The influence of stratification on heat and momentum turbulent transfer in antarctica

Data from the Antarctic winter at Halley Base have been used in order to evaluate qualitatively and quantitatively how the stratification in the low atmosphere (evaluated with the gradient Richardson number, Ri) influences the eddy transfers of heat and momentum. Vertical profiles of wind and temperature up to 32 m, and turbulent fluxes ( , and ) measured from three ultrasonic thermo-anemometers installed at 5, 17 and 32 m are employed to calculate Ri, the friction velocity (u _*) and the eddy diffusivities for heat (K _h ) and momentum (K _m ). The results show a big dependence of stability onK _m ,K _h andu _*, with a sharp decrease of these turbulent parameters with increasing stability. The ratio of eddy diffusivities (K _h /K _m ) is also analyzed and presents a decreasing tendency as Ri increases, reaching values even less than 1, i.e., there were situations where the turbulent transfer of momentum was greater than that of heat. Possible mechanisms of turbulent mixing are discussed.     

A comparison of several diffuse solar radiation models for Greece

Summary This paper attempts to test the applicability of existing correlation models to the estimation of diffuse radiation with respect to measured values at a station. There are two types of model: The first type depends on the fraction of monthly average daily diffuse radiation to total solar radiation, , as a function of the clearness index, . The second type expresses the fraction or as a function of the sunshine fraction Therefore, it presents statistically based correlations between global radiation and its diffuse component on a horizontal surface and suggests two equations to determine the ratio of diffuse radiation to total radiation received on a horizontal surface. The results of these correlation equations are compared with other accepted equations.With 3 Figures     

Comparison of a computer-simulated stratus-topped boundary layer with aircraft observations

To assess the realism of large-eddy simulation (LES) of the stratus-topped boundary layer and its predicted turbulent structure, we performed detailed data analyses on a LES (which has a 12.5 m grid size in all three directions), in a manner similar to those used by Nicholls (1989) on aircraft measurements. The first analysis retrieves the primary convective elements, i.e., the negatively buoyant downdrafts, which are driven mainly by cloud-top radiative cooling, through a conditional sampling technique. Comparison shows that the LES of this resolution reflects most of the observed downdraft features; most of the discrepancies that exist between the obervations and the LES can be explained by decoupling of the cloud layer from the underlying flow that exists in the former but not in the latter. The second analysis shows the vertical velocity spectrum and its agreement with the measurements. In the third analysis, showing the turbulent kinetic energy budgets, the discrepancy in the turbulent transport term (i.e., the divergence of the third-moment quantity , the turbulent-kinetic-energy flux) between the LES and measurements exists even with such a fine resolution LES. This discrepancy is related mainly to the different behavior in between the LES and observations, which may again be associated with decoupling.An advantage of LES over aircraft observations is that the former can provide three-dimensional flow structure at any instant. In this paper, we examined the instantaneous flow structure and observed closed cellular patterns near the cloud top in which updrafts occupy the broad centers and relatively strong downdrafts occur in the narrow edges. In the intersections of these cell boundaries, there exist weak downdrafts, consisting of relatively cold and dry air, that are the most likely origins of the strong downdrafts extending throughout the mixed layer.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Oxidized Nitrogen in the Remote Pacific: The Role of Electrical Discharges over the Oceans

Simulations of oxidized nitrogen performed withglobal transport tracer models systematicallyunderestimate the concentrations of total nitrate atremote marine locations in the Mid-Pacific. Higheremission rates in the models of nitrogen oxides( ) from continental sources or alarger influx from the stratosphere do not seem tobe able to account for the shortfall. We are led toconclude that there has to be a substantial sourcelocated in oceanic areas. We speculate that -emission from electrical discharges overthe oceans could be the source we are looking for.Airborne observations of atmospheric concentrationsof oxidized nitrogen and ozone in the remoteatmosphere and observations of nitrate wetdeposition in remote sites are used as additionalconstraints to check the plausibility of thishypothesis. We find that a larger emission of due to lightning activity over the oceansindeed results in a much improved simulation oftotal nitrate in the remote Pacific, particularly inthe Equatorial and Tropical South Pacific and thatsuch a scenario is generally consistent withavailable observations of nitrate wet deposition andatmospheric concentrations of oxidized nitrogen andozone. An alternative hypothesis is that there is ahitherto unknown in situ source of over thePacific Ocean.     

