jet streak in the presence of a low-level inversion.The simulation results indicate that, unlike semi-geostrophic frontogenesis, a front (which is alligned perpendicular to the axis of the jet stream) may form when significant adiabatic heating occurs within a stratified shear flow over horizontal length scales shorter than the Rossby radius of deformation. The mechanism responsible for the frontogenesis is the growth of the divergent along-stream wind velocity component which becomes coupled to the front's along-stream pressure gradient force. This nonlinear interaction produces hydrostatic mesoscale frontogenesis as follows: 1) vertical wind shear in the along-stream plane strengthens resulting in the increasingly nonuniform vertical variation of horizontal temperature advection as the ageostrophic wind component grows in magnitude downstream of the meso-scale terrain-induced adiabatic heating, 2) increasing along-stream differential vertical motions (i.e., along-stream thermally indirect circulation with warm air sinking to the west and cold air rising to the east) tilt the vertical gradient of isentropes into the horizontal as the vertical temperature gradient increases due to the previous process in proximity to horizontal gradients in the along-stream component of the ageostrophic wind, 3) as tilting motions act to increase the along-stream horizontal temperature gradient, the along-stream confluence acts to nonuniformly increase the along-stream frontal temperature gradient which increases the along-stream pressure gradient force resulting in further accelerations, ageostrophy, and frontal steepening as part of a scale contraction process.The evolution of the aforementioned processes results in the three-dimensional hydrostatic frontogenesis accompanying the overturning of isentropic surfaces. These adjustments act to turn air parcels to the right of the southwesterly geostrophic wind vector at successively lower atmospheric levels as the scale contraction continues. This simulated along-stream front is verified from diagnostic analysis of the profiler-derived temperature and wind fields.With 17 Figures 相似文献
9.
Summary The effects of varying horizontal and vertical grid resolution on the numerical simulation of upper-tropospheric frontal structures are examined using a, two-dimensional, dry, hydrostatic sigma-coordinate primitive equation model. These effects are illustrated with the results of 72 h model integrations in which frontogenesis is forced solely by confluence. Four different horizontal grid spacings, ranging from 100 km to 12.5 km, and four different vertical, grid resolutions, varying from 10 to 70 layers (90 mb to 13 mb), are considered.The intensity and integrity of the frontogenesis as a function of grid resolution are diagnosed in terms of time histories and spatial distributions of frontal parameters, such as the speed of the along-front jet, maxima of absolute vorticity and potential temperature gradient, and ageostrophic motions. The time histories show that, provided the vertical resolution is sufficient, increasing horizontal resolution leads to better-defined frontal structure due to the decrease in cross-frontal scale. They also indicate that for a given horizontal resolution there exists an optimal vertical resolution beyond which frontal parameters change only slightly. This optimal vertical resolution increases with increasing horizontal resolution, and apparently is related to the horizontal resolution through the slope of the frontal zone.The time histories for simulations combining low vertical resolution with high horizontal resolution exhibit substantial high-frequency variability. Cross sections show that this temporal variability appears to be manifested spatially in the form of gravity waves characterized by wavelengths on the order of 200 km and periods of 12 h, over the range of horizontal and vertical resolution that is considered. Although the source mechanism for these waves cannot be established definitively, it likely involves a grid-induced ageostrophic component of the along-front wind which disrupts thermal wind balance. This results thus demonstrates the potential risk of flawed frontal simulations, when incompatible combinations of horizontal and vertical resolution are used.With 7 Figures 相似文献
10.
凝结潜热对锋生的影响 总被引：1，自引：2，他引：1
本文在二维半地转峰生模式中，考虑了水汽凝结加热和边界层抽吸作用对锋生及越锋环流的影响。水汽凝结加热考虑了大尺度凝结加热和对流凝结加热两种方式。计算结果表明：水汽凝结加热和边界层抽吸对峰区环流具有显著的作用，凝结加热使锋生加快，使锋区上升运动倍增，并使其水平尺度明显减少。边界层抽吸作用也使垂直环流加强。现在 相似文献
11.
