一次冬季锋面暴风雪天气过程的斜压边界层特征的观测分析

对STORM-FESTIOP17一次冬季锋面暴风雪天气过程的斜压边界层结构演变及特征进行了分析。发现：暖湿空气沿锋面抬升凝结成云，产生降水过程中释放的大量潜热显著增加锋两侧的水平温度差异，产生锋生。与锋生相伴，在锋前产生低空急流和高空急流。当锋生至最强时，锋两侧温差可达20K，锋前低空急流开始减弱，锋后低空急流增强，锋后冷平流开始主导锋两侧的环流系统。该冷平流削弱锋两侧的温度水平梯度，产生锋消作用。对这次锋面斜压对流边界层的湍流特征分析表明：在边界层之上切应力wv明显增大；湍能收支分析表明在边界层之上的风切变产生项很强，即大尺度天气系统有利于斜压对流边界层的发展，边界层内各量充分混合。这次冬季锋面暴风雪天气过程，冷锋前的低空南风急流从墨西哥湾携带来的充足水汽及锋区边界层大气的强斜压性是其产生的关键因子：冷锋过后，大尺度高空急流的作用更有利于对流边界层的充分发展。     

高空急流加速与低层锋生

本文从动力学上找到了低层锋生函数同急流区内纬向平均风加速的关系,指出高空急流加速有利于大气低层锋生,并从天气学的角度给出了高空急流加速同低层大气锋生的环流形态特征。即高空波的动量和热量通量在急流区的辐合,促使高空急流加速,并引起急流入口区上下层之间的质量调整,在急流轴高度以下出现一个反环流,反环流的上升支同其下原有的弱锋前的上滑爬升气流发生耦合,造成上滑气流加强,低层锋面坡度变陡,冷暖对比明显,引起大气低层锋生。     

急流加速产生的高空锋生和低空锋生

本文用数值模拟方法证实了曾庆存提出的一种锋生机制,即高空急流加速或高空动量输入通过地转适应过程造成高空锋生.我们采用二维非弹性数值模式模拟了五种急流加速分布产生的锋生过程:三个低空锋生,分别由中尺度地面、中尺度低空和次天气尺度低空急流加速产生;二个高空锋生,分别由高空次天气尺度和天气尺度急流加速产生.模拟的结果表明:(1)通过地转适应产生的位温梯度的大小依赖于急流的空间结构以及Rossby变形半径和急流扰动尺度的比值,比值越大,适应锋生越强.(2)在相同的尺度条件下急流加速产生的高空锋生比低空锋生更强.(3)地转适应过程将加速的高空急流动量向下层传输,使锋区变陡并向对流层中、下层延伸,这种对流层高低层的非地转耦合可能是低空急流形成的另一种机制.(4)在模拟的五个过程中急流振荡的主频率约是惯性频率的1.7倍.     

滇黔准静止锋诱发贵州春季暴雨的锋生机制分析

利用卫星云图、常规地面观测资料和逐6 h的1°×1°NCEP再分析资料,对2003-2006年春季滇黔准静止锋背景下,出现的5次贵州暴雨天气过程进行了诊断分析和总结。结果表明:在准静止锋背景下,贵州春季暴雨是由高低空急流、高空槽、冷空气与准静止锋的共同作用产生的。低空急流将大量的水汽从孟加拉湾和北部湾输送到贵州,不断积累对流有效位能;高空急流的加速增强了"高层辐散、低层辐合"的大尺度上升运动,并通过急流下侧的正环流圈带动冷空气南下,使得准静止锋活跃锋生,是暴雨天气过程的触发机制。锋生现象分析表明,高空急流加速导致对流层中高层极锋锋区内锋生和对流层中层正环流圈的形成,加强了准静止锋附近的水平变形和垂直运动,进而促使锋生加强。水平变形和垂直运动对暴雨的产生也有直接影响:水平变形项范围越大则降雨强度越强,与垂直运动相关的倾斜项移动与在准静止锋附近生成的强对流云团的移动方向一致。准静止锋与贵州春季的暴雨过程关系密切,暴雨落区集中分布在准静止锋南侧1个纬距带内。高空急流加速度、冷锋附近的水汽辐合强度以及对流有效位能的高能舌区范围对暴雨范围和强度有指示作用。基于以上锋生机制,提炼了滇黔准静止锋诱发贵州春季暴雨的物理模型。     

