Experimental study of atmospheric gravity waves and visibility oscillations in a fog episode

Summary A fog episode characterized by quasi-periodic oscillations in visibility is described. Gravity waves are also present, and frequencies at which pressure and transparency fluctuate are compared. The existence of intervals during which they are synchronized and the presence of strong winds aloft suggest that shear instabilities in the troposphere can influence the fog behaviour at ground.     

Negatively buoyant plume dispersion: a comparison of simple mathematical models, hydraulic simulations and field data

Summary The ground-level concentration distribution for negatively buoyant gases of various densities, emitted from stacks with heights, diameters and exit velocities varying in the typical range of geothermal drilling activities, was studied by means of hydraulic simulations performed in neutral conditions at different flow speeds. These measurements, and a few others taken in the field in nearly neutral and unstable atmosphere during tests of CO₂ release from a geothermal well, were compared with the values expected from a Gaussian formula corrected for denser-than-air, tilted plumes. For calculating theoretical concentrations, three couples of horizontal and vertical plume standard deviations as functions of downwind distance from source were employed: those given by Singer-Smith and Briggs and a third one obtained by our group during a previous water channel experiment. Also the maximum ground-level concentration and its distance from the source were measured in each simulation and compared with the prediction of our Gaussian formula, as well as with a best-fit expression given by Meroney. The concentration pattern χ(x, y, 0) given by the corrected Gaussian formula proved to be in good agreement with experimental data, especially in association with Singer-Smith's or our sigmas. When only maximum g.l. concentrations are needed, also the simple formula suggested by Meroney can give a good approximation.

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Riassunto Una serie di simulazioni in vasca senza stratificazione, condotte a varie velocità della vena idraulica, ha permesso di studiare la distribuzione di concentrazione al suolo dovuta a gas con varie densità maggiori di quella dell'aria, emessi da ciminiere con altezze, diametri e velocità di efflusso variabili entro intervalli tipici delle attività di perforazione geotermica. Queste misure, ed alcune altre ottenute in campo in condizioni di atmosfera circa neutra e instabile durante prove di rilascio di CO₂ da un pozzo geotermico, sono state confrontate con i valori attesi in base ad una formula gaussiana corretta per pennacchi inclinati. Nel calcolo delle concentrazioni teoriche si sono impiegate tre diverse coppie di espressioni per le deviazioni standard orizzontale e verticale del pennacchio: quelle date da Singer-Smith e Briggs ed una terza ottenuta dal nostro gruppo durante una precedente esperienza in canale idraulico. In ciascuna prova si è misurata anche la concentrazione massima al suolo e la sua distanza dalla sorgente; queste quantità sono state raffrontate con le previsioni della formula gaussiana e con una espressione empirica proposta da Meroney. Il campo di concentrazione χ(x, y, 0) dato dalla formula gaussiana corretta è risultato in buon accordo con le misure, specialmente se associato alle sigma di Singer-Smith o alle nostre. Quando sia sufficiente una valutazione della concentrazione massima, anche la semplice formula di Meroney può dare una buona approssimazione.

     

An analysis by ARMA stochastic processes of the daily average temperatures collected during 1984 in Udine and Lignano Sabbiadoro

Summary The daily average air temperatures collected during 1984 in Udine and Lignano are studied using stochastic processes. In order to obtain the seasonal component, a computational method has been developed involving the Fourier expansion, while the study of the stationary residual series is carried out by means of ARMA recursive filters. It was found that an autoregressive model of the third order represents the residual data fairly well. This method may also be applied to other meteorological parameters with significant frequencies and with a time behaviour which can be made stationary at least in the wide sense. This work was carried out in the scope of the MICMET program which is aimed at obtaining a detailed analysis of micrometeorological phenomena in the region of Friuli-Venezia Giulia.

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Riassunto La temperatura media giornaliera misurata a Udine e Lignano Sabbiadoro durante il 1984 è analizzata usando la tecnica dei processi stocastici. La stima della componente stagionale è rappresentata dall'armonica fondamentale dello sviluppo in serie di Fourier che, in media, ricopre oltre il 90% della varianza totale. Le serie residue, ottenute sottraendo la componente stagionale, sono studiate con le tecniche dei filtri ricorsivi lineari di tipo ARMA. Un modello stocastico autoregressivo del terzo ordine, AR(3), è risultato il piú adatto a rappresentare le serie residue stazionarie. Si danno ampi dettagli del metodo in quanto può essere generalmente impiegato per lo studio di altre grandezze meteorologiche che presentano frequenze significative e un andamento temporale che può essere reso stazionario almeno in senso lato. Questo lavoro è stato eseguito nell'ambito del progetto MICMET che è finalizzato a ottenere una dettagliata analisi dei fenomeni micrometeorologici nella Regione Friuli-Venezia Giulia.

