Stable Boundary-Layer Scaling Regimes: The Sheba Data

Turbulent and mean meteorological data collected at five levels on a 20-m tower over the Arctic pack ice during the Surface Heat Budget of the Arctic Ocean experiment (SHEBA) are analyzed to examine different regimes of the stable boundary layer (SBL). Eleven months of measurements during SHEBA cover a wide range of stability conditions, from the weakly unstable regime to very stable stratification. Scaling arguments and our analysis show that the SBL can be classified into four major regimes: (i) surface-layer scaling regime (weakly stable case), (ii) transition regime, (iii) turbulent Ekman layer, and (iv) intermittently turbulent Ekman layer (supercritical stable regime). These four regimes may be considered as the basic states of the traditional SBL. Sometimes these regimes, especially the last two, can be markedly perturbed by gravity waves, detached elevated turbulence (‘upside down SBL’), and inertial oscillations. Traditional Monin–Obukhov similarity theory works well in the weakly stable regime. In the transition regime, Businger–Dyer formulations work if scaling variables are re-defined in terms of local fluxes, although stability function estimates expressed in these terms include more scatter compared to the surface-layer scaling. As stability increases, the near-surface turbulence is affected by the turning effects of the Coriolis force (the turbulent Ekman layer). In this regime, the surface layer, where the turbulence is continuous, may be very shallow (< 5 m). Turbulent transfer near the critical Richardson number is characterized by small but still significant heat flux and negligible stress. The supercritical stable regime, where the Richardson number exceeds a critical value, is associated with collapsed turbulence and the strong influence of the earth’s rotation even near the surface. In the limit of very strong stability, the stress is no longer a primary scaling parameter.     

Radiative Processes in the Stable Boundary Layer: Part II. The Development of the Nocturnal Boundary Layer

The interaction between radiation and turbulence in the stable boundary layer over land is explored using an idealized model, with a focus on the surface layer after the evening transition. It is shown that finer vertical resolution is required in transitional boundary layers than in developed ones. In very light winds radiative cooling determines the temperature profile, even if similarity functions without a critical Richardson number are used; standard surface similarity theory applied over thick layers then yields poor forecasts of near-surface air temperatures. These points are illustrated with field data. Simulations of the developing nocturnal boundary layer are used to explore the wider role of radiation. Comparatively, radiation is less significant within the developed stable boundary layer than during the transition; although, as previous studies have found, it remains important towards the top of the stable layer and in the residual layer. Near the ground, reducing the surface emissivity below one is found to yield modest relative radiative warming rather than intense cooling, which reduces the potential importance of radiation in the developed surface layer. The profile of the radiative heating rate may be strongly dependent on other processes, leading to quite varied behaviour.     

Nocturnal Boundary-Layer Regimes

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.     

Extremely Weak Mixing in Stable Conditions

Transport by extremely weak turbulence occurring on nights with clear skies and weak winds is examined from seven tower levels of eddy-correlation data taken from each of two field programs. The very small flux is systematic, provided that the perturbations are computed from a record-dependent averaging length, which must be as small as 10 s in very stable conditions. With traditional methods for computing the flux, these fluxes were considered too small to estimate, in that the computed values behaved erratically. For extremely weak turbulence, the fluxes decrease systematically with height and often indicate very shallow boundary-layer depths on the order of 10 m. However, in one field program, the turbulence slowly increases with height above the surface flux-based boundary layer apparently due to horizontal advection of stronger turbulence driven by modest surface heterogeneity. For very weak turbulence, the eddy diffusivity for momentum is systematically greater than that for heat in both field programs. The dependence of the turbulence strength and its variability with stability is examined in some detail.     

Contrasting vertical structures of nocturnal boundary layers

This study analyzes eight levels of sonic anemometerdata collected on a 60-m towerduring CASES-99, toward the goal of understanding thevertical structure of thenocturnal boundary layer. Several different regimesare found. Thin boundarylayers are often observed where fluxes decrease with height and approximately vanish between 20 and 30 m aboveground. The flow above the thin boundary layeraccelerates and increasing shear oftengenerates significant turbulence in the middle ofthe night. Thisshear-generated turbulence is often stronger thanthat near the surface corresponding to an upside-downboundary layer. During these conditions,the turbulent transport of turbulence is downwardtoward the surface. The turbulence in this regimeshows features of z-less turbulence to the extentthat neither the height above groundnor the boundary-layer depth are primary scalingvariables. This layer isdifferent from a `residual layer' in thatturbulence is actively generated byshear associated with nocturnal accelerationsand often is stronger than that inthe surface-based boundary layer.In many cases, the turbulence does not varysignificantly across the towerlayer, implying that the boundary layer ismuch deeper than the 60-m towerlayer. Several case studies are presentedto illustrate the largevariation of vertical structure betweennights.     

