Revisit to frozen-in property of vorticity

Considering some simple topological properties of vorticity vector, the frozen-in property of vorticity herein is revisited. A vortex line, as is analogous to velocity vector along a streamline, is defined as such a coincident material(curve) line that connects many material fluid elements, on which the local vorticity vector for each fluid element is also tangent to the vortex line. The vortex line evolves in the same manner as the material line that it is initially associated with. The vortex line and the material line are both oriented to the same directions, and evolve with the proportional magnitude, just like being ‘frozen' or ‘glued' to the material elements of the fluid under the barotropic assumption. To relax the limits of incompressible and barotropic atmosphere, the frozen-in property is further derived and proved in the baroclinic case. Then two effective usages are given as examples. One is the derivation of potential vorticity conservation from the frozen-in property in both barotropic and baroclinic atmospheres, as a theory application, and the other is used to illuminate the vorticity generation and growth in ideal cases and real severe weather process, e.g., in squall line, tornado, and other severe convection weather with vortex. There is no necessity to derive vorticity equation, and this method is very intuitive to explain vorticity development qualitatively, especially for fast analysis for forecasters. Certainly, by investigating the evolution of vortex line, it is possible to locate the associated line element vector and its development on the basis of the frozen-in property of vorticity. Because it is simple and visualized, it manifests broad application prospects.     

An Approximation of the Steady Gravity Wave Induced by Mountainous Topography

An approximation of the orography gravity wave, which is induced by mountainous topography, is considered in this study. By assuming that the horizontal wind is a linear function with respect to the height, the approximating equation for the orography gravity waves is obtained. Four topography functions are considered in this study and the orography gravity wave are obtained. The dynamics of the orography gravity wave is then discussed by considering the effect of the surface topography and background horizontal wind.     

Another look at the moist baroclinic Ertel–Rossby invariant with mass forcing

Due to the importance of the mass forcing induced by precipitation and condensation in moist processes, the Lagrangian continuity equation without a source/sink term utilized to prove the Ertel–Rossby invariant(ERI) and its conservation property is re-derived considering the mass forcing. By introducing moist enthalpy and moisture entropy, the baroclinic ERI could be adapted to moist flow. After another look at the moist ERI, it is deployed as the dot product between the generalized velocity and the generalized vorticity in moist flow, which constitutes a kind of generalized helicity.Thus, the baroclinic ERI is further extended to the moist case. Moreover, the derived moist ERI forumla remains formally consistent with the dry version, no matter whether mass forcing is present. By using the Weber transformation and the Lagrangian continuity equation with a source/sink effect, the conservation property of the baroclinic ERI in moist flow is revisited. The presence or absence of mass forcing in the Lagrangian continuity equation determines whether or not the baroclinic ERI in moist flow is materially conserved. In other words, it would be qualified as a quasi-invariant but only being dependent on the circumstances. By another look at the moist baroclinic ERI, it is surely a neat formalism with a simple physical explanation, and the usefulness of its anomaly in diagnosing atmospheric flow is demonstrated by case study.     

Physical Mechanism of Formation of the Bimodal Structure in the Meiyu Front System

The bimodal structure of the Meiyu front system is readdressed after Zhou et al.（2005）. The physical mechanism of the formation of the bimodal distribution is discussed. The bimodal structure of the Melyu front system considerably results from atmospheric moisture gradients, though atmospheric temperature gradients are also not negligible. According to the definition of equivalent potential temperature, and by scale analysis, we find that atmospheric equivalent potential temperature gradients, which could be regarded as an indicator of the Meiyu front system, could be mainly attributed to the variations of atmospheric potential temperature gradients with a scaling factor of 1 and moisture gradients multiplied by a scaling factor of an order of about 2.5 × 10^3, which means that small variations of atmospheric moisture gradients could lead to large variations of equivalent potential temperature gradients, and thus large variations of the Meiyu front system. Quantitative diagnostics with a mesoscale simulation data in the vicinity of the Meiyu front system show that moisture gradients contribute to equivalent potential temperature gradients more than potential temperature gradients.     

