The influence of nonstationarity on the turbulent flux–gradient relationship for stable stratification

2007 ◽  
pp. 89-108
Author(s):  
L. Mahrt
Keyword(s):  
Turbulent Flux ◽  
Stable Stratification ◽  
Flux Gradient

A data-driven stochastic parametrisation of intermittent turbulence in the stably stratified atmospheric boundary layer

2021 ◽  
Author(s):  
Vyacheslav Boyko ◽  
Sebastian Krumscheid ◽  
Nikki Vercauteren
Keyword(s):  
Boundary Layer ◽  
Time Series Data ◽  
Stable Stratification ◽  
Data Driven ◽  
Series Data ◽  
Similarity Function ◽  
Intermittent Turbulence ◽  
Stochastic Perturbations ◽  
Model Based Clustering ◽  
Flux Gradient

<p>We present results on the modelling of intermittent turbulence in the nocturnal boundary layer using a data-driven approach. In conditions of high stratification and weak wind, the bulk shear can be too weak to sustain continuous turbulence, and the sporadic submeso motions play an important role for the turbulence production. We show a way to stochastically parametrise the effect of the unresolved submeso scales and include it into a 1.5-order turbulence closure scheme. This is achieved by introducing a stochastic equation, which describes the evolution of the non-dimensional flux-gradient stability correction for momentum ($\phi_m$). The unperturbed equilibrium solution of the equation follows the functional form of the universal similarity function. The stochastic perturbations reflect the instantaneous excursions from its equilibrium state, and the distribution of values covers the scatter found in observations at high stability.</p><p>The non-stationary parameters of this equations are estimated from a time-series data of the FLOSS2 experiment using a model-based clustering approach. The clustering analysis of the parameters shows a scaling relationship with the local gradient Ri number, leading to a suggested closed-form model for the stochastic flux-gradient stability correction. The spatial correlation in height of the perturbations is included in the model as well. The resulting equation captures the transition of the stability correction across and beyond the critical Ri up to a value of 10. The out-of-sample prediction shows a valid transient dynamics into and within the regime of strongly-stable stratification.</p>

Turbulent flux estimation in the atmospheric surface layer through flux-gradient similarity during the ALEX17 field campaign

2020 ◽  
Author(s):  
Belén Martí ◽  
Daniel Martínez-Villagrasa ◽  
Joan Cuxart
Keyword(s):  
Heat Flux ◽  
Surface Layer ◽  
Eddy Covariance ◽  
Friction Velocity ◽  
Turbulent Flux ◽  
Sensible Heat Flux ◽  
Sensible Heat ◽  
Flux Gradient ◽  
Eddy Covariance System ◽  
Flux Estimation

<p>Turbulent flux measurements require high frequency sampling in order to characterize appropriately all the variability scales of the atmosphere. A 3D sonic anemometer coupled with a gas detector allows for applying the eddy-covariance method which has become the standard. However, the high cost of this system often implies to look for alternative methods, specially when multiple stations are required. Turbulent fluxes can also be estimated through the flux-gradient similarity theory, requiring observations of mean quantities of (at least) air temperature and humidity at two levels and wind at one height. This approach is more sensitive to the disturbing influence of heterogeneous and complex surfaces and a comparison between methodologies is required under these conditions.<br><br>The data used in this study is part of the ALaiz EXperiment 2017-2018 (ALEX17). This campaign was the last within the New European Altas project. It had a duration of over a year with measurements in complex terrain. The location of the experiment is a valley bounded by two mountain ranges that rise 150 m north and over 600 m south. A central site in the centre of the valley was instrumented with a sodar-RASS, an 80-m tower, a surface energy balance (SEB) station with an eddy-covariance system and a surface-layer station (SLS) with the necessary measurements to estimate the turbulent fluxes. In addition, eight supplementary SLS were deployed along the longitudinal and transverse valley axes to characterize the surface layer variability within the valley.<br><br>This communication will present a comparison of the friction velocity and sensible heat flux obtained from both the eddy-covariance system and the flux-gradient method at the central site for a time series of 8 months. Friction velocity is highly comparable between methodologies with a correlation of 0.92 and a standard deviation of 0.05. The performance of the sensible heat flux estimation differs between stable and unstable cases, with a correlation of 0.70 and 0.89, respectively, after applying a quality control procedure. The poorer results obtained under stable conditions points out the need for alternative estimations of the sensible heat flux for these cases.</p>