东亚春季强冷锋结构及其动力学诊断研究 I. 东亚春季强冷锋结构

本文利用常规探空资料和华东中尺度试验的部分资料，对1983年春季一次快速南下，并在江淮地区产生大范围强对流天气的冷锋进行了三维结构的分析。通过研究发现，这次冷锋过程主要有以下特征: （1） 与冷锋相对应的高空槽前存在一支下沉（DVM）气流；（2）有一强的辐合区出现在对流层中层，锋前上升运动的最大值也出现在对流层中层；（3）比较强的锋生过程主要集中于对流层中下层；（4）存在一支明显的热力直接环流（TDC），即暖湿空气沿冷锋倾斜上升；（5）在冷锋后存在一支较强的下沉气流（DVM），这支DVM对冷锋逆温层（或等温层）的形成可能有重要作用。并将此次东亚春季强冷锋个例与小仓义光（Ogura）等分析的北美春季冷锋（SESAME）个例作了对比，发现此次冷锋个例中，锋区的温度密集区主要在对流层中层，而北美SESAME个例温度密集区主要在对流层低层。这可能是由于东亚高空急流较强，动力强迫而引发锋生所致。     

2009年呼伦贝尔市一次寒潮大风天气分析

利用地面、高空等常规气象观测资料,对2009年5月28日发生在呼伦贝尔市一次全市性寒潮大风天气过程进行总结分析。结果表明:贝加尔湖冷涡和蒙古气旋及冷锋是造成这次寒潮大风天气的主导系统;高空强锋区及低层强温度平流是寒潮天气爆发的关键;高低空急流为大风天气提供能量;气压梯度大和冷锋后较大的3h正变压与大风天气有很好的相关性。     

2010年5月6日重庆强对流过程的天气学分析

利用常规观测、NCEP/NCAR再分析、FY-2E卫星、多普勒雷达以及自动站观测资料,对2010年5月6日重庆强风雹、暴雨天气过程进行了天气学分析。详细讨论了各尺度天气系统的发展演变及相互作用关系,并在此基础上提出了此次强对流发生的天气背景概念模型。研究结果表明:这次强对流过程发生在中纬度气旋、反气旋和高空槽增强发展的背景下,地面气旋与反气旋之间的冷锋也在南移中逐渐增强,而中纬度系统与西南地区低压和副热带高压间构成的鞍形场在重庆北侧造成锋生,一方面使得冷锋西段增强,另一方面通过次级环流与中层急流正反馈相互作用,加剧重庆中低层辐合抬升运动,既降低了大气稳定度,又为对流的触发形成提供了有利条件。与此同时,重庆低层始终位于西南地区低压东北部,偏南气流不仅输送了充足的水汽,其伴随的暖平流还与太阳辐射共同造成了重庆低层的显著升温,形成了条件不稳定层结。5月5日夜间冷锋移至重庆地区,锋面附近的强辐合造成了对流的初始发展,这一过程很可能与冷锋进入四川盆地后的下坡地形有关。另外,此次天气过程中副热带高空急流造成的强垂直风切变和高空辐散也为对流的强烈发展提供了有利条件。     

我国北方地区一次沙尘暴天气特征分析

使用美国大气中心6h一次的NCEP再分析格点资料(1.0°×1.0°)对2002年3月18—22日发生在我国北方的大范围沙尘暴天气成因进行分析。从环流形势、物理量诊断、高空急流等方面进行研究分析,结果表明:蒙古气旋是这次沙尘暴天气的主要影响系统,这次气旋发生发展在斜压区,气旋的发展阶段温度平流作用明显。沙尘暴过程主要是由气旋冷锋及锋后地面大风触发的,地面大风的形成与气旋发展、锋后冷平流及高空急流动量下传有关。     

