A proposed adiabatic formulation of 3‐dimensional global atmospheric models based on potential vorticity

A 2‐time‐level finite difference atmospheric general circulation model based on the semi‐Lagrangian advection of pseudo potential vorticity (which becomes potential vorticity in that part of the domain where the hybrid vertical coordinate becomes isentropic) has been formulated. At low levels, the hybrid vertical coordinate is terrain following. The problem of isentropic potential vorticity possibly becoming ill‐defined in the regions of planetary boundary layer is thus circumvented. The divergence equation is a companion to the (pseudo) potential vorticity equation and the model is thus called a PV‐D model. Many features of a previously developed shallow water PV‐D model are carried over: a modification of the PV equation needed to give computational stability of long Rossby waves; a semi‐Lagrangian semi‐implicit treatment of both the linear and the nonlinear terms; the use of an unstaggered grid in the horizontal; the use of a nonlinear multigrid technique to solve the nonlinear implicit equations. A linear numerical stability analysis of the model's gravity–inertia waves indicates that the potential temperature needs to be separated into horizontal mean and perturbation parts. This allows an implicit treatment of the vertical advection associated with the mean in the thermodynamic equation. Numerical experiments with developing baroclinic waves have been carried out and give realistic results.     

A method for second-order diffraction potential from an axisymmetric body

The paper provides a detailed analysis for the second-order diffraction of monochromatic waves. For the second-order potential on the free surface, the paper proposed a forward prediction method for computing the integration on the free surface. By this method we only need to run the infinity integration on the free surface directly for a few points; a one-step quadrature can then be applied successively outward from the body for potentials at other points. For wave diffraction from a body of revolution with a vertical axis, the paper derives a new integral equation, which can cancel the leading singularity in the derivative of ring Green's functions automatically. To obtain accurate results, different approaches are also used to deal with singularities in the ring Green's functions in the integration on both the body surface and free surface. The method has been implemented for bodies of revolution with vertical axes, but the theory is also available for arbitrary bodies.A numerical examination is made to validate the numerical code by comparing the second-order forces and moments on uniform and truncated cylinders and second-order diffraction potentials on the free surface with some published results. The comparison shows that the present results are in good agreement with those published. The method is also used to compute the second-order wave elevation around uniform and truncated cylinders.     

Bifurcation of barotropic flow and homogenization of potential vorticity in a rotating laboratory model

Laboratory experiments with a rotating tank confirm the bifurcation character of a barotropic flow driven by an inflow and an outflow described by Sakai (1986). The model, a circular basin with a topographic β-effect, simulates a mid-latitude oceanic feature. At a low Rossby number, stationary Rossby waves are observed which are symmetrical with a line connecting the inlet and the outlet. As the Rossby number increases, a bifurcation occurs and two kinds of vortex flows are observed. In the vortex, potential vorticity is almost uniform. In addition to the two vortex flows, a jet-like inertial flow can also be observed. In general, thre results of these experiments agree well with those of a low-order model and a numerical model.     

Cyclostrophic adjustment and nonlinear oscillations in the core of an intense atmospheric vortex

Intense atmospheric vortices are characterized by a regime of cyclostrophic balance, i.e., the balance between the pressure gradient and centrifugal force. To describe motions in the core of an axisymmetrical vortex, a class of exact solutions to the equations of gas dynamics with a linear dependence on radius is derived for the velocity components and with a quadratic dependence for temperature. It is shown that small deviations from the balance state give rise to oscillations of the hydrothermodynamic fields in the vortex core with a frequency proportional to the angular velocity of the rotation of the core. For fairly large initial deviations, oscillations are clearly anharmonic and, under the conditions of the prevailing centrifugal force, result in a significant temperature decrease on the vortex axis. The application of this class of solutions to describing the Ranque vortex effect (the intense cooling of gas during rapid rotations) and the acoustic radiation from tornadoes is discussed.     

Dynamic parameters in models of atmospheric vortex structures

Vortex simulation and the computation of fields of dynamic parameters of vortex structures (velocity, rotor velocity, and helicity) are carried out with the use of exact hydrodynamic equations in a cylindrical coordinate system. Components of centripetal and Coriolis accelerations are taken into account in the initial equations. Internal and external solutions are defined. Internal solutions ignore the disturbances of the pressure field, but they are considered in external solutions. The simulation is carried out so that the effect of accounting for spatial coordinates on the structure of the above fields is pronounced. It is shown that the initial kinetic energy of rotating motion transforms into the kinetic energy of radial and vertical velocity components in models with centripetal acceleration. In models with Coriolis acceleration, the Rossby effect is clearly pronounced. The method of an “inverse problem” is used for finding external solutions, i.e., reconstruction of the pressure field at specified velocity components. Computations have shown that tangential components mainly contribute to the velocity and helicity vortex moduli at the initial stage.     

