共查询到20条相似文献，搜索用时 46 毫秒
1.
Igor Petenko Giangiuseppe Mastrantonio Angelo Viola Stefania Argentini Ilaria Pietroni 《Boundary-Layer Meteorology》2012,143(1):125-141
Some characteristics of wavelike motions in the atmospheric boundary layer observed by sodar are considered. In an experiment
carried out in February 1993 in Milan, Italy, Doppler sodar measurements were accompanied by in situ measurements of temperature
and wind velocity vertical profiles using a tethered balloon up to 600 m. The oscillations of elevated wavy layers containing
intense thermal turbulence, usually associated with temperature-inversion zones, were studied by using correlation and spectral
analysis methods. The statistics of the occurrence of wavelike and temperature-inversion events are presented. The height
distributions of Brunt–Vaisala frequency and wind shear and their correlation within elevated inversion layers were determined,
with a strong correlation observed between the drift rate of the wavy layers and the vertical velocity measured by Doppler
sodar inside these layers. Spectral analysis showed similarities regarding their frequency characteristics. The phase speed
and propagation direction of waves were estimated from the time delay of the signals at three antennae to provide estimates
of wavelength. Moreover, wavelengths were estimated from the intrinsic frequency obtained from sodar measurements of the Doppler
vertical velocity and oscillations of wavy turbulent layers. The two wavelength estimates are in good agreement. 相似文献
2.
Summary The temporal variability of thermal turbulence and vertical velocity derived from sodar measurements during periods of atmospheric
free convection is studied using both spectral and wavelet analysis. A promising approach to analyse atmospheric processes,
an advanced high-resolution spectrum estimation technique is described. Variance spectra of meteorological and turbulent parameters
are shown to have their specific comb shape at a low-frequency range. Spectra and wavelet transforms of the data obtained
at different sites both indicate the existence of some representative predominant temporal scales in time variations of the
convective boundary layer structure. The most evident temporal scales revealed are centered around 7–9 and 18–22 minutes.
Received October 16, 1998 Revised April 15, 1999 相似文献
3.
High-resolution measurements of thermodynamic, microphysical, and turbulence properties inside a turbulent inversion layer above a marine stratocumulus cloud layer are presented. The measurements are performed with the helicopter-towed measurement payload Airborne Cloud Turbulence Observation System (ACTOS), which allows for sampling with low true air speeds and steep profiles through cloud top. Vertical profiles show that the turbulent inversion layer consists of clear air above the cloud top, with nearly linear profiles of potential temperature, horizontal wind speed, absolute humidity, and concentration of interstitial aerosol. The layer is turbulent, with an energy dissipation rate nearly the same as that in the lower cloud, suggesting that the two are actively coupled, but with significant anisotropic turbulence at the large scales within the turbulent inversion layer. The turbulent inversion layer is traversed six times and the layer thickness is observed to vary between 37 and 85 m, whereas the potential temperature and horizontal wind speed differences at the top and bottom of the layer remain essentially constant. The Richardson number therefore increases with increasing layer thickness, from approximately 0.2 to 0.7, suggesting that the layer develops to the point where shear production of turbulence is sufficiently weak to be balanced by buoyancy suppression. This picture is consistent with prior numerical simulations of the evolution of turbulence in localized stratified shear layers. It is observed that the large eddy scale is suppressed by buoyancy and is on the order of the Ozmidov scale, much less than the thickness of the turbulent inversion layer, such that direct mixing between the cloud top and the free troposphere is inhibited, and the entrainment velocity tends to decrease with increasing turbulent inversion-layer thickness. Qualitatively, the turbulent inversion layer likely grows through nibbling rather than engulfment. 相似文献
4.
Z. Petkovšek 《Meteorology and Atmospheric Physics》1992,47(2-4):237-245
Summary Problems of turbulent dissipation of a cold air lake (CAL) and the inversion layer bordering CAL on the upper boundary are presented and studied with the compound model. In wintertime such cold air lakes can persist for days even if rather strong winds are blowing above them. The required conditions for CAL dissipation are removed processes of its formation or maintenance, as well as a sufficiently strong invasion of turbulence in the inversion layer from above down-wards. By this, the inversion layer at first becomes stronger and dissipation is stopped, until the increase of turbulent kinetic energy of the upper flow enables further dissipation. Such turbulent dissipation process is shown by the model for typical conditions and for different initial values of the relevent variables.With 6 Figures 相似文献
5.