本文建立了一个考虑天气尺度水平变形场的二维、滞弹性、非静力平衡锋生模式，对冷锋过山时的锋生锋消机制进行了数值研究，试验结果表明:当无地形影响时，由于变形场的锋生作用与摩擦等因子的锋消作用相互抵消，冷锋最终达到其强度的准定常状态；当考虑地形影响时，地形强迫环流与锋面环流相互作用，使得锋面强度在山前和山后发生变化，在冷锋开始爬坡时锋生，爬过半山腰后开始锋消，过山顶后锋面强度变化不明显或者有微弱锋生，到达山脚后开始强烈锋生，并在地形下游其强度达到最大；总之，冷锋在翻越山脉或从高原移到平原之后，锋面发展的强度比冷锋不遇到地形时的更大。 相似文献
12.
13.
三种温湿参数下昆明准静止锋锋面位置及锋生函数诊断的对比分析 总被引：3，自引：2，他引：1
针对2008年1~2月昆明准静止锋天气过程,讨论了位温、相当位温和广义位温三个温湿参数及其对应的锋生函数,对比分析结果表明:（1）广义位温因其湿度因子权重过大,不适用于分析以温度梯度定义的昆明准静止锋锋面位置和强度;（2）理想状态下的干大气位温和饱和大气相当位温仅与气压和温度有关,均能较好地反映大气温度场特征,适用于昆明准静止锋的分析,比较而言相当位温对温度锋区的描述更为显著;（3）用位温或相当位温计算锋生函数所得结果直接表达了因温度场变化导致的锋面生消状况,而用广义位温计算锋生函数所得结果同时包含温度锋生（消）和湿度锋生（消）两种情况,不能简洁地判断昆明准静止锋锋面生消状况。 相似文献
14.
Prof. Dr. J. Egger 《Meteorology and Atmospheric Physics》1992,48(1-4):131-137
Summary A three-layer version of Davies' (1984) model is used to investigate orographic impact on fronts. Uniform potential vorticity is assumed for each layer. The uppermost layer represents the flow ahead and above a frontal zone approaching a mountain. The frontal zone is represented by the second layer and the cold air behind it by the third layer of the model. The upper layer induces weak frontogenesis when the frontal zone is moving towards the mountain. However, rapid frontogenesis sets in when the zone's leading edge is climbing up the slope while part of the frontal zone is still in the plain. When, finally, the whole frontal zone is moving up the slope, the flow evolution strongly depends on the structure of the frontal zone: frontolysis as well as frontogenesis can occur.With 5 Figures 相似文献
15.
利用ERA-Interim 0.125°×0.125°高分辨率逐6小时再分析数据,选取昆明准静止锋2008年1~2月长时间维持过程诊断分析了锋生函数及其各分项在锋面生消过程中的作用与贡献,并结合2016年1月20~27日锋面增强西进过程和1979年1月14~20日锋面减弱东退过程进行了比较分析。结果表明:（1）非绝热加热项对锋面生消作用较小,但存在显著的日变化。在辐射作用下,非绝热加热项日间表现为锋消,夜间为锋生。（2）西移冷气团在被高原大地形抬升过程中存在局地地形迫使冷气团下沉的情况,并导致垂直运动倾斜项表现为锋生。由于地形固定不变,局地锋生形成次级锋生带。次级锋生带与昆明准静止锋的形成、维持和东西摆动有密切的联系。（3）辐合辐散项和水平形变项是锋生函数变化的主要贡献项,其中水平形变项中的切变变形作用以锋生为主,而伸缩变形项在纬向产生锋生,在经向产生锋消。 相似文献
16.