Interannual variations in low potential vorticity water and the subtropical countercurrent in an eddy-resolving OGCM

Interannual-to-decadal variations in the subtropical countercurrent (STCC) and low potential vorticity (PV) water and their relations in the North Pacific Ocean are investigated on the basis of a 60-year-long hindcast integration of an eddy-resolving ocean general circulation model. Although vertically coherent variations are dominant for STCC interannual variability, a correlation analysis shows that an intensified STCC vertical shear accompanies lower PV than usual to the north on 25.5- to 26.1-σ_θ isopycnal surfaces, and intensified meridional density gradient in subsurface layers, consistent with Kubokawa’s theory (J Phys Oceanogr 29:1314–1333, 1999). The low-PV signals appear at least 2 years before peaks of STCC, propagating southwestward from the subduction region.     

Mixed layer depth front and subduction of low potential vorticity water in an idealized ocean GCM

It is known that there is a front-like structure at the mixed layer depth (MLD) distribution in the subtropical gyre, which is called the MLD front, and is associated with the formation region of mode water. In the present article, the generation mechanism of the MLD front is studied using an idealized ocean general circulation model with no seasonal forcing. First, it is shown that the MLD front occurs along a curve where u _g ·∇T _s = 0 is satisfied (u _g is the upper ocean geostrophic velocity vector, T _s is the sea surface temperature and ∇ is the horizontal gradient operator). In other words, the front is the boundary between the subduction region (u _g ·∇T _s > 0) and the region where subduction does not occur (u _g ·∇T _s < 0). Second, we have investigated subduction of low potential vorticity water at the MLD front, which has been pointed out by past studies. Since u _g ·∇T _s = 0 at the MLD front, the water particles do not cross the outcrop at the MLD front. The water that is subducted at the MLD front has come from the deep mixed layer region where the sea surface temperature is higher than that at the MLD front. The temperature of the water in the deep mixed layer region decreases as it is advected eastward, attains its minimum at the MLD front where u _g ·∇T _s = 0, and then subducts under the warmer surface layer. Since the deep mixed layer water subducts beneath a thin stratified surface layer, maintaining its thickness, the mixed layer depth changes abruptly at the subduction location.     

Rossby number dependence on meander dynamics of a potential vorticity front

Semi-geostrophic dynamics of jets are studied using a potential vorticity front in an equivalent barotropic model. Meandering processes of the front are examined in the thin-jet limit on a -plane by a curvilinear coordinate system. For calculated along-front velocity fields, asymmetrical profiles are caused by meandering. This asymmetry of the velocity profile is enhanced as the Rossby number becomes large. Using the along-front velocity fields, the normal velocity of front is expressed so that the Rossby number is explicitly included. This expression can be rewritten in the form of the mKdV equation.     

Mixed layer depth front and subduction of low potential vorticity water under seasonal forcings in an idealized OGCM

The mixed layer depth (MLD) front and subduction under seasonal variability are investigated using an idealized ocean general circulation model (OGCM) with simple seasonal forcings. A sharp MLD front develops and subduction occurs at the front from late winter to early spring. The position of the MLD front agrees with the curve where \({\rm D}T_{\rm s}/{\rm D}t = \partial T_{\rm s} /\partial t + {\user2{u}}_{\rm g} \cdot \nabla T_{\rm s} = 0\) is satisfied (t is time, \({\user2{u}}_{\rm g}\) is the upper-ocean geostrophic velocity, \(T_{\rm s}\) is the sea surface temperature (SST), and \(\nabla\) is the horizontal gradient operator), indicating that thick mixed-layer water is subducted there parallel to the SST contour. This is a generalization of the past result that the MLD front coincides with the curve \({\user2{u}}_{\rm g} \cdot \nabla T_{\rm s} = 0\) when the forcing is steady. Irreversible subduction at the MLD front is limited to about 1 month, where the beginning of the irreversible subduction period agrees with the first coincidence of the MLD front and \({\rm D}T_{\rm s}/{\rm D}t =0\) in late winter, and the end of the period roughly corresponds to the disappearance of the MLD front in early spring. Subduction volume at the MLD front during this period is similar to that during 1 year in the steady-forcing model. Since the cooling of the deep mixed-layer water occurs only in winter and SST can not fully catch up with the seasonally varying reference temperature of restoring, the cooling rate of SST is reduced and the zonal gradient of the SST in the northwestern subtropical gyre is a little altered in the seasonal-forcing case. These effects result in slightly lower densities of subducted water and the eastward shift of the MLD front.     