Boundary-Layer Similarity Under an Axisymmetric,Gradient Wind Vortex

We present similarity solutions for the mean boundary-layer profiles under an axisymmetric vortex that is in gradient wind balance; the similarity model includes the nonlinear momentum advection and curvature terms. These solutions are a generalization of the Ekman layer mean flow, which is the canonical boundary-layer basic state under a uniform, geostrophically-balanced flow. Near-surface properties such as inflow angle, surface wind factor, diffusive transport of kinetic energy into the surface layer and dissipational heating are calculated and shown to be sensitive to the choice of turbulence parameterization.     

论大气边界层的局地相似性

本文利用日本气象厅研究所在筑波市213m气象塔1983年观测的湍流资料验证了大气边界层的局地相似性,求出了相似性函数的经验常数.进一步建立了局地湍流统计量同近地面层和边界层顶湍流通量之间的关系.     

夜间近地面稳定边界层湍流间歇与增温

在夜间晴空条件下，近地面大气湍流表现出很强的间歇性，这种间歇现象导致夜间气温短时的急剧下降，随后大幅度增温。近地面大幅度增温表明此时存在着很大的湍流热通量散度，常通量层的概念这时不存在。从各高度层温度和风速变化的曲线上分析，我们发现湍流大多在距离地面较高一点的高度上发生发展，然后向下层传递，尽管上层的湍流可能是由于下层的某一触发机制向上传递而引起的。湍流偶尔也出现自下向上传递的过程，但这一过程较少发生。湍流的这种上下传递说明稳定边界层大气经常处于非平衡状态，在运用相似理论研究稳定边界层大气结构时要特别注意这一情形。     

Turbulence Characteristics Of The Stable Boundary Layer Over A Mid-Latitude Glacier. Part Ii: Pure Katabatic Forcing Conditions

Observations obtained over a glacier surface in a predominantlykatabatic flow and with a distinctwind maximum below 13-m height are presented. The data werecollected using a 13-m high profilemast and two sonic anemometers (at about 2.5-m and 10-m heights).The spectra at frequencies belowthat of the turbulence range appear to deviate considerably fromthe curves obtained by Kaimal andco-workers during the 1968 Kansas experiment. The characteristicsof these deviations are compared tothe observations of others in surface-layers disturbed by anykind of large-scale outer-layer (orinactive) turbulence. In our case the disturbances arelikely to be induced by the highmountain ridges that surround the glacier. Moreover, the deviationsobserved in the cospectra seemto result from an, as yet, unspecified interaction between theinactive outer-layer turbulenceand the local surface-layer turbulence. Near the distinctwind maximum turbulence production ceasedwhile turbulence itself did not, probably the result ofturbulence transport from other levels. Consequently, we studied thelocal similarity relations using _w instead of u_* as an alternative velocity scale. Wellbelow the wind maximum, and for relatively low stability(0< Ri_g <0.2), the flow behaves accordingto well established local-scaling similarity relationshipsin the stable boundary layer. For higherstability (Ri_g > 0.2), and near or above the wind maximum, the boundary-layer structure conforms tothat of z-less stratification suggesting that the eddy sizeis restricted by the local stability ofthe flow. In line with this we observed that the sensibleheat fluxes relate remarkably well to thelocal flow parameters.     

On the Kinetic Energy Budget of the Unstable Atmospheric Surface Layer

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.     