低空西南急流与冷锋快速南移的关系

该文提出了冷锋的不连续传播是引起冷锋快速南移的直接原因的观点，即主冷锋南侧的锋生及其发展代替了主冷锋，在南方的锋生过程中，西南低空急流起了重要作用。     

2020年初内蒙古一次暴雪天气过程的成因分析

利用常规观测资料和NCEP（1°x1°）再分析资料，对2020年2月发生在内蒙古的一次地面回流与倒槽共同作用下的暴雪天气过程进行详细分析。结果表明：本次暴雪过程的主要影响系统是高空槽、700hPa切变线、高低空急流、地面冷高压、倒槽和冷锋。在高空下沉气流及1000~800hPa上东北急流的共同作用下，干冷气流形成“冷垫”，迫使暖湿空气沿冷垫抬升，同时不断的有干冷空气向中低层暖湿气流下方入侵，与中高层的西南急流形成深厚的锋生区和锋面次级环流，二者的正反馈作用为暴雪提供增幅作用。700hPa西南急流不断输送水汽，暴雪区位于比湿、水汽通量和水汽通量散度辐合的大值区。低层辐合高层辐散，配合显著的上升气流，有利于水汽积聚与输送和上升运动。强锋生落区与暴雪区域相对应，其中水平变形作用项对锋生的贡献最大，垂直运动项对锋生的贡献最小。湿位涡在强降雪落区内MPV1>0, MPV2<0，有利于本次暴雪过程的发生，高空下传的正MPV1会引起低层冷空气加强，冷暖空气对比度加大，有利于锋生，同时湿斜压性增强，诱发气旋式环流，进一步增强降雪。     

Ekman动量近似下中间边界层模式中的风场结构

发展了一个准三维的、中等复杂的边界层动力学模式，该模式包含了EKman动量近似下的惯性加速度和Blackadar的非线性湍流粘性系数，它进一步改进了Tan和Wu(1993）提出的边界层理论模型。该模型在数值计算复杂性上与经典Ekman模式相类似，但由于包含了Ekman动量近似下的惯性项，使得该模式比传统Ekman模式更近于实际过程。中详细地比较了该模式与其他简化边界层模式在动力学上的差异，结果表明：在经典的Ekman模式中，由于忽略了流动的惯性项作用，导致在气旋性切变气流（反气旋性切变气流）中风速和边界层顶部的垂直速度的高估（低估），而在半地转边界层模式中，由于高估了流动惯性项的作用，结果与经典Ekman模式相反。同样，该模式可以应用于斜压边界层，对于Ekman动量下的斜压边界层风场同时具有经典斜压边界层和Ekman动量近似边界层的特征。     

边界层急流与北京局地强降水关系的数值研究

应用3 km分辨率的中尺度模式,成功模拟出了2005年8月3日凌晨北京地区一次局地强降水过程,在此基础上探讨了地形热力作用和局地强降水与边界层急流的关系,结果表明,局地强降水与边界层急流之间存在正反馈作用,降水过程通过改变局地大气温度场的分布,导致边界层气流加速和急流的形成。而边界层气流的加速又加强了急流前方的风速辐合,为降水提供更多的水汽和更有利的动力条件。     

An Approximate Analytical Solution For The Baroclinic And Variable Eddy Diffusivity Semi-Geostrophic Ekman Boundary Layer

The WKB method has been used to develop an approximate solutionof the semi-geostrophic Ekman boundary layer with height-dependenteddy viscosity and a baroclinic pressure field. The approximate solutionretains the same simple form as the classical Ekman solution. Behavioursof the approximate solution are discussed for different eddy viscosityand the pressure systems. These features show that wind structure inthe semi-geostrophic Ekman boundary layer depends on the interactionbetween the inertial acceleration, variable eddy viscosity and baroclinicpressure gradient. Anticyclonic shear has an acceleration effect on theair motion in the boundary layer, while cyclonic shear has a decelerationeffect. Decreasing pressure gradient with height results in a super-geostrophicpeak in the wind speed profile, however the increasing pressure gradient withheight may remove the peak. Anticyclonic shear and decreasing the variableeddy viscosity with height has an enhanced effect on the peak.Variable eddy viscosity and inertial acceleration has an important role in thedivergence and vorticity in the boundary layer and the vertical motion at the top of the boundary layer that is called Ekman pumping. Compared to the constanteddy viscosity case, the variable eddy diffusivity reduces the absolute value ofEkman pumping, especially in the case of eddy viscosity initially increasing with height. The difference in the Ekman pumping produced by different eddy diffusivity assumptions is intensified in anticyclonic flow and reduced in cyclonic flow.     

Aircraft and ship observations of the mean structure of the marine boundary layer over the Arabian Sea during MONEX 79

Analysis of the mean wind, equivalent potential temperature and virtual potential temperature profiles observed by the National Center for Atmospheric Research (NCAR) Electra aircraft and obtained from dropwindsondes and ship-launched radiosondes were made in conjunction with synoptic observations to study the structure of the monsoon boundary layer over the Arabian Sea during MONEX 79. Comparison of mean profiles indicates the monsoon boundary layer to be much different from the trade wind boundary layer. Results confirm the existence of a boundary-layer jet known as East African or Somali Jet. Regions of multiple cloud layers at roughly the height of the capping inversion layer were associated with the jet. Regions in which a more well-mixed layer was observed showed a jet structure depressed in height. A free-jet surface-layer model appears to describe the mean wind structure of this jet observed during the present study and by others. An approximate balance of forces was found in the monsoon boundary layer between friction, advective acceleration, Coriolis and pressure gradient forces. Friction and advective acceleration terms were significant in the lower levels of the boundary layer. Forces in a typical trade wind boundary layer were found to be approximately one order of magnitude smaller than those observed in the monsoon boundary layer.     

The Nighttime Accelerations of the Wind in the Boundary Layer

Blackadar’s inertial relaxation of the nighttime boundary layer is modified to account for the finite scale of the pressure gradient normal to the jet. Even though this horizontal dimension may be several thousand times the depth of the daytime convective layer, it severely reduces the period of the free oscillation in the relaxation process and replaces the harmonic acceleration of the jet with a linear growth rate.     

The influence of the geostrophic wind advection approximation on a well-mixed layer

Numerical results indicate that advection of momentum in the boundary layer may significantly alter both the structure of the planetary boundary layer and its influence on the overlying free atmosphere. However, due to the nonlinearity of the inertial terms, it is always difficult to obtain the analytical solution of the boundary-layer model that retains the flow acceleration. In order to overcome this difficulty, the geostrophic momentum (hereafter GM) approximation has been introduced into boundary-layer models. By replacing the advected momentum with the geostrophic wind, the effect of the flow acceleration is partially considered and the original nonlinear partial differential equation set is converted to ordinary differential equations, the solutions of which can be obtained easily with standard techniques. However, the model employing GM fails to capture the features of the boundary layer when the spatio-temporal variation of the boundary-layer flow cannot be properly approximated by the geostrophic wind. In the present work, a modified boundary-layer model with the inertial acceleration in a different approximate form is proposed, in which the advecting wind instead of the advected momentum is approximated by the geostrophic wind (hereafter GAM).Comparing the horizontal velocity and boundary-layer pumping obtained from the classical Ekman theory, and the model incorporating (i) GM and (ii) GAM, it is found that the model with GAM describes most facets of the steady well-mixed layer beneath a north-westerly flow with embedded mesoscale perturbations that is considered in the present work. Inspection of the solution of the model with GAM shows that, within the limit of the validation of the model (i.e., the Rossby number R_O is not very large and the drag coefficient C_D is not too small), the horizontal convergence (divergence) is strengthened by the effect of the inertial acceleration in the region of maximum positive (negative) geostrophic vorticity. Consequently, the boundary-layer pumping there is intensified. It is found that the intensification is firstly strengthened and then weakened as R_O or C_D increases.     

