共查询到20条相似文献，搜索用时 31 毫秒
1.
S. S. Zilitinkevich 《Boundary-Layer Meteorology》1973,3(4):416-423
The paper discusses convection in the presence of wind shear, a condition analysed previously by Zilitinkevich (1971). This region (between the forced and fully developed convective layers) was also considered by Betchov and Yaglom (1971). In the present paper the author endeavors to develop a consistent analysis from the basic hypothesis of a very weak interaction between the vertical convective motions and mechanical turbulence, employing a new similarity model of the turbulent regime. Additional experimental data are introduced. Unlike the notation used in the references quoted above, this regime is termed shear convection rather than free convection. The latter is traditionally regarded as synonymous with the terms pure or fully developed convection. 相似文献
2.
Summary A study of the dependence features of the relative optical mass functions for air, water vapour, ozone and nitrogen dioxide on the apparent solar zenith angle was performed by calculating these optical parameters by means of the well-known computer code LOWTRAN 7 at several values of and for nine atmospheric models characterized by different latitudes and seasons. Moreover, other investigations were performed on the dependence features of (i) the relative optical air mass on the thermal characteristics of the low troposphere, (ii) the relative optical water vapour mass on the vertical distribution characteristics of absolute humidity in the troposphere, and (iii) the relative optical mass function for ozone and nitrogen dioxide on the shape characteristics of the vertical profiles of the two gaseous concentrations and the concentration peak altitudes. The results are compared with the values given by the two simple formulas proposed by Kasten (1966) for air and water vapour and the formulas defined by Young (1969) and Staehelin et al. (1995) for ozone and nitrogen dioxide. From this comparison, a wide set of correction factors were obtained which can be conveniently used in the analysis of multispectral sun-radiometric measurements for calculating, with a very high precision, the values of the four optical mass functions at all the angles in the 0° to 87° range, corresponding to the various latitudinal and seasonal conditions described by the nine atmospheric models.With 9 Figures 相似文献
3.
Dr. Ch. H. Schraff 《Meteorology and Atmospheric Physics》1997,64(1-2):21-50
Summary A number of problems related to mesoscale numerical prediction of low stratus in the Alpine region are formulated, and addressed in a series of experiments for two wintertime cases. These problems include modelling aspects and issues of data assimilation which are relevant particularly in relation to the observation nudging technique. A focus is on the influence of orography.A comparison of operational optimum interpolation, and nudging of routine rawinsonde and surface-level data reveals that nudging often yields better analyses and forecasts of low stratus, and notably of the sharp vertical temperature and humidity gradients. However, the humidity advection scheme of the model and, near steep terrain, particularly the horizontal diffusion along the model's -levels are identified to contribute to spurious vertical smoothing which can result in erroneous cloud dissipation. On occasions, forecasts succeeding a nudging period are more sensitive to this process due to the sharper initial vertical gradients.Specific problems of representiveness arise when low-level rawinsonde information is spread laterally along the sloping -levels from low to high terrain. A new concept for -layer models is introduced by speading the observational information along isentropic surfaces, and this tends to improve the low stratus prediction over steep and even moderate orography. A partly successful attempt to take advantage of the steep Alpine orography is made by applying this concept to surface-level humidity data from a high-resolution network of Alpine surface stations which are distributed relatively uniformly in the vertical.With 19 Figures 相似文献
4.