Giampietro Casasanta Ilaria Pietroni Igor Petenko Stefania Argentini 《Boundary-Layer Meteorology》2014,151(3):597-608
The mixing-layer height is estimated using measurements from a high resolution surface-layer sodar run at the French-Italian station of Concordia at Dome C, Antarctica during the summer 2011–2012. The temporal and spatial resolution of the sodar allows the monitoring of the mixing-layer evolution during the whole diurnal cycle, i.e. a very shallow nocturnal boundary layer followed by a typical daytime growth. The behaviour of the summer mixing-layer height, variable between about 10- and 300 m, is analyzed as a function of the mean and turbulent structure of the boundary layer. Focusing on convective cases only, the retrieved values are compared with those calculated using a one-dimensional prognostic equation. The role of subsidence is examined and discussed. We show that the agreement between modelled and experimental values significantly increases if the subsidence is not kept fixed during the day. A simple diagnostic equation, which depends on the time-averaged integral of the near-surface turbulent heat flux, the background static stability and the buoyancy parameter, is proposed and evaluated. The diagnostic relation performance is comparable to that of the more sophisticated prognostic model. 相似文献
6.
In this work, the thermic structure of the atmospheric boundary layer is analyzed by means of direct measurements with radiosonde equipment, remote exploration with a three-monostatic Doppler sodar, and a boundary layer model of order one-and-a-half. Intercomparisons of radiosonde data, sodar data, and model results are made through the study of radiative nocturnal inversion, subsidence inversion, development and height of the mixing layer, and calculus of the temperature structure parameter. The ability of sodar to find the mixing layer height and to detect stable layers is enhanced when these layers are low enough. 相似文献
7.
M. A. Novitskii D. E. Gaitandzhiev N. F. Mazurin M. K. Matskevich 《Russian Meteorology and Hydrology》2011,36(9):580-589
Turbulence characteristics in the surface layer of the coastal area of Akhtopol (Bulgaria) under conditions of the breeze
circulation are measured. The measurements were carried out at the level of 4.5 m by means of three-component ultrasonic anemometer.
To estimate the wind regime in the atmospheric boundary layer, the sodar data and synoptic charts were used. All turbulent
characteristics except the correlation coefficient of the friction flow have an appreciable daily course in the case of the
sea breeze. In the frontal zone, some characteristics demonstrate sometimes short-term variations of their numerical values.
The sea-land front of the breeze circulation is effectively detected from the measurements of wind speed, wind direction,
and temperature in the surface layer. It is also possible to judge about the breeze circulation type and turbulence structure
in this layer on the basis of these measurements. 相似文献
8.
The high-power Defford radar has been upgraded to provide Doppler information regarding the motion of echoes from weak refractive index inhomogeneities within the optically clear atmosphere. A case study is presented in which data from the radar are used to derive the detailed velocity structure in and above the planetary boundary layer. These data are analysed to show how convective circulations in the boundary layer can perturb the height of a shallow inversion overlying it, thereby producing local enhancements of wind shear and a decrease in dynamic stability within the inversion. The measurements were obtained as part of a Boundary-Layer Project in which simultaneous measurements were made using fast-response instruments suspended from a tethered balloon within the region scanned by the radar. The balloon-borne probes showed that the most intense turbulence and fluctuations of temperature and refractivity were encountered when radar-detected hummocks in the height of the inversion were advected through the probes. The fine-scale turbulence measurements within the perturbed inversion are consistent with the existence of Kelvin-Helmholtz billows. 相似文献
9.
A complex marine experiment was conducted in autumn 1991 on the research vessel Dmitry Mendelev in association with the Atlantic Stratocumulus Transition Experiment (ASTEX). A three-axis Doppler sodar designed at the Institute of Atmospheric Physics, Moscow, was used in this experiment. Total observation time was about 770 hours from 6 October to 23 November. Besides facsimile records illustrating spatial and temporal structure of the turbulence distribution in the atmospheric boundary layer (ABL), routine quantitative measurements of profiles of wind and echo-signal strength were taken. Some main characteristics of the ABL behavior over the ocean were revealed through an analysis of these data as well as the results of other kinds of measurements. An important peculiarity of the ABL observed between the Canary Islands and the Azores was the presence of diurnal variation of convective turbulence strength having a maximum between 04:00 and 07:00 LT. A similar diurnal variation was observed for low-level cloud cover. Occurrence of various types of thermal stratification and their diurnal variation were obtained. Comparison of elevated stable layers and low-level cumulus showed that the lower boundary of clouds correlates well with the height of the bottom of elevated inversion layers (at heights of 200–600 m). Canary and Cabo Verde observations showed that islands strongly affect the ABL structure. The strong effect of a surface water temperature gradient on the ABL stability was observed when crossing the Canary, Azores, and Labrador currents and the Gulf Stream. 相似文献
10.