Note on turbulence statistics in z-less stratification

In boundary layer meteorology, surface layer similarity theory plays a critical role in measuring and modeling biospheric fluxes. In stable boundary layer, surface layer similarity called z-less stratification has been one of main research topics for over than two decades and the issue has yet to be settled in micrometeorology. In this scientific discussion on z-less turbulence, different turbulence statistics were used inconsistently and it was argued that z-less turbulence was valid if only any turbulence statistics were constant with different atmospheric stabilities. Consequently, such inconsistently tested turbulence statistics and misconception on z-less turbulence hinder us from correctly understanding turbulence structure in the stable boundary layer. This note revisits z-less turbulence and emphasizes that different dimensionless turbulence statistics generally do not exhibit a common behavior in the limit of z-less stratification.     

A Note on Wave-Turbulence Interaction and the Possibility of Scaling the Very Stable Boundary Layer

Success in scaling the very stable boundary layer has been mixed. One possible reason for this is that wave-turbulence interaction can result in significant energy coupling between turbulence within the planetary boundary layer and the mean flow outside the PBL. Three regimes are described where wave-turbulence interaction forms a significant part of the dynamics. In the first, energy and momentum flows are confined within the PBL and do not preclude local scaling. In the other two, which involve topographically forced waves and propagating waves respectively, energy/momentum transfers across the PBL make success in local scaling unlikely.     

Aircraft Observations Of The Mean And Turbulent Structure Of A Shallow Boundary Layer Over The Persian Gulf

Stable internal boundary layers form when warm air isadvected over a cooler surface, a common occurrence incoastal areas. The internal boundary layer deepenswith distance along-wind, eventually reachingequilibrium with the surface and becoming a fullydeveloped marine boundary layer. We presentobservations of the late stages of internalboundary-layer evolution made bythe U.K. Meteorological Office'sC-130 Hercules research aircraft over the Persian Gulfin April 1996. Northwesterly winds brought warm dryair from the surrounding desert landmass across thecooler waters of the Gulf. Loss of heat to the surfaceresulted in the formation of a shallow, stableinternal boundary layer downwind of the coast. The aircraftmeasurements were made several hundred kilometresdownwind, by which point the original deep convectiveboundary layer had been eroded away and the internalboundary layer was well developed, effectively a newmarine atmospheric boundary layer. Throughout most ofits depth the boundary layer was statically stable anda downward heat flux of approximately 15 W m^-2was observed; however, an exceptionally strong latentheat flux, in excess of 250 W m^-2 near thesurface, was sufficient to overcome the downwards heatflux and maintain weak buoyant convection in the lower30–50% of the boundary layer.Scaling of boundary-layer turbulence statistics usinglocal similarity theory produces results in agreementwith previous studies. Because of the strong humiditycontribution to the buoyancy flux, however, care isrequired with the definition of the similarity scales.It is usual for either the sensible heat or buoyancyflux to be used in the definitions of both thetemperature and length scales; the latter being usedover water where humidity plays a significant role indetermining stability. In the present case we findthat while the buoyancy flux is appropriate in thedefinition of the length scale, the temperature scalemust be defined in terms of the sensible heat flux.     

Vertical Structure Of Turbulence In Offshore Flow During Rasex

The adjustment of the boundary layer immediately downstream froma coastline is examined based on two levels of eddy correlation data collected on a mast at the shore and six levels of eddy correlation data and profiles of mean variables collected from a mast 2 km offshore during the Risø Air-Sea Experiment. The characteristics of offshore flow are studied in terms of case studies and inter-variable relationships for the entire one-month data set. A turbulent kinetic energy budget is constructed for each case study.The buoyancy generation of turbulence is small compared to shear generation and dissipation. However, weakly stable and weakly unstable cases exhibit completely different vertical structure. With flow of warm air from land over cooler water, modest buoyancy destruction of turbulence and reduced shear generation of turbulence over the less rough sea surface cause the turbulence to rapidly weaken downstream from the coast. The reduction of downward mixing of momentum by the stratification leads to smaller roughness lengths compared to the unstable case. Shear generation at higher levels and advection of stronger turbulence from land often lead to an increase of stress and turbulence energy with height and downward transport of turbulence energy toward the surface.With flow of cool air over a warmer sea surface, a convective internal boundary layer develops downstream from the coast. An overlying relatively thick layer of downward buoyancy flux (virtual temperature flux) is sometimes maintained by shear generation in the accelerating offshore flow.     

Wind-Tunnel Study Of Atmospheric Stable Boundary Layers Over A Rough Surface

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.     

