共查询到20条相似文献,搜索用时 31 毫秒
1.
Weiguo Wang Kenneth J. Davis Chuixiang Yi Edward G. Patton Martha P. Butler Daniel M. Ricciuto Peter S. Bakwin 《Boundary-Layer Meteorology》2007,124(2):305-314
The dimensionless bottom-up and top-down gradient functions in the convective boundary layer (CBL) are evaluated utilizing
long-term well-calibrated carbon dioxide mixing ratio and flux measurements from multiple levels of a 447-m tall tower over
a forested area in northern Wisconsin, USA. The estimated bottom-up and top-down functions are qualitatively consistent with
those from large-eddy simulation (LES) results and theoretical expectations. Newly fitted gradient functions are proposed
based on observations for this forested site. The integrated bottom-up function over the lowest 4% of the CBL depth estimated
from the tower data is about five times larger than that from LES results for a ‘with-canopy’ case, and is smaller than that
from LES results for a ‘no-canopy’ case by a factor of 0.7. We discuss the uncertainty in the evaluated gradient functions
due to stability, wind direction, and uncertainty in the entrainment flux and show that while all of these have a significant
impact on the gradient functions, none can explain the differences between the modelled and observed functions. The effects
of canopy features and atmospheric stability may need to be considered in the gradient function relations. 相似文献
2.
Sergej Zilitinkevich 《Boundary-Layer Meteorology》1994,70(1-2):51-78
During the last two decades, different scalings for convective boundary layer (CBL) turbulence have been proposed. For the shear-free regime, Deardorff (1970) introduced convective velocity and temperature scales based on the surface potential temperature flux,Q
s
, the buoyancy parameter, , and the time-dependent boundary-layer depth,h. Wyngaard (1983) has proposed decomposition of turbulence into two components, bottom-up (b) and top-down (t), the former characterized byQ
s
, the latter, by the potential temperature flux due to entrainment,Q
h
. Sorbjan (1988) has devised height-dependent velocity and temperature scales for both b- and t-components of turbulence.Incorporating velocity shear, the well known similarity theory of Monin and Obukhov (1954) has been developed for the atmospheric surface layer. Zilitinkevich (1971, 1973) and Betchov and Yaglom (1971) have elaborated this theory with the aid of directional dimensional analysis for a particular case when different statistical moments of turbulence can be alternatively attributed as being of either convective or mechanical origin.In the present paper, we attempt to create a bridge between the two approaches pointed out above. A new scaling is proposed on the basis of, first, decomposition of statistical moments of turbulence into convective (c), mechanical (m) and covariance (c&m) contributions using directional dimensional analysis and, second, decomposition of these contributions into bottom-up and top-down components using height-dependent velocity and temperature scales. In addition to the statistical problem, the scaling suggests a new approach of determination of mean temperature and velocity profiles with the aid of the budget equations for the mean square fluctuations.Notation ATL
alternative turbulence layer
- CBL
convective boundary layer
- CML
convective and mechanical layer
- FCL
free convection layer
- MTL
mechanical turbulence layer 相似文献
3.
Flux Footprints in the Convective Boundary Layer: Large-Eddy Simulation and Lagrangian Stochastic Modelling 总被引:1,自引:1,他引:0
We investigated the flux footprints of receptors at different heights in the convective boundary layer (CBL). The footprints
were derived using a forward Lagrangian stochastic (LS) method coupled with the turbulent fields from a large-eddy simulation
model. Crosswind-integrated flux footprints shown as a function of upstream distances and sensor heights in the CBL were derived
and compared using two LS particle simulation methods: an instantaneous area release and a crosswind linear continuous release.
We found that for almost all sensor heights in the CBL, a major positive flux footprint zone was located close to the sensor
upstream, while a weak negative footprint zone was located further upstream, with the transition band in non-dimensional upwind
distances −X between approximately 1.5 and 2.0. Two-dimensional (2D) flux footprints for a point sensor were also simulated. For a sensor
height of 0.158 z
i, where z
i is the CBL depth, we found that a major positive flux footprint zone followed a weak negative zone in the upstream direction.
