首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到10条相似文献，搜索用时 78 毫秒
1.
This study focuses on the behaviour of the turbulent Prandtl number, Pr t , in the stable atmospheric boundary layer (SBL) based on measurements made during the Surface Heat Budget of the Arctic Ocean experiment (SHEBA). It is found that Pr t increases with increasing stability if Pr t is plotted vs. gradient Richardson number, Ri; but at the same time, Pr t decreases with increasing stability if Pr t is plotted vs. flux Richardson number, Rf, or vs. ζ = z/L. This paradoxical behaviour of the turbulent Prandtl number in the SBL derives from the fact that plots of Pr t vs. Ri (as well as vs. Rf and ζ) for individual 1-h observations and conventional bin-averaged values of the individual quantities have built-in correlation (or self-correlation) because of the shared variables. For independent estimates of how Pr t behaves in very stable stratification, Pr t is plotted against the bulk Richardson number; such plots have no built-in correlation. These plots based on the SHEBA data show that, on the average, Pr t decreases with increasing stability and Pr t < 1 in the very stable case. For specific heights and stabilities, though, the turbulent Prandtl number has more complicated behaviour in the SBL.  相似文献   

2.
Measurements of atmospheric turbulence made over the Arctic pack ice during the Surface Heat Budget of the Arctic Ocean experiment (SHEBA) are used to determine the limits of applicability of Monin–Obukhov similarity theory (in the local scaling formulation) in the stable atmospheric boundary layer. Based on the spectral analysis of wind velocity and air temperature fluctuations, it is shown that, when both the gradient Richardson number, Ri, and the flux Richardson number, Rf, exceed a ‘critical value’ of about 0.20–0.25, the inertial subrange associated with the Richardson–Kolmogorov cascade dies out and vertical turbulent fluxes become small. Some small-scale turbulence survives even in this supercritical regime, but this is non-Kolmogorov turbulence, and it decays rapidly with further increasing stability. Similarity theory is based on the turbulent fluxes in the high-frequency part of the spectra that are associated with energy-containing/flux-carrying eddies. Spectral densities in this high-frequency band diminish as the Richardson–Kolmogorov energy cascade weakens; therefore, the applicability of local Monin–Obukhov similarity theory in stable conditions is limited by the inequalities Ri < Ri cr and Rf < Rf cr. However, it is found that Rf cr  =  0.20–0.25 is a primary threshold for applicability. Applying this prerequisite shows that the data follow classical Monin–Obukhov local z-less predictions after the irrelevant cases (turbulence without the Richardson–Kolmogorov cascade) have been filtered out.  相似文献   

3.
Wind-tunnel simulations of theatmospheric stable boundary layer (SBL) developedover a rough surface were conducted by using athermally stratified wind tunnel at the Research Institutefor Applied Mechanics (RIAM), Kyushu University. Thepresent experiment is a continuation of the workcarried out in a wind tunnel at Colorado StateUniversity (CSU), where the SBL flows were developed over asmooth surface. Stably stratified flows were createdby heating the wind-tunnel airflow to a temperature ofabout 40–50°and by cooling the test-section floor toa temperature of about 10°. To simulate therough surface, a chain roughness was placed over thetest-section floor. We have investigated the buoyancyeffect on the turbulent boundary layer developed overthis rough surface for a wide range of stability,particularly focusing on the turbulence structure andtransport process in the very stable boundary layer.The present experimental results broadly confirm theresults obtained in the CSU experiment with the smoothsurface, and emphasizes the following features: thevertical profiles of turbulence statistics exhibitdifferent behaviour in two distinct stability regimes with weak and strong stability,corresponding to the difference in the verticalprofiles of the local Richardson number. The tworegimes are separated by the critical Richardsonnumber. The magnitudes in turbulence intensities andturbulent fluxes for the weak stability regime aremuch greater than those of the CSU experiments becauseof the greater surface roughness. For the very stableboundary layer, the turbulent fluxes of momentum andheat tend to vanish and wave-like motions due to theKelvin–Helmholtz instability and the rolling up andbreaking of those waves can be observed. Furthermore,the appearance of internal gravity waves is suggestedfrom cross-spectrum analyses.  相似文献   

4.
The experiment IGLOS (Investigation of the Greenland Boundary Layer Over Summit) was conducted in June and July 2002 in the central plateau of the Greenland inland ice. The German research aircraft Polar2, equipped with the turbulence measurement system Meteopod, was used to investigate turbulence and radiation flux profiles near research station “Summit Camp”. Aircraft measurements are combined with measurements of radiation fluxes and turbulent quantities made from a 50 m tower at Summit Camp operated by Eidgenössische Technische Hochschule (ETH) Zürich. During all six flight missions, well-developed stable boundary layers were found. Even in high-wind conditions, the surface inversion thickness did not exceed roughly 100 m. The turbulent height of the stable boundary layer (SBL) was found to be much smaller than the surface inversion thickness. Above the surface layer, significant turbulent fluxes occurred only intermittently in intervals on the order of a few kilometres. Turbulent event fraction in the upper SBL shows the same dependence on gradient Richardson number as reported for near-surface measurements. Clear-air longwave radiation divergence was always found to contribute significantly to the SBL heat budget. In low-wind cases, radiative cooling even turned out to be dominant.  相似文献   