     

Speckle statistical properties of Gaussian beam from a semi-rough target in the atmospheric turbulence

Based on extended Huygens–Fresnel principle and Goodman model for target surface, analytic expression is developed for the mutual coherent function (MCF) of a reflected Gaussian-beam from a semi-rough target in single-pass atmospheric turbulence. Then according to the MCF we derive expression about the mean intensity and average speckle size at the receiver. The analysis indicates that the mean intensity is closely related to the ratio of root mean square (rms) height to the lateral correlation length; in addition, the speckle size is associated with turbulence strength and roughness of target. Our results agree with well-known the limiting cases of perfectly smooth and Lambertian target.     

Low-frequency dielectric properties and conductivity of a polyvinylcaprolactam-water system

The dielectric relaxation and conductivity of polyvinylcaprolactam (PVCL)-water systems of different compositions (with water contents of ∼1, 5, and 24 wt %) were studied by means of dielectric spectroscopy (10⁻¹–10⁷ Hz) at −100−50°C. It was found that the conductivity of samples with a low content of water upon cooling has an activation character, whereas a cooperative mechanism of conduction holds for the PVCL-water system with 24 wt % water, which switches to an activation character of interactions of water with polymer changes in the course of a glass transition.     

Characteristics of high-frequency atmospheric eddies over the mediterranean area in different seasons

Summary We examine the properties of atmospheric eddies over Europe and the Mediterranean, as revealed by an analysis of high-pass filtered time series of geopotential height. The data used are seven years of 6-hourly initialized analyses, prepared at ECMWF. The application of a time-filter with cut-off around 6 days allows us to isolate the variability associated with travelling eddies, which generally come from the Atlantic and whose properties (spatial structure, propagation characteristics) are deeply modified when they pass over the Mediterranean region. Different statistical indicators have been derived to reveal these modifications, that are particularly evident in the lower troposphere. It appears that the orography surrounding the Mediterranean region is the main factor influencing eddy behaviour. This conclusion is supported, among other considerations, by the comparison between winter and summer statistics: notwithstanding the different climatological properties of the mean atmospheric circulation and of the surface heat fluxes in the two seasons, some peculiar properties of the high-frequency components over the European and Mediterranean region, such as the vertical and horizontal structure, do not vary much as a function of the season.

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Riassunto Sono esaminate le proprietà dei vortici atmosferici sull'Europa e il Mediterraneo, mediante un'analisi di serie temporali di altezza di geopotenziale, filtrate con un filtro passa alto. I dati utilizzati sono costituiti da sette anni di analisi inizializzate, disponibili ad intervalli di 6 ore, preparate dal Centro Europeo di Previsione Meteorologica a Medio Termine (ECMWF). L'applicazione di un filtro temporale con un periodo di taglio di circa 6 giorni permette di isolare la variabilità associata con vortici viaggianti che generalmente provengono dall'Atlantico e le cui proprietà (struttura spaziale, caratteristiche di propagazione) sono profondamente modificate allorquando essi transitano sopra la regione mediterranea. Sono stati calcolati diversi indicatori statistici allo scopo di rivelare tali modifiche, che sono particolarmente evidenti nella bassa troposfera. L'orografia che circonda la regione mediterranea rappresenta il fattore principale che influenza il comportamento dei vortici. Questa conclusione è rafforzata, fra l'altro, dal confronto tra le statistiche riferite ai periodi invernale ed estivo: nonostante le differenti proprietà climatologiche della circolazione atmosferica media e dei flussi di calore alla superficie durante le due stagioni, alcune proprietà peculiari delle componenti di alta frequenza della circolazione sulla regione europea e mediterranea, quali la struttura orizzontale e verticale, non variano molto in funzione della stagione.

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Резюме Мы исследуем свойства атмосферных вихрей над Европой и над Средиземным морем, которые были обнаружены при анализе фильтрованных временных последовательностей для геопотенциальной высоты. Данные собирались в течение семи лет. Применение временного фильтра с обрезанием в 6 дней позволяет нам выделить изменчивость, связанную с перемещаюимися вихрями, которые обычно приходят из Атлантики и чьи свойства (пространственная структура, характеристики распространения) сильно изменяются, когда вихри проходят над областью Средиземного моря. Используются различные статистические индикаторы, чтобы обнаружить эти изменения, которые наблюдаются в нижней части тропосферы. Оказывается, что орография, окружающая область Средиземного моря, является основным фактором, влияющим на поведение вихрей. Это утверждение подтверждается также сравнением зимней и летней статистик: несмотря на различные климатологические свойства средней атмосферной циркуляции и поверхностных потоков в течение двух сезонов, некоторые свойства высокочастотных компонент над Европой и областью Средиземного моря, такие как вертикальная и горизонтальная структуры, существенно не изменяются в зависимости от сезона.

