EDWARD - Extreme Downslope Winds And Resonant mountain Drag enhancement in structured atmospheres

Among the many processes whose scale falls below the grid spacing of meteorological models and must be parameterized, gravity wave drag is one of the most important. This force, which is produced in stratified flow over orography, and leads to a deceleration of the atmospheric circulation at the global scale, receives a disproportionate contribution from flow configurations called high-drag states. In these configurations, which are often associated with downslope windstorms, the drag force is enhanced by a large factor as compared to typical leading-order estimates. Therefore, it is essential to understand high-drag states in order to correctly parameterize the effect of gravity waves in weather prediction and climate models. A better ability to predict downslope windstorms, which can cause severe local damage, and may constitute a serious aviation hazard, is also of vital interest.

Project EDWARD addresses the problem of downslope windstorms and high-drag states produced in stratified flow over orography. Both these phenomena are manifestations of a single physical process: high-amplitude orographic internal gravity waves. These phenomena have been partially explained by previous authors using the concepts of linear resonance for multilayered atmospheres, by performing fully nonlinear numerical simulations of high-drag states, and by solving Long’s equation (which commands the behaviour of moderately nonlinear 2D mountain waves) subject to various conditions. However, there are still many gaps in our knowledge. For example, the drag associated with lee waves, which are quite common in Nature, has seldom been calculated. Additionally, Long’s equation has almost always been solved with constant wind and stability profiles, even though shear is known to be important for the dynamics of high-drag states. On the other hand, given that wave breaking and flow stagnation are also known to be associated with high-drag states, it is important to understand the conditions under which they occur. This has been achieved almost exclusively for unidirectional flows. Finally, the resonance mechanism discovered for the ocean back in, but shown only recently to be important for drag amplification in the atmosphere by, is almost totally unexplored. In this project, we aim to fill these gaps. First, we will proceed with our study of drag enhancement in two-layer atmospheres by considering both variable shear and variable static stability. We will also calculate the drag produced by lee waves for complex atmospheric profiles and orography shapes, conducive to resonance. Second, we will explore in detail the drag enhancement mechanism re-discovered by, in which the mountain waves are amplified by the presence of an atmospheric Scorer parameter that oscillates with height. All these calculations, to be carried out using different variants of linear theory, will be compared with numerical simulations so as to assess the importance of nonlinearity. Additional numerