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1.
Remote sensing of the lower planetary boundary layer in the vicinity of a meteorological tower on many occasions reveals the existence of shear instability (Kelvin-Helmholtz) waves. In general, such waves are found within shallow strata which are marked by strong thermal stability and large vertical wind shear. The independent and concurrent measurements of the vector wind and temperature, made on a 152-m high tower, allow the construction of wind and temperature profiles. From such measurements, the Richardson number profile is constructed as well as the instability regime according to Drazin's criterion. The results show that regions of shear-instability waves as depicted by the remote sensor (an acoustic sounder) agree well with Drazin's instability regime, and that within such regions the Richardson number is indeed 0.25.  相似文献   

2.
In an experiment investigating the planetary boundary layer (PBL) wind and temperature fields, and PBL inversion height recorded by various instruments, the results reveal the presence of organized large eddies (OLE) or rolls. The measurements by lidars, anemometers, soundings and sodar gave an overview of the characteristics of the rolls and sources of energy production that maintain them. The experimental results obtained on two consecutive days are compared to model outputs. The agreement is excellent, showing that thermal stratification and wind shear are important factors in the structure and dynamics of OLE. A heterodyne Doppler lidar (HDL) is shown to be a useful tool in the study of OLE.  相似文献   

3.
Gravity waves associated with stably stratified layers were observed in the planetary boundary layer at two locations in France. Using an array of three monostatic Doppler acoustic sounders, the wave speeds and directions were found. A quantitative study of the waves has been performed to determine their physical characteristics. Based on their dynamics, a classification into three types was possible. Most were found to be trapped beneath a critical level.  相似文献   

4.
Large-Eddy Simulation Of The Stably Stratified Planetary Boundary Layer   总被引：3，自引：1，他引：2  
In this work, we study the characteristics of a stably stratifiedatmospheric boundary layer using large-eddy simulation (LES).In order to simulate the stable planetary boundary layer, wedeveloped a modified version of the two-part subgrid-scalemodel of Sullivan et al. This improved version of themodel is used to simulate a highly cooled yet fairly windy stableboundary layer with a surface heat flux of(W)o = -0.05 m K s-1and a geostrophic wind speed of Ug = 15 m s-1.Flow visualization and evaluation of the turbulencestatistics from this case reveal the development ofa continuously turbulent boundary layer with small-scalestructures. The stability of the boundary layercoupled with the presence of a strong capping inversionresults in the development of a dominant gravity wave atthe top of the stable boundary layer that appears to be relatedto the most unstable wave predicted by the Taylor–Goldsteinequation. As a result of the decay of turbulence aloft,a strong-low level jet forms above the boundary layer.The time dependent behaviour of the jet is compared with Blackadar'sinertial oscillation analysis.  相似文献   

5.
R.E. Munn 《大气与海洋》2013,51(4):144-147
An acoustic echo sounder situated in downtown Toronto has been used to detect convective plumes in the planetary boundary layer and to measure, by means of the Doppler effect, the vertical air motions associated with them. The plumes observed were the order of 390 m in horizontal extent, were detectable to a height of about 400 m, and were characterized by peak upward velocities in excess of 1 m s?1. The sounder measurements are shown to be consistent with surface meteorological parameters, and suggest that free convection over an urban area of considerable surface roughness and non‐uniformity is not greatly different from that over uniform land surfaces or water.  相似文献   

6.
The structure of turbulence in an inversion layer and in an homogeneous convective field of the planetary boundary layer is described. In the first part of the paper, we validate the sodar estimates of turbulent dissipation, by using measurements with an hot-wire anemometric system in situ. Limitations of an ε measurement technique using structure function calculations are given, taking account of atmospheric properties and acoustic Doppler instrumental effects. By comparison between isopleths of backscattering intensity and of turbulent dissipation rates, we observe that in the early morning, turbulence is advected by mechanical turbulence generated by wind shear. The same mechanism seems to be operating in the case of an inversion layer capping thermal instability, when the convective activity is not too greatly developed. A turbulent kinetic energy budget is examined using aircraft, sodar, and tower measurements. This indicates a constant turbulent dissipation profile through a deep convective layer.  相似文献   