5.
Utilizing aircraft sounding data collected from the Surface Heat Budget of the Arctic Ocean (SHEBA, 1998) campaign, the authors evaluated commonly-used profile methods for Arctic ABL height estimation by validating against the’true’ABL height from aircraft sounding profiles, where ABL height is defined as the top of the layer with significant turbulence. Furthermore, the best performing method was used to estimate ABL height from the one-year GPS soundings obtained during SHEBA (October 1997-October 1998). It was found that the temperature gradient method produces a reliable estimate of ABL height. Additionally, the authors determined optimal threshold values of temperature gradient for stable boundary layer (SBL) and convective boundary layer (CBL) to be 6.5 K/100 m and 1.0 K/100 m, respectively. The maximum ABL height during the year was 1150 m occurred in May. Median values of Arctic ABL height in May, June, July, and August were 400 m, 430 m, 180 m, and 320 m, respectively. Arctic ABL heights are clearly higher in the spring than in the summer.  相似文献   

6.
Nocturnal Boundary-Layer Regimes   总被引：17，自引：6，他引：11  
This study analyzes turbulence data collected over a grassland site in the nocturnal boundary layer. Examination of the dependence of the nocturnal boundary layer on stability suggests three regimes: a) the weakly stable case, b) a transition stability regime where many of the variables change rapidly with increasing stability and c) the very stable case. The value of z/L where the downward heat flux is a maximum defines the stability boundary between the weakly stable and transition regimes, where L is the Obukhov length. In the present analysis, the downward heat flux reaches a maximum at z/L approximately equal to 0.05 for 10 m, although comparison with other data indicates that this is not a universal value. For weaker stability, the heat flux decreases with decreasing z/L due to weaker temperature fluctuations. In the transition stability regime, the heat flux decreases rapidly with increasing stability due to restriction of vertical velocity fluctuations by the increasing stratification.For weakly stable conditions, the variances scale according to Monin-Obukhov similarity theory. For very stable conditions, the variances are contaminated by non-turbulent horizontal motions and do not follow the scaling laws. An alternative length scale based on variances is developed which explains more of the variance of the transfer coefficients compared to the Obukhov length.  相似文献   

7.
We document numerical experiments with a single-column, high-resolution model of the stable boundary layer. The model resolves the logarithmic layer, and does not require inverting the Monin–Obukhov similarity functions in order to calculate the surface fluxes. The turbulence closure is based on the K-theory approach, with a new form of stability functions of the Richardson number, evaluated by using the Surface Heat Budget of the Arctic Ocean (SHEBA) and the Cooperative Atmosphere-Surface Exchange Study (CASES-99) data. A comparison with two, high-resolution large-eddy simulation models shows very good agreement. The reported numerical experiments test the effects of shear, surface cooling, the Coriolis parameter, subsidence, and baroclinicity. The time evolution of the drag coefficient, the heat-transfer coefficient, and the cross-isobar angle is also evaluated.  相似文献   

8.
Detailed measurements of profiles of mean and turbulent variablesthrough the nocturnal stable boundary layer over a valley arepresented. Two nights of data are analysed in detail, one with only aweakly stable boundary layer and one with a strongly stable boundarylayer. The weakly stratified night shows high levels of turbulence inwhich the flow remains attached to the valley and the boundary layeracts as a single coherent layer. On the strongly stratified night, twoflow regimes are identified: attached flow, as on the weaklystratified night, and decoupled flow in which the air in the valleybecomes so dynamically stable that there is no turbulent interactionwith the mean flow aloft. Because the valley is sloping, the decoupledlower stagnant air then forms a drainage current. We find that theFroude number evaluated at the hill height, FH = U(H)/N(H) H,diagnoses the flow regime: when FH = 2, the flow remainsattached and when FH 2 the flow in the valley becomesdecoupled from the flow aloft. The dynamics of the flow regimes areshown to be understandable in terms of the gradient Richardson number,which indicates the turbulent mixing. We show that the gradientRichardson number is a key parameter in diagnosing each flow regime.  相似文献   

9.
Measurements of atmospheric turbulence made during the Surface Heat Budget of the Arctic Ocean Experiment (SHEBA) are used to examine the profile stability functions of momentum, φ m , and sensible heat, φ h , in the stably stratified boundary layer over the Arctic pack ice. Turbulent fluxes and mean meteorological data that cover different surface conditions and a wide range of stability conditions were continuously measured and reported hourly at five levels on a 20-m main tower for 11 months. The comprehensive dataset collected during SHEBA allows studying φ m and φ h in detail and includes ample data for the very stable case. New parameterizations for φ m (ζ) and φ h (ζ) in stable conditions are proposed to describe the SHEBA data; these cover the entire range of the stability parameter ζ = z/L from neutral to very stable conditions, where L is the Obukhov length and z is the measurement height. In the limit of very strong stability, φ m follows a ζ 1/3 dependence, whereas φ h initially increases with increasing ζ, reaches a maximum at ζ ≈ 10, and then tends to level off with increasing ζ. The effects of self-correlation, which occur in plots of φ m and φ h versus ζ, are reduced by using an independent bin-averaging method instead of conventional averaging.  相似文献   

10.
Stratified Atmospheric Boundary Layers   总被引：32，自引：24，他引：8  
Various features of different stability regimes of the stable boundary layer are discussed. Traditional layering is examined in terms of the roughness sublayer, surface layer, local similarity, z-less stratification and the region near the boundary-layer top. In the very stable case, the strongest turbulence may be detached from the surface and generated by shear associated with a low level jet, gravity waves or meandering motions. In this case, similarity theory and the traditional concept of a boundary-layer break down. The elevated turbulence may intermittently recouple to the surface. Inability to adequately measure turbulent fluxes in very stable conditions limits our knowledge of this regime.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号