Operation of a monostatic acoustic sounder on the campus of The University of Calgary for the period March to September, 1976, has provided data on the atmospheric boundary layer over Calgary. The formation and dissipation of the nocturnal temperature inversion layer leaves a clear signature in the sounder records. A trend is discerned in which the nocturnal layer tends to form at sunset, whereas convective instability is established some 2–3 hours after sunrise. Comparison of sounder records with an extensive set of radiosonde temperature profiles has shown that the height of the inversion layer can be determined reliably when a single, dark, ground-based return is visible on the sounder charts. On many occasions, however, the sounder detects multiple structures in the atmosphere. In these cases, it is difficult to establish a systematic correspondence of the turbulent structure as observed in the sounder records with the temperature profile as observed by the radiosonde. 相似文献
11.
The ECLATS experiment was conducted in order to investigate the influence of radiative processes on the dynamics of the atmospheric boundary layer during its diurnal evolution. This experiment was carried out over Niger, near Niamey, by measuring continuously the energy balance at ground level and by using an instrumented aircraft for turbulence, radiative fluxes and aerosol measurements in the boundary layer during dusty conditions (brumes sèches). This paper is restricted to an analysis of the turbulent structure in the homogeneous and stationary convective boundary layer. The turbulence moments for kinetic energy and the spectral characteristics of the vertical velocity are discussed. These results are compared with a set of data obtained for clear convective boundary layers. The differences observed are quite important and seem, at least in part, due to radiative processes (infrared radiative divergence in the surface layer and absorption of solar radiation in the boundary layer). 相似文献
12.
Zbigniew Sorbjan 《Boundary-Layer Meteorology》2007,123(1):121-142
Results of large-eddy simulations of shallow, quasi-steady, shear-less convection in the Martian boundary layer are presented
and discussed. In the considered three cases, turbulence is forced by the radiative flux divergence, prescribed as given functions
of height, and the strength of the surface heat flux. It is constrained by the temperature inversion at the boundary-layer
top. The resulting convective boundary layer exhibits horizontal cellular structures. The presence of radiative heating causes
dimensionless statistics of turbulence to depend on the parameter F, defined in terms of the integrated radiative and turbulent heating rates in the boundary layer. 相似文献
13.
The spectral structure of turbulence (spectra and cospectra) and water content characteristics for convective clouds in the tropical region was analyzed using aircraft observations in the meteorological testbed over Cuba; the results are presented. The cospectra for vertical turbulent heat fluxes allow classifying convective clouds based on their development stage and revealing the stages of growth, stabilization, and dissipation of clouds. It is shown that the value of cloud air overheating as compared to the ambient space is a parameter defining the cloud development stage. The interrelation is revealed between the integral characteristics of turbulence and water content (the mean values and distribution functions of parameters) for different stages of development of convective clouds. The recommendations are formulated on using the data on the dynamic structure of clouds for weather modification activities. A method is proposed for the instrumental estimation of cloud suitability for seeding that is aimed at precipitation augmentation. 相似文献
14.
The measurement of turbulent surface-layer fluxes by use of bichromatic scintillation 总被引：4，自引：2，他引：4
Scintillation measurements with a HeNe and a CO2 laser were used to derive turbulent fluxes of heat and momentum in the surface layer. This was achieved by the structure constant or dissipation technique, i.e., by relating the measured structure constants and inner scales of refractive index fluctuations to structure constants of temperature fluctuations and dissipation rates of turbulent kinetic energy, respectively, and then assuming Monin-Obukhov similarity.The resulting heat fluxes agree well with measurements using the eddy correlation technique but for averaging periods of 10 min, the optical data show a much smoother and physically more plausible behaviour. The optically derived friction velocities are in good agreement with estimates derived from wind velocity and surface roughness. It was also observed that for stationary conditions, 1-min averaged optical measurements already provide good estimates for longer averaged heat and momentum fluxes.Even though some uncertainty remains about the empirical constants and Monin-Obukhov similarity expressions used, the method clearly proves to be of great value for monitoring surface-layer turbulence. 相似文献
15.
