Abstract

In this article, we present an overview of the HyIV-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.

Highlights

  • Topography covering the Earth’s surface is mostly complex

  • Afterwards, we examine the results from experiments with double obstacles and focus on the different aspects of the influence of the secondary obstacle on the flow

  • In the HyIV-CNRS-SecORo stratified water tank experiments, we have investigated the influence of of double obstacles on the characteristics of stably stratified flow characterized by a profile with double obstacles on the characteristics of stably stratified flow characterized by a profile with a density jump

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Summary

Introduction

Topography covering the Earth’s surface is mostly complex. Isolated ridges or mountains, a useful idealization for process understanding, are rarely found in reality, though notable exceptions exist, e.g., [1,2,3,4]. Most of the literature on stable flow over mountains still focuses on mountain waves forming over isolated obstacles, e.g., [5,6,7,8,9,10,11,12]. Earliest studies of flow over double obstacles [13,14,15] suggest that a systematic variation of wave amplitude can occur when waves generated by the first obstacle are in or out of phase with those generated by the second obstacle. This phenomenon was termed as lee-wave interference. Tampieri and Hunt [16] recognized that when the lee-wave wavelength is similar to or larger than the ridge separation distance, the valley atmosphere can be flushed (or ventilated) out of the valley by terrain-following shooting flow

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