During spring and autumn, many lakes in temperate latitudes experience intensive convective mixing in the vertical, which leads to almost isothermal conditions with depth. Thus the regime of turbulence appears to be similar with that characteristic of convective boundary layers in the atmosphere. In the present paper a simple analytical approach, based on boundary-layer theory, is applied to convective conditions in lakes. The aims of the paper are firstly to analyze in detail the temperature distribution during these periods, and secondly to investigate the current system, created by the horizontal temperature gradient and wind action. For these purposes, simple analytical solutions for the current velocities are derived under the assumption of depth-constant temperatures. The density-induced current velocities are shown to be small, in the order of a few mm/sec. The analytical model of wind-driven currents is compared with field data. The solution is in good qualitative agreement with observed current velocities under the condition that the wind field is steady for a relatively long time and that residual effects from former wind events are negligible.The effect of the current system on an approximately depth-constant temperature distribution is then checked by using the obtained current velocity fields in the heat transfer equation and deriving an analytical solution for the corrected temperature field. These temperature corrections are shown to be small, which indicates that it is reasonable to describe the temperature distribution with vertical isotherms.Notation
T
temperature
-
t
time
-
x, y, z
cartesian coordinates
-
molecular viscosity
-
h
,
v
horizontal and vertical turbulent viscosity
-
K
h
,K
v
horizontal and vertical turbulent conductivity
-
Q
heat flux through the water surface
-
D
depth
-
u, v, w
average current velocity components inx, y andz directions
-
f
Coriolis parameter
-
p
pressure
-
density
-
g
gravity acceleration
-
a
constant in the freshwater state equation
-
h
s
deviation from the average water surface elevation
-
L
*,H
*
length and depth scale
-
U
*,W
*
horizontal and vertical velocity scale
- T
temperature difference scale
-
bottom slope
-
u
*
friction velocity at the water surface
-
von Karman constant
-
L
Monin-Obukhov length scale
-
buoyancy parameter
-
l
turbulence length scale
-
C
1,C
2,C
3
dimensionless constants in the expressions for the vertical turbulent viscosity
- ,
dimensionless vertical coordinate and dimensionless local depth
-
angle between surface stress direction andx-axis
-
T
bx
,T
by
bottom stress components
-
c
bottom drag coefficient 相似文献
5.
J. R. Salmon A. J. Bowen A. M. Hoff R. Johnson R. E. Mickle P. A. Taylor G. Tetzlaff J. L. Walmsley 《Boundary-Layer Meteorology》1988,43(3):247-271
This is one of a series of papers on the Askervein Hill Project. It presents results on the variations in mean wind speed at fixed heights (z) above the ground from linear arrays of anemometer posts and towers. Most of the data are for z = 10 m but some are for z = 3 m. Selected and directionally grouped data from the 55 Mean Flow runs are presented together with mean flow data from Askervein '83 Turbulence runs. Comparisons are made between the data and guideline estimates of fractional speed-up ratio at hilltop locations and between the data and MS3DJH/3 model predictions along the tower lines. There is good agreement in most cases. 相似文献
6.
Richard S. Lindzen 《Boundary-Layer Meteorology》1973,4(1-4):327-343
The nature of internal gravity waves is described with special emphasis on their ability to transport energy and momentum. The conditions under which these fluxes interact with the mean state of the atmosphere are described and the results are applied to various problems of the upper atmosphere, including the quasi-biennial oscillation, the heat budget of the thermosphere, the general circulation of the mesosphere, turbulence in the mesosphere, and even the 4-day circulation of the Venusian stratosphere.Gravity waves have long been recognized as an important element in the dynamics of the upper atmosphere - largely due to their very large amplitudes in this region. It is the implied purpose of this paper to convey some of the thinking that has gone on about the role of gravity waves in the large-scale circulation of the upper atmosphere. Insofar as gravity waves are now being identified as important entities in the lower atmosphere as well, it is possible that mechanisms identified in the upper atmosphere will prove relevant to the troposphere.Alfred P. Sloan Foundation Fellow. Now at Harvard University. 相似文献
7.
Ferenc Ács 《Boundary-Layer Meteorology》2003,109(2):139-162
Transpiration Ev and bare soil evaporation Eb processes are comparatively analysed assuming homogeneous and inhomogeneous areal distributions of volumetric soil moisture content . For a homogeneous areal distribution of we use a deterministic model, while for inhomogeneous distributions a statistical-deterministic diagnostic surface energy balance model is applied. The areal variations of are simulated by Monte-Carlo runs assuming normal distributions of .The numerical experiments are performed for loam. In the experiments we used different parameterizations for vegetation and bare soil surface resistances and strong atmospheric forcing. According to the results theEv()-Eb() differences are great, especially in dry conditions. In spite of this, the available energy flux curves of vegetation Av() and bare soil Ab() surfaces differ much less than the Ev() and Eb() curves. The results suggest that Ev is much more non-linearly related to environmental conditions than Eb. Both Ev and Eb depend on the distribution of , the wetness regime and the parameterization used. With the parameterizations, Eb showed greater variations than Ev. These results are valid when there are no advective effects or mesoscale circulation patterns and the stratification is unstable. 相似文献
8.