étude expérimentale de la couche limite au-dessus d'un relief modéré proche d'une chaîne de montagne
A. Druilhet J. Noilhan B. Benech G. Dubosclard D. Guedalia J. Frangi 《Boundary-Layer Meteorology》1983,25(1):3-16
For the heterogeneous site described in the first part, some aspects of the turbulent structure of the planetary boundary layer are studied. Using mixed-layer scaling, the normalized profiles are compared with those obtained over flat terrain during convective conditions. The measurements were made with the same instrumented aircraft at both sites. The dissipative and spectral length scales are smaller over complex terrain within the whole boundary layer. This is due to the shifting of the wavelength peak toward the high frequencies by dynamic turbulence.This last effect can also explain the increase of the dissipation rate over the heterogeneous site during strong wind conditions. The vertical profiles of sensible heat flux and temperature-water vapor correlation show a lack of entrainment process at the top of the boundary layer. This fact suggests that the investigated boundary layer is advected from the neighbouring plain over the complex site (plateau de Lannemezan). 相似文献
16.
Stefania Argentini Giangiuseppe Mastrantonio Igor Petenko Ilaria Pietroni Angelo Viola 《Boundary-Layer Meteorology》2012,143(1):177-188
Measurements in the atmospheric surface layer are generally made with point sensors located in the first few tens of metres.
In most cases, however, these measurements are not representative of the whole surface layer. Standard Doppler sodars allow
a continuous display of the turbulent thermal structure and wind profiles in the boundary layer up to 1000 m, with a few points,
if any, in the surface layer. To overcome these limitations a new sodar configuration is proposed that allows for a higher
resolution in the surface layer. Because of its capabilities (echo recording starting at 2 m, echo intensity vertical resolution
of approximately 2 m, temporal resolution of 1 s) this sodar is called the surface-layer mini-sodar (SLM-sodar). Features
and capabilities of the SLM-sodar are described and compared with the sodar. The comparison of the thermal vertical structure
given by the SLM-sodar and the sodar provides evidence that, in most cases, the surface layer presents a level of complexity
comparable to that of the entire boundary layer. Considering its high vertical resolution, the SLM-sodar is a promising system
for the study of the nocturnal surface layer. The nocturnal SLM-sodar measurements have shown that, depending on wind speed,
the structure of the surface layer may change substantially within a short time period. At night, when the wind speed is greater
than 3 m s−1, mechanical mixing destroys the wavy structure present in the nocturnal layer. Sonic anemometer measurements have shown that,
in such cases, also the sensible heat flux varies with height, reaching a peak in correspondence with the wind speed peak.
Under these conditions the assumption of horizontal homogeneity of the surface layer and the choice of the averaging time
need to be carefully treated. 相似文献
17.
Daniel F. Nadeau Eric R. Pardyjak Chad W. Higgins Harinda Joseph S. Fernando Marc B. Parlange 《Boundary-Layer Meteorology》2011,141(2):301-324
A simple model to study the decay of turbulent kinetic energy (TKE) in the convective surface layer is presented. In this
model, the TKE is dependent upon two terms, the turbulent dissipation rate and the surface buoyancy fluctuations. The time
evolution of the surface sensible heat flux is modelled based on fitting functions of actual measurements from the LITFASS-2003
field campaign. These fitting functions carry an amplitude and a time scale. With this approach, the sensible heat flux can
be estimated without having to solve the entire surface energy balance. The period of interest covers two characteristic transition
sub-periods involved in the decay of convective boundary-layer turbulence. The first sub-period is the afternoon transition,
when the sensible heat flux starts to decrease in response to the reduction in solar radiation. It is typically associated
with a decay rate of TKE of approximately t
−2 (t is time following the start of the decay) after several convective eddy turnover times. The early evening transition is the
second sub-period, typically just before sunset when the surface sensible heat flux becomes negative. This sub-period is characterized
by an abrupt decay in TKE associated with the rapid collapse of turbulence. Overall, the results presented show a significant
improvement of the modelled TKE decay when compared to the often applied assumption of a sensible heat flux decreasing instantaneously
or with a very short forcing time scale. In addition, for atmospheric modelling studies, it is suggested that the afternoon
and early evening decay of sensible heat flux be modelled as a complementary error function. 相似文献
18.