Two even weaker positive zones were also present on either side of the footprint axis, where the latter was rotated slightly
from the geostrophic wind direction. Using CBL scaling, the 2D footprint result was normalized to show the source areas and
was applied to real parameters obtained using aircraft-based measurements. With a mean wind speed in the CBL of U = 5.1 m s−1, convective velocity of w
* = 1.37 m s−1, CBL depth of z
i = 1,000 m, and flight track height of 159 m above the surface, the total flux footprint contribution zone was estimated to
range from about 0.1 to 4.5 km upstream, in the case where the wind was perpendicular to the flight track. When the wind was
parallel to the flight track, the total footprint contribution zone covered approximately 0.5 km on one side and 0.8 km on
the other side of the flight track. 相似文献
4.
The Role of Shear in the Morning Transition Boundary Layer 总被引:1,自引:1,他引:0
Robert J. Beare 《Boundary-Layer Meteorology》2008,129(3):395-410
We use large-eddy simulation (LES) to better define the early stages of the morning transition boundary layer. Previous LES
studies relating to the morning transition boundary layer focus on the role of the entraining convective boundary layer (CBL).
By using a combination of different domain sizes and grid lengths, the full evolution from the stable boundary layer (SBL)
to the CBL is modelled here. In the early stages of the morning transition the boundary layer is shown to be a combination
of a shallow mixed layer capped by a significant shear driven stable boundary layer (the so-called mixed CBL–SBL state). The
mixed CBL–SBL state is the key to understanding the sensitivity to shear. Turbulent kinetic energy budgets also indicate that
it is shear driven. The negative flux from the mixed CBL–SBL state extends much further above the minimum than is typically
found for the CBL later in the day, and the depth of penetration scales as w
m
/N
i
, where w
m
is the combined friction and convective velocity scale and N
i
the static stability at the inversion top. 相似文献
5.
M. G. Hadfield 《Boundary-Layer Meteorology》1994,69(4):417-448
We examine vertical and horizontal diffusion of a passive scalar puff from a surface point source in a convective boundary layer (CBL). Numerical results are presented from a large-eddy simulation (LES) with embedded subgrid Lagrangian particle simulation (LPS). There is good agreement in most respects with previous laboratory and numerical studies. Analytical approximations for the concentration, horizontal flux and vertical flux are found to work reasonably well; they are based on the assumption that the concentration follows a Gaussian function in the horizontal and vertical, and that the dimensionless width and height scales of the puff follow simple functions of time. Fluxes and concentration gradients are related through a continuity relationship, without the need for an eddy diffusivity assumption. The instantaneous, point-source fields can be integrated for any source geometry. We compare predictions from the LES/LPS model for a sinusoidal surface flux with previous results from an LES with sinusoidal buoyancy flux and confirm that the buoyancy perturbations diffuse like a passive scalar. We also consider a continuous point source and derive footprint functions for vertical flux measurements above the surface layer. 相似文献
6.
Regional-Scale Heat and Water Vapour Fluxes in an Agricultural Landscape: An Evaluation of CBL Budget Methods at OASIS 总被引:1,自引:0,他引:1
Helen A. Cleugh Michael R. Raupach Peter R. Briggs Peter A. Coppin 《Boundary-Layer Meteorology》2004,110(1):99-137
This paper evaluates convective boundary layer (CBL) budget methods as a tool for estimating regionally averaged sensible and latent heat fluxes for the study region used in OASIS (Observations at Several Interacting Scales). This is an agricultural region of mixed cropping and grazing extending about 100 km west of the town of Wagga Wagga, NSW, Australia.The analysis proceeds in three stages: first, a simpleone-dimensional model of the well-mixed layer (the CBL slab model), forced with measurements of the surface heat and evaporation fluxes, is evaluated by comparing measured and modelled CBL temperature, humidity and depths. A comparison of several entrainment schemes shows that a simple model, where the entrainment kinetic energy is parameterised as a fraction (3) of the surface sensible heat flux, works well if is set to 0.5. Second, the slab model is coupled to a Penman–Monteith model of surface evaporation to predict regional scale evaporation and thence heat fluxes. Finally, the integral CBL budget approach, which is an inverse method using theone-dimensional slab model, is used to infer regional heat and evaporation fluxes from measured time series of CBL temperature and humidity.We find that the simple CBL slab model works reasonably well for predicting CBL depth and very well for CBL temperature, especially if approximate estimates of subsidence velocity and warming due to advection are included. Regional sensible heat fluxes estimated from the integral CBL method match those measured, although the method is very sensitive to measurement errors. Measurement-model differences were larger for short integration times, because the well-mixed assumptions are violated at particular times of the day. The corollary is that `whole-day' (0530–1530 h) estimates are in reasonable agreement with measured values. Integral methods could not be used to infer the regional evaporation flux directly because CBL humidity profiles were complex and often not well mixed until mid-afternoon. We recommend that regional evaporation fluxes be predicted either from a coupled Penman–Monteith – CBL slab model, or inferred as a residual term from estimates of the regionally averaged available energy and sensible heat flux. Furthermore, we show that inferring fluxes via integral methods will always be difficult when the scalar concentrations have either a large surface source and free atmosphere sink (in the case of water vapour and methane), or a large surface sink and upper level source (in the case of CO2). 相似文献
7.