Statistical properties of 49 years of rainfall rate events

Summary The paper presents a detailed investigation of magnitudes and properties pertaining to the large population of rainfall events recorded during 49 years at the Fabra Observatory in Barcelona. The study includes a statistical analysis of event durationT and rainfall quantityQ together with the statistical rainfall rate parameters: , ²(R) and maximum Rm within an event. The decorrelation time is also analysed. It is found thatQ, T, and can be well modelled by a lognormal distribution, but , ² and Rm are only so for a limited range of precipitation ratesR. The regression analysis between pairs of logarithms of the magnitudes investigated is generally good and a regression coefficient is often better than 0.9. Comparison with published work is also carried out. An attempt is made to discriminate between heavy and non-heavy rainfall rates, and the 50 mm/h threshold is used for the study. The twelve-monthly running average indicates that the rainfall amount has a small increasing trend over the fifty year period. However, this trend is reversed when considering heavy rains. Finally, the return period in years to exceeding a thresholdR within an event is also investigated and the distribution of the population of annual extremes is found to be Gumbel II.With 6 Figures     

A multifractal analysis of lidar measured water vapour

Accurate and fast-response measurements of space-time observations of specific humidity were made above a drying land surface at the University of California at Davis, using the Los Alamos water Raman-lidar. In an attempt to quantify the space-time intermittency features of turbulent flows in the lower atmosphere, a multifractal analysis of these water vapour measurements was performed. The structure of the specific humidity, (x, t), was analyzed quantifying a scalar gradient measure both in time and space, for all possible one-dimensional cuts, i.e. and . The results confirm the multifractal nature of this scalar gradient measure (a type of scalar dissipation rate) and show that humidity measurements at fixed times (x) are more intermittent (e.g. have less entropy dimension) than those at fixed locations in space (t). Similar multifractal behaviour of the spatial data, with and without a transformation from the observed wind velocities, supports the validity of Taylor's hypothesis for the studied fields.     

Numerical simulation of flow data over two-dimensional hills

We have studied a neutrally-stratified flow over two-dimensional hills using a two-dimensional, non-hydrostatic version of the Regional Atmospheric Modeling System (RAMS). We have implemented three different turbulence closure models: the standardE- model, an Algebraic Reynolds Stress Model (ARSM) and a new model. Model predictions for the mean and turbulence flows using different closure schemes are compared with the data of a wind tunnel experiment containing isolated two-dimensional hills of varying slope. From the comparison, it is concluded that all three models predict the mean flow velocities equally well while only the new closure model accurately predicts the turbulence data statistics.The research reported in this paper was conducted while the first author held a National Research Council (NRC) Associateship.     

Effects of water vapour on the structure parameter of the refractive index for near-infrared radiation

The refractive index of air (n) mainly depends on temperature andwater vapour content. For near-infrared radiation, temperature isthe main determining factor. To determinethe structure parameter of temperature ( ) fromthe structure parameter of the refractive index( ), the influence ofwater vapour content on n needs to be taken into account as acorrection. Three levels of approximation are presented. The firstinvolves the standard deviations of T and q (_T and_q) as well as the correlation coefficient betweenT and q (R_Tq). The second approximation involvesR_Tq and the Bowen ratio (), and the last uses only the Bowen ratio.The latter is the classical Bowen ratio correction.Evaluation of the validity of the assumptions used in the derivationreveals that a large error may be introduced (for situations withR_Tq < 0.8) by replacing _T/_qwith the Bowen ratio.Consequently, estimating from using the Bowen ratiocan contain errors of 5–40% for || < 1. Adding R_Tqas extra information reduces the errors only marginally. For || > 1,the correction is small, and all three approximations give errors of lessthan 1% in . When is used to compute the sensible heat flux,the influence of the quality of the correction forwater vapour fluctuations on the measured energy balance is small: forsmall ||, the correction is large, but the absolute value ofthe heat flux is small, whereas for large ||, the correctionis insignificant.     

Transformation of Irradiance Measurements into Spectral Actinic Flux for Photolysis Rates Determination

The actinic flux is the only radiometric quantity suitable for photolysisfrequency determination. It is derived from solar spectral irradiancemeasurements performed by a portable spectroradiometer in the 300–850nm wavelength range. The spectral irradiance is first divided into a directpart and a diffuse part, according to the atmospheric conditions, and thenconverted into the corresponding actinic flux quantity. As an intermediary,the ratio of diffuse actinic flux to diffuseirradiance is calculated by the spherical harmonics radiative code withrespect to wavelength, solar zenith angle, surface albedo, and aerosolproperties.The results of extensive sensitivity studies of asa function of the main atmospheric parameters are discussed and lead to theconclusion that aerosol optical depth is the major critical value for aprecise determination. The global algorithm totransform spectral irradiance into actinic flux is finally applied forphotodissociation rate calculations by convolution of the obtained actinicflux spectra with the absorption cross-sections and quantum yields of themolecule studied. Photolysis rates of different atmospheric photooxidants havebeen measured with this spectroradiometric method during the summers of 1993and 1994 in Brittany and in Portugal. The ozone and nitrogen dioxidephotodissociation rates obtained present a good agreement with thej(O₃) and j(NO₂) actinometerresults, for the same experimental conditions in Brittany.     