7.
The stable planetary boundary layer at the baseof the residual layer supports internalwaves that are unambiguously boundary layer incharacter. Some of these wavesare instabilities and some are neutrally stable modes, but they all have critical levelsin the residual layer. These waves exist for a broad range of conditions and should bea major component of any ducted disturbance that propagates within ninety degreesof the wind direction. The wave properties can be computed without the numericaldifficulties usually associated with critical-level systems.  相似文献   

8.
The theoretical developments yielding the resistance law for the planetary boundary layer are summarized, including the refinements of the ageostrophic method. This leads to the hypothesis that the resistance law for the simple non-stationary, advective boundary layer is obtained from the one for a stationary, horizontally homogeneous boundary layer, if the modulous of the geostrophic wind in the latter is replaced with a generalized frictionless wind in the former. The generalized frictionless wind includes the inertial terms.Contribution from the Sonderforschungsbereich Meeresforschung Hamburg of the Deutsche Forschungsgemeinschaft.  相似文献   

9.
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   

10.
Summary Within the Mesoscale Alpine Programme MAP conducted in autumn 1999, the vertical structure and the temporal evolution of the planetary boundary layer (PBL) in the Rhine Valley 2km south of Lake Constance were observed with a Remtech PA2 sodar (sound-detection-and-ranging instrument) rendering half-hour averages of the three-dimensional wind profile within the lowest kilometre above ground. During Foehn events, tethered balloon soundings and wind profiler measurements were conducted in addition to the rawinsonde network which was built up for the MAP field campaign.The remote sensing instrument renders a surprisingly high number of valid data during south Foehn. Due to the frequent formation of a cold air pool with stable conditions below the Foehn flow with near-neutral static stability, even more sodar data is valid during Foehn periods than during no Foehn periods. A significant reduction of the sodar data quality is only observed during Foehn events with grounding of the Foehn at the sodar site due to high background noise. At higher levels, a Foehn signal can be detected from the sodar wind and turbulence intensitiy information. With Foehn, higher wind speeds and larger turbulence intensities occur than without Foehn. Comparisons to rawinsonde and tethersonde soundings and wind profiler measurements at sites nearby reveal the spatial inhomogeneity of the Foehn flow within this part of the valley as well as instrumental short-comings. Different methods to determine the mixing height using the vertical sounding devices lead to some uncertainty of mixing height estimates which however can reasonably be explained.  相似文献   

11.
Three-dimensional air motion, observed with radar in a convective planetary boundary layer under light wind conditions, is organized in cellular structures scaled from 2 to 5 km; simultaneous acoustic echosonde measurements indicate that near the ground there are smaller cells than those observable by the radar. At heights of 400 m, cell scales become as large as those observable by the radar. Pressure fluctuations at the ground measured with spatial pressure transducers, at the same time as the radar and echosonde observations, traveled at the same speed and direction as the radar volume-averaged wind. The length scale determined at the ground with pressure fluctuation data was between 1.2 and 2.4 km, smaller than the largest radar-measured motion scales.  相似文献   

12.
Summary The effects of internal waves on the propagation of acoustic pulses in the lower atmosphere were studied theoretically and by acoustic pulse sounding of the stable atmospheric boundary layer. Due to a control in the experiments of the stratification and time variations of meteorological parameters, such as wind speed, temperature and atmospheric pressure, we were able to observe the influence of the variations of these parameters on a pulse wave form, travel time and time duration. For the travel time and wind speed variations we obtained statistical characteristics (variances, frequency spectra and coherences) in the range of periods from 1 min to 1h and found several dominant periods, which are inherent to the trapped internal waves in the lower atmosphere. Using a nonlinear model of internal wave spectrum in the atmosphere described here we have made the calculations of variances, frequency spectra and structure functions of travel time fluctuations, which allowed us to interpret some of the observed data.  相似文献   