Verifying Inventory Predictions of Animal Methane Emissions with Meteorological Measurements 总被引:1,自引:0,他引:1
O. T. Denmead R. Leuning D. W. T. Griffith I. M. Jamie M. B. Esler L. A. Harper J. R. Freney 《Boundary-Layer Meteorology》2000,96(1-2):187-209
The paper examines the strengths and weaknesses of a rangeof meteorological flux measurement techniques that mightbe used to verify predictions of greenhouse gas inventories.Recent research into emissions of methane (CH4)produced by enteric fermentation in grazing cattle and sheepis used to illustrate various methodologies. Quantifying thisimportant source presents special difficulties because the animalsconstitute moving, heterogeneously distributed, intermittent, pointsources. There are two general approaches: one, from the bottom up,involves direct measurements of emissions from a known number ofanimals, and the other, from the top down, infers areal emissions ofCH4 from its atmospheric signature. A mass-balance methodproved successful for bottom-up verification. It permits undisturbedgrazing, has a simple theoretical basis and is appropriate for fluxmeasurements on small plots and where there are scattered pointsources. The top-down methodologies include conventional flux-gradientapproaches and convective and nocturnal boundary-layer (CBL and NBL)budgeting schemes. Particular attention is given to CBL budget methods inboth differential and integral form. All top-down methodologies require ideal weather conditions for their application, and they suffer from the scattered nature of the source, varying wind directions and low instrument resolution. As for mass-balance, flux-gradient micrometeorological measurements were in good agreement with inventory predictions of CH4 production by livestock, but the standard errors associated with both methods were too large to permit detection of changes of a few per cent in emission rate, which might be important for inventory, regulatory or research purposes. Fluxes calculated by CBL and NBL methods were of the same order of magnitude as inventory predictions, but more improvement is needed before their use can be endorsed. Opportunities for improving the precision of both bottom-up and top-down methodologies are discussed. 相似文献
8.
Large-eddy simulations (LESs) are employed to investigate the turbulence characteristics in the shear-free convective boundary
layer (CBL) driven by heterogeneous surface heating. The patterns of surface heating are arranged as a chessboard with two
different surface heat fluxes in the neighbouring patches, and the heterogeneity scale Λ in four different cases is taken as 1.2, 2.5, 5.0 and 10.0 km, respectively. The results are compared with those for the
homogeneous case. The impact of the heterogeneity scale on the domain-averaged CBL characteristics, such as the profiles of
the potential temperature and the heat flux, is not significant. However, different turbulence characteristics are induced
by different heterogeneous surface heating. The greatest turbulent kinetic energy (TKE) is produced in the case with the largest
heterogeneity scale, whilst the TKE in the other heterogeneous cases is close to that for the homogeneous case. This result
indicates that the TKE is not enhanced unless the scale of the heterogeneous surface heating is large enough. The potential
temperature variance is enhanced more significantly by a larger surface heterogeneity scale. But this effect diminishes with
increasing CBL height, which implies that the turbulent eddy structures are changed during the CBL development. Analyses show
that there are two types of organized turbulent eddies: one relates to the thermal circulations induced by the heterogeneous
surface heating, whilst the other identifies with the inherent turbulent eddies (large eddies) induced by the free convection.