Similarity theory of diffusion and the observed vertical spread in the diabatic surface layer

The extension of Lagrangian similarity theory of diffusion to stratified flow is examined, to improve its prediction of the vertical spread of a passive substance. In the basic equation, where is the average height of a cluster of particles,u _* is the friction velocity andL is Monin-Obukhov length. It is shown theoretically, under the assumption of an equivalence between the diffusivities of heat and matter, that the unspecified function is the reciprocal of a more familiar meteorological parameter _n , the dimensionless temperature gradient. The universal constantb is found to be approximately equal to von Karman's constant for various stability conditions. The predicted effect of stability on vertical spread shows excellent agreement with that of the published data from the O'Neill experiments.     

Zur Struktur der Wolken

Zusammenfassung Auf dem Feldberg (1493 m NN) im südlichen Schwarzwald wurden von Januar 1953 bis Mai 1953 und von September 1954 bis Februar 1955 Messungen von Tropfengröße, Wassergehalt und Sichtweite in Wolken durchgeführt. Die Tropfen der einzelnen Spektren haben einen mittleren Durchmesser zwischen 6 und 30 ; die Durchmesser der größten gemessenen Tropfen liegen zwischen 50 und 60 . Der Wassergehalt hat Werte zwischen 0,003 und 1,54 g/m³, wobei über 90% aller Messungen einen Wassergehalt unter 0,50 g/m³ zeigen. Ein einfacher Zusammenhang zwischen Sichtweite, Tropfengröße und Wassergehalt nach derTrabertschen Formel besteht nicht.Eine enge beziehung wurde zwischen der Form der Spektren und dem Niederschlag gefunden. In Zeiten ohne Niederschlag und bei geringfügigem Niederschlag überwiegen schmale ( < 12,0 ), in Zeiten mit länger andauerndem Niederschlag breite Spektren ( > 12,0 ). Eine Ausnahme machen alle Messungen in Hochnebel, die breite Spektren mit zwischen 13 und 17 zeigen, ohne daß Niederschlag fällt.Das gleichzeitige Vorhandensein der drei Phasen Wasserdampf, flüssiges Wasser und Eisteilchen über längere Zeiträume hinweg wurde festgestellt; somit kann die Eiskristall-Niederschlagstheorie vonBergeron und vonFindeisen für den Mechanismus der Niederschlagsbildung nicht im allgemein angenommenen Ausmaß zutreffen. Es erscheint notwendig, die Frage der Wolkenphysik nicht nur thermodynamisch, sondern auch in vermehrtem maß kolloidphysikalisch im Sinne vonSchmauss zu untersuchen.

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Summary Measurements of drop size, water content and visibility in clouds have been carried out from January till May 1953 and from September 1954 to February 1955 on the Feldberg (1493 m. a. s. 1.) in the southern Black Forest. The drops of the different spectra have a mean diameter between 6 and 30 ; the diameters of the largest measured drops lie between 50 and 60 . The water content shows values between 0.003 and 1.54g/m³ whereby more than 90 per cent of all measurements lead to a water content below 0.50 g/m³. A simple relation between visibility, drop size and water content according to the formula ofTrabert does not exist.A closed connection between the form of the spectra and the precipitations has been found. In periods without or with only slight precipitation narrow spectra ( < 12.0 ) prevail, in periods with persistent precipitation broad ones ( > 12.0 ). All measurements in elevanted fog make an exception, showing broad spectra with between 13 and 17 without any precipitation.The simultaneous existance of the 3 phases: water vapour, liquid water and ice particles over alonger periods has been stated; the ice crystall precipitation theory ofBergeron andFindeisen for the mechanism of precipitation can therefore not hold in the generally accepted extent. The examination of the problems of cloud physics appears to be necessary, not only in thermodynamical respect, but also in the sense of coloid physics according toSchmauss.

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Résumé On a effectué de janvier à mai 1953 et de septembre 1954 à février 1955 au Feldberg (1493 m.) en Forêt-Noire des mesures de grosseur de gouttelettes, de teneur en eau et de visibilité dans les nuages. Les gouttelettes ont und diamètre moyen compris entre 6 et 30 ; les plus grosses mesurent 50 à 60 . La teneur en eau varie de 0,003 à 1,54 g/m³; dans plus de 90% des mesures, elle reste inférieure a 0,5g/m³. Il n'y a pas de relation simple, qui suivrait la formula deTrabert, entre visibilité, grosseur des gouttelettes et teneur en eau.Il existe par contre une relation étroite entre la forme du spectre de fréquence et les précipitations. Lorsque celles-ci sont nulles ou très faibles ce sont les spectres étroit ( < 12,0 ) qui dominent; par précipitations durables, ce sont les spectres larges ( > ). Dans le stratus, sans pluie, les spectres larges ( entre 13 et 17 ) prédominent.On a constaté l'existence simultanée des trois phases solide, liquide et gazeuse pendant d'assez longs intervalles de temps, de sorte que le théorie deBergeron et deFindeisen concernant la genène des précipitations n'a pas la généralité qu'on lui prête. Il apparaît nécessaire d'aborder les problèmes de la physique des nuages non seulement du point de vue thermodynamique, mais aussi de celui de l'état colloïdal au sens deSchmauss.

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Mit 9 Textabbildungen.

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Hinsichtlich der Wiedergabe von Tabellen und Tropfenspektren stark verkürzte Fassung einer bei der Technischen Hochschule in Karlsruhe eingereichten und genehmigten Dissertation; die hier weggelssene Dokumentation kann zur Einsichtnahme bezogen werden bei den Bibliotheken der Technischen Hochschule Karlsruhe und des Deutschen Wetterdienstes, Offenbach (Main).     

Eine Theorie der anisotropen und inhomogenen Turbulenz in der bodennahen Luftschicht

Zusammenfassung Ausgehend von denMonin-Obuchowschen Ansätzen [1] wird mit Hilfe eines Mischungswegkonzepts für das vertikale Wind-, Temperatur- und Feuchteprofil in der Bodenschicht der Atmosphäre die analytische Form bestimmt. Es ergibt sich dabei auch eine Bedingung für das Verhältnis zwischen den Austauschkoeffizienten für Wärme und Impuls. Letzteres kann bei stationärer und in derz-Richtung symmetrischer Turbulenz zwischen 1/2 und 2 schwanken; Werte <1 sind nachts, solche >1 tagsüber zu erwarten. Für dieses Ergebnis ist der praktische Nachweis beiLettau [16] zu finden. — Im Gegensatz zuLettau [16] wird mit Begründung vermutet, daß der Austauschkoeffizient für Wasserdampf jenem für Wärme gleicht.Bei konstantem Gradientwind existiert in der Bodenschicht der Atmosphäre ein (festes) Niveau (z=h) maximaler Tagesschwankung des Windes. Nur in diesem äquivalenthomogenen Niveau sind konstante kritische Werte der Richardsonzahl möglich. Die spezifische kinetische Turbulenzenergie sowie die Anisotropie der Turbulenz entspricht dort dem für die Reibungsschicht zutreffenden Mittelwert. —Es wird auch nachgewiesen, daß dievon Kármánsche Konstante eine Funktion der Meßdauer ist.

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Summary Starting from the equations ofMonin andObuchow [1], and using a mixing length concept, an analytical expression for the vertical profiles of wind, temperature, and humidity near the ground is derived. The calculations also render a condition for the ratio of the exchange coefficients for heat and momentum. For stationary and vertically symmetrical turbulence, this ratio can vary between the limits 1/2 and 2; values <1 are to be expected during the night, and >1 during the day. The practical proof of this result can be given using data published byLettau [16]. Contrary toLettau [16], there is reason to suspect that the exchange coefficients for water vapour and heat are equal.At constant gradient wind, there exists a (fixed) level (z=h) in the ground layer of the atmosphere in which the daily variation of wind velocity has a maximum. Only in this equivalent-homogeneous level, critical values of theRichardson number are possible. Atz=h, the specific kinetic energy as well as the anisotropy of turbulence, , correspond to the respective averages in the entire friction layer. It is shown thatvon Kármán's constant is a function of the duration of measurement.

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Résumé Au moyen des dispositions deMonin-Obuchow [1] et de la hypothèse du parcours de mélange, on détermine la forme analytique du profil vertical du vent, de la température et de l'humidité dans les couches voisines du sol. Il en découle également une limitation quant au rapport existant entre les coefficients d'échange de la chaleur et des bouffées. Ce rapport peut osciller entre 1/2 et 2 dans le cas d'une turbulence stationnaire et symétrique par rapport à l'axe desz. Les valeurs inférieures à 1 se rencontrent de nuit, celles dépassant 1 le jour. La preuve pratique de ce résultat est contenue dansLettau [16]. Au contraire de cet auteur, on admet, avec preuves à l'appui, que le coefficient d'échange est le même pour la vapeur d'eau que pour la chaleur.Si le vent de gradient est constant, il existe dans les couches de l'atmosphère voisines du sol un niveau (constant) (z=h) où les variations journalières du vent sont maximum. Ce n'est qu'à ce niveau «d'homogénéité équivalente» que les valeurs critiques des constantes deRichardson sont possible. L'énergie cinétique spécifique de la turbulence ainsi que l'anisotropie de la turbulence sont alors égales à la moyenne correspondante de la couche de friction. On prouve ainsi que la constante devon Kármán est une fonction de la durée de la mesure.