THE GENERALIZED E-P FLUX OF WAVEMEANFLOW INTERACTIONS

This paper deals with the generalized E-P flux relation, containing the stratification effect and directly diagnosing the mean zonal acceleration from data. Its advantages are that the mean zonal acceleration may also be diagnosed, even if there is the strong baroclinity in the atmosphere and the heat transportion of the transient disturbances obviously varies with time and spsee. Meanwhile, both the ill-condition /(u'v')_y existing in the E-P flux and the terms correlative with the disturbance displacement η' not easily measured in Andrews' E-P flux are automatically eliminated in the generalized E-P flux. Therefore, it is suitable for directly diagnobing the mean zonal acceleration from observational data. In order to examine the applicability of the generalized E-P flux, it is illustrated that an example is correctly diagnosed by using the generalized E-P flux instead of the E-P flux.     

计算涡度的新方法

提出应用一维数值微分算法求取观测风场的涡度场,并与中央差分方法作了比较.结果表明,一维数值微分算法稳定、可行且计算精度较高,对较小尺度的天气系统具有较强的识别能力. 关键词： 数值微分 吉洪诺夫正则化 正则化解     

Ageostrophic Generalized E－P Flux in Baroclinic Atmosphere

Aimed at limitation and deficiency of the traditional Eliassen-Palm (E-P) flux associated with wave-meanflow interaction and its subsequent generalization based on the Boussinesq approximation or quasi-geostrophic approximation, we develop an ageostrophic Generalized E-P flux in baroclinic stratified atmosphere. This generalized E-P flux can be conveniently used to diagnose and analyse some important phenomena related to wave-meanflow interaction of the baroclinic atmosphere with observational data, such as the upper-level jet acceleration, gravity wave breaking-up and stratospheric erupt warming.     

Effects of sea surface temperature, cloud radiative and microphysical processes, and diurnal variations on rainfall in equilibrium cloud-resolving model simulations

The effects of sea surface temperature(SST),cloud radiative and microphysical processes,and diurnal variations on rainfall statistics are documented with grid data from the two-dimensional equilibrium cloud-resolving model simulations.For a rain rate of higher than 3 mm.h 1,water vapor convergence prevails.The rainfall amount decreases with the decrease of SST from 29℃ to 27℃,the inclusion of diurnal variation of SST,or the exclusion of microphysical effects of ice clouds and radiative effects of water clouds,which are primarily associated with the decreases in water vapor convergence.However,the amount of rainfall increases with the increase of SST from 29℃ to 31℃,the exclusion of diurnal variation of solar zenith angle,and the exclusion of the radiative effects of ice clouds,which are primarily related to increases in water vapor convergence.For a rain rate of less than 3 mm.h 1,water vapor divergence prevails.Unlike rainfall statistics for rain rates of higher than 3 mm.h 1,the decrease of SST from 29℃ to 27℃ and the exclusion of radiative effects of water clouds in the presence of radiative effects of ice clouds increase the rainfall amount,which corresponds to the suppression in water vapor divergence.The exclusion of microphysical effects of ice clouds decreases the amount of rainfall,which corresponds to the enhancement in water vapor divergence.The amount of rainfall is less sensitive to the increase of SST from 29℃ to 31℃ and to the radiative effects of water clouds in the absence of the radiative effects of ice clouds.     

Moist Potential Vorticity and Up—Sliding Slantwise Vorticity Development

By using the moist potential vorticity equation derived from complete atmospheric equations including the effect of mass forcing,the theory of up-sliding slantwise voriticity development (USVD) is proposed based on the theory of slantwise vorticity development.When an air parcel slides up along a slantwise isentropic surface,its vertical component of relative vorticity is developed.Based on the theory of USVD,a complete vertical vorticity equation is expected with mass forcing ,which explicitly includes the effect of both internal forcings and external forcings.     

表征波与流相互作用的广义E-P通量

本文给出了含有大气层结效应的、并用观测资料可直接去诊断纬向平均流加速的广义E-P通量关系。它的优点在于,即使在大气斜压性较强,瞬变扰动的热量输送随时间和空间有明显变化的情况下,波对纬向平均流的加速作用也能较好地被诊断出来。同时广义E-P通量中,自动解除了过去E-P通量中存在的病态条件及扰动位移η’的不确定性,它能较准确地用观测资料去判别。文中用实例说明了广义E-P通量能诊断出一般E-P通量所不能正确诊断出来的结果。