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1.
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.  相似文献   

2.
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.  相似文献   

3.
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  相似文献   

4.
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 andq) as well as the correlation coefficient betweenT and q (RTq). The second approximation involvesRTq 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 withRTq < 0.8) by replacing T/qwith the Bowen ratio.Consequently, estimating from using the Bowen ratiocan contain errors of 5–40% for || < 1. Adding RTqas 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.  相似文献   

5.
The applicability of the log-linear profile relationship over rough terrain to a height of 126 m is investigated. Simultaneous hourly averaged mean wind and temperature profiles measured at the Brookhaven meteorological tower during stable conditions are used in the analysis. The tower was surrounded by fairly homogeneous vegetation to a height of about 8 m. The results indicate that the log-linear profile relationship is valid at least for a height of 126 m for stabilities with Richardson numbers less than the critical value of 0.25. The mean value of in is found to be about 5.2 for these stabilities. The log-linear profile relation is found to be applicable for profiles observed beyond the critical stability; but the height of validity seems to decrease to about 100 m and the mean value of is about 1.6.Research performed under the auspices of the United States Energy Research and Development Administration (Contract E(30-1)-16).  相似文献   

6.
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, ra 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.  相似文献   

7.
In this study, profile functions for flux calculations during unstable conditions are proposed and examined. These functions are based on a direct interpolation for the dimensionless wind speed and temperature gradients between the standard Businger–Dyer formulae, , , and free convection forms, , being the Monin–Obukhov stability parameter. A previously presented interpolation between the corresponding profile relationships, in attempting to provide a general relationship for the whole unstable regime, leads to serious restrictions for the values of in the free convection forms. These restrictions rendered available experimental data almost inapplicable, since the behaviour of the formulae in the near-neutral range controls the values of those parameters. The proposed interpolation provides functions that, firstly, fit the standard Businger–Dyer forms for near-neutral conditions and, secondly, satisfy the asymptotic behaviour as , permitting wider ranges of possible values. This step is very important, taking into account the large spread of the experimental data. Thus, as further and more accurate observations at strong instability become available, this approach could prove very efficient in fitting these data while retaining correct near-neutral behaviour.  相似文献   

8.
Summary The efficiency of successive satellite-observed stages in the development of extratropical frontal cyclones for transporting eddy sensible heat at 850 mb on the Southern Hemisphere, is determined for the five winters (June through September) of 1973–1977. An inventory of cloud vortex data is allied with independent estimates of the zonally-averaged transient eddy heat flux, , using a statistical technique known aspath analysis. Path analysis is an improvement over single equation linear regression models since it controls for the temporally continuous nature of cyclone evolution. The results indicate that different cloud vortex types are associated with characteristic regimes of . The quite strong poleward flux of heat effected in the cyclogenetic stages weakens substantially by the onset of dissipation. These results are in general agreement with those for the summer and transition seasons derived using linear regression by Tucker (1979). However, a significant netequatorward flux of is found to be connected with the short-lived mature (spiral) cloud vortex type. In association with a southward displacement of the maximum latitude of cyclogenesis and a reduced crosslatitude motion of systems in winter, this apparently results in the strong lower-level heat flux convergence characteristic of the circumpolar trough at that time. A systematic error, by year, in the statistical model specification of heat transport related to cloud vortex type is found to be associated with phase of the Southern Oscillation (SO). Incorporating this effect into the model greatly improves the goodness-of-fit of the heat flux estimates and indicates that the efficiency of the mcridional eddy heat transport by cyclones is greatest (least) when the SOI is low (high). The implications of these results in the context of other, related studies are discussed.With 7 Figures  相似文献   

9.
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  相似文献   

10.
The study focuses on a way to parameterize the effect of subgrid scale convective motions on surface fluxes in large scale and regional models for the case of light surface winds. As previously proposed, these subgrid effects are assumed to scale with the convection intensity through the relationship: where is the mean velocity of the wind, U0 the velocity of the mean wind, w* the free convection velocity, and an empirical coefficient to be determined. Both observations and numerical simulation are presently used to determine the free convection coefficient .Large eddy simulation of a fair weather convective boundary layer case observed during TOGA-COARE is performed. Comparisons between observations and the simulation of surface properties and vertical profiles in the planetary boundary layer are presented. The simulated vertical turbulent fluxes of heat, moisture and buoyancy range well within estimates from aircraft measurements.The most important result is that the true free convection coefficient , directly estimated from simulation, leads to a value of 0.65, smaller than the ones estimated from temporal and spatial variances. Using observations and simulation, estimates of from temporal and spatial variances are obtained with similar values 0.8. From both theoretical derivations and numerical computations, it is shown that estimates of the true from variances are possible but only after applying a correction factor equal to 0.8. If this correction is not used, is overestimated by about 25%. The time and space sampling problem is also addressed in using numerical simulations.  相似文献   

11.
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.  相似文献   

12.
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.
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.
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.