At the early stage of the CBL development, the dominant scale of the organized turbulent eddies is controlled by the scale
of the surface heterogeneity. With time increasing, the original pattern breaks up, and the vertical velocity eventually displays
horizontal structures similar to those for the homogeneous heating case. It is found that after this transition, the values
of λ/z
i
(λ is the dominant horizontal scale of the turbulent eddies, z
i
is the boundary-layer height) ≈1.6, which is just the aspect ratio of large eddies in the CBL. 相似文献
9.
Volker Wulfmeyer Sandip Pal David D. Turner Erin Wagner 《Boundary-Layer Meteorology》2010,136(2):253-284
High-resolution water vapour measurements made by the Atmospheric Radiation Measurement (ARM) Raman lidar operated at the
Southern Great Plains Climate Research Facility site near Lamont, Oklahoma, U.S.A. are presented. Using a 2-h measurement
period for the convective boundary layer (CBL) on 13 September 2005, with temporal and spatial resolutions of 10 s and 75 m,
respectively, spectral and autocovariance analyses of water vapour mixing ratio time series are performed. It is demonstrated
that the major part of the inertial subrange was detected and that the integral scale was significantly larger than the time
resolution. Consequently, the major part of the turbulent fluctuations was resolved. Different methods to retrieve noise error
profiles yield consistent results and compare well with noise profiles estimated using Poisson statistics of the Raman lidar
signals. Integral scale, mixing-ratio variance, skewness, and kurtosis profiles were determined including error bars with
respect to statistical and sampling errors. The integral scale ranges between 70 and 130 s at the top of the CBL. Within the
CBL, up to the third order, noise errors are significantly smaller than sampling errors and the absolute values of turbulent
variables, respectively. The mixing-ratio variance profile rises monotonically from ≈0.07 to ≈3.7 g2 kg−2 in the entrainment zone. The skewness is nearly zero up to 0.6 z/z
i
, becomes −1 around 0.7–0.8 z/z
i
, crosses zero at about 0.95 z/z
i
, and reaches about 1.7 at 1.1 z/z
i
(here, z is the height and z
i
is the CBL depth). The noise errors are too large to derive fourth-order moments with sufficient accuracy. Consequently,
to the best of our knowledge, the ARM Raman lidar is the first water vapour Raman lidar with demonstrated capability to retrieve
profiles of turbulent variables up to the third order during daytime throughout the atmospheric CBL. 相似文献
10.
Temperature variance and temperature power spectra in the unstable surface layer have always presented a problem to the standard
Monin-Obukhov similarity model. Recently that problem has intensified with the demonstration by Smedman et al. (2007, Q J
Roy Meteorol Soc 133: 37–51) that temperature spectra and heat-flux cospectra can have two distinct peaks in slightly unstable
conditions, and by McNaughton et al. (2007, Nonlinear Process Geophys 14: 257–271) who showed that the wavenumber of the peak
of temperature spectra in a convective boundary layer (CBL), closely above the surface friction layer (SFL), can be sensitive
to the CBL depth, z
i. Neither the two-peak form at slight instability nor the dependence of peak position on z
i at large instability is compatible with the Monin-Obukhov model. Here we examine the properties of temperature spectra and
heat-flux cospectra from between these extremes, i.e. from within the unstable SFL, in two experiments. The analysis is based
on McNaughton’s model of the turbulence structure in the SFL. According to this model, heat is transported through most of
the SFL by sheet plumes, created by the action of impinging outer eddies. The smallest and most effective of these outer eddies
have sizes that scale on SFL depth, z
s. The z
s-scale eddies and plumes are organised within the overall convection pattern in the CBL, and in turn they organise the motion
of smaller eddies within the SFL, whose sizes scale on height, z. The main experimental results are: (1) the peak amplitudes of the temperature spectra in the SFL are collapsed with a scaling
factor (zsz)1/3eo2/3{(z_{\rm s}z)^{1/3}\varepsilon_{\rm o}^{2/3}} divided by the square of the surface temperature flux, where eo{\varepsilon_{\rm o}} is the dissipation rate of turbulent energy in the outer CBL (above the SFL); (2) the peak wavenumbers of the temperature
spectra are collapsed with the mixed length scale (z
i
z
s)1/2; (3) the peak wavenumbers of the heat-flux cospectra are collapsed with the doubly-mixed length scale (z
i
z
s)1/4
z
1/2; (4) for z/z
s < 0.03, the peak in the cospectrum is replaced by another peak at a wavenumber about a magnitude larger. This peak’s position
scales on z; (5) all these findings are consistent with the observations of Smedman et al. 相似文献
11.
