On the onset of separation in turbulent boundary layer flow over two-dimensional humps of various size

Summary The available laboratory data of turbulent boundary layer flow over two-dimensional obstacles have been examined in order to identify the parameters (such as the aspect ratio or the surface roughness) driving the onset of separation. A comparison with some linearized models suitable for atmospheric applications is also made. It results that i) the model response can be highly sensitive to the detailed shape of the obstacle; ii) the ratio between obstacle length and boundary layer thickness is relevant in determining the pressure perturbation near the surface; iii) the surface shear stress is poorly described in most cases and in particular in the obstacle wake.

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Riassunto I dati di laboratorio disponibili, relativi a strati limite turbolenti su ostacoli bidimensionali, sono stati esaminati al fine di identificare i parametri guida della separazione (quali la pendenza o la rugosità della superficie). Inoltre è stato fatto un confronto con i risultati di alcuni modelli linearizzati adatti per applicazioni atmosferiche. Ne risulta che i) la risposta del modello è molto sensibile alla forma dettagliata dell’ostacolo; ii) il rapporto fra la lunghezza dell’ostacolo e lo spessore dello strato limite turbolento incidente è importante nel determinare la perturbazione della pressione vicino alla superficie; iii) lo ?shear stress? superficiale non è riprodotto con accuratezza nella maggioranza dei casi, in particolare nella scia dell’ostacolo.

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Резюме Анализируются имеющиеся лабораторные данные по турбулентному течению в пограничном слое над двумерными препятствиями, чтобы идентифицировать параметры (такие как аспектное отношение или шероховатость поверхности), определяющие возникновение отрыва. Проводится сравнение с некоторыми линеаризованными моделями, удобными для атмосферных приложений. Получены следующие результаты: 1) модельный отклик может быть очень чувствительным к детальной форме препятствия; 2) отношение между длиной препятствия и толщиной поверхностного слоя является существенным при определении возмущения давления вблизи поверхности; 3) напряжение поверхностного сдвига плохо описывается в большинстве случаев и, в частности, в спутной струе.

     

Wind climatology between 950 and 500 hPa over the Mediterranean area above Italy

Summary In order to improve the simulation of wind probability distributions at ground level, eight years of wind data of the multivariate analyses at 850, 700, 500 hPa levels over Italy, and eleven years of wind radiosoundings performed at 5 Italian Air Force stations at 950, 900, 800, 700, 600, and 500 hPa have been processed. The vertical profiles of the distribution parameters of wind speed present predictable behaviour, with the exception of Brindisi. The frequency distribution profiles of the wind directions are more complex. Temporarily seconded to the RISOE National Laboratory (DK) as part of the EEC-DGXII Programme ?Stimulation Action?.     

Scattering of electromagnetic waves from a half-space of randomly distributed discrete scatterers and polarized backscattering ratio law

Summary The effective-medium approximation is applied to investigate scattering from a half-space of randomly and densely distributed discrete scatterers. Starting from vector wave equations, an approximation, called effective-medium Born approximation, a particular way, treating Green's functions, and special coordinates, of which the origin is set at the field point, are used to calculate the bistatic- and back-scatterings. An analytic solution of backscattering with closed form is obtained and it shows a depolarization effect. The theoretical results are in good agreement with the experimental measurements in the cases of snow, multi- and first-year sea-ice. The root product ratio of polarization to depolarization in backscattering is equal to 8; this result constitutes a law about polarized scattering phenomena in the nature.     

The missing part of the greenhouse effect

Summary Global average temperatures from 1860 to 1989 were analysed with a simple fitting procedure. The temperature records can be fitted with a rising exponential up to 1946 and with the same exponential minus an increasing cooling action from 1946 to 1989. It is shown that the results are compatible with the hypothesis of an albedo increase caused by SO₂ emissions. This hypothesis gives satisfactory explanations of the temperature drop in the years 1940–1970, of the different warming of the northern and southern hemispheres and of the slower warming observed over North America. The results give an indication that the greenhouse warming in the next years could be faster than predicted until now. Due to the relevance of its scientific content, this paper has been given priority by the Journal Direction.     

Measurements of climatic parameters and ground-level ozone at Terra Nova Bay in Antarctica

Summary The results of local meteorology, solar radiation and ground-level ozone measurements taken in Antarctica during the second Italian expedition (December 86–February 87) at Terra Nova Bay are presented. During the summer months the site of the Italian base camp is characterized by a thin strip of deglaciated ground, along which the temperature measurements close to the ground and up to 6 m high show a strongly superdiabatic profile. This irregular trend of the temperature in the surface layer is mainly due to the notable incoming amounts of radiation and to the extreme transparency of the atmosphere. This is also shown by the low values of the ratio between total radiation and diffuse radiation. The ground is thus subjected to intense heating, especially in the warmer hours of the day, while the surface layer of the atmosphere will be characterized by strong upward heat fluxes and by turbulent convective movements. Vertical-temperature-profile measurements show an almost forced persistence in the superdiabatic trend, which tends towards isothermic values only as a resultof rapid variations in the direction and intensity of the wind, connected to the downward flux of cold air masses, shown also by the simultaneous increases in ground-level ozone concentrations which would support the presence of dry-deposition processes in the lower layer. However, the complex local orography and the horizontal discontinuity (sea, deglaciated coast, snow-covered surfaces) do not permit a correct application of profile flux relations, normally used in studies on groundlevel dry-deposition. Paper presented at the IV Congresso del Gruppo Nazionale per la Fisica dell'Atmosfera e dell'Oceano, June 22–24, 1987, Rome.     

On the analytical solutions of unstable surface layer similarity equations

Summary An approximate relationship between the gradient, Ri, and the bulk Richardson number, Ri_b, for unstable atmospheric conditions, is suggested. The proposed relation shows much better agreement with the estimation provided by a numerical iterative method than the usual approximations suggested by Louis and Byun.     

Climatologies of tides at mid-latitudes in the lower thermosphere

Summary Time variations of the semi-diurnal and diurnal tides observed at Budrio (45°N, 12°E) in the wind structure of the lower thermosphere ((80÷110) km) throughout the period 1976–1990 are herewith represented as monthly contours of both amplitudes and phasesvs. height. The semi-diurnal tide has more marked seasonal variations, showing amplitudes generally larger than the diurnal ones ((10÷30) m/svs. (5÷15) m/s). The vertical wavelengths are longer in summer than in winter, at least below 100 km. Agreement with new numerical models for semi-diurnal tide at 50°N appears to be satisfactory.     

An analytical model for a screen evaluation of the environmental impact from a single point source

Summary A screening model (CISP) is presented for the calculation of maximum predicted concentrations from a single point source, which is particularly useful in situations where on-site meteorological measurements are limited or unavailable. CISP utilizes an analytical solution of the advection diffusion equation which is non-Gaussian in form. CISP performance in evaluating maximum ground-level concentrations was compared with that of the Environmental Protection Agency (EPA) Regulatory PTPLU2 Gaussian model.     

Detection of urban heat island in Istanbul, Turkey

Summary The aim of this study is to detect the urban-heat-island effects in Istanbul which is the largest and most important megapolis of Turkey. The mean monthly minimum, normal and maximum temperatures are derived from daily ones. The method of linear regression and Mann-Kendall test are used in the analysis of trend. Despite Istanbul is surrounded by the Black Sea and Marmara Sea, the urban-heat-island intensity stands out quite clearly.     

Analysis of 11 carefully selected Italian stations. Choice of an interpolation method for probability papers

Summary Eleven wind stations, evenly distributed over Italy, were selected and the data from them carefully checked. Asymptotic analysis (first-type distribution) was applied to the largest wind values. Three different methods of probability papers were used in order to estimate the distribution parameters from which the return periods are computed. Comparing the ordinary least squares, Lieblein's and Tukey's methods, the latter appears to be the most suitable for mapping extreme winds in Italy.