D. K. Pissimanis V. A. Notaridou P. S. Viglas 《Theoretical and Applied Climatology》1994,49(3):121-128
Summary The integral aerosol optical depths (k
) at the hour of 08:20 Local Standard Time (LST), are compared with those calculated previously at 11:20 and 14:20 LST, for clear days during summer in Athens over the period 1962–1988. The mean values at 08:20 LST were consistently lower than the values at 11:20 and 14:20 LST. The influence of the vertical wind profile on the values ofk
was also investigated. A comparison was made of the wind profiles at 02:00 and 14:00 LST, for days in which the 11:20 and 14:20 LST values ofk
were 0.200 andk
0.350, respectively. The corresponding bulk wind shear s was also found for the period 1980–1988. The most significant results occurred with the first category of days. The resultant wind velocities from the surface to the 900 hPa level, in each hour were higher by 2–4 m·s–1 with respect to the corresponding values for the second category. At 02:00 LST the bulk wind shear showed a considerable difference (1.8) between the two categories of days in the surface to 700 hPa layer at 02:00 LST. Finally, the associated weather conditions that appear to initiate a period of low values ofk
(k
0.200) at 11:20 and 14:20 LST were examined for the period 1980–1988. Fifteen such cases were identified and it was found that they all occurred after the passage of weak cold fronts.With 6 Figures 相似文献
9.
The system transfer function ¦H(v)¦2 at frequencyv (units of Hz) for a vertical velocity propeller anemometer in a statistically stationary and horizontally homogeneous turbulent flow is determined from: (1) experimental estimates of propeller velocity spectra; and (2) estimates of Eulerian vertical velocity spectra based on the hypothesis that degradation of the input vertical velocity Fourier components occurs in the inertial subrange. The experimental estimates of ¦H(v)¦2 were adequately summarized with the mathematical expression for the system transfer function of a first-order system with parameterT which has units of time and is analogous to the time constant of a horizontal velocity propeller anemometer. Dimensional analysis techniques and the Monin-Obukhov similarity hypothesis were used to construct a model for the system parameterT which yielded the result that
w
/D
1 (
w
/)1/3, where
w
, andD
1 denote the standard deviation of the input vertical velocity fluctuations, the horizontal mean wind speed, and the diameter of the propeller, respectively. The system parameterT is interpreted in terms of the time required for the propeller velocity statistics to become asymptotically independent of time upon being released from rest in a statistically stationary turbulent flow.Currently on leave of absence from the Indian Institute of Technology, New Delhi, India. 相似文献
10.
Beljaars et al. (1987) developed a model for neutrally stratified boundary-layer flow over roughness changes and topography. It has been discovered that a constant parameter, , was missing in the algebraic-stress closure equations of their paper. This omission also occurred in the MSFD model code but only affects the Askervein Hill shear-stress results for the E-- turbulence closure in Beljaars et al. It also changes the stress results of Karpik (1988), but not his conclusions regarding the robustness of his improved numerical scheme. The present paper demonstrates the effect of the omission of the parameter, , and tests the sensitivity of the model to variations in its value. The new results are compared with the data and model results of Zeman and Jensen (1987). 相似文献
11.
In the summer of 1988/89 flights were carried out in the Coorong coastal area of South Australia to investigate sea-breeze fronts. The flights yielded data sets of the structure of the fronts in the cross-frontal direction with a spatial resolution of approximately 3 m. The study is focused on the budgets of sensible and latent heat in the vicinity of the front and on frontogenesis/frontolysis processes which are closely related to budget considerations.The frontogenesis relationships and the budgets were established on a 2 km length scale by low-pass filtering of the space series. As the wind components were measured with high accuracy, all processes which determine frontogenesis could be evaluated and are displayed in x,z-cross-sections: these are the confluence, shear and diabatic effects, all of which play a role in q/x-, q/z-, /x- as well as /z-frontogenesis. A detailed analysis is given for two different states of frontal development. The presented results shed much light on the governing physical processes in the frontal region with strong emphasis on the effects of confluence-generated updrafts, on shear instabilities causing bulges and clefts in the frontal surface as well as producing the elevated frontal head, and on processes related to differential heating and moistening. 相似文献
12.