Zbigniew Sorbjan 《Boundary-Layer Meteorology》2007,123(3):365-383
We examine daily (morning–afternoon) transitions in the atmospheric boundary layer based on large-eddy simulations. Under
consideration are the effects of the stratification at the top of the mixed layer and of the wind shear. The results describe
the transitory behaviour of temperature and wind velocity, their second moments, the boundary-layer height Z
m
(defined by the maximum of the potential temperature gradient) and its standard deviation σ
m
, the mixed-layer height z
i
(defined by the minimum of the potential temperature flux), entrainment velocity W
e, and the entrainment flux H
i
. The entrainment flux and the entrainment velocity are found to lag slightly in time with respect to the surface temperature
flux. The simulations imply that the atmospheric values of velocity variances, measured at various instants during the daytime,
and normalized in terms of the actual convective scale w*, are not expected to collapse to a single curve, but to produce a significant scatter of observational points. The measured
values of the temperature variance, normalized in terms of the actual convective scale Θ*, are expected to form a single curve in the mixed layer, and to exhibit a considerable scatter in the interfacial layer. 相似文献
12.
Arnold F. Moene Berenice I. Michels Albert A. M. Holtslag 《Boundary-Layer Meteorology》2006,120(2):257-274
For the presentation and analysis of atmospheric boundary-layer (ABL) data, scales are used to non-dimensionalise the observed quantities and independent variables. Usually, the ABL height, surface sensible heat flux and surface scalar flux are used. This works well, so long as the absolute values of the entrainment ratio for both the scalar and temperature are similar. The entrainment ratio for temperature naturally ranges from −0.4 to −0.1. However, the entrainment ratio for passive scalars can vary widely in magnitude and sign. Then the entrainment flux becomes relevant as well. The only customary scalar scale that takes into account both the surface flux and the entrainment flux is the bulk scalar scale, but this scale is not well-behaved for large negative entrainment ratios and for an entrainment ratio equal to −1. We derive a new scalar scale, using previously published large-eddy simulation results for the convective ABL. The scale is derived under the constraint that scaled scalar variance profiles are similar at those heights where the variance producing mechanisms are identical (i.e., either near the entrainment layer or near the surface). The new scale takes into account that scalar variance in the ABL is not only related to the surface flux of that scalar, but to the scalar entrainment flux as well. Furthermore, it takes into account that the production of variance by the entrainment flux is an order of magnitude larger than the production of variance by the surface flux (per unit flux). Other desirable features of the new scale are that it is always positive (which is relevant when scaling standard deviations) and that the scaled variances are always of order 1–10. 相似文献
13.
14.
James M. Godowitch 《Boundary-Layer Meteorology》1990,50(1-4):375-404
The spatial variability and temporal behavior of the vertical flux of ozone have been investigated from turbulence measurements collected on aircraft flight legs in the daytime period during two consecutive summer experimental field programs. The data were obtained during horizontal flight legs conducted over agricultural crops and forested land in three different regions of the eastern United States.Results from individual experimental cases and statistics derived from all cases in each region are presented. Ozone flux generally exhibited a significant height dependency. The strongest negative (downward) fluxes in the lowest-level flight legs were primarily attributed to the uptake of ozone by the surface and vegetative cover. Fluxes were near-zero in the middle of the convective boundary layer (CBL) in the afternoon period. As ozone flux was proportional to concentration, slightly stronger fluxes were found in low-level urban plume segments where ozone concentrations were 10–20 ppb higher than in the surrounding area. The derived deposition velocity showed no such bias as a function of position across the urban plume. Ozone flux differences were not apparent between the more heavily forested sections and the primarily agricultural cropland areas in these regions. During the afternoon period when no clear temporal trend was evident, means from values obtained below 0.15Z
i
(Z
i
being the CBL height) were -0.421 and -0.431 ppb m-2 s-1 for ozone flux and 0.81 and 0.82 cm s-1 for the derived mean deposition velocity in the southeastern Pennsylvania and central Ohio areas, respectively. These experimental results for ozone provide support to a dry deposition parameterization module which computes grid-area averaged deposition velocities for use in regional-scale models.On assignment from the National Oceanic and Atmospheric Administration, U.S. Department of Commerce. 相似文献
15.