Mit 16 Textabbildungen  相似文献   

13.
To investigate tubulence characteristics and organized motion within and above an urban canopy, field observations were conducted in July 1991 and November 1992 in Sapporo, Japan. The measurement heights were 5.4, 10.3, 18, 35 and 45 m above ground; the canopy height was 7 m. The profiles of u peaked slightly above the canopy, while v and w had nearly uniform profiles. Vertical profiles of Reynolds stress - peaked slightly at 1.5 times the canopy height and decreased slowly with height thereafter. A four-quadrant analysis showed that sweep and ejection motions caused high-velocity fluid from above moves downward toward the surface and low-velocity fluid from below moves upward. An ensemble-averaging technique was used to isolate typical features of the flow and temperature fields. A time-height cross-section of velocity vectors and temperature contours showed details of the flow structures associated with temperature ramps. It has been noted that the organized motions play important roles in the transport of heat near the urban canopy, where the sweep motion causes negative temperature fluctuations and the ejection motion causes positive temperature fluctuations.  相似文献   

14.
We describe one-dimensional (1D) simulations of the countergradient zone of mean potential temperature observed in the convective boundary layer (CBL). The method takes into account the third-order moments (TOMs) in a turbulent scheme of relatively low order, using the turbulent kinetic energy equation but without prognostic equations for other second-order moments. The countergradient term is formally linked to the third-order moments and , and a simple parameterization of these TOMs is proposed. It is validated for several cases of a dry CBL, using large-eddy simulations that have been realized from the MESO-NH model. The analysis of the simulations shows that TOMs are responsible for the inversion of the sign of in the higher part of the CBL, and budget analysis shows that the main terms responsible for turbulent fluxes and variances are now well reproduced.  相似文献   

15.
Summary The sums of negative daily means of temperature (centigrade degrees) for the winter season appear to be suitable criteria of the severity or the mildness of winters.K. Knoch published such temperature totals for the homogenized 180 year's temperature series for Berlin, Germany. By means of the standard deviation ()objective numerical limits are established between normal and abnormal winters. Wedefine: winters with a temperature total smaller than the arithmetic mean of the entire series, diminished by aremild; those with a temperature total aresevere. With these definitions a series of severe winters and one of mild winters are determined and investigated.The main result showed that frequencies, average numerical severity (or mildness), average time interval between abnormal winters, and therefore also contingent cyclic intervals depend, to a great extent, upon the magnitude of the usually arbitrarily fixed limits (between normal and abnormal). Theobjective limits, here used, do not offer a reason to surmise a cyclical recurrence of extreme winters — mild or severe — in the case of the Berlin temperature series.Avoiding any deformation of the actual series — as can be caused by intense smoothing procedures — a simple method revealed clearly the existence of asecular climatic change. During the first portion of the 180 years' period, severe winters were prevalent, while during the second portion mild ones prevailed.
Zusammenfassung Summen negativer Mittel der Tagestemperatur (°C) für die Wintersaison sind wohl geeignet zur Charakteristik der Strenge des Winters.K. Knoch publizierte solche Temperatursummen für die homogen gemachte 180 jährige Berliner Reihe. Mit Hilfe des mittleren, Fehlers werdenobjektive numerische Grenzen zwischen normalen und abnormalen Wintern festgelegt. Winter mit einer negativen Temperatursumme kleiner als das arithmetische Mittel, vermindert um den mittleren Fehler , werden alsmild, solche, bei denen die Temperatursumme den Betrag überstieg, alsstreng definiert. Die zwei so festgelegten Reihen,strenge Winter undmilde Winter wurden in verschiedener Weise untersucht. Als Hauptresultat ergab sich, daß Häufigkeiten, mittlere Strenge, (Milde) und mittleres Zeitintervall zwischen abnormalen Wintern in hohem Grade von der Höhe der üblicherweisewillkürlich angenommenen fixen Grenzen (zwischen normal und abnormal) abhängen. Die hier angewendetenobjektiven Grenzen bieten keinen Grund, bei der Berliner Reihe eine zyklische Wiederkehr einer der beiden extremen Wintertypen zu vermuten.Unter Vermeidung jedweder Verzerrung der Beobachtungsreihe, wie sie eventuell infolge intensiver Glättungsverfahren auftreten kann, führte eine einfache Methode zu dem klaren Bild einer säkularen Klimaänderung. Im ersten Teil der Beobachtungsperiode überwiegen unzweifelhaft die strengen Winter, im zweiten Teil die milden.
Résumé La rigueur de l'hiver peut se caractériser par la somme des températures moyennes journalières négatives (°C).K. Knoch a publié de telles sommes pour la séries rendue homogène de 180 ans à Berlin. A l'aide de l'erreur moyenne il est possible de fixer des limites numériques objectives entre hivers normaux et anormaux. Par définition un hiver est doux si la somme des températures est inférieure à la moyenne arithmétique diminuée de l'erreur moyenne ; il est rigoureux si cette somme dépasse la valuer . L'auteur étudie à divers points de vue les séries d'hivers doux et rigoureux ainsi définis et aboutit au résultat principal selon lequel les fréquences, la rigueur moyenne et l'intervalle, moyen de temps séparant des hivers anormaux dépendent largement des limites arbitraires adoptées en général entre le normal et l'anormal. Les limitesobjectives adoptées ici ne permettent pas de supposer que la série berlinoise présente un retour périodique de l'un ou l'autre des types extrêmes d'hivers.Es évitant de fausser d'une façon quelconque les séries d'observations comme cela peut se produire par un procédé d'adoucissement abusif, une méthode simple de l'auteur met en évidence une variation climatique séculaire. Dáns la première partie de la période d'observations les hivers rigoureux l'emportent indiscutablement; dans la seconde partie, ce sont les hivers doux.

With 1 Figure.  相似文献   

16.
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.  相似文献   

17.
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.  相似文献   

18.
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.  相似文献   

19.
The standard deviation of temperature T is proposed as a temperature scale and as a velocity scale to describe the behaviour of turbulent flows in the Atmospheric Surface Layer (ASL), instead of * andu * of the Monin—Obukhov similarity theory, and ofT f andU f used for free convection stability conditions. On the basis of experimental evidence reported in the literature, it is shown that T T f andv * U f in the free convection region, and T * andv * U * in nearneutral and stable conditions. This implies that the proposed scales can be applied for all stabilities. Furthermore, a new length scale is proposed and its relation with Obukhov length is given. Also, a simple semi-empirical expression is presented with which T andv * can be evaluated in a rather simple way. Some examples of practical applications are given, e.g., a stability classification for unstable conditions.  相似文献   

20.
Summary Rainfall in West Africa is examined in relation to monthly mean equivalent potential temperature ( e )at the earth's surface. The study revealed that monthly mean equivalent potential temperature ( e ) and monthly rainfall (R) generally decreased northwards from the equator.A good relationship existed betweenR and e in the northern zone of West Africa (i.e., north of 7.5° N). No definite relationship existed in the southern zone. In the northern zone, the departure of e from its annual mean ( ) first became positive about a month before the onset of the rains. Positive departures from ) generally resulted in more than normal (or average) rainfall in this zone. In general, little or no rainfall occurred in West Africa whenever e was less than 320 K.
Zusammenfassung Der Niederschlag (MonatssummeR) in Westafrika wird in Zusammenhang mit der mittleren monatlichen Äquivalent-temperatur ( e ) an der Erdoberfläche untersucht. Es zeigte sich, daß die Monatswerte beider Elemente im allgemeinen vom Äquator nach Norden abnehmen.ZwischenR und e ergab sich für das nördliche Westafrika (nördlich von 7.5° N) eine gute, für die südliche Zone jedoch keine beweisbare Übereinstimmung. In der nördlichen Zone übertraf e das Jahresmittel erstmals etwa einen Monat vor Beginn der Regenzeit. Positive Abweichungen vom mittleren e hatten immer übernormalen Niederschlag in dieser Zone zur Folge. Dagegen gab es wenig oder keinen Niederschlag in Westafrika, wenn e unter 320 K lag.

With 7 Figures  相似文献   

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