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Mit 2 Textabbildungen     

A study of the seasonal migration of ITCZ and the quasi-periodic oscillations in a simple monsoon system using an energy balance model

Summary A zonally averaged global energy balance model with feedback mechanisms was constructed to simulate (i) the poleward limits of ITCZ over the continent and over the ocean and (ii) a simple monsoon system as a result of differential heating between the continent and the ocean. Three numerical experiments were performed with lower boundary as (1) global continent, (2) global ocean and (3) continent-ocean, with freezing latitudes near the poles. Over the continent, midlatitude deserts were found and the ITCZ migrates 25° north and south with seasons. Over a global swamp ocean results do not show migration of ITCZ with time but once the ocean currents are introduced the ITCZ migrates 5° north and south with seasons. It was found that the seasonal migration of ITCZ strongly depends on the meridional distribution of the surface temperature. It was also found that continent influences the location of the oceanic ITCZ. In the tropics northward progression of quasi-periodic oscillations called events are found during the pre- and post-monsoon periods with a period of 8 to 15 days. This result is consistent with the observed quasi-periodic oscillations in the tropical region. Northward propagation of the surface temperature perturbation appears to cause changes in the sensible heat flux which in turn causes perturbations in vertical velocity and latent heat flux fields.List of Symbols vertical average - ₀ zonal average - vertical mean of the zonal average - _0s zonal average at the surface - _0a zonal average at 500 mb level - latitude We now define the various symbols used in the model rate of atmospheric heating due to convective cloud formation (K/sec) - dp/dt (N/m²/sec) - density - potential temperature (K) - rate of rotation of the earth (rad/sec) - empirical constant - humidity mixing ratio - ^* saturated humidity mixing ratio - opacity of the atmosphere - ₁,₂ factors for downward and upward effective black body long wave radiation from the atmosphere - Stefan-Boltzmann constant - emissivity of the surface - _D subsurface temperature (K) - a specific volume - _0xs ,_0ys eastward and northward components of surface frictional stress - ^* vertical velocity at the top of the boundary layer (N/m²/sec) - P Thickness of the boundary layer (mb) - nondimensional function of pressure - P pressure - P _a pressure of the model atmosphere (N/m²) - P _s pressure at the surface (N/m²) - t time (sec) - U eastward wind speed (m/sec) - V northward wind speed (m/sec) - surface water availability - T absolute temperature (K) - heat addition due to water phase changes - g acceleration due to gravity (m²/sec) - a radius of the earth (m) - R gas constant for dry air (J/Kg/K) - C _p specific heat of air at constant pressure (J/Kg/K) - k R/C _p - L latent heat of condensation (J/Kg) - f coriolis parameter (rad/sec) - H _s H _0s ⁽¹⁾ +H _0s ⁽²⁾ +H _0s ⁽³⁾ +H _0s ⁽⁴⁾ +H _0s ⁽⁵⁾ (J/m²/Sec)=sum of the rates of vertical heat fluxes per unit surface area, directed toward the surface - H _a H _0a ⁽¹⁾ +H _0a ⁽²⁾ +H _0a ⁽³⁾ +H _0a ⁽⁴⁾ (J/m²/Sec)=sum of the rates of heat additions to the atmospheric column per unit horizontal area by all processes - H _0s ⁽¹⁾ ,H _0a ⁽¹⁾ heat flux due to short wave radiation - H _0s ⁽²⁾ ,H _0a ⁽²⁾ heat flux due to long wave radiation - H _0s ⁽³⁾ ,H _0a ⁽³⁾ heat flux due to small scale convection - H _0s ⁽⁴⁾ heat flux due to evaporation - H _0a ⁽⁴⁾ heat flux due to condensation - H _0s ⁽⁵⁾ heat flux due to subsurface conduction and convection - e ^* saturation vapor pressure - R solar constant (W/m²) - r _a albedo of the atmosphere - r _s albedo of the surface - b ₂ empirical constant (J/m²/sec) - c ₂ empirical constant (J/m²/sec) - e ₂ nondimensional empirical constant - f ₂ empirical constant (J/m₂/sec) - factor proportional to the conductive capacity of the surface medium - a _s constant used in Sellers model - b _s positive constant of proportionality used in the Sellers model (kg m²/J/sec²) - K _HT coefficient for eddy diffusivity of heat (m²/sec) - K _HE exchange coefficient for water vapor (m²/sec) - h depth of the water column (m) - z height (m) - V _0ws meridional component of surface current (m/sec) - n cloud amount - G _0,n long wave radiation form the atmosphere for cloud amount n (W/m²) - B ₀ long wave radiation from the surface (W/m²) - S _0,n short wave radiation from the atmosphere for cloud amount n (W/m²) - A _n albedo factor for a cloud amount n - R _f1 large scale rainfall (mm/day) - R _f2 small scale rainfall (mm/day) With 22 Figures     

