一个大气边界层非线性三维数值模式及其在太湖流域的应用

本模式用以K半经验理论为基础的闭合方程组求解,引进了地形作用、动量、热量和水汽的侧向扩散和垂直湍流交换以及凝结加热等物理过程,其中湍流交换系数K是地表粗糙度、几何高度、大气稳定度及风速切变等因子的函数。本模式还考虑了山地、平原和水面在地形高度、地表粗糙度、辐射、蒸发及热交换等方面的差异,在下垫面上建立热平衡方程与边界层内控制方程耦合。将模式应用于太湖流域,计算得到的该流域边界层内温、压、湿、风的分布特征与实际情况相似,模式具有一定的实用性。     

一个大气边界层非线性三维数值模式及其在太湖流域的应用

本模式用以K半经验理论为基础的闭合方程组求解,引进了地形作用、动量、热量和水汽的侧向扩散和垂直湍流交换以及凝结加热等物理过程,其中湍流交换系数K是地表粗糙度、几何高度、大气稳定度及风速切变等因子的函数。本模式还考虑了山地、平原和水面在地形高度、地表粗糙度、辐射、蒸发及热交换等方面的差异,在下垫面上建立热平衡方程与边界层内控制方程耦合。将模式应用于太湖流域,计算得到的该流域边界层内温、压、湿、风的分布特征与实际情况相似,模式具有一定的实用性。     

Formation of strong stationary vortex turbulence in the terrestrial magnetosheath

The microscopic consequences of the presence of nonlinear vortex structures in the near-Earth plasma dispersive medium are studied in this work. In dispersive media, strongly localized vortex structures contain trapped particles, cause pronounced density fluctuations, and intensify transfer processes, mixing in a medium; i.e., they can form strong vortex turbulence. Turbulence is represented as a gas in the ensemble of strongly localized (therefore, weakly interacting) identical vortices composing the ground state. Vortices with different amplitudes are randomly located in space (since they interact with one another) and are described statistically. It is assumed that the steady turbulent state is formed through a balance of mutually competing effects: spontaneous generation of vortices due to nonlinear steepening of the disturbance front, ^noise transfer to small scales, and collisional or collisionless damping of disturbances in the HF region. Noise scaling in the inertial interval takes place since structures merge during their collision. A magnetized plasma medium in the magnetosheath is considered. A new type of turbulent fluctuation spectra with respect to wavenumbers k ^−8/3, which is in satisfactory agreement with satellite observations in space plasma, has been determined. The medium particle diffusion on an ensemble of vortices has also been studied. It has been established that the interaction between structures themselves and between structures and medium particles causes anomalous diffusion in the medium. The effective diffusion coefficient square roothly depends on the noise stationary level.     

Character of turbulence in the boundary regions of the Earth’s magnetosphere

The statistical features of the magnetic field and ion flux fluctuations in the boundary regions of the Earth’s magnetosphere have been studied on different timescales based on the Interball satellite measurements. Changes in the form and parameters of the probability density function have been studied for the periods when the satellite was in the solar wind plasma, different magnetosheath regions, and the turbulent boundary layer (TBL) at the polar cusp outer boundary. Variations in the probability density function maximum (P ₀) and the kurtosis value as characteristics of the turbulence property evolution on different timescales have been studied. Two asymptotic regimes of P ₀, which are characterized by different power laws, have been found. The structural functions of different orders and the types of diffusion processes in different regions, depending on time variations in the generalized diffusion coefficient, have been studied in order to analyze the character of diffusion processes. For the magnetosheath regions, TBL, and polar cusp, it has been found that the diffusion coefficient increases in the course of time (i.e., the regime of superdiffusion has been obtained). In the foreshock region before the main shock, turbulent processes are described by the Kolmogorov model of classical diffusion.     

On Production of Long-term Mean Zonal Current and Eddy Momentum and Heat Transports in Atmosphere

— The mean zonal velocity in the atmosphere is taken as being created continually by the global scale Hadley circulation produced by the differential solar heating through the balance between the Coriolis effect and vertical diffusion, and not by conservation of absolute momentum. Hence a proper determination of the diffusion coefficient becomes the key to the solution of the zonal flow problem. In this study we take the flow field as composed of a primary global scale Hadley circulation, and a secondary flow created by the convergences of the eddy transports of heat and momentum and surface friction, which give rise to the classical three cell structure of the meridional circulation but which only modifies the zonal velocity distribution slightly.¶Finally, we use the equilibrium solution of the perturbation potential vorticity equation to obtain the eddy transports of momentum and heat, with the zonal velocity given by the primary Hadley flow as the basic flow, and we found that they are close to the statistically observed values, demonstrating that the system can maintain itself.     

Steady state solutions for axially-symmetric climatic variables

Summary Equations governing the axially-symmetric time-average state of the atmosphere and the transient departures from this mean state are set down. As a first step toward a solution of this system for seasonal average conditions, a model is formulated based on the thermodynamical energy equation for the vertical average of the mean state, and on the perturbation solutions of the linearized equations governing the baroclinic growth of transient eddies. All forms of non-adiabatic heating within the atmosphere and at the earth's surface are parameterized. The resulting differential equation governing the axially-symmetric mean potential temperature distribution takes the form of a steadystate diffusion equation in surface spherical coordinates, with a variable Austausch coefficient which is to be determined iteratively as a dependent variable.Global solutions, for winter and summer equilibrium conditions, are obtained for the thermal structure, the heat balance components, the transient eddy variances of temperature and meridional wind speed, and the covariance representing the meridional eddy heat transport. These solutions are for different types of surface conditions (ocean, land), and for a successively more complete variety of modes of heat transfer ranging from pure radiation to a combination of radiation, latent heat processes, and conduction and convection within the atmosphere and the subsurface layers. The results for this latter complete case seem to be a reasonable first order approximation to the observed distributions. Suggestions are made for improving and generalizing the study.     

Pixel‐oriented land use classification in energy balance modelling

Mass and energy transfer between soil, vegetation and atmosphere is the process that allows to maintain an adequate energy and water balance in the earth–atmosphere system. However, the evaluation of the energy balance components, such as the net radiation and the sensible and latent heat fluxes, is characterized by significant uncertainties related to both the dynamic nature of heat transfer processes and surfaces heterogeneity. Therefore, a detailed land use classification and an accurate evaluation of vegetation spatial distribution are required for an accurate estimation of these variables. For this purpose, in the present article, a pixel‐oriented supervised classification was applied to obtain land use maps of the Basilicata region in Southern Italy by processing three Landsat TM and ETM+ satellite images. An accuracy analysis based on the overall accuracy index and the agreement Khat of Cohen coefficient showed a good performance of the applied classification methodology and a good quality of the obtained maps. Subsequently, these maps were used in the application of a simplified two‐source energy balance model for estimating the actual evapotranspiration at a regional scale. The comparison between the simulations made by applying the simplified two‐source energy balance model and the measurements of evapotranspiration at a lysimetric station located in the study area showed the applicability and the validity of the proposed methodology. Copyright © 2012 John Wiley & Sons, Ltd.     

Turbulent transport of magnetic fields. V. Distribution of magnetic energy in a simple α2-dynamo

Abstract

In this paper we analyse the stationary mean energy density tensor T_ij = B_iB_j for the x ²-sphere. This model is one of the simplest possible turbulent dynamos, originally due to Krause and Steenbeck (1967): a conducting sphere of radius R with homogeneous, isotropic and stationary turbulent convection, no differential rotation and negligible resistivity. The stationary solution of the (linear) equation for T_ij is found analytically. Only T_rr , T _θθ and T _φφ are unequal to zero, and we present their dependence on the radial distance r.

The stationary solution depends on two coefficients describing the turbulent state: the diffusion coefficient β≈?u²?τ_c/3 and the vorticity coefficient γ ≈ ?|?×u|²?τ_c/3 where u(r, t) is the turbulent velocity and _c its correlation time. But the solution is independent of the dynamo coefficient α≈??u·?×u?τ_c/3 although α does occur in the equation for T_ij . This result confirms earlier conclusions that helicity is not required for magnetic field generation. In the stationary state, magnetic energy is generated by the vorticity and transported to the boundary, where it escapes at the same rate. The solution presented contains one free parameter that is connected with the distribution of B over spatial scales at the boundary, about which T_ij gives no information. We regard this investigation as a first step towards the analysis of more complicated, solar-type dynamos.     

The effect of Newtonian cooling on vertically propagating magneto-acoustic waves in a thermally conducting isothermal atmosphere (II)

In this article, we examine reflection, dissipation and attenuation of vertically propagating waves in an isothermal atmosphere under the combined effect of Newtonian cooling, thermal conduction and viscosity with a weak horizontal magnetic field. We consider the case in which the combined effect of viscosity and magnetic field is dominated by that of the thermal conduction and for small values of the Newtonian parameter. As a result, the atmosphere can be divided into three distinct regions that are connected by two transition regions. The lower and middle regions are connected by a semi-transparent barrier and the middle and upper regions are connected by an absorbing and reflecting barrier. In the connecting barriers the reflection and transmission of the waves takes place. The presence of Newtonian cooling effects on the adiabatic region, produces attenuation in the amplitudes of the waves and reduces the energy absorption in the transition regions. The reflection coefficient is determined in the lower and middle regions and the results are discussed in the context of the heating of the solar atmosphere.     

Quantifying the effect of thermospheric parameterization in a global model

Global climate models have become useful tools for studying the important physical processes that affect the Earth's upper atmosphere. However, the results produced by all models contain uncertainty that stems for the manner in which the model is driven, as well as in the treatment of the internal physics and numerics. In order to fully understand the scientific value of the model results then, it is necessary to have a quantitative understanding of the uncertainty in the model. In this study, the global ionosphere–thermosphere model is used to investigate how uncertainty in the use of parameters in a large scale model can affect the model results. Eight parameters are studied that ultimately have an effect on the thermospheric temperature equation. It is found that among these, uncertainty in the thermal conductivity, NO cooling, and NO binary diffusion coefficients most strongly translate to uncertainty in the temperature and density results. In addition, variations in the eddy diffusion coefficient are shown to result in significant uncertainty in the thermospheric composition, and ultimately the electron density.     

Equation de transport et de diffusion pour l'atmosphère turbulente avec des masses d'air différemment échauffées

Summary Generalization of the diffusion and transport equation are given. They refer to an atmosphere in which in addition to irregular turbulent motion exists also regular motion of the heated and cooled air masses. The equations are briefly discussed.     

A universal field equation for dispersive processes in heterogeneous media

When formulated properly, most geophysical transport-type process involving passive scalars or motile particles may be described by the same space–time nonlocal field equation which consists of a classical mass balance coupled with a space–time nonlocal convective/dispersive flux. Specific examples employed here include stretched and compressed Brownian motion, diffusion in slit-nanopores, subdiffusive continuous-time random walks (CTRW), super diffusion in the turbulent atmosphere and dispersion of motile and passive particles in fractal porous media. Stretched and compressed Brownian motion, which may be thought of as Brownian motions run with nonlinear clocks, are defined as the limit processes of a special class of random walks possessing nonstationary increments. The limit process has a mean square displacement that increases as t^α+1 where α > −1 is a constant. If α = 0 the process is classical Brownian, if α < 0 we say the process is compressed Brownian while if α > 0 it is stretched. The Fokker–Planck equations for these processes are classical ade’s with dispersion coefficient proportional to t^α. The Brownian-type walks have fixed time step, but nonstationary spatial increments that are Gaussian with power law variance. With the CTRW, both the time increment and the spatial increment are random. The subdiffusive Fokker–Planck equation is fractional in time for the CTRW’s considered in this article. The second moments for a Levy spatial trajectory are infinite while the Fokker–Planck equation is an advective–dispersive equation, ade, with constant diffusion coefficient and fractional spatial derivatives. If the Lagrangian velocity is assumed Levy rather than the position, then a similar Fokker–Planck equation is obtained, but the diffusion coefficient is a power law in time. All these Fokker–Planck equations are special cases of the general non-local balance law.     

A theory of atmospheric tides generated by the differential heating between land and sea

Summary The differential heating between land and sea is incorporated into the theory of atmospheric tides. This involves the representation of the land and sea distribution by a set of Fourier series.The theory postulates the existence of waves of angular speeds different from the angular velocity of the earth with some of the waves travelling from West to East instead of the usual East to West.By considering the diurnal variation of eddy diffusion of heat energy absorbed close to the surface, the semidiurnal standing waves at the poles was calculated by the application of this theory. The order of magnitude of the calculated tides agreed well with observation, though, contrary to observation, the calculated amplitude at the South pole is larger than that at the North pole.It is also shown that the polar standing oscillation is caused mainly by the land and sea distribution between 75°N and 45°S.     

Large-scale motions of the tropics in observations and theory

Recent observations of the tropical and subtropical atmosphere are interpreted in terms of scaling arguments and wave propagation theory advanced byCharney (1963, 1969).Charney’s idealizations describe the tropical atmosphere in terms of large regions of quasi-nondivergent flow containing small subdomains of heavy convection and divergence, and place emphasis upon the quasi-rotational regions. FGGE (First GARP Global Experiment) observations suggest that strongly divergent local tropical circulations are forced by latent heating and produce important direct modifications of the total wind field. We describe the extent to which the resulting field consists of divergent and rotational components in different analyses of the FGGE data, and present independent supporting documentation of the results in terms of heating estimates and rainfall observations. Local tropical heating rates on the order of 10°C/day are apparently due to latent heat release associated with precipitation rates as large as 6 cm/day during extended periods. The large contribution of the divergent wind is generally underestimated in models that do not retain such energetic local forcings, and this deficiency may be related to general underestimation of tropical-extratropical connections of many linear models. Such connections are commonly cited in relation to El Niño events, the Southern Hemisphere stationary-wave pattern, and in FGGE studies, but are not well simulated in most linear theories. It is not yet clear whether this is an inherent limitation of linear models, or whether the linear models have not yet explored all the potentially relevant ambient states. We explore the latter possibility by construction of a basic state that allows reasonable latitudinal evolution of the wave field. This basic state has zero absolute vorticity gradient throughout the tropics, and deviations linearized about this state are dynamically analogous to a “local” Hadley cell. To the extent that it is appropriate to regard the results in terms of wave propagation, our analysis suggests a prominent role for gravity-inertia waves in the tropics and for the extratropical connections. The relevance of gravity modes to observations and the theoretical explanation of the flat vorticity field remain to be established.     

Temperature and heat flux spectra in the turbulent buoyancy subrange

The physical nature of motions with scales intermediate between approximately isotropic turbulence and quasi-linear internal gravity waves is not understood at the present time. Such motions play an important role in the energetics of small scales processes, both in the ocean and in the atmosphere, and in vertical transport of heat and constituents. This scale range is currently interpreted either as a saturated gravity waves field or as a buoyancy range of turbulence.We first discuss some distinctive predictions of the classical (Lumley, Phillips) buoyancy range theory, recently improved (Weinstock, Dalaudier and Sidi) to describe potential energy associated with temperature fluctuations. This theory predicts the existence of a spectral gap in the temperature spectra and of an upward mass flux (downward buoyancy and heat fluxes), strongly increasing towards large scales. These predictions are contrasted with an alternate theory, assuming energetically insignificant buoyancy flux, proposed by Holloway.Then we present experimental evidences of such characteristic features obtained in the lower stratosphere with an instrumented balloon. Spectra of temperature, vertical velocity, and cospectra of both, obtained in homogeneous, weakly turbulent regions, are compared with theoretical predictions. These results are strongly consistent with the improved classical buoyancy range theory and support the existence of a significant downward heat flux in the buoyancy range.The theoretical implications of the understanding of this scale range are discussed. Many experimental evidences consistently show the need for an anisotropic theory of the buoyancy range of turbulence.     

Reducing the biases in shortwave cloud radiative forcing in tropical and subtropical regions from the perspective of boundary layer processes

Biases in shortwave cloud radiative forcing (SWCF), which cause overestimates in tropical regions and underestimates in subtropical marine stratocumulus regions, are common in many climate models. Here, two boundary layer processes are investigated in the atmospheric model GAMIL2, entrainment at the top of the boundary layer and longwave radiative cooling at the top of stratocumulus clouds, in order to reduce biases and reveal the mechanisms underlying these processes. Our results show that including the entrainment process in the model can reduce negative SWCF biases in most tropical regions but increases positive SWCF biases in subtropical marine stratocumulus regions. This occurs because entrainment reduces the low-level cloud fraction and its cloud liquid water content by suppressing the vertical turbulent diffusion in the boundary layer and decreasing the relative humidity when warm and dry free atmosphere is entrained in the boundary layer. Longwave radiative cooling at the top of stratocumulus clouds can enhance turbulent diffusion within the stratocumulus-topped boundary layer. When combined with the entrainment process, longwave radiative cooling reduces the positive SWCF biases in subtropical marine stratocumulus regions that are observed using the entrainment process alone. The incorporation of these two boundary layer processes improves the simulated SWCF in tropical and subtropical regions in GAMIL2.     

CO2 and H2S concentrations in the atmosphere at the Solfatara of Pozzuoli

The CO₂ and H₂S concentration in the Solfatara atmosphere has been measured. The concentrations of both gases are higher neraby the more active areas and decrease away from them. A sharp horizontal and vertical gradient of the CO₂ content has been recognized.Such gradient is assumed to result from a diffusion of gas from the ground to the atmosphere.The total output of CO₂ has been computed based on a turbulent diffusion model. The obtained value is in good agreement with previously abserved values (Italiano et al., 1984).The feasibility of monitoring the atmosphere of Solfatara for either gas hazard and surveillance of volcanic activity has also been evaluated.     

Estimates of heat and pyroclast discharge by volcanic eruptions based upon the eruption cloud and steady plume observations

Fumarolic steam plumes and eruption clouds rise like convetive turbulent columns into the atmosphere. Formulae are presented here for estimating the heat power of plumes, the production rate of juvenile pyroclasts ejected during eruptions and the heat output of fumaroles. Their accuracy is tested using the well-studied examples of eruptions of Kamchatkan volcanoes.The Briggs (1969) formula may be used in observing the ascending part of a plume in crosswinds. The best results have been obtained using the CONCAWE formula which permits estimation of the heat power in crosswinds based on the axis height of a horizontal part of a maintained plume. Three connected equations have been suggested for a stable atmosphere and calm weather conditions. The first one, which is applicable for heights ranging from 100 m to 1 km, is the formula proposed by Morton et al. (1956). This equation changes for higher layers of the troposphere (1–10 km) and stratosphere (10–55 km).A classification scale was constructed allowing us to compare volcanic eruptions and fumarolic activity in terms of the intensity of their plumes.The described method is useful for volcano surveillance; it helps in the study of the energetics and mechanics of volcanic and magmatic processes.