     

中国极端干旱事件的年代际变化及其成因

研究了河北山西、黄河中下游、江淮和西北东部地区531年极端干旱事件的概率,采用古里雅冰芯δ¹⁸O含量和500年来太平洋年代际涛动指数进行了对比分析.Morlet小波分析发现四个地区的极端干旱事件概率和古里雅冰芯δ¹⁸O含量都存在着350年左右的准周期,且四个地区近500年来极端干旱事件概率的变化与古里雅冰芯δ¹⁸O含量的变化相反,在δ¹⁸O含量较高的时期,发生极端干旱事件的概率较低,反之亦然.对太平洋年代际涛动指数进行小波变换,发现80年尺度的准周期在逐渐衰弱而50年尺度的准周期却在逐渐增强.在80年或50年尺度的准周期处于控制地位时,四个地区处于极端干旱事件多发期,而在二者处于交替时期,即二者周期性均较弱的时期,处于极端干旱事件偏少期. 关键词： 干旱 冰芯 太平洋年代际涛动     

A simple model of summer atmospheric circulation in the Po Valley

Summary Through numerical integration of the primitive equations, we study the summer circulation in the Po Valley, Italy. We investigate separately the mountain slope effect and the sea-breeze effect. The results show that, because of the mountain slope, there is convergence early in the morning and divergence during the day in the lower atmosphere. Further-more, the sea breeze generated by the Adriatic Sea is confined to the eastern side of the Valley. These two phenomena are expected. However, the model suggests that the convergence due to the slope is organized in an area as large as the valley and that, even if the sea breeze is a boundary layer circulation, the perturbation introduced triggers a train of gravity waves as deep as the troposphere. These effects might be very important for the distribution of the summer thunderstorms. Further specific investigations. Experimental and theoretical, are needed. Paper presented at the 1° Congresso del Gruppo Nazionale per la Fisica dell'At-mosfera dell'Oceano, June 19–22, 1984, Rome.     

Reduced models of atmospheric low-frequency variability: Parameter estimation and comparative performance

Low-frequency variability (LFV) of the atmosphere refers to its behavior on time scales of 10–100 days, longer than the life cycle of a mid-latitude cyclone but shorter than a season. This behavior is still poorly understood and hard to predict. The present study compares various model reduction strategies that help in deriving simplified models of LFV.Three distinct strategies are applied here to reduce a fairly realistic, high-dimensional, quasi-geostrophic, 3-level (QG3) atmospheric model to lower dimensions: (i) an empirical–dynamical method, which retains only a few components in the projection of the full QG3 model equations onto a specified basis, and finds the linear deterministic and the stochastic corrections empirically as in Selten (1995) [5]; (ii) a purely dynamics-based technique, employing the stochastic mode reduction strategy of Majda et al. (2001) [62]; and (iii) a purely empirical, multi-level regression procedure, which specifies the functional form of the reduced model and finds the model coefficients by multiple polynomial regression as in Kravtsov et al. (2005) [3]. The empirical–dynamical and dynamical reduced models were further improved by sequential parameter estimation and benchmarked against multi-level regression models; the extended Kalman filter was used for the parameter estimation.Overall, the reduced models perform better when more statistical information is used in the model construction. Thus, the purely empirical stochastic models with quadratic nonlinearity and additive noise reproduce very well the linear properties of the full QG3 model’s LFV, i.e. its autocorrelations and spectra, as well as the nonlinear properties, i.e. the persistent flow regimes that induce non-Gaussian features in the model’s probability density function. The empirical–dynamical models capture the basic statistical properties of the full model’s LFV, such as the variance and integral correlation time scales of the leading LFV modes, as well as some of the regime behavior features, but fail to reproduce the detailed structure of autocorrelations and distort the statistics of the regimes. Dynamical models that use data assimilation corrections do capture the linear statistics to a degree comparable with that of empirical–dynamical models, but do much less well on the full QG3 model’s nonlinear dynamics. These results are discussed in terms of their implications for a better understanding and prediction of LFV.     

Correlation properties of the backscattered field in a nonstationary statistical problem

The statistical problem of the scattering of wideband pulses by a random layered medium at normal incidence is considered in the framework of the wave approach in the space—time domain. Simulated correlation functions and power spectral densities of the backscattered field are presented. They extend the earlier findings concerning the backscattered field formation and also confirm and refine a number of conclusions drawn earlier from the behavior of the field’s statistical moments. The simulation technique is free from approximations commonly used in the statistical analysis of the propagation problems and can be used to study the statistical properties of the scattered field in a wide range of time intervals, as well as to find the limits of applicability of the approximate methods.     

Annual variability in a conceptual climate model: snapshot attractors, hysteresis in extreme events, and climate sensitivity

In a conceptual model of global atmospheric circulation, the effects of annually periodic driving are investigated. The driven system is represented in terms of snapshot attractors, which may remain fractal at all times. This is due to the transiently chaotic behavior in the regular parameter regimes of the undriven system. The driving with annual periodicity is found to be relatively fast: There is a considerable deviation from the undriven case. Accordingly, the existence of a hysteresis loop is identified, namely, the extremal values of a given variable depend not only on the actual strength of the insolation but also on the sign of its temporal change. This hysteresis is due to a kind of internal memory. In the threshold-dependence of mean return times of various extreme events, a roughly exponential scaling is found. Climate sensitivity parameters are defined, and the measure of certain types of extremal behavior is found to be strongly susceptible to changes in insolation.     

Topological and statistical properties of a constrained Voronoi tessellation

Voronoi tessellation has been used widely to approximate and model various cellular structures and stochastic patterns appearing in nature. In this work, we present an extended version of the Voronoi tessellation method that partitions the space with certain constraints commonly encountered in either experimental measurements or theoretical models, such as cell volume or size distribution. The new Voronoi method is implemented using an inverse Monte Carlo method. We calculate the topological and statistical properties of tessellated Voronoi cells in several model systems with cell volumes obeying lognormal and bimodal distributions. We also compare the results with those obtained using the conventional Poisson–Voronoi method. We observed systematic changes in the topological properties as well as deviations from some established topological relations as the parameters in the constraint were varied. The application of this constrained Voronoi method in microstructure modelling and characterization in poly- and nano-crystalline materials is briefly discussed.     

正压大气环流中的曲面周期波和孤波

大尺度正压大气环流的波动特征对理解气候变化具有重要的意义,而非线性浅水波方程组是描述大尺度正压大气环流的原始控制方程.本文对线性方程的复变函数解,通过二次适当的移植,求得浅水波方程组的发展方程的扰动位势的实变函数解,该实变函数解析解由基流项和波动项两部分组成.其中基流由波数、波速、β效应、变形半径和时间的任意函数共同决定;波动项与β效应有关.分析表明,在大尺度正压大气环流中扰动位势存在曲面的周期波和孤波的现象,周期波与孤波相互调制而呈现不稳定性;当多个周期孤波同时出现时,则彼此独立传播;扰动位势波动项中的时间任意函数对曲面周期孤波的波幅有调制作用,可控制波的产生、发展和消失.所得结果对研究大气波动现象和气候变化具有一定的理论参考价值.     

Spectral and statistical properties of fluorescence from N atoms in a cavity

Spectral and statistical properties of fluorescence from N two-level atoms interacting with an intense cavity mode are discussed. Quite different results from collective resonance fluorescence in a free space are shown.     

Effect of ion sputtering on the statistical properties of a surface

The modification of a gallium arsenide surface during irradiation by heavy cesium ions Cs⁺ is investigated by measuring the surface height distribution with an atomic force microscope. Both increases and decreases in the rms height σ, an integral parameter of the surface, are observed to occur. It is established that for all experimental samples the roughness of the gallium arsenide surface increases in a 1–100 nm lateral range. Analysis of the structure function yields an estimate of the characteristic lateral dimensions of the surface structures arising during ion etching. Zh. Tekh. Fiz. 69, 107–111 (February 1999)     

Two-point statistical properties of a passive tracer in a turbulent compressible medium

The two-point statistics of a tracer in a turbulent compressible medium is presented. The equations for Lagrange and Euler probability distributions of the density field of the tracer have been obtained and analyzed. The time behavior of dispersion of the logarithm of the density field Jacobian is considered. The power law, which is similar to the Kolmogorov-Obukhov law for an actual turbulence, has been obtained theoretically. Lobachevsky State University, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 40, No. 8, pp. 1019–1034, August, 1997.     

The properties of a dynamo wave in the case of intense meridional circulation

The solution of a Parker dynamo was constructed for the case of intensive meridional circulation using a method similar to WKB. We show how to build a solution in the transition from the traveling-wave regime to the regime of stationary magnetic field configuration. A solution of the Hamilton-Jacobi equations was discovered for the problem of a dynamo containing a triple point in the complex plane wave vector.