The numerical experiments on applicating geostrophic momentum approximation to the baroclinic and non-neutral PBL

In this paper, Wu and Blumen’s boundary layer geostrophic momentum approximation model (Wu and Blumen, 1982) is applied to baroclinic and non-neutral PBL, the motion equations for the PBL under the geostrophic momentum approximation are solved, in which the eddy transfer coefficient is a function of the distributions of the wind and temperature. The results are compared with those in barotropic and neutral conditions with the geostrophic momentum approximation. It is found that in the baroclinic condition, the wind distribution has both the characteristics of a steady, homogeneous and baroclinic PBL and those caused by the geostrophic momentum approximation. Those in non-neutral conditions show that they retain the intrinsic characteristics for the wind in non-neutral PBL, at the same time, the effects of the large-scale advection and local variation are also included. We can predict the wind in the non-neutral and baroclinic PBL by use of the geostrophic mo-mentum approximation when the temporal and spatial distributions of the geostrophic wind, as well as the po-tential temperatures and their variation rates at the upper and lower boundary of the PBL are given by large-scale model. Finally, the model is extended to the case over sea surface.     

地转动量近似下边界层中风场的调整

本文利用地转动量近似,并假设气压场为定常的圆形涡旋和初始风场不满足四力平衡(气压梯度力、科里奥利力、湍流粘性力和半地转惯性力)的条件下,求解了正压边界层中风场向气压场调整的初边值问题,得到了一些初步结论。本工作为利用四力平衡下的风速分布来诊断预报边界层风场提供了理论依据。     

A theoretical test of the geostrophic momentum approximation

A previously developed model of a 2-dimensional air flow with constant horizontal shear is used to estimeate the accuracy of the geostrophic momentum approximation by comparison of exact solutions when exist with approxi-mate ones.     

A barotropic model of the Ekman planetary boundary layer based on the geostrophic momentum approximation

A model of a stationary planetary boundary layer is proposed based on the equation of motion with the advective term retained. The latter is modeled by means of the so-called geostrophic momentum approximation in two versions — original and modified. New expressions for the vertical velocity W at the top of the boundary layer are derived and analyzed. They underestimate W compared to the classical expression.     

The influences of orography upon the flow within Ekman boundary layer under the approximation of geostrophic momentum

In this paper, the influences of orography on the boundary layer flow with the approximation of geostrophic momentum are studied. The wind velocity at the lower boundary will not always be zero when the orography exists. So the structure of the boundary layer flow, as well as the vertical velocity at the top of the boundary layer, is affected. There are three factors affecting the vertical motion at the top of the boundary layer: lifting due to orography; divergence due to Ekman flow, and advection of the geostrophic momentum. These effects and the features of the flow within the boundary layer are discussed in detail.     

A numerical study of geostrophic adjustment and frontogenesis

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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.     

A numerical experiment with wind variations in the planetary boundary layer

Summary The equations of motion applied to the planetary boundary layer are numerically integrated for certain special eddy viscosity distributions. Accelerations are retained, eddy viscosity varied with height and, to a lesser extent, with time. The resulting inertial oscillations, although very limited in generality, show agreement with observations. Not only clockwise but also counterclockwise rotation of the end point of the wind vector can occur in some cases. Some solutions agree and others disagree with the analytical solutions for unbounded eddy viscosity.

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Zusammenfassung Die auf die planetare Grenzschicht angewandten Bewegungsgleichungen werden für spezielle Verteilungen des Austauschkoeffizienten numerisch integriert. Beschleunigungen werden beibehalten, vertikale, und, in geringerem Maße, zeitliche Variationen des Austauschkoeffizienten angenommen. Die resultierenden Trägheitsschwingungen sind zwar im allgemeinen sehr beschränkt, zeigen jedoch gute Übereinstimmung mit den Beobachtungen. In einzelnen Fällen kann eine Drehung des Windvektors nicht nur im Uhrzeigersinn, sondern auch im Gegenuhrzeigersinn auftreten. Einige Lösungen stimmen gut, andere nicht mit den analytischen Lösungen für einen unbegrenzten Austauschkoeffizienten überein.

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Résumé Les équations du mouvement appliquées à la couche limite planétaire sont intégrées pour des distributions particulières du coefficient d'échange turbulent. On tient compte des accélérations et on admet des variations verticales et, en moindre mesure, temporelles du coefficient d'échange. Les oscillations d'inertie qui en résultent sont, il est vrai, très limitées mais s'accordent bien avec les observations. Dans certains cas une rotation de la résultante du vecteur vent peut apparaître, non seulement dans le sens des aiguilles d'une montre, mais aussi en sens contraire. Quelques solutions concordent avec celles que l'on obtient en admettant un coefficient d'échange illimité, d'autres pas.

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With 5 Figures     

A numerical experiment of cyclogenesis and the development of disturbances

A numerical experiment has been carried out to study the mechanism of cyclogenesis and the develop-ment of disturbances. First, an idealized field of temperature and pressure is designed, which is very similar to the actual synoptic situation, consisting of the jet-like zonal circulation with some nonzonal waves superim-posed on it. Prediction is made by using a six-level splitting primitive model with the idealized field as an initial one. The results show that if a disturbance like a frontal zone in the lower troposphere is superim-posed on the zonal circulation, a frontal cyclone will quickly develop and then gradually become an oc-cluded one. Its life cycle is similar to that of the actual frontal cyclone on the synoptic map. However, if there is a disturbance superimposed on the zonal circulation in the middle troposphere, the cyclone with weaker intensity will be slowly formed near the surface. Finally, if the initial disturbance is located at the high-level, a situation like a cut-off low rather than a frontal cyclone will develop.     