Large-eddy simulations (LES) are performed to investigate the entrainment andthe structure of the inversion layer of the convective boundary layer (CBL) withvarying wind shears. Three CBLs are generated with the constant surface kinematicheat flux of 0.05 K m s-1 and varying geostrophic wind speeds from 5 to 15m s-1. Heat flux profiles show that the maximum entrainment heat flux as afraction of the surface heat flux increases from 0.13 to 0.30 in magnitude withincreasing wind shear. The thickness of the entrainment layer, relative to the depthof the well-mixed layer, increases substantially from 0.36 to 0.73 with increasingwind shear. The identification of vortices and extensive flow visualizations nearthe entrainment layer show that concentrated vortices perpendicular to the meanboundary-layer wind direction are identified in the capping inversion layer for thecase of strong wind shear. These vortices are found to develop along the mean winddirections over strong updrafts, which are generated by convective rolls and to appearas large-scale wavy motions similar to billows generated by the Kelvin–Helmholtzinstability. Quadrant analysis of the heat flux shows that in the case of strong windshear, large fluctuations of temperature and vertical velocity generated by largeamplitude wavy motions result in greater heat flux at each quadrant than that inthe weak wind shear case. 相似文献
16.
An evaluation of neutral and convective planetary boundary-layer parameterizations relative to large eddy simulations 总被引:2,自引:1,他引:2
Keith W. Ayotte Peter P. Sullivan Anders Andrén Scott C. Doney Albert A. M. Holtslag William G. Large James C. McWilliams Chin-Hoh Moeng Martin J. Otte Joseph J. Tribbia John C. Wyngaard 《Boundary-Layer Meteorology》1996,79(1-2):131-175
This paper compares a number of one-dimensional closure models for the planetary boundary layer (PBL) that are currently in use in large-scale atmospheric models. Using the results of a large-eddy simulation (LES) model as the standard of comparison, the PBL models are evaluated over a range of stratifications from free convective to neutral and a range of surface shear stresses. Capping inversion strengths for the convective cases range from weakly to strongly capped. Six prototypical PBL models are evaluated in this study, which focuses on the accuracy of the boundary-layer fluxes of momentum, heat, and two passive scalars. One scalar mimics humidity and the other is a top-down scalar entrained into the boundary layer from above. A set of measures based on the layer-averaged differences of these fluxes from the LES solutions is developed. In addition to the methodological framework and suite of LES solutions, the main result of the evaluation is the recognition that all of the examined PBL parameterizations have difficulty reproducing the entrainment at the top of the PBL, as given by the LES, in most parameter regimes. Some of the PBL models are relatively accurate in their entrainment flux in a subset of parameter regimes. The sensitivity of the PBL models to vertical resolution is explored, and substantive differences are observed in the performance of the PBL models, relative to LES, at low resolution typical of large scale atmospheric models. 相似文献
17.