The flow in a turbulent boundary layer upstream of a change in surface roughness

Modification of a turbulent flow upstream of a change in surface roughness has been studied by means of a stream function-vorticity model.A flow reduction is found upstream of a step change in surface roughness when a fluid flows from a smooth onto a rough surface. Above that layer and above the region of flow reduction downstream of a smooth-rough transition, a flow acceleration is observed. Similar flow modification can be seen at a rough-smooth transition with the exception that flow reduction and flow acceleration are reversed. Within a fetch of –500 < x/z ₀< + 500 (z ₀ is the maximum roughness length, the roughness transition is located at x/z ₀ = 0), flow reduction (flow acceleration) upstream of a roughness transition is one order of magnitude smaller than the flow reduction (flow acceleration) downstream of a smooth-rough (rough-smooth) transition. The flow acceleration (flow reduction) above that layer is two orders of magnitude.The internal boundary layer (IBL) for horizontal mean velocity extends to roughly 300z ₀ upstream of a roughness transition, whereas the IBL for turbulent shear stress as well as the distortion of flow equilibrium extend almost twice as far. For the friction velocity, an undershooting (overshooting) with respect to upstream equilibrium is predicted which precedes overshooting (undershooting) over new equilibrium just behind a roughness transition.The flow modification over a finite fetch of modified roughness is weaker than over a corresponding fetch downstream of a single step change in roughness and the flow stays closer to upstream equilibrium. Even in front of the first roughness change of a finite fetch of modified roughness, a distortion of flow equilibrium due to the second, downwind roughness change can be observed.     

A Homogeneous Langevin Equation Model, Part i: Simulation of Particle Trajectories in Turbulence with a Skewed Velocity Distribution

A Lagrangian stochastic model for the time evolution of the velocity of a fluid particle is presented. This model is based on a one-dimensional generalized Langevin equation, and assumes the velocity probability distribution of the turbulent fluid is skewed and spatially homogeneous. This has been shown to be an effective approach to simulating vertical dispersion in the convective boundary layer. We use a form of the Langevin equation that has a linear (in velocity) deterministic acceleration and a random acceleration that is a non-Gaussian, skewed process. For the case of homogeneous fluid velocity statistics, this 'linear-skewed' Langevin equation can be integrated explicitly, resulting in an efficient numerical simulation method. Model simulations were tested using cases for which exact, analytic statistical properties of particle velocity are known. Results of these tests show that, for homogeneous turbulence, a linear-skewed Langevin equation model can overcome the difficulties encountered in applying a Langevin equation with a skewed random acceleration. The linear-skewed Langevin equation model results are compared to results of a 'nonlinear-Gaussian' Langevin equation model, and show that the linear-skewed model is significantly more efficient.     

Time-dependent, coupled, Ekman boundary layer solutions incorporating Stokes drift

Recent technological advances in current measuring devices has resulted in a large observational database related to wind-driven motions in the upper ocean mixed layer. This has served to highlight the fact that transient motions make up a substantial contribution of the resulting Ekman currents. At the same time, certain discrepancies have emerged between the observed angular deflections of the steady-state currents from the surface wind stress, both at the surface and at sub-surface depths, which cannot be reconciled using the classical Ekman model. This paper seeks to tackle these two issues.First a general analytical method is presented for solving the time dependent horizontal momentum Ekman equations. Analysis of the unsteady terms that arise from simple special cases shows how the evolution proceeds through three stages. At early times, the Coriolis acceleration is insignificant, and the current is unidirectional and deepens through downward diffusion of momentum. Later Coriolis acceleration deflects the current vectors in the upper layers, whilst downward diffusion of momentum continues to deepen the layer. Finally, once diffusion has penetrated down to the depth of the steady-state current, then the transients decay on the inertial or diffusive timescale, depending upon the boundary conditions of the particular problem.In the second half of the paper, a new steady-state model is developed that includes the effects of wind-generated waves, through the action of their Stokes drift on the planetary vorticity. Comparisons between observations and the theoretical predictions, demonstrate that inclusion of the Stokes drift is the key to reconciling the discrepancies in the angular deflections of the steady-state currents. This leads to the conclusion that Ekman layer currents are significantly influenced by the surface waves.