Le Ngoc Ly 《Boundary-Layer Meteorology》1991,54(4):327-346
An E- turbulence model is used to study air-sea interaction characteristics and turbulence structure using a coupled model for air-sea boundary layers. The E- turbulence model consists of equations for the turbulent kinetic energy, the energy-dissipation, and for the turbulent exchange coefficient expressed in terms of turbulent kinetic energy and energy-dissipation. The energy-dissipation equations for the air-sea interface are solved analytically to obtain boundary conditions for energy-dissipation at the interface. The air-sea interaction and turbulence characteristics of the E- model are compared with those of the mixing-length model and with available observations.The simulations demonstrate that the air-sea interaction parameters obtained by the E- model agree well with observations. The numerical studies also show that the E- turbulence model with appropriate constants can give good results in modeling coupled air-sea boundary-layer flows. 相似文献
13.
This paper considers the ground area which affects the properties of fluid parcels observed at a given spot in the Planetary Boundary Layer (PBL). We examine two source-area functions; the footprint, giving the source area for a measurement of vertical flux: and the distribution of contact distance, the distance since a particle observed aloft last made contact with the surface. We explain why the distribution of contact distance extends vastly farther upwind than the footprint, and suggest for the extent of the footprint the inequalities: % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqaqpepeea0xe9qqVa0l% b9peea0lb9Lq-JfrVkFHe9peea0dXdarVe0Fb9pgea0xa9W8qr0-vr% 0-viWZqaceaabiGaciaacaqabeaadaqaaqaaaOabaeqabaGaamyvam% aalaaabaGaamiAaaqaaiabeo8aZnaaBaaaleaacaWGxbaabeaakiaa% cIcacaWGObGaaiykaaaacqGH8aapcaWG4bGaeyipaWJaamyvaKazaa% iadaGabaqaamaaDaaajqwaacqaaiaadIgacaGGVaGabmOEayaacaGa% aiilaiaabccacaGGVbGaaiiDaiaacIgacaGGLbGaaiOCaiaacEhaca% GGPbGaai4CaiaacwgaaeaacaWGubWaaSbaaKazcaiabaGaamitaaqa% baqcKfaGaiaacIcacaWGObGaaiykaiaabYcacaqGGaGaaeiAaiaabc% cacaGGHbGaaiOyaiaac+gacaGG2bGaaiyzaiaabccacaGGZbGaaiyD% aiaackhacaGGMbGaaiyyaiaacogacaGGLbGaeyOeI0IaaiiBaiaacg% gacaGG5bGaaiyzaiaackhaaaaajqgaacGaay5EaaaakeaaaeaacaGG% 8bGaamyEaiaacYhacqGH8aapcqaHdpWCdaWgaaWcbaGaamODaaqaba% GccaGGOaGaamiAaiaacMcadaWcaaqaaiaadIhaaeaacaWGvbaaaaaa% aa!7877!\[\begin{array}{l} U\frac{h}{{\sigma _W (h)}} < x < U\left\{ {_{h/\dot z,{\rm{ }}otherwise}^{T_L (h){\rm{, h }}above{\rm{ }}surface - layer} } \right. \\ \\ |y| < \sigma _v (h)\frac{x}{U} \\ \end{array}\] where U is the mean streamwise (x) velocity, h is the observation height, L is the Lagrangian timescale,
v
and
w
are the standard deviations of the cross-stream horizontal (y) and vertical (z) velocity fluctuations, and is the Lagrangian Similarity prediction for the rate of rise of the centre of gravity of a puff released at ground.Simple analytical solutions for the contact-time and the footprint are derived, by treating the PBL as consisting of two sub-layers. The contact-time solutions agree very well with the predictions of a Lagrangian stochastic model, which we adopt in the absence of measurements as our best estimate of reality, but the footprint solution offers no improvement over the above inequality. 相似文献
14.
