30 research outputs found
Composite hodographs and inertial oscillations in the nocturnal boundary layer
No full textIn this work the dynamic behaviour of the wind in the nocturnal boundary layer is studied, with a particular focus on systematic behaviour of the near-surface wind. Recently, an extension of the well-known Blackadar model for frictionless inertial oscillations above the nocturnal boundary layer was proposed by Van deWiel et al., which accounts for frictional effects within the nocturnal boundary layer. It appears that the nocturnal wind velocity profile tends to perform an inertial oscillation around an equilibrium wind profile, rather than around the geostrophic wind vector (as in the Blackadar model). In the present studywe propose the concept of ‘composite hodographs’ to evaluate the ideas and assumptions of the aforementioned analytical model. Composite hodographs are constructed based on a large observational dataset from the Cabauw observatory. For comparison and deeper analysis, this method is also applied to single-column model simulations that represent the same dataset. From this, it is shown that winds in the middle and upper part of the nocturnal boundary layer closely follow the dynamics predicted by the model by Van de Wiel et al. In contrast, the near-surface wind shows more complex behaviour that can be described by two different stages: (1) a decelerating phase where the wind decreases rapidly in magnitude due to enlarged stress divergence in the transition period near sunset (an aspect not included in the analytical model), and (2) a regular type of inertial oscillation, but with relatively small amplitude as compared to the oscillations in the middle and upper parts of the nocturnal boundary laye
The cessation of continuous turbulence as precursor of the very stable nocturnal boundary layer
No full textThe mechanism behind the collapse of turbulence in the evening as a precursor to the onset of the very stable boundary layer is investigated. To this end a cooled, pressure-driven flow is investigated by means of a local similarity model. Simulations reveal a temporary collapse of turbulence whenever the surface heat extraction, expressed in its nondimensional form h/L, exceeds a critical value. As any temporary reduction of turbulent friction is followed by flow acceleration, the long-term state is unconditionally turbulent. In contrast, the temporary cessation of turbulence, which may actually last for several hours in the nocturnal boundary layer, can be understood from the fact that the time scale for boundary layer diffusion is much smaller than the time scale for flow acceleration. This limits the available momentum that can be used for downward heat transport. In case the surface heat extraction exceeds the so-called maximum sustainable heat flux (MSHF), the near-surface inversion rapidly increases. Finally, turbulent activity is largely suppressed by the intense density stratification that supports the emergence of a different, calmer boundary layer regime.Multi-Scale PhysicsApplied Science
Daily cycle of Skewness and Kurtosis charateristics within and just above a crop canopy
No full textA measurement program is carried out within as well as above a maize crop canopy. Statistical characteristics are analyzed for the velocity components as well as for temperature for a clear weather day as well as a cloudy day. During daytime it appears that the above and within-canopy characteristics are dominated by sweeps, which means a positive u-skewness and a negative w-skewness. During daytime the within-canopy w-kurtosis is extremely high due to strong turbulence events of sweeps as well as ejections. During night-time, hot plumes released from the soil surface dominate the extremes, which results in a positive w-skewness, above as well as within the canopy. The temperature skewness is mostly positive within as well as above the canopy. After mid-day on the clear day with low wind conditions, however, the within-canopy skewness for temperature is negative nearly throughout the whole canopy due to sweeps from a relatively high and cool level
Approximate solution for the Obukhov lenght and the surface fluxes in terms of bulk Richardson numbers
No full textSimple analytic approximate solutions arepresented for the set of equations that follows fromthe Monin–Obukhov flux-profile relationships using thestability functions of Dyer (unstable case) andBeljaars–Holtslag (stable case). Several publicationsare devoted to the same subject, however the currentapproach contains some new features, namely: (a) itappears to be more accurate for unstable situationsand (b) it applies also to the general case where windspeed (u) and potential temperature(¿) are given at different levels. In order toillustrate the accuracy of the approach a comparisonwith the actual solutions is presented for someselected combinations of ¿ and u levelstypical for various practical applications
Modelling the artic stable boundary layer and its coupling to the surface
No full textThe impact of coupling the atmosphere to the surface energy balance is examined for the stable boundary layer, as an extension of the first GABLS (GEWEX Atmospheric Boundary-Layer Study) one-dimensional model intercomparison. This coupling is of major importance for the stable boundary-layer structure and its development because coupling enables a realistic physical description of the interdependence of the surface temperature and the surface sensible heat flux. In the present case, the incorporation of a surface energy budget results in stronger cooling (surface decoupling), and a more stable and less deep boundary layer. The proper representation of this is a problematic feature in large-scale numerical weather prediction and climate models. To account for the upward heat flux from the ice surface beneath, we solve the diffusion equation for heat in the underlying ice as a first alternative. In that case, we find a clear impact of the vertical resolution in the underlying ice on boundary-layer development: coarse vertical resolution in the ice results in stronger surface cooling than for fine resolution. Therefore, because of this impact on stable boundary-layer development, the discretization in the underlying medium needs special attention in numerical modelling studies of the nighttime boundary layer. As a second alternative, a bulk conductance layer with stagnant air near the surface is added. The stable boundary-layer development appears to depend heavily on the bulk conductance of the stagnant air layer. This result re-emphasizes the fact that the interaction with the surface needs special attention in stable boundary-layer studies. Furthermore, we perform sensitivity studies to atmospheric resolution, the length-scale formulation and the impact of radiation divergence on stable boundary-layer structure for weak windy conditions
Exploring self-correlation in flux–gradient relationships for stably stratified conditions
No full textIn this paper, the degree of scatter in flux–gradient relationships for stably stratified conditions is analyzed. It is generally found that scatter in the dimensionless lapse rate ¿h is larger than in the dimensionless shear ¿m when plotted versus the stability parameter z/¿ (where ¿ is the local Obukhov length). Here, this phenomenon is explained to be a result of self-correlation due to the occurrence of the momentum and the heat flux on both axes, measurement uncertainties, and other possibly relevant physical processes left aside. It is shown that the ratio between relative errors in the turbulent fluxes influences the orientation of self-correlation in the flux–gradient relationships. In stable conditions, the scatter in ¿m is largely suppressed by self-correlation while for ¿h this is not the case (vice versa for unstable stratification). An alternative way of plotting is discussed for determining the slope of the linear ¿m function
Local Similarity in the Stable Boundary Layer and Mixing-Length Approaches: Consistency of Concepts
No full textIn stably stratified flows vertical movement of eddies is limited by the fact that kinetic energy is converted into potential energy, leading to a buoyancy displacement scale z B . Our new mixing-length concept for turbulent transport in the stable boundary layer follows a rigid-wall analogy, in the sense that we assume that the buoyancy length scale is similar to neutral length scaling. This implies that the buoyancy length scale is: ? B = ? B z B , with ? B ? ?, the von Karman constant. With this concept it is shown that the physical relevance of the local scaling parameter z/? naturally appears, and that the ? coefficient of the log-linear similarity functions is equal to c/? 2, where c is a constant close to unity. The predicted value ? ? 1/? 2 = 6.25 lies within the range found in observational studies. Finally, it is shown that the traditionally used inverse linear interpolation between the mixing length in the neutral and buoyancy limits is inconsistent with the classical log-linear stability functions. As an alternative, a log-linear consistent interpolation method is proposed.Department of Multi-Scale PhysicsApplied Science