Zbigniew Sorbjan 《Boundary-Layer Meteorology》1997,82(3):503-517
Results of a large-eddy simulation of a decaying convective mixed layer over land are presented. The time evolution of the mixed layer is forced by the surface heat flux gradually decreasing with time. The results obtained show that the decay of the turbulent kinetic energy is governed by two scales, the external time scale controlling the surface heat flux changes, and the convective time scale. During the simulation, large eddies persist even when the heat flux at the surface becomes negative. A decoupled residual layer of active turbulence is developed above the stable surface layer. The residual layer is marked by large-scale updrafts that are able to penetrate the capping inversion layer and induce entrainment. 相似文献
19.
Estimates from semiempirical models that characterize surface heat flux, mixing depth, and profiles of temperature, wind, and turbulence are compared with observations from atmospheric field studies conducted in Colorado, Illinois, Indiana, and Minnesota. Sodar observations are compared with tower measurements at the Colorado site, for wind and turbulence profiles. The median surface heat flux, as calculated using surface-layer flux-profile relationships and an energy budget model, was consistently overestimated by 20 to 80%. Several mixing-depth models were evaluated: (1) integration of the hourly surface heat flux and friction velocity, (2) solving for the time rate of change of profiles of virtual potential temperature, and (3) an interpolation scheme used by the U.S. Environmental Protection Agency in regulatory dispersion models. For the late afternoon, 80 to 90% of the estimates from the first and third models were within 40% of the observed values. For the morning hours after sunrise, all were less accurate. Temperature estimates from surface-layer flux-profile relationships compared well with observations within the mixed layer, but were too low for the inversion layer aloft. Wind profiles were derived using surface-layer flux-profile relationships, a windprofile power-law based on Pasquill stability category, and sodar measurements. The sodar measurements were superior to both types of model estimates. Turbulence profiles were derived from sodar measurements and from semiempirical similarity relationships based on mixing depth and Obukhov length. The scatter in the comparisons with the sodar observations is twice that seen in the comparisons with empirical profile relationships. Overall, it appears that uncertainty of as low as 20 to 30% in the characterization of the diffusion meteorology is the exception rather than the rule.On assignment from the National Oceanic and Atmospheric Administration, U. S. Department of Commerce.Disclaimer: Although the research described in this article has been supported by the United States Environmental Protection Agency, it has not been subjected to Agency review and therefore does not necessarily reflect the views of the Agency and no official endorsement should be inferred. Mention of trade names or commercial products does not constitute endorsement or recommendation for use. 相似文献
20.
K. G. Mcnaughton 《Boundary-Layer Meteorology》2006,118(1):83-107
We present a new account of the kinetic energy budget within an unstable atmospheric surface layer (ASL) beneath a convective
outer layer. It is based on the structural model of turbulence introduced by McNaughton (Boundary-Layer Meteorology, 112:
199–221, 2004). In this model the turbulence is described as a self-organizing system with a highly organized structure that
resists change by instability. This system is driven from above, with both the mean motion and the large-scale convective
motions of the outer layer creating shear across the surface layer. The outer convective motions thus modulate the turbulence
processes in the surface layer, causing variable downwards fluxes of momentum and kinetic energy. The variable components
of the momentum flux sum to zero, but the associated energy divergence is cumulative, increasing both the average kinetic
energy of the turbulence in the surface layer and the rate at which that energy is dissipated. The tendency of buoyancy to
preferentially enhance the vertical motions is opposed by pressure reaction forces, so pressure production, which is the work
done against these reaction forces, exactly equals buoyant production of kinetic energy. The pressure potential energy that
is produced is then redistributed throughout the layer through many conversions, back and forth, between pressure potential
and kinetic energy with zero sums. These exchanges generally increase the kinetic energy of the turbulence, the rate at which
turbulence transfers momentum and the rate at which it dissipates energy, but does not alter its overall structure. In this
model the velocity scale for turbulent transport processes in the surface layer is (kzɛ)1/3 rather than the friction velocity, u*. Here k is the von Kármán constant, z is observation height, ɛ is the dissipation rate. The model agrees very well with published experimental results, and provides
the foundation for the new similarity model of the unstable ASL, replacing the older Monin–Obukhov similarity theory, whose
assumptions are no longer tenable. 相似文献