Correlation methods in fingerprint detection studies

This investigation addresses two general issues regarding the role of pattern similarity statistics in greenhouse warming detection studies: normalization, and the relative merits of centered versus uncentered statistics. A pattern correlation statistic is used to search for the greenhouse warming signals predicted by five different models in the observed records of land and ocean surface temperature changes. Two forms of this statistic were computed: R (t), which makes use of nonnormalized data, and (t), which employs point-wise normalized data in order to focus the search on regions where the signal-to-noise ratio is large. While there are no trends in the R (t) time series, the time series of (t) show large positive trends. However, it is not possible to infer from the (t) results that the observed pattern of temperature change is, in fact, becoming increasingly similar to the model-predicted signal. This is because point-wise normalization of the observed and simulated mean change fields by a single common field introduces a common factor effect, which means that the quantities being compared should show some similarity a priori. This does not necessarily make normalization inapplicable, because the detection test involves seeking a trend in the similarity statistic. We show, however, that trends in (t) must arise almost completely from the observed data, and cannot be an indicator of increasing observed data/signal similarity. We also compare the information provided by centered statistics such as R(t) and the uncentered C(t) statistic introduced by Barnett. We show that C(t) may be expressed as the weighted sum of two terms, one proportional to R(t) and the other proportional to the observed spatial mean. For near-surface temperatures, the spatial average term dominates over the R(t) term. In this case the use of C(t) is equivalent to the use of spatial-mean temperature. We conclude that at present, the most informative pattern correlation statistic for detection purposes is R(t), the standard product-moment correlation coefficient between the observed and model fields. Our failure to find meaningful trends in R(t) may be due to the fact that the signal is being obscured by the background noise of natural variability, and/or because of incorrect model signals or sensitivities.     

Weather modification reagents. I. On the presentation and interpretation of the cold chamber ice-nucleating reagent activity measurement

New activity coefficients for ice-forming characteristics of the pyro-technical mixtures are proposed. For activity-temperature relationship, we propose the following empirical expression:

On the profiles of wind velocity in the roughness sublayer above a coniferous forest

Wind velocity and temperature measurements from a 200 m tower, locatedin a forest near Karlsruhe were used to investigatethe modified profile function of the wind velocity in theroughness sublayer.To avoid determination of the friction velocity we introduced analternative analysis with the expression instead of From the observed Fm* profiles we evaluated the profile function m*. The wind profiles observed under neutral conditions were well representedby a modified non-dimensional profile function with physically based boundary values at the top and at the bottom of theroughness sublayer.The results of our analysis can be used to take into consideration themomentum exchange between the atmosphere and a forest in mesoscaleatmospheric models in a refined way.     

Theorie der räumlichen und zeitlichen Mittelbildung einer beliebigen Feldfunktion und ihre Anwendung in der Meteorologie