利用中国气象局热带气旋年鉴、FY-2D（0.1°×0.1°）云顶亮温、逐时自动气象站降雨量、常规观测资料和NCEP/NCAR再分析资料,运用锋生函数对台风“麦德姆”（Matmo,1410）影响辽东半岛和山东半岛期间的降水特征进行了诊断分析。结果表明：1）Matmo影响辽东半岛和山东半岛期间,其低压环流与西风带高空槽相互作用,在其西侧和东北侧分别有冷锋和暖锋锋生,两条锋带均向东移。强锋生区首先在低层生成,随后尽管高空锋区向下延伸,但并未与低层冷锋重合,低层冷锋锋生强度减弱。2）山东半岛和辽东半岛的降水均发生在台风低压环流的锋生过程中,但山东半岛的降水明显多于辽东半岛。这与锋生强度密切相关,辽东半岛的锋生强度和垂直运动较山东半岛明显偏弱。3）强降水与台风环流内冷、暖平流活动密切相关,冷暖平流交汇之处对强降水有较好的示踪作用。山东半岛始终处于冷暖平流交汇处,其西侧斜压不稳定加强,上升运动发展,强降水出现在冷锋带上暖平流区内;而辽东半岛由冷平流转为暖平流时,对流运动向其东北方向发展,强降水位于辽东半岛东北部。 相似文献
17.
Helicity Dynamics of Atmospheric Flow 总被引：17，自引：0，他引：17
Helicity is an important physical variable which is similar to the energy and enstrophy in three-dimensional fluid. It can be used to describe the motion in the direction of fluid rotation and also can be regarded as a new physi-cal variable in turbulence theory. In recent years, it has been used in atmospheric dynamics. In this paper, helicity of atmospheric flow, especially helicity in the boundary layer and in the vicinity of front was discussed. These results show that helicity is usually positive in the boundary layer due to the effect of friction. The helicity of boundary layer flow is larger in anticyclone than that in cyclone, resulting from the different wind structures of boundary layers in an-ticyclone and cyclone under the geostrophic momentum approximation. It is possible that the helicity is negative at certain height in the baroclinic boundary layer. The influences of nonlinearity and baroclinity on the helicity are im-portant. The so called “Cloud Street” in the boundary layer is related to the dynamics of helicity. Helicity in the at-mosphere can be expressed as the temperature advection under some conditions, so helicity would be allowed to des-cribe the frontogenesis and development of frontal structure. The amplitude of helicity increases with time in the frontogenesis. A large gradient of helicity is generated in the region located to the northeast of the surface low and in which the front is formed. In warm frontal region, as well as behind the trough of temperature, the helicity is positive, while the helicity is negative in cold frontal sector and in the ahead ridge of temperature. The largest helicity occurs in the boundary. 相似文献
18.
利用1981～2016年气象常规观测和自动站资料对南充大风的基本气候特征进行统计分析,重点探讨不同类型区域雷暴大风的天气系统配置和环境物理量基本特征。结果表明:(1)南充雷暴大风按照形成原因主要分为高空冷平流强迫类和斜压锋生类,按落区出现情况分为全市型、东部型和西部型,东部型雷暴大风主要由高空冷平流强迫所致,全市型和西部型雷暴大风过程则多为斜压锋生所造成。(2)斜压锋生类雷暴大风主要发生在显著冷暖平流导致的斜压锋生与锋面动力强迫共同作用的形势下,高空冷平流强迫类则主要是高空强干冷平流的作用。(3)雷暴大风过程发生前大气环境呈上干下湿、湿层浅薄或为“喇叭口”形态,对于不同类型雷暴大风过程发生前的环境物理结构不同,斜压锋生类雷暴大风产生时大气环境多为明显斜压特征,高空常伴有强锋区,低层不稳定能量大,因此热力因子比较重要。高空冷平流强迫类主要发生在川陕槽后强烈冷平流形势下,水平风垂直切变大、要求低层增温快,故热力和动力因子都重要。 相似文献
19.
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 相似文献
20.
大尺度凝结加热与中尺度位温扰动对冷锋锋生的作用 总被引：1，自引：0，他引：1
文中导得简化的包括大尺度凝结潜热的半地转锋生模式，利用该模式讨论了大尺度凝结加热和中尺度位温扰动对冷锋锋生的影响，给出了锋生过程中各物理量的演变图。计算结果表明，大尺度凝结加热对冷锋锋生具有加强作用，增大锋生率，增大上升运动速度，缩小上升运动区的范围，使之更具有中尺度系统特征，使非地转越锋环流增强并发生倾斜，凝结加热和中尺度位温扰动的结合是锋前暖区多重雨带形成的可能机制之一。 相似文献