We have studied the evolution of the planetary boundary layer using both oceanic and continental stations. This has been possible through the analysis of twice-daily temperature radiosondes at three different oceanic stations (Point A; Point K; and Azores) and also at three continental stations (Berlin, Trappes and Madrid) all situated in the Northern Hemisphere. We have studied the annual evolution of mixing layers and of temperature inversion levels; an annual evolution presenting a minimum in winter and a maximum in summer has been observed in the continental stations; the oceanic stations present a lesser variation and in the opposite sense. As for the elevated inversion layers, their maximum frequency can be observed in summer at the oceanic stations and in winter at the continental stations. We have shown that considering the studied stations, both the level and the frequency of the elevated inversion layers are similar at 00 h and at 12 h; such a result is important because these layers regulate the intensity of the exchanges between the ground and the free atmosphere. Using the equivalent coefficient, we have determined the annual variation of the vertical exchanges between the surface and different altitudes. At the continental stations, the vertical exchanges are more important in summer than they are in winter; the opposite behaviour occurs at the oceanic stations. 相似文献
15.
R. A. Antonia 《Boundary-Layer Meteorology》1977,12(3):351-364
Horizontal u and vertical w velocity fluctuations have been measured together with temperature fluctuations in the atmospheric surface layer, at a small height above a wheat crop canopy. Marginal probability density functions are presented for both individual fluctuations u, w, and for the instantaneous Reynolds stress uw, and heat fluxes w and u. Probability density functions of the velocity fluctuations deviate less significantly from the Gaussian form than the probability density of temperature. There appears to be closer similarity between statistics of the instantaneous heat fluxes than between the momentum flux and either of the heat fluxes investigated. The mean momentum flux receives equal contributions from the events referred to as ejections and sweeps in laboratory boundary layers. Sweeps provide the largest contribution to the heat fluxes. 相似文献
16.
Turbulence measurements from the city of Uppsala, Sweden, are analysed. Measurements were taken at two sites: one in the central area, ca. 6 m above roof level, the average building height being ca. 15 m; the other at ca. 8 and 50 m above the ground on a tower situated 100 m downwind of a sharp discontinuity between the densely built-up urban area and flat grass-covered land. The average stability was close to neutral, the range being -0.2 < z/L < 0.2. The main emphasis of the study is on the non-dimensional standard deviations of the velocity components
i
/u
*t
and on the corresponding non-dimensional energy spectra, u
*t
being a local velocity scale defined as
i
/(
l
is the local momentum flux). Comparison with results obtained from surface-layer measurements at ideal sites (with u
*, being the ordinary friction velocity) shows good general agreement. The most complete agreement is found for the tower 50 m measurements, a result which is notable as this measurement point is found to be within a distinctly transitional zone between the urban and post-urban boundary layers. The results from the central city measurement point are also fairly close to the ideal results, the deviations found being small in view of the fact that the site is probably inside the layer in which the roughness elements (the buildings) have direct influence. The measurements at the tower 8 m level show certain distinct deviations from ideal results: all three
i
/u
*l
, are higher by ca. 10%, the excessive energy being found at the low frequency end of the spectrum. Arguments are presented for this feature to be due to a spectral lag effect. 相似文献
17.
Field measurements of snow-drift threshold and mass fluxes,and related model simulations 总被引：2，自引：2，他引：0
Field measurements were carried out to calculate the threshold friction velocity for snow saltation, and mass fluxes during snow drift. The wind was measured in three components by an ultrasonic anemometer, and the mass fluxes were determined using an optical sensor (snow particle counter), acoustic sensors (Flowcapt) and mechanical traps. The threshold friction velocity was found to be correlated to the grain size (R2=0.75). The mass flux measurements were compared with numerical simulations of snow drift, and it was demonstrated that the maximum snow transport takes place at shear stress values of roughly two times the average shear stress over 20 min. By implementing a probability distribution for the shear stress the mass flux was simulated with only the mean measured value of the shear stress as input. This procedure enables the future use of the numerical model for operational applications. 相似文献
18.
