Similarity Scaling Over a Steep Alpine Slope

Abstract

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.

Exchange Processes in the Atmospheric Boundary Layer Over Mountainous Terrain

“…These factors include the inherent horizontal heterogeneity [15,47], the corresponding local advection processes [57,61], slope effects on the available radiant energy [62,63], the potential failure of the vertically-constant-flux assumption in the SL [64], and others.…”

“…Since many of the pre-requisites of this theory are almost invariably not fulfilled over mountainous terrain, no universal scaling behaviour according to MOST (that is, correspondence to the well-established universal MOST functions) has been reported over complex orography so far. Local scaling has been suggested as an alternative [64,82], but it is a much less powerful approach than MOST, because the scaling functions need to be established for each site separately [83]. For this reason, the use of local SL scaling in numerical models is not yet feasible.…”

On Turbulent Fluxes During Strong Winter Bora Wind Events

“…Högström 1996), g = 9.81 m s −2 is the acceleration due to gravity, w θ is the local vertical heat flux, u * is the local friction velocity and θ v is the virtual potential temperature. Since no direct measurement of relative humidity was made, we make the assumption that the sonic temperature is a relatively good approximation to the virtual potential temperature (Fortuniak et al 2013;Nadeau et al 2013;Večenaj and De Wekker 2014). Plotting all the variables in Fig.…”

The near‐surface evening transition

“…Nadeau et al . () examined the early evening calm period in detail over a steep slope (>25°). Busse and Knupp () also revealed details of the early‐evening calm period.…”