Zusammenfassung Durch Untersuchung des Zusammenhanges zwischen dem Zeit- und dem Raummittelfeld einer beliebigen Feldfunktion wurde folgende Beziehung gefunden: Existiert zwischen dem Zeit- und Raummittel einer Feldfunktion ein Äquivalenzprinzip, so genügt diese Funktion einer homogenen Wellengleichung mit der Geschwindigkeit der Welle (im zweidimensionalen Fall). s und t sind hiebei die Raumbzw. Zeitinkremente der räumlichen bzw. zeitlichen Mittelung der Feldfunktion. Diese Feststellung gilt auch umgekehrt: falls eine Raum-Zeitfunktion einer Wellengleichung mit genügt, so gilt für das Raum- und Zeitmittel der Funktion mit s und t als die dazugehörigen Inkremente ein Äquivalenzprinzip. Ist die Geschwindigkeit der Welle , so besteht zwischen dem Raum- und Zeitmittel einer Feldfunktion eine allgemeinere lineare Beziehung, die genau bestimmbar ist. DaH. Reuter für das Geopotential einer Druckfläche ein Äquivalenzprinzip von Raum- und Zeitmittel statistisch nachweisen konnte, muß auf Grund der oben erwähnten Ergebnisse geschlossen werden, daß in der Atmosphäre Wellen- bzw. Schwingungsvorgänge (lineare Vorgänge) sehr häufig vorkommen. Es kann nachgewiesen werden, daß gewisse partikuläre Integrale der divergenzfreien Vorticitygleichung (z. B. Rossby-Lösung) gleichzeitig auch Lösungen der homogenen Wellengleichung sind. Für diese Lösungen werden die Bedingungen für ein Äquivalenzprinzip von Raum- und Zeitmittel ausführlich diskutiert.Weiters kann gezeigt werden, daß man mittels des Äquivalenzprinzipes die homogene Wellengleichung sehr einfach numerisch oder graphisch integrieren kann. Diese Tatsache wird zur Konstruktion von Vorhersagekarten im 500 mb-Niveau verwendet. Allerdings muß bei einer solchen Integration die Geschwindigkeit des Feldes bekannt sein. Diese Geschwindigkeit wird daher entweder durch Extrapolation oder mittels derRossby-Formel unter Zuhilfenahme der zonalen Windkomponenten ermittelt. Naturgemaß können aber durch eine solche Methode der Vorhersage barokline Entwicklungen nicht erfaßt werden. Ferner werden noch Vorschläge zur Verbesserung der Methode zur Vorhersage von Mittelkarten nachH. Reuter ausgearbeitet.

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Summary An investigation of the correlation between time means and space means of an arbitrary field function renders the following relationship: If the time and space means of a field function follow a principle of equivalence, this field function satisfies a homogeneous wave equation, with a wave velocity (in the two-dimensional case). s and t are the grid distance and time interval used in calculating the space and time averages of the field. The above statement may also be reversed: if a function of space and time satisfies a wave equation with , the space and time means (increments s and t) follow a principle of equivalence. If the wave velocity , the space and time means of the field function can be represented accurately by a linear general equation.The existence of a principle of equivalence between space and time means of the geopotential field of a constant pressure level has been proved statistically byH. Reuter. In view of the present results, this equivalence suggests that wave processes must be rather common in the atmosphere. It can be shown that certain particular integrals of the non-divergence vorticity equation (e. g. theRossby-solution) are also solutions of the homogeneous wave equation. For these solutions, the conditions of a principle of equivalence of space and time means are discussed in detail.Further, it can be shown that, by means of the principle of equivalence, the homogeneous wave equation may easily be integrated, either numerically or graphically. These findings were utilized in constructing prognostic charts of the 500 mb level. Integration of the wave equation is only possible if the velocity of the field is known. It can be found either by extrapolation in time, or by means of theRossby formula and the zonal wind component. Of course, the prognostic method described here does not cover baroclinic developments. Finally, several suggestions are made to improveH. Reuter's method of extended forecasting.

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Résumé En recherchant la connexion entre les champs des moyennes temporelles et spatiales d'une fonction de champ quelconque on atrouvé le rapport suivant: Si la moyenne temporelle et la moyenne spatiale d'une fonction de champ obéissent à un principe d'équivalence, cette fonction satisfait une équation des ondes, la vitesse de l'onde (dans le cas à deux dimensions) étant . Ici, s indique la dimension du carrelage et t la différence du temps au calcul des moyennes spatiales et temporelles dans le champ. Cette règle est aussi reversible: si une fonction spatiale et temporelle satisfait une équation des ondes avec . un principe d'équivalence est applicable pour les moyennes spatiales et temporelles en employant les différences s et t. Si la vitesse de l'onde , une relation linéaire et plus générale existe entre la moyenne spatiale et temporelle, laquelle peut être déterminée précisément.CommeH. Reuter pouvait prouver par la statistique un principe d'équivalence entre des moyennes spatiales et temporelles dans un champ du géopotentiel d'un niveau isobarique, il faut conclure en raison des résultats mentionnés ci-dessus que dans l'atmosphre les événements ondulatoires ou de l'oscillation (évéments linéaires) sont très fréquents. On peut prouver que certaines intégrales particulières de l'équation de vorticity sans divergence (par exemple: solution deRossby) sont simultanément aussi des solutions de l'équation homogéne des ondes. Pour ces solutions-là les conditions pour un principe de l'équivalence entre les moyennes spatiales et temporelles sont discutées en détail.En outre on peut montrer qu'on peut intégrer l'équation homogène des ondes bien facilement numériquement ou d'une manière graphique au moyen du principe de l'équivalence. Ce fait est appliqué pour la construction des cartes de prévision au niveau de 500 mb. Cependant il faut connaître la vitesse du champ pour une telle intégration. On trouve cette vitesse par extrapolation ou au moyen de la formule deRossby en s'appuyant sur les composantes zonales du vent. Mais il faut dire que les évolutions baroclines ne peuvent pas être saisies par une telle méthode de prévision. De plus l'auteur propose des amendements de la méthode de prévision des cartes moyennes selonReuter.