13.
Use of a High-Resolution Sodar to Study Surface-layer Turbulence at Night   总被引：1，自引：1，他引：0  
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   

14.
Summary Air quality forecasting and emergency response are receiving increasing emphasis in the US and elsewhere; both share similar but not identical needs for advanced meteorological observations. At the same time, there has been a steady increase in the performance characteristics of ground-based remote sensing systems as well as ground-based and airborne in situ measurement systems. The structure of the planetary boundary layer is summarized in the context of implications for transport and diffusion, along with the measurement requirements for dispersion modeling. We then review the current state-of-the-art of operational and quasi-operational measurement systems with a focus on boundary layer measurements. Measurement systems discussed include: meteorological radar, radar wind profilers, radio acoustic sounding systems, lidar, sodar, GPS receivers, microwave radiometers, radiosonde systems, commercial aircraft measurements, and traditional near-surface in situ sensors.  相似文献   

15.
Models of the wave boundary layer   总被引：2，自引：1，他引：1  
A general approach to model the structure of the wave boundary layer, based on the nonlinear Reynolds equations in a curvilinear system of coordinates, is described. Both spectral and numerical grid models are used. The energetic interactions between wind and wave in terms of Miles' parameter are studied. For waves outrunning or running against the wind, the range of the inverse flux of energy is found. For waves running slower than the wind, quadratic growth of is established. Vertical profiles of the wave momentum flux for different fetches are given. Following P. Janssen, a one-dimensional analytical model of the wave boundary layer is suggested. The effect of waves on the drag coefficient is analyzed.  相似文献   

16.
The simultaneous operation of a three-axis Doppler sodar system in the centralurban area of Rome and two similar systems in the suburban area, forming atriangle about 20 km on each side, provided evidence of solitary-type wavesin the urban boundary layer. Three events, each lasting from a few minutes toabout 30 min, and ranging in depth from the minimum range of the sodar (39 m) to over 500 m, are reported here. Two events were recognizable onall three sodar records while the third event could be observed at the urbanlocation only. Time-height acoustic echo intensity records showed no-echoregions within the wave indicating transport of trapped recirculating air.This is typical of large amplitude solitary waves. The time series plots ofsodar-derived vertical wind velocity revealed a maximum peak-to-peakvariation of about 5 m s-1 during periods of wave-associated disturbance.The vertical velocity is found to increase with height up to the top of the closedcirculation within the wave and decreases further above. The normalisedamplitude-wavelength relationship for the two events indicates that theobserved waves are close to a strongly nonlinear regime.  相似文献   

17.
We consider the assumption postulated by Deusebio and Lindborg (J Fluid Mech 755:654–671, 2014) that the helicity injected into the Ekman boundary layer undergoes a cascade, with preservation of its sign (right- or alternatively left-handedness), which is a signature of the system rotation, from large to small scales, down to the Kolmogorov microscale of turbulence. At the same time, recent direct field measurements of turbulent helicity in the steppe region of southern Russia near Tsimlyansk Reservoir show the opposite sign of helicity from that expected. A possible explanation for this phenomenon may be the joint action of different scales of atmospheric flows within the boundary layer, including the sea-breeze circulation over the test site. In this regard, we consider a superposition of the classic Ekman spiral solution and Prandtl’s jet-like slope-wind profile to describe the planetary boundary-layer wind structure. The latter solution mimics a hydrostatic shallow breeze circulation over a non-uniformly heated surface. A 180°-wide sector on the hodograph plane exists, within which the relative orientation of the Ekman and Prandtl velocity profiles favours the left rotation with height of the resulting wind velocity vector in the lowermost part of the boundary layer. This explains the negative (left-handed) helicity cascade toward small-scale turbulent motions, which agrees with the direct field measurements of turbulent helicity in Tsimlyansk. A simple turbulent relaxation model is proposed that explains the measured positive values of the relatively minor contribution to turbulent helicity from the vertical components of velocity and vorticity.  相似文献   