Numerical study of stable stratification effects on the wind over simplified tall building models using large-eddy simulations

2021 ◽  
Vol 193 ◽  
pp. 107625
Author(s):  
Zhenqing Liu ◽  
Weipeng Li ◽  
Lian Shen ◽  
Yan Han ◽  
Zhiwen Zhu ◽  
...  
Keyword(s):  
Numerical Study ◽  
Stable Stratification ◽  
Tall Building ◽  
Large Eddy Simulations ◽  
Building Models ◽  
Large Eddy

scholarly journals Flux-gradient relation and atmospheric wind profiles — an exploration using WRF and lidars

2020 ◽  
Vol 1618 ◽  
pp. 032032
Author(s):  
Pedro Santos ◽  
Alfredo Peña ◽  
Jakob Mann
Keyword(s):  
Flux Gradient ◽  
Wind Profiles

scholarly journals A Climatology of Nocturnal Low-Level Jets at Cabauw

2009 ◽  
Vol 48 (8) ◽  
pp. 1627-1642 ◽  
Author(s):  
P. Baas ◽  
F. C. Bosveld ◽  
H. Klein Baltink ◽  
A. A. M. Holtslag
Keyword(s):  
Wind Speed ◽  
Ground Level ◽  
Geostrophic Wind ◽  
Stable Stratification ◽  
Radiative Cooling ◽  
Inertial Oscillation ◽  
Stable Boundary Layers ◽  
Above Ground Level ◽  
Low Level ◽  
Low Level Jets

Abstract A climatology of nocturnal low-level jets (LLJs) is presented for the topographically flat measurement site at Cabauw, the Netherlands. LLJ characteristics are derived from a 7-yr half-hourly database of wind speed profiles, obtained from the 200-m mast and a wind profiler. Many LLJs at Cabauw originate from an inertial oscillation, which develops after sunset in a layer decoupled from the surface by stable stratification. The data are classified to different types of stable boundary layers by using the geostrophic wind speed and the isothermal net radiative cooling as classification parameters. For each of these classes, LLJ characteristics like frequency of occurrence, height above ground level, and the turning of the wind vector across the boundary layer are determined. It is found that LLJs occur in about 20% of the nights, are typically situated at 140–260 m above ground level, and have a speed of 6–10 m s−1. Development of a substantial LLJ is most likely to occur for moderate geostrophic forcing and a high radiative cooling. A comparison with the 40-yr ECMWF Re-Analysis (ERA-40) is added to illustrate how the results can be used to evaluate the performance of atmospheric models.

scholarly journals Turbulent fluxes of entropy and internal energy in temperature stratified flows

2015 ◽  
Vol 81 (5) ◽  
Author(s):  
I. Rogachevskii ◽  
N. Kleeorin
Keyword(s):  
Mach Number ◽  
Internal Energy ◽  
Turbulent Flux ◽  
Stratified Flows ◽  
Fluid Temperature ◽  
Stratified Turbulence ◽  
Convective Flux ◽  
Low Mach Number ◽  
Linear Approach ◽  
The Mean

We derive equations for the mean entropy and the mean internal energy in low-Mach-number temperature stratified turbulence (i.e. for turbulent convection or stably stratified turbulence), and show that turbulent flux of entropy is given by$\boldsymbol{F}_{s}=\overline{{\it\rho}}\,\overline{\boldsymbol{u}s}$, where$\overline{{\it\rho}}$is the mean fluid density,$s$is fluctuation of entropy and overbars denote averaging over an ensemble of turbulent velocity fields,$\boldsymbol{u}$. We demonstrate that the turbulent flux of entropy is different from the turbulent convective flux,$\boldsymbol{F}_{c}=\overline{T}\,\overline{{\it\rho}}\,\overline{\boldsymbol{u}s}$, of the fluid internal energy, where$\overline{T}$is the mean fluid temperature. This turbulent convective flux is well-known in the astrophysical and geophysical literature, and it cannot be used as a turbulent flux in the equation for the mean entropy. This result is exact for low-Mach-number temperature stratified turbulence and is independent of the model used. We also derive equations for the velocity–entropy correlation,$\overline{\boldsymbol{u}s}$, in the limits of small and large Péclet numbers, using the quasi-linear approach and the spectral${\it\tau}$approximation, respectively. This study is important in view of different applications to astrophysical and geophysical temperature stratified turbulence.

Sign in / Sign up

Export Citation Format

Share Document