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Die Verteilung des SO2-Gehaltes der Luft im Stadtgebiet von Wien

Zusammenfassung An zahlreichen über das ganze Stadtgebiet von Wien verteilten Beobachtungsstellen wurden vom September 1956 bis September 1957 an je vier Wochen lang ausgesetzten, mit einem Absorptionsmittel getränkten Baumwollappen SO₂-Ablagerungen gesammelt. Die gefundenen SO₂-Mengen zeigen eine stark unterschiedliche Verteilung im Stadtbereich und einen deutlichen Jahresgang mit einem Maximum im Winter und einem Minimum im Juli. Der Einfluß meteorologischer Faktoren auf den SO₂-Gehalt der Luft und seine Änderungen wird unter besonderer Berücksichtigung der orographischen Verhältnisse und örtlicher SO₂-Quellen eingehend diskutiert. Abschließend wird auch über eine Analyse von Schneeproben auf berichtet.

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Summary During the period September 1956 to September 1957 the amount of SO₂ deposited from the atmosphere was observed at numerous locations in the town of Vienna. The observations were made by means of cotton rags, soaked with an absorbing chemical, and each rag was exposed for four weeks. The amounts of SO₂ show great differences within the area, and a pronounced annual variation, with a maximum in winter and a minimum in July. The influence on the SO₂ content of the air of meteorological factors is discussed in detail, with special regard to variations by orographic conditions and local SO₂ sources. Additional determinations on snow samples are reported.

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Résumé Des prélèvements de SO₂ par absorption sur des bandes de coton imprégné, laissées sur place pendant quatre semaines, ont été faits en de nombreux endroits de l'agglomération viennoise de septembre 1956 à septembre 1957. Les quantités de SO₂ varient beaucoup selon les endroits et la saison; elles sont maximum en hiver et minimum en juillet. Etude des facteurs météorologiques agissant sur la répartition du gaz en tenant compte en particulier du relief et des sources locales d'anhydride sulfureux. On a également déterminé la teneur de d'échantillons de neige.

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Aircraft Observations of Air-mass Modification Over the Sea of Okhotsk during Sea-ice Growth

In order to quantitatively investigate the role of leads and sea-ice in air-mass modification, aircraft observations were conducted over the partially ice-covered Sea of Okhotsk. We investigated two cold-air outbreak events with different sea-ice concentrations. In both cases, the difference between the temperatures of surface air and the sea surface (ΔT) dropped rapidly with the accumulated fetch-width of leads up to about 35-40 km, and then decreased very slowly. The surface sensible heat flux originating from open water was about 300 W m⁻² within a few kilometres from the coast and decreased with increasing accumulated fetch-width. The sensible heat flux was about 100 W m⁻² on average. These results indicate that the downwind air-mass modification depends mainly on the total (accumulated) extent of open water. The total buoyancy flux calculated by the joint frequency distribution method correlated very well with ice concentration. Such a relationship was not clear in the case of the moisture flux . The ratio between rising thermals and cold downdrafts differed significantly between upwind and downwind regions; that is, the buoyancy flux was dominated by in the developing stage of the boundary layer, while also became important after the development of the boundary layer.     

A THEORETICAL BASIS FOR THE PRIESTLEY-TAYLOR COEFFICIENT

The relationship between potential evaporation and arealevaporation is assessed using a closed-box model of the convectiveboundary layer (CBL). Potential evaporation is defined as theevaporation that would occur from a hypothetical saturated surface,with radiative properties similar to those of the whole area, and smallenough that the excess moisture flux does not modify thecharacteristics of the CBL. It is shown that the equilibrium rate ofpotential evaporation is given by Ep0=E0,where E0 is the equilibrium evaporation (radiative termof the Penman formula), and is a coefficient similar to thePriestley-Taylor coefficient. Its expression is , where is the areal surface resistance, r_a is the localaerodynamic resistance, and is the dimensionless slope of thesaturation specific humidity at the temperature of the air. Itscalculated value is around 1 for any saturated surface surrounded bywater, about 1.3 for saturated grass surrounded by well-watered grassand can be greater than 3 over saturated forest surrounded by forest.The formulation obtained provides a theoretical basis to the overallmean value of 1.26, empirically found by Priestley and Taylor for thecoefficient . Examining, at the light of this formulation, thecomplementary relationship between potential and actual evaporation(as proposed by Bouchet and Morton), it appears that the sum ofthese two magnitudes is not a constant at equilibrium, but depends onthe value of the areal surface resistance.