18.
Summary This case study describes the decay of a low-level jet in the alpine foreland as a planetary boundary layer phenomenon. Few measurements are known, which document this transition period of the boundary layer from night to day. Analysis of 1 and 20 Hz data of temperature and the three wind components of the ELECTRA aircraft mission on April 16, 1982 between 5 and 11 GMT allow an assessment of the temporal and spatial fine structure during the decay phase of the jet in the morning. Using the flight technique of horizontal and vertical zigzagging, the coupling of thermal stability, turbulence intensity, topography and behavior of the jet is shown.Vertical profiles and cross-sectional analysis of mean and turbulent parameters document three separate jet decay phases: decoupling, transition and erosion. During the first phase, the jet maximum is situated between surface and synoptic inversions, decoupling from surface friction and prohibiting momentum transport towards the free atmosphere. During the transition phase, the jet maximum increases in altitude. In the erosion phase, the jet covers the top of a developing well mixed layer. Turbulent mixing and entrainment on top of the layer are responsible for the decay of the jet. This is confirmed by calculating the decrease of the wind speed maximum from the turbulent momentum flux and the growth rate of the mixing layer by means of a mixed layer model in comparison to the measured wind speed jump and to other observations.With 7 Figures  相似文献   

19.
Analysis of profiles of meteorological measurements from a 160 m high mast at the National Test Site for wind turbines at Høvsøre (Denmark) and at a 250 m high TV tower at Hamburg (Germany) shows that the wind profile based on surface-layer theory and Monin-Obukhov scaling is valid up to a height of 50–80 m. At higher levels deviations from the measurements progressively occur. For applied use an extension to the wind profile in the surface layer is formulated for the entire boundary layer, with emphasis on the lowest 200–300 m and considering only wind speeds above 3 m s?1 at 10 m height. The friction velocity is taken to decrease linearly through the boundary layer. The wind profile length scale is composed of three component length scales. In the surface layer the first length scale is taken to increase linearly with height with a stability correction following Monin-Obukhov similarity. Above the surface layer the second length scale (L MBL ) becomes independent of height but not of stability, and at the top of the boundary layer the third length scale is assumed to be negligible. A simple model for the combined length scale that controls the wind profile and its stability dependence is formulated by inverse summation. Based on these assumptions the wind profile for the entire boundary layer is derived. A parameterization of L MBL is formulated using the geostrophic drag law, which relates friction velocity and geostrophic wind. The empirical parameterization of the resistance law functions A and B in the geostrophic drag law is uncertain, making it impractical. Therefore an expression for the length scale, L MBL , for applied use is suggested, based on measurements from the two sites.  相似文献   

20.
We investigate the effects of an isolated meso-\(\gamma \)-scale surface heterogeneity for roughness and albedo on the atmospheric boundary-layer (ABL) height, with a case study at a semi-arid forest surrounded by sparse shrubland (forest area: \(28~\text{ km }^2\), forest length in the main wind direction: 7 km). Doppler lidar and ceilometer measurements at this semi-arid forest show an increase in the ABL height over the forest compared with the shrubland on four out of eight days. The differences in the ABL height between shrubland and forest are explained for all days with a model that assumes a linear growth of the internal boundary layer of the forest through the convective ABL upwind of the forest followed by a square-root growth into the stable free atmosphere. For the environmental conditions that existed during our measurements, the increase in ABL height due to large sensible heat fluxes from the forest (\(600~\text {W~m}^{-2}\) in summer) is subdued by stable stratification in the free atmosphere above the ABL, or reduced by high wind speeds in the mixed layer.  相似文献   

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