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Trapped lee waves form downwind of a mountain ridge under conditions conducive to wave energy confinement within the lower troposphere. Placing a secondary ridge downwind of the first leads to constructive and destructive interference of lee waves. Idealized high-resolution two-dimensional simulations with the NRL COAMPS model are used to investigate the trapped lee wave interference over a double ridge in the linear and nonlinear flow regimes as well as in the presence and absence of surface friction. The upstream atmospheric structure and terrain profiles derive from observations obtained in the Terrain-induced Rotor Experiment (T-REX). The results show that the interference pattern is described well by linear interference theory for all mountain heights. The pattern is determined by the ratio of the intrinsic trapped lee wave wavelength and the ridge separation distance. On the contrary, the wave amplitude is not predicted well by linear interference theory. For the double ridge, the primary horizontal lee wave wavelength is determined by the upstream wind shear profile and modulated by the ridge separation distance. In the presence of surface friction, boundary-layer separation develops and rotors form for mountain heights exceeding a certain critical value. For a double ridge this critical mountain height is strongly dependent on interference. In the lee of the downstream ridge, changes in the wave amplitude due to interference affect the rotor strength most significantly. Only for the strongest nonlinear regime examined is the rotor strength amplified by constructive interference beyond that obtained in the lee of a single ridge, while destructive interference significantly diminishes the rotor strength. The strength of rotors in the valley attains a constant value for highly nonlinear regimes and is significantly diminished compared to their strength in the lee of a single ridge. Destructive interference is seen to be highly nonlinear even for cases of very low mountain heights in absence of surface friction. Total destructive interference, in which waves almost completely cancel out in the lee of the downstream ridge, develops for certain ridge separation distances but only for the height of the downstream ridge being less than the upstream one.

At main focus of the Terrain-induced Rotor Experiment (T-REX) is a tight coupling of mountain-waves and boundary layer displayed in the formation of atmospheric rotors. In our earlier study of lee wave resonances over double bell-shaped obstacles, in which the profiles of wind speed and stability as well as terrain were derived from the T-REX observations of lee wave events over the Sierra Nevada and Inyo Mountains, we focused on the mountain-wave part of the problem, examining the interaction of trapped lee waves with downstream orography in absence of friction. It was shown that the displayed variation of wave amplitude and drag with the ridge separation distance can be attributed to nonlinear wave resonance. In this study, effects of the frictional boundary layer on lee wave resonance are investigated by means of high-resolution 2D numerical simulations with the NRL COAMPS model, using the same idealized representation of the T-REX environment from our earlier study. Due to complex nonlinear interactions between the lee waves and boundary layer, new results show a different resonance pattern, which no longer displays itself in wave amplitudes or drag but rather in the amplitude ratio and steady-state averaged reversed flow strength underneath the primary lee-wave crest. The total destructive interference, in which waves completely cancel out in the lee of the downstream peak, is also observed for certain ridge separation distances when the height of the downstream obstacle is 2/3 the height of the upstream one. The flow within the valley exhibits a highly complex structure. Regions of reversed flow form at the surface underneath the lee wave crests and are lifted above the ground, extending up to 2 km AGL for 3 km high obstacles. The flow within a rotor is unsteady with multiple sub-vortices. The location of the inversion relative to the mountaintop is shown to significantly affect the resonance pattern, reversed flow strength, and the wave steadiness.