We have conducted large-eddy simulations (LES) of the atmospheric boundary layer with surface heat flux variations on a spatial scale comparable to the boundary layer depth.We first ran a simulation with a horizontally homogeneous heat flux. In general the results are similar to those of previous large-eddy simulations. The model simulates a field of convective eddies having approximately the correct velocity and spatial scales, and with the crucial property that kinetic energy is transported vigorously upwards through the middle levels. However, the resolved temperature variance is only about half what is observed in the laboratory or the atmosphere. This deficiency — which is shared by many other large-eddy simulations — has dynamic implications, particularly in the pressure/temperature interaction terms of the heat flux budget. Recent simulations by other workers at much higher resolution than ours appear to be more realistic in this respect.The surface heat flux perturbations were one-dimensional and sinusoidal with a wavelength equal to 1.3 times the boundary-layer depth. The mean wind was zero. Results were averaged over several simulations and over time. There is a mean circulation, with ascent over the heat flux maxima (vertical velocity ~0.1w
*) and descent over the heat flux minima. Turbulence is consistently stronger over the heat flux maxima. The horizontal velocity variance components (calculated with respect to the horizontal average) become unequal, implying that convective eddies are elongated parallel to the surface heat flux perturbations.A consideration of the budgets for temperature and velocity suggests several simplifying concepts.The research reported in this paper was conducted while the first author was on study leave at Colorado State University. 相似文献
18.
J. Wieringa 《Boundary-Layer Meteorology》1972,2(4):406-426
For 390 ten-minute samples of turbulent flux, made with a trivane above a lake, the vertical alignment is determined within 0.1 ° through azimuth-dependent averaging. One degree of instrumental misalignment is found to produce an average tilt error of 9 ± 4% for momentum flux, and 4 ± 2% for heat flux. The tilt error in the vertical momentum flux depends mainly ons
u/u*, and cannot be much diminished with impunity by high-pass pre-filtering of the turbulence signals. The effects of rain on trivane measurements of vertical velocity are shown to be negligible at high wind speeds, and adaptable to correction in any case.The normalized vertical velocity variance,s
w/u*, appears to be proportional to the square root ofz/L for unstable stratification. For a wind speed range of 2 to 15 m s–1, the eddy correlation stresses measured at 4- and 8-m heights can be reasonably well estimated by using a constant drag coefficientC
d=1.3 X 10-3, while cup anemometer profile measurements give an overestimate of eddy stress at high wind speeds. A good stress estimate is also obtained from the elevation variance; it is suggested that trivane measurement of this variance might be made from a mobile platform, e.g., a moderately stabilized spar buoy. 相似文献
19.
Dissimilarity of Scalar Transport in the Convective Boundary Layer in Inhomogeneous Landscapes 总被引:1,自引:1,他引:0
A land-surface model (LSM) is coupled with a large-eddy simulation (LES) model to investigate the vegetation-atmosphere exchange
of heat, water vapour, and carbon dioxide (CO2) in heterogeneous landscapes. The dissimilarity of scalar transport in the lower convective boundary layer is quantified
in several ways: eddy diffusivity, spatial structure of the scalar fields, and spatial and temporal variations in the surface
fluxes of these scalars. The results show that eddy diffusivities differ among the three scalars, by up to 10–12%, in the
surface layer; the difference is partly attributed to the influence of top-down diffusion. The turbulence-organized structures
of CO2 bear more resemblance to those of water vapour than those of the potential temperature. The surface fluxes when coupled with
the flow aloft show large spatial variations even with perfectly homogeneous surface conditions and constant solar radiation
forcing across the horizontal simulation domain. In general, the surface sensible heat flux shows the greatest spatial and
temporal variations, and the CO2 flux the least. Furthermore, our results show that the one-dimensional land-surface model scheme underestimates the surface
heat flux by 3–8% and overestimates the water vapour and CO2 fluxes by 2–8% and 1–9%, respectively, as compared to the flux simulated with the coupled LES-LSM. 相似文献
20.
The heights of the daytime convective boundary layer (CBL), computed by a one-dimensional model for a bare soil surface at a semi-arid station,Anand, during the dry and hot summer month of May 1997, are presented. As input, the model requires surface heat flux, friction velocity and air temperature as functions of time. Temperature data at the one-metre level from a tower and sonic anemometer data at 9.5 m collected during the period 13–17 May 1997 in the Land Surface Processes Experiment (LASPEX-97) are used to compute hourly values of surface heat flux, friction velocity and Obukhov length following the operational method suggested by Holtslag and Van Ulden [J. Climate Appl. Meteorol.
22,517–529 (1983)]. The model has been tested with different values for the potential temperature gradient ( ) above the inversion. The model-estimated CBL heights comparefavourably with observed heights obtained from radiosonde ascents. 相似文献