Spectra and Cospectra of Turbulence in an Internal Boundary Layer over a Heterogeneously Irrigated Cotton Field

During the Energy Balance Experiment, patch-to-patch irrigation generated gradients in soil moisture in a north-south oriented cotton field. An internal boundary layer (IBL) developed as a result of strong horizontal advection from relatively dry upstream patches to relatively wet downstream patches associated with the prevailing northerly winds. This generated large eddies of multiple sizes, which had significant influences on the structure of turbulence in the IBL. The power spectra and cospectra of wind speed, temperature, humidity, and energy fluxes measured at two heights within the IBL are presented and used to investigate the influence of the IBL on surface layer turbulence. The spectra and cospectra were greatly enhanced by external disturbances at low frequencies. The peak frequencies of these disturbances did not change with height. The spectra and cospectra typically converged and were parallel to the Kansas spectrum at high frequencies (in the inertial subrange). A clear gap in the spectra of horizontal wind velocity existed at intermediate frequencies when the surface layer was stable. The results indicate that large eddies that originated in the upstream convective boundary layer had considerable impacts on the spectra and cospectra of surface layer turbulence. The influence of these large eddies was greater (1) when the IBL was well-developed in the near surface layer than when the IBL did not exist, (2) at higher levels than at lower levels, and (3) when the atmospheric surface layer (ASL) was unstable than when the ASL was stable. The length scales of these large eddies were consistent with the dominant scales of surface heterogeneity at the experiment site.     

Helicopter-Borne Flux Measurements in the Nocturnal Boundary Layer Over Land – a Case Study

This case study introduces measurements of turbulent fluxes in a nocturnal boundary layer in North Germany with the new helicopter-borne turbulence measurement system HELIPOD, a detailed data analysis and examination in regard of systematic errors of the instrument, and some comparison with local similarity theory and experiments of the past, in order to confirm the occurrence of small vertical turbulent fluxes. The examined nocturnal boundary layer offered excellent conditions to analyse the quality of the measurement system. In this connection, a detailed look at a strong ground-based inversion disclosed small turbulent fluxes with a spectral maximum at ten metres wavelength or less, embedded in intermittent turbulence. For verification of these fluxes, the measurements were compared with well established results from past experiments. Local similarity theory was applied to calculate dimensionless variances of the turbulent quantities, which were found in good agreement with other observations. Since shear and stratification varied significantly on the horizontal flight legs due to global intermittency, a method was developed to determine vertical gradients on a horizontal flight pattern, by use of small fluctuations of the measurement height. With these locally determined gradients, gradient transport theory became applicable and the turbulent diffusivities for heat and momentum, the Richardson number, and the flux Richardson number were estimated within isolated strong turbulent outbursts. Within these outbursts the flux Richardson number was found between 0.1 and 0.2. The functional relationship between the gradient Richardson number and the turbulent Prandtl number agreed well with observations in past experiments and large eddy simulation. The impact of the stratification on the vertical turbulent exchange, as already described for the surface layer using Monin–Obukhov similarity, was analogously observed in the very stably stratified bulk flow when local scaling was applied.     

Analysis of Model Results for the Turning of the Wind and Related Momentum Fluxes in the Stable Boundary Layer

The turning of wind with height and the related cross-isobaric (ageostrophic) flow in the thermally stable stratified boundary layer is analysed from a variety of model results acquired in the first Global Energy and Water Cycle Experiment (GEWEX) Atmospheric Boundary Layer Study (GABLS1). From the governing equations in this particular simple case it becomes clear that the cross-isobaric flow is solely determined by the surface turbulent stress in the direction of the geostrophic wind for the quasi-steady state conditions under consideration. Most models indeed seem to approach this relationship but for very different absolute values. Because turbulence closures used in operational models typically tend to give too deep a boundary layer, the integrated total cross-isobaric mass flux is up to three times that given by research numerical models and large-eddy simulation. In addition, the angle between the surface and the geostrophic wind is typically too low, which has important implications for the representation of the larger-scale flow. It appears that some models provide inconsistent results for the surface angle and the momentum flux profile, and when the results from these models are removed from the analysis, the remaining ten models do show a unique relationship between the boundary-layer depth and the surface angle, consistent with the theory given. The present results also imply that it is beneficial to locate the first model level rather close to the surface for a proper representation of the turning of wind with height in the stable boundary layer.