Abstract (eng)
Lee waves excited by a single mountain range have been studied extensively in the past. Flow over a mountain range is associated with downslope windstorms and turbulence, both of which may pose a hazard for aircraft. A particularly turbulent phenomenon associated with large-amplitude lee waves or hydraulic jumps is an atmospheric rotor, characterized by a reversal of surface flow on the lee of the mountain range and turbulent internal structure.
In reality, mountain ridges are rarely isolated. Flow over double or multiple ridges can lead to lee wave interference, which can either enhance or diminish the amplitude of lee waves downstream of the ridges. A similar effect on the flow field can be expected between the individual ridges.
To test the influence of secondary topography on lee wave amplitudes, wavelengths, steadiness, and boundary layer separation, water-flume experiments are planned with a double orography setup. In preparation for these experiments, several possible laboratory setups were simulated numerically with the CM1 model (Bryan Cloud Model 1, version 16) with two- and three-dimensional (2D and 3D) large-eddy simulations (LES). To classify the simulation results, non-dimensional parameters such as the Froude number and the mountain/inversion height ratio are used.
For the 2D simulations, both with a single and double-ridge topography, a series of sensitivity tests were conducted to examine the dependence of the flow field on a number of parameters, including the inversion height and strength, horizontal wind speed, and mountain shape. For double-ridge topography, a lee wave interference pattern can be determined for setups including inversions, and the results mostly agree with previous findings by Stiperski and Grubišić (2011) for nonlinear flow regimes. Increasing the mountain height and width leads to mountain wave breaking and unsteady flow in lee wave regimes, which is governing the formation and location of rotors in both single- and double ridge setups. Hydraulic jump like flows display more steadiness. However, when the mountain wave breaks at low levels, the jump region gains intensity in terms of reverse flow and turbulence.
From the 2D sensitivity tests, special cases were selected for the 3D LES simulations. In the analysis, a special focus was placed on the difference between the 2D and 3D simulations in terms of rotor dynamics and turbulence. In the 3D LES, the horizontal vorticity field shows that the rotor consists of several smaller-scale subrotors, which are more intense than in the corresponding 2D simulations. As for turbulence, the 3D LES allows a direct calculation of the turbulent kinetic energy (TKE) from the variances of the horizontal and vertical wind speeds. The distribution of the TKE field also gives insight into the highly turbulent regions, such as rotors, hydraulic jumps, and breaking mountain waves.