The mesoscale atmospheric vortex as a manifestation of the Novorossiysk bora

The WRF-ARW regional atmosphere circulation model has been used to reproduce a few episodes of cold intrusion and the Novorossiysk bora accompanied by the formation of the mesoscale cyclonic vortex over the Black sea, which can be clearly observed from satellite images of cloudiness. It has been shown that the vortex development is associated with the specific features of air flow around the northwestern edge of the Caucasus Mountains. We have estimated the vertical vorticity associated with the alongshore horizontal gradient of temperature. We have considered the field structure of wind velocity and temperature of the axisymmetric quasi-two-dimensional vortex generated in the coastal zone and displaced seaward after separating from the coast. With the background northerly wind, the coastal cyclonic circulation is not accompanied by the vortex separation from the coast. The specific feature of the development of the cyclonic vortex is the southeastern wind with velocities of up to 10 m/s in the Caucasus coastal area from Sochi to Sukhum.     

Laboratory model of the Obukhov geostrophic vortex

The results of laboratory modeling of geostrophic adjustment in a shallow-water layer in rotating paraboloid are presented. According to the Rossby-Obukhov theory, this process excites nonstationary wave and stationary vortex (geostrophic) components of motion in a rotating fluid. In our experiments, the wave and vortex components were excited by extracting a preliminarily imbedded hemisphere (which made the initial distribution of the depth of the fluid inhomogeneous) from the central area of a rotating vessel with a parabolic base. Under this excitation technique, a prominent cyclonic eddy is formed in the central portion; the structure of this eddy is satisfactorily described within the linear theory of adjustment. Along with the shallow-water experiments, the published experimental data on modeling geostrophic adjustment in a two-layer medium are analyzed. A simple analytic solution to the corresponding problem of the adjustment theory is obtained, and this solution agrees with the experiment.     

Simulation of the seasonal atmospheric circulation with the new version of the semi-Lagrangian atmospheric model

Simulation of the atmospheric circulation on the seasonal scale with the new version of the global semi-Lagrangian model is considered. The new version includes land surface processes parameterization taking into account influence of the vegetation and also freezing and melting of soil moisture. The new version also includes improved parameterization for short and long wave radiation, cloudiness and atmospheric boundary layer.     

Three-parameter model of turbulence for the atmospheric boundary layer over an urbanized surface

A modified three-parameter model of turbulence for a thermally stratified atmospheric boundary layer (ABL) is presented. The model is based on tensor-invariant parametrizations for the pressure-strain and pressure-temperature correlations that are more complete than the parametrizations used in the Mellor-Yamada model of level 3.0. The turbulent momentum and heat fluxes are calculated with explicit algebraic models obtained with the aid of symbol algebra from the transport equations for momentum and heat fluxes in the approximation of weakly equilibrium turbulence. The turbulent transport of heat and momentum fluxes is assumed to be negligibly small in this approximation. The three-parameter E ? ε ? 2> model of thermally stratified turbulence is employed to obtain closed-form algebraic expressions for the fluxes. A computational test of a 24-h ABL evolution is implemented for an idealized two-dimensional region. Comparison of the computed results with the available observational data and other numerical models shows that the proposed model is able to reproduce both the most important structural features of the turbulence in an urban canopy layer near the urbanized ABL surface and the effect of urban roughness on a global structure of the fields of wind and temperature over a city. The results of the computational test for the new model indicate that the motion of air in the urban canopy layer is strongly influenced by mechanical factors (buildings) and thermal stratification.     

零航速减摇鳍升力模型研究

为了解决船舶在零航速下减摇的问题,采用一种新的运动控制方法,使减摇鳍的工作不再受航速的限制。根据零航速条件下的特殊运动方式,对减摇鳍在非定常流中的受力情况进行分析,详细讨论了各种流体作用力的产生机理,并给出定量计算公式。结果表明,零航速减摇鳍的升力与它的几何尺寸、旋转角速度和角加速度有关。在此基础上建立了零航速减摇鳍的升力模型,通过仿真结果与实验数据的对比,验证了该模型的正确性。     

Effect of mesoscale atmospheric vortex processes on the upper atmosphere and ionosphere of the Earth

The mechanisms of incipience and intensification of dangerous atmospheric vortex processes such as tropical cyclones (TCs) and their interaction with the Earth’s ionosphere are considered. Different models of TCs are analyzed, including models taking into account the ionization processes. The mechanisms taking into account the spiral field of velocities during TC formation are analyzed, as are the physical mechanism that explains the statistical correlation between short-term variations in galactic cosmic rays (Forbush decreases) and the frequency of incipience and the intensification of TCs. It is shown that such an effect is conditioned by a decrease in the ion-production rate during Forbush decreases against the tropopause and, hence, a decrease in the temperature upon the top of the ionosphere altitude because of a decrease in the latent heat release due to water-vapor condensation on the newly formed ions. This process leads to an increase in the temperature difference between the ocean surface and the top level of TCs and, respectively, to the intensification of vertical convection, which results in cyclone intensification. It is concluded that the study of these mesoscale vortex processes requires taking into account not only the hydrodynamical features of these formations, but also their thermodynamical and electrodynamical properties. The results are important for the organization of studying and monitoring TCs with the use of spaceborne techniques.