A coupled ocean-atmosphere climate model: temperature versus salinity effects on the thermohaline circulation 总被引：2，自引：0，他引：2
G Edward Birchfield 《Climate Dynamics》1989,4(1):57-71
A simple nonlinear three-box ocean model of the North Atlantic Ocean including the rudiments of eddy mixing, vertical stratification and thermohaline circulation is first presented. It is subject to uniform latitudinal differential heating, q, and net evaporation m
e
, and includes a linear equation of state. Two quite different limiting steady-state solutions exist. The first has a warm saline surface water and a cold, low-salinity deep ocean; deep water is primarily formed in higher latitudes by the prevalence of differential heating. A second limiting solution consists of a warm saline deep ocean underlying a cool, low-salinity surface ocean; deep water is formed primarily in lower latitudes as a consequence of large differential evaporation. A coupled ocean-atmosphere model, in which the oceanic surface heat fluxes are determined internally but with differential evaporation at the ocean surface m
e
remaining an external parameter, is next presented. The atmosphere component is a simple energy balance model that emphasizes the vertical fluxes of radiative, sensible and latent heat fluxes but does not include temperature-albedo feedback. Model response depends on the external parameters m
e
and , controlling the magnitude of the thermohaline-driven circulation, and on the magnitudes of the eddy mixing coefficients and the solar constant. For small m
e
, a steady-state solution corresponding to a cold fresh deep ocean is found, qualitatively similar to the modern ocean. For large m
e
, a steady-state solution with a warm saline deep ocean occurs; this solution resembles conceptual models that have been proposed for the warm saline Cretaceous ocean. There exists an intermediate region of values of m
e
for which the solutions are more complex. On the lower end of this region, both the cold fresh deep-ocean and warm saline deep-ocean circulations coexist as stable equilibria. On the upper end, the cold-deep ocean becomes unstable, manifesting oscillations with growing amplitude, and ultimately reaches the warm saline deep-ocean solution. In the neighborhood of a cusp on the , m
e
plane, that is, for relatively small , more complex behaviour occurs, which has not yet been fully analyzed. The model response in the region of complexity is not sensitive to changes in the solar constant but is sensitive to the eddy mixing coefficients. 相似文献
19.
The nocturnal structure of the lower troposphere is studied using aerosol profile data (50–2800 m AGL) obtained with a bistatic, continuous wave, Argon ion lidar system during October 1986–September 1989 at Pune (18°32 N, 73°51 E, 559m AMSL), India. The top of the nocturnal groundbased inversion is taken as the height above ground where the negative vertical gradient in aerosol concentration first reaches a maximum. During the post-sunset period over this station, this height is as low as 160m and frequently lies around 550m. Greater heights are observed in pre-monsoon months and smaller ones during the southwest monsoon season. Positive vertical gradients in aerosol concentration, indicative of stable/elevated layers, appear frequently around 750m. Temporal variations of aerosol concentration gradients in two adjacent air layers, 920–1000m and 100–1100m, provide evidence that stability increases downward in the early night hours. 相似文献
20.
Prof. S. Hastenrath 《Meteorology and Atmospheric Physics》1997,64(1-2):51-59
Summary During an expedition to the high Andes of Southern Peru in June–July 1977, measurements of direct solar radiation in four spectral bands (0.270–0.530–0.630–0.695–2.900 ) were conducted at six sites in elevations ranging from sea level to 5645 m. These measurements were evaluated in Langley plots to determine total optical depths () and irradiances at the top of the atmosphere. In addition, water vapor optical depths (wv) were calculated from the mean radiosounding over Lima during the expedition, and Rayleigh (ray) and ozone (oz) optical depths were obtained from published tabulations. Subtracting ray, oz, and wv from yielded estimates of aerosol optical depth aer. The components ray and oz decrease from the shorter towards the longer wavelength bands and from the lower towards the higher elevation sites; aer also decreases towards the higher elevations. Particularly pronounced is the decrease of aer and from the lowlands of the Pacific coast to the highlands of the interior, reflecting the effect of a persistent lower-tropospheric inversion and the contrast from the marine boundary layer to the clear atmosphere of the high Andes.With 4 Figures 相似文献