气旋形成与扰动发展机理的数值试验

应用六层全隐式原始方程模式,对地面气旋形成与扰动发展的机制进行了数值试验. 首先,构造了接近实际的理想带状温压场和相应的扰动场.试验表明在带状环流上如果在低空加上一个类似锋区的扰动,便会有锋面气旋很快地发展起来,并逐渐趋于锢囚阶段,和实际大气的锋面气旋生命史很相似.但若扰动在对流层中层,则地面气旋形成过程较慢,强度也弱.如果初始扰动位于对流层高层,则地面形成的气旋在结构上和切断低压类似,不像锋面气旋.     

一个描述河陆风变化的数值模式及其数值实验

本文设计了一个包括水平及垂直扩散、牛顿冷却的二维46层非弹性运动方程模式来模拟及研究河陆风的变化。在模式中包括了太阳辐射;地-气系统的长波辐射;地表面、河表面向大气的感热与潜热输送以及地表面向土壤层的热传导等物理过程。 本模式采用分解算法及隐式时间差分方案,计算结果表明,这个模式无论对稳定层结或不稳定层结其计算都是稳定的,且所需计算时间也是很节省的,其计算结果是符合实际的。     

A numerical experiment of mesolow on the eastern side of the taihang mountains

A numerical experiment on the formation and decay process of a mesolow on the plain east to the Taihang Mountains has been conducted. The dynamical effect of the special topography of the Taihang Mountains and the Yanshan Mountains on the formation of the mesolow is very important. Namely, the difference of the heating be-tween the Taihang Mountains and the North China plain plays an important role in the formation and decay of the mesolow.     

贵州中尺度数值预报系统及其模拟试验

主要介绍了贵州中尺度数值预报系统、中尺度模式MM5以及业务情况。业务运行实践证明，系统具有较强的短期天气预报能力，对降水的量级和落区预报效果较好。选取4种积云对流参数化方案和两种边界层参数化方案对贵州汛期11个降水个例进行36h的降水模拟，经检验、分析和比较发现：各种方案预报效果的优劣在不同条件下有所不同，没有哪一种方案的预报效果是绝对最优的，总的来说：对50mm以下的降水，GR方案的预报效果较好，而KF方案则较差；对50mm以上的降水，BM、KF方案的预报效果一般较好；降水的模拟对参数化方案敏感。     

建筑物对城市通风自净能力影响的数值试验

应用城市气候数值模式，分别计算了三栋6层住宅楼和一栋20层住宅楼两类建筑形式影响下的风场，以及在此风场中街道汽车尾气的浓度分布。计算结果表明在总建筑面积基本相同的情况下，高层建筑周围环境的通风自净能力要优于多层建筑。     

动力初值预报试验

本文提出一种动力初值方案,这一方案考虑了不同纬度大气适应过程的不同特征:在低纬度,主要调整气压场和温度场,而对流场只作适当调整;在中高纬度,则主要调整流场,而对气压场和温度场只作适当调整。此方案计算收敛很快,只需相当于做2至3小时预报的时间。用此初值做48小时预报,得到了较好的预报结果。     

复杂地形下冰雹预报的数值试验研究

根据威宁地区的地形、气候特征，建立三维冰雹云数值模式，对冰雹研究有一定参考价值。     

土壤湿度影响中国夏季气候的数值试验

利用"全球土壤湿度计划第2阶段"提供的土壤湿度资料强迫区域气候模式RegCM3,通过数值试验讨论了土壤湿度对东亚夏季气候模拟效果的影响。结果表明,合理考虑土壤湿度的作用,能够提高区域气候模式对中国夏季降水和2 m气温的空间分布型及逐日变化的模拟效果;模拟结果与观测的相关分析显示,降水和2 m气温的年际变化都得到了有效改进,这种改进在气温上尤为明显。不过上述改进具有区域依赖性。数值试验结果表明,气温对土壤湿度的敏感性强于降水,这也从一个侧面说明提高降水模拟效果的难度。总体而言,合理的土壤湿度能够提高区域气候模式对中国夏季气候的模拟能力。因此,合理描述土壤湿度的变化,是提高中国夏季气候预报技巧的潜在途径之一。     

跨季度汛期气候数值预测试验

利用国家气候中心全球大气环流模式BCC_AGCM2.0.1开展季节预测,对模式预测与观测值进行相关分析。结果表明:该模式对夏季大尺度环流场、降水及气温等都有一定的跨季度预测能力,但对中国不同区域的要素预测能力有所不同。总体来说,夏季大气环流场的预测效果在海洋上优于陆地上,在低纬优于高纬,在新疆及蒙古也较好。从距平相关系数指标看,气温预测效果在中国西北部分地区较好,降水在中国北方及西南部分地区预测效果较好。由预测效果的年际变化看,在不同年份模式预测效果不同。另外,在不同海温情况下,该模式对不同区域的汛期气候预测效果不同,进而说明春季海温预测的正确性对汛期预测的重要性。     

A numerical world ocean general circulation model: Part II. A baroclinic experiment

A new six-layer world ocean general circulation model based on the primitive system of equations is described in detail and its performance in the case of a homogeneous ocean is described. These test integrations show that the model is capable of reproducing the observed mean barotropic or vertically-integrated transport, as well as the seasonal variability of the major ocean gyres. The surface currents, however, are dominated by the Ekman transport, and such non-linear features as the western boundary currents and the equatorial countercurrents are poorly represented. The abyssal boundary countercurrents are also absent due to the lack of thermohaline forcing. The most conspicuous effect of the bottom topography on a homogeneous ocean is seen in the Southern ocean where the calculated Antarctic circumpolar transport through the Drake passage ( ≈ 10 Sv, with bathymetry included) greatly underestimates the observed transport (≈ 100 Sv).