The low‐level katabatic jet height versus Monin–Obukhov height

Based on gradient transport theory or K-theory, turbulent transport in the atmosphere has long been parameterized using the eddy diffusivity. Due to its simplicity, this approach has often been applied in many numerical models but rarely tested with observations. Here, the widely used O'Brien cubic polynomial approach has been validated together with an exponential approach against eddy diffusivity profiles determined from measurements and from large-eddy simulation data in stable conditions. Verification is completed by analyzing the variability effects on pollutant concentrations of two different vertical diffusion (K (z)) schemes incorporated in an atmospheric chemical model. It is shown that the analytical, exponential solution agrees better with observations than the O'Brien profile and should be used henceforth in practical applications.

Section: Implementation Of the Grisogono Methods In The Unified Emep Msupporting

confidence: 80%

Improvement of Vertical Diffusion Analytic Schemes Under Stable Atmospheric Conditions

Jeričević

Večenaj

2009

“…Indirect evidence for fine-scale turbulence includes the close relationship of the turbulence and the momentum flux to local shear on vertical scales of a metre or less, in the presence of a near-surface wind maximum (Mahrt et al 2014). A minimum of turbulence at the wind maximum suggests that the turbulence above the wind maximum is partially decoupled from the surface turbulence (Horst and Doran 1988;Conangla and Cuxart 2006;Grisogono et al 2007). Turbulence that does not directly interact with the ground can be referred to as a form of "z-less" turbulence (e.g.…”

Section: Introductionmentioning

confidence: 99%

Dependence of Turbulent Velocities on Wind Speed and Stratification

Mahrt

Sun

Stauffer

2014

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.

“…Grisogono et al (2007) recently provided a theoretical analysis on the reliability of MOST for drainage flows over small slopes and proposed a modified Obukhov length that incorporates the height of the katabatic jet. Park and Park (2006) studied surface-layer turbulence characteristics over a university campus surrounded by sloping terrain, but the maximum slopes were still relatively small (14 • ).…”

Section: Introductionmentioning

confidence: 99%

Similarity Scaling Over a Steep Alpine Slope

Nadeau

Pardyjak

Higgins

et al. 2012

In this study, we investigate the validity of similarity scaling over a steep mountain slope (30-41 • ). The results are based on eddy-covariance data collected during the Slope Experiment near La Fouly (SELF-2010); a field campaign conducted in a narrow valley of the Swiss Alps during summer 2010. The turbulent fluxes of heat and momentum are found to vary significantly with height in the first few metres above the inclined surface. These variations exceed by an order of magnitude the well-accepted maximum 10 % required for the applicability of Monin-Obukhov similarity theory in the surface layer. This could be due to a surface layer that is too thin to be detected or to the presence of advective fluxes. It is shown that local scaling can be a useful tool in these cases when surface-layer theory breaks down. Under convective conditions and after removing the effects of self-correlation, the normalized standard deviations of slope-normal wind velocity, temperature and humidity scale relatively well with z/Λ, where z is the measurement height and Λ(z) the local Obukhov length. However, the horizontal velocity fluctuations are not correlated with z/Λ under all stability regimes. The non-dimensional gradients of wind velocity and temperature are also investigated. For those, the local scaling appears inappropriate, particularly at night when shallow drainage flows prevail and lead to negative wind-speed gradients close to the surface.