Numerical Simulation of Katabatic Flow with Changing Slope Angle

Abstract

A large eddy simulation (LES) model and the Advanced Regional Prediction System (ARPS) model, which does not resolve turbulent eddies, are used to study the effect of a slope angle decrease on the structure of katabatic slope flows. For a simple, uniform angle slope, simulations from both models produce turbulence kinetic energy and momentum budgets that are in good overall agreement. Simulations of a compound angle slope are compared to a uniform angle slope to demonstrate how a changing slope angle can strongly affect the strength of katabatic flows. Both ARPS and the LES model show that slopes with a steep upper slope followed by a shallower lower slope (concave shape) generate a rapid acceleration on the upper slope followed by a transition to a slower evolving structure characterized by an elevated jet over the lower slope. In contrast, the case with uniform slope (having the same total height change) yields a more uniform flow profile with stronger winds at the slope bottom. Higher average slope in the uniform slope angle case generates greater gravitational potential energy, which is converted to kinetic energy at the bottom of the slope. Analysis of the total energy budget of slope flows indicates a consistent structure where potential energy generated at the top of the slope is transported downslope and converted into kinetic energy near the slope base.

Energetics of Slope Flows: Linear and Weakly Nonlinear Solutions of the Extended Prandtl Model

“…Smith and Skyllingstad, 2005;Mauritsen et al, 2007). The three terms on the right side are described as diffusion (DIF), dissipation (DIS), and interaction (INT) terms: DIF represents the diffusion of TE by the turbulent flow, DIS represents the dissipation of TE, and INT represents the interaction of the slope flow with the background atmosphere in the case of the weakly nonlinear model.…”

“…This type of flow is often observed in regions of complex orography and substantially affects the weather and climate in these regions (e.g., Poulos and Zhong, 2008). The topic of katabatic and anabatic wind is being actively explored and the work on its understanding includes the application of numerical models (direct numerical simulations (DNS): (e.g., Shapiro and Fedorovich, 2008); large eddy simulations (LES): e.g., Skyllingstad, 2003;Smith and Porté-Agel, 2013); mesoscale models: (e.g., Smith and Skyllingstad, 2005;Zammett and Fowler, 2007); and analytical models (e.g., Prandtl, 1942;Defant, 1949;Grisogono and Oerlemans, 2001;Zardi and Serafin, 2014). Continued interest in katabatic and anabatic winds stems from the important effects of this type of orographic flows on visibility and fog formation, air pollutant dispersion, agriculture and energy use, fire-fighting operations, sea-ice formation, etc.…”

“…The difference, when compared to Mauritsen et al (2007), is in our focus not being on the turbulent part of the flow but on the wind and temperature finite amplitude deviations from the background state coming from katabatic/anabatic flows. In this sense, our approach is similar to the energy framework of katabatic winds applied by Smith and Skyllingstad (2005). While, Smith and Skyllingstad (2005) define kinetic energy in the same way as Mauritsen et al (2007), their potential energy is defined as a linear function of both temperature perturbations and the height above the slope.…”

“…In this sense, our approach is similar to the energy framework of katabatic winds applied by Smith and Skyllingstad (2005). While, Smith and Skyllingstad (2005) define kinetic energy in the same way as Mauritsen et al (2007), their potential energy is defined as a linear function of both temperature perturbations and the height above the slope. Although there are some differences in the literature concerning the definition of potential energy, it is typically a function of potential temperature perturbations.…”

The Impact of Different Terrain Configurations on the Formation and Dynamics of Katabatic Flows: Idealised Case Studies

“…Normally, they affect local weather by inducing a stabilisation of the atmosphere within the nocturnal boundary layer, which has been consistently examined over the last few decades (Fleagle 1950;Thyer 1966;Gutman 1983;Smith and Skyllingstad 2005;Yu and Cai 2006;Poulos et al 2007). Such investigations comprised measurement campaigns as well as model studies for a better comprehension of their characteristics and more realistic prediction of their occurrence.…”