The Cessation of Continuous Turbulence as Precursor of the Very Stable Nocturnal Boundary Layer

Wiel, van de Bjh Bas; Moene, Arnold F.; Jonker, Harm J. J.

doi:10.1175/jas-d-12-064.1

Cited by 81 publications

(59 citation statements)

References 72 publications

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“…Further theoretical analysis shows that is impossible for the traditional parameters to make such distinction. Shear Capacity, however, appears the right normalisation to build a dimensionless theoretical framework to generalise the hypothesis of [3,4]. The fact that both results from atmospheric flows as well as theoretical model analysis point in the same direction, indicates that the Shear Capacity is a suitable indicator of this turbulent-to-laminar transition.…”

Section: Resultsmentioning

confidence: 94%

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Shear Capacity as Prognostic for Nocturnal Boundary Layer Regimes

Hooijdonk

Donda

Clercx

et al. 2015

Journal of the Atmospheric Sciences

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In some nights, the near-surface temperature can drop dramatically and turbulence in the stably stratified boundary layer becomes very weak, such that the lfow reaches a (quasi-) laminar state. In other cases, however, the atmosphere remains in a turbulent state and temperatures stay relatively high. Recently, the appearance of two distinct boundary layer regimes was explained by a new theoretical framework. This theory builds on the fact that the turbulent heat flux in stably stratified flow is limited to a maximum for given wind shear. This introduces a characteristic flux-based velocity scale, which can be used to predict the regimes. This hypothesis is consistent with field observations and numerical results. Also, the hypothesis is generalised to a dimensionless framework.

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Section: Resultsmentioning

confidence: 94%

“…Recently, a mechanism was proposed by [3,4]. It is based on the fact that the turbulent heat flux (heat supply to the surface energy budget) is limited to a maximum.…”

Section: Conceptmentioning

confidence: 99%

Shear Capacity as Prognostic for Nocturnal Boundary Layer Regimes

Hooijdonk

Donda

Clercx

et al. 2015

Journal of the Atmospheric Sciences

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“…Such nonstationarity can be internally forced through interactions of the turbulence with the mean flow (van de Wiel et al 2012a) or forced by submeso motions (Mahrt 2010b). Acevedo et al (2014) find that the submeso motions introduce a site dependency for the relationship between the turbulence and the mean flow for stable conditions.…”

Section: Introductionmentioning

confidence: 99%

Dependence of Turbulent Velocities on Wind Speed and Stratification

Mahrt

Sun

Stauffer

2014

Boundary-Layer Meteorol

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We examine the dependence of several turbulence quantities on the wind speed and stability using nocturnal data from the Shallow Cold Pool Experiment. The turbulent quantities (velocities) are defined in terms of the standard deviation of the horizontal and vertical velocity fluctuations, two different calculations of the friction velocity, and two turbulent velocities based on the heat flux. The dependence of the turbulent velocities on the wind speed shows a transition between the weak-wind regime of small slope and the stronger wind regime of larger slope, as found in previous studies. This transition occurs for all of the turbulent velocities examined and occurs for a wide range of averaging times. Although this study concentrates primarily on data over a flat surface above the valley, the transition also occurs at the other 18 stations that have non-zero local slopes up to about 10 %. At the same time, the relationship between the turbulence and the wind speed cannot be universal because of the influence of stratification and site-dependent non-stationarity in the weak-wind regime. The wind speed of the transition increases with increasing stratification at a rate that is an order of magnitude slower than that predicted by a constant transition bulk Richardson number. For the weakest winds, the impact of stratification is unexpectedly small.

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“…Numerical simulations play an increasing role in the understanding of these complex processes, and in quantifying the dual problem of the increased stability [11] and the spontaneous generation of bursts. However, modeling of the PBL in weather and climate codes is often inadequate, resulting, for example, in an inaccurate evaluation of the extension of the ice sheet, as is the case over Greenland [12], and in a faulty estimate of the overall energy balance in long-term climate systems, since it affects for example mixing, frost occurrence, aerosol dispersion, and air quality [13].…”

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confidence: 99%

Turbulence comes in bursts in stably stratified flows

2014

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There is a clear distinction between simple laminar and complex turbulent fluids. But in some cases, as for the nocturnal planetary boundary layer, a stable and well-ordered flow can develop intense and sporadic bursts of turbulent activity which disappear slowly in time. This phenomenon is ill-understood and poorly modeled; and yet, it is central to our understanding of weather and climate dynamics. We present here a simple model which shows that in stably stratified turbulence, the stronger bursts can occur when the flow is expected to be more stable. The bursts are generated by a rapid non-linear amplification of energy stored in waves, and are associated with energetic interchanges between vertical velocity and temperature (or density) fluctuations. Direct numerical simulations on grids of 2048 3 points confirm this somewhat paradoxical result of measurably stronger events for more stable flows, displayed not only in the temperature and vertical velocity derivatives, but also in the amplitude of the fields themselves.PACS numbers: 47.55. Hd, 47.35.Bb, 47.27.ek Large fluctuations are common in physical systems with long-range correlations, and have been found to be linked to so-called "1/f" noise [1]. They take the form of sporadic and localized events, as observed in many instances in critical phenomena and in turbulent flows, and are diagnosed through non-Gaussian Probability Distribution Functions (PDFs) [2]. In turbulence, strong events occur in field gradients, with the velocity itself being nearly Gaussian. There are however exceptions to this last rule for shear flows [3,4], quantum fluids [6,7], and subtropical current systems [5]. Extreme events associated with random plumes have also been diagnosed in the atmospheric convective boundary layer [8], or when linked with coherent structures, e.g., storm tracks.The occurrence of intermittent strong activity is a signature of fully developed turbulence and is therefore more surprising in stable flows. Intermittency makes the stable nocturnal planetary boundary layer (PBL) highly unpredictable: as night sets in, this layer between the atmosphere and land or sea stabilizes due to the radiative cooling of the land and ocean masses. It is still unclear how stable the nocturnal PBL becomes. Three regimes have been observed [9]: very stable, weakly stable with turbulent motions persisting and competing with internal gravity waves, and transitory. Even in the very stable case, the PBL is subject to intense sporadic bursts of turbulence which die out after many wave periods [9,10].Numerical simulations play an increasing role in the understanding of these complex processes, and in quantifying the dual problem of the increased stability [11] and the spontaneous generation of bursts. However, modeling of the PBL in weather and climate codes is often inadequate, resulting, for example, in an inaccurate evaluation of the extension of the ice sheet, as is the case over Greenland [12], and in a faulty estimate of the overall energy balance in long-term climate syste...

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The Cessation of Continuous Turbulence as Precursor of the Very Stable Nocturnal Boundary Layer

Cited by 81 publications

References 72 publications

Shear Capacity as Prognostic for Nocturnal Boundary Layer Regimes

Shear Capacity as Prognostic for Nocturnal Boundary Layer Regimes

Dependence of Turbulent Velocities on Wind Speed and Stratification

Turbulence comes in bursts in stably stratified flows

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