The strong bora winds are a common occurrence along the eastern mountainous Adriatic coast, especially in winter. Bora is known to be particularly severe in the lee of the Velebit range in the northern Adriatic, where leeward slopes are particularly steep and mountain passes scarce but prominent. The Velebit range is highest at its southern part, where it abruptly ends, resulting in steep terrain slopess. The highest bora wind gust ever recorded in Croatia (69 m/s) was measured in the lee of the southern tip of Velebit. This is also the area where bora winds are extremely spatially variable. Located only a short distance away from the locus of the bora maximum, the area of the city of Zadar, in the lee of one of the highest Velebit peaks (1757m), is characterized climatologically by weak winds, considerably weaker compared to its surroundings. The primary aim of this study is to investigate small-scale characteristics and spatial variability of the severe Bora flow in the wide Zadar area and to identify reason for the “Zadar calm”. The complex flow structure in the lee of southern Velebit is investigated with very high-resolution numerical simulations carried out with the NRL COAMPS model. The focus is placed on two wintertime severe bora episodes. A synoptically-induced critical level at the altitude ranging from 3 to 5 km depending on the time period defines the upstream bora flow layer and leads to the formation of a hydraulic jump in the lee of the highest terrain. The complex and temporally highly variable three-dimensional flow structure is characterized by a pronounced wake in the Zadar area surrounded by two jets, one emanating from a pass at the northern end of southern Velebit and the second one originating as a southern tip jet. The wake flow is highly nonstationary and is influenced by both the change in the upstream profile as well as boundary layer evolution. Reversed flow and rotors develop within the wake region in connection with the hydraulic jump. Sensitivity experiments were conducted to examine the effects of the Zadar peninsula topography and the height of southern Velebit on the structure of the bora flow. The influence of Velebit is particularly strong, governing the onset and strength of the bora flow within the domain. The terrain of the Zadar peninsula, although significantly lower than Velebit, is also shown to influence the characteristics of the developed bora flow, especially the location of the point of flow separation. Certain types of gravity wave response within the bora flow appear to be particularly sensitive to the small scale topography of the Zadar peninsula.

The Sierra Nevada– White-Inyo system is an almost perfect two-dimensional double barrier well known for the generation of large-amplitude trapped lee waves and rotors, which were the focus of the recent Terrain-induced Rotor Experiment (T-REX in 2006) and its pilot Sierra Rotors Project (SRP in 2004). Observational evidence from T-REX as well as attendant numerical studies indicate that trapped lee waves and the process of wave-induced boundary-layer separation play an important role in the formation of rotors. From the observations it appears that the largest amplitude lee waves are those with the wavelength close to the ridge separation distance, suggesting a resonant wave response of the atmosphere to the surrounding terrain. Our earlier idealized free-slip simulations confirm that the presence of a second mountain exercises a profound influence on the wavelength as well as the amplitude of trapped lee waves over the double mountain barrier. The ridge separation distance was shown to be a key element that controls the trapped lee wave wavelength. Moreover, the oscillatory character of the quasi-state-state value of gravity-wave drag as a function of the ridge separation distance revealed the presence of constructive and destructive non-linear interferences and identifies separation distances for which these interferences occur. Here we report on further idealized high-resolution numerical simulations of the double-barrier problem using a no-slip lower boundary condition. Simulations, carried out using the NRL Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS), are otherwise two-dimensional, irrotational and dry. In this study, we examine in more detail the effect of the frictional boundary layer on the previously established sensitivities of the double barrier flow to the valley geometry and upstream atmospheric structure. Our preliminary results indicate that surface friction significantly affects the lee wavelengths, reduces lee wave amplitudes and enhances trapping. The interference pattern changes and destructive (constructive) interference appears for different ridge separation distances than in the free slip simulation. Boundary layer separation is also observed together with regions of reversed flow. While these regions developed in the free slip simulations as well due to the strong pressure gradients induced by the large amplitude waves, in the no slip simulations they are enhanced.

In this article,we present an overviewof theHyIV-CNRS-SecORo (Hydralab IV-CNRS-Secondary Orography and Rotors Experiments) laboratory experiments carried out in the CNRM (Centre National de Recherches Météorologiques) large stratified water flume. The experiments were designed to systematically study the influence of double obstacles on stably stratified flow. The experimental set-up consists of a two-layer flow in the water tank, with a lower neutral and an upper stable layer separated by a sharp density discontinuity. This type of layering over terrain is known to be conducive to a variety of possible responses in the atmosphere, from hydraulic jumps to lee waves and highly turbulent rotors. In each experiment, obstacles were towed through the tank at a constant speed. The towing speed and the size of the tank allowed high Reynolds-number flow similar to the atmosphere. Here, we present the experimental design, together with an overview of laboratory experiments conducted and their results. We develop a regime diagram for flow over single and double obstacles and examine the parameter space where the secondary obstacle has the largest influence on the flow. Trapped lee waves, rotors, hydraulic jumps, lee-wave interference and flushing of the valley atmosphere are successfully reproduced in the stratified water tank. Obstacle height and ridge separation distance are shown to control lee-wave interference. Results, however, differ partially from previous findings on the flow over double ridges reported in the literature due to the presence of nonlinearities and possible differences in the boundary layer structure. The secondary obstacle also influences the transition between different flow regimes and makes trapped lee waves possible for higher Froude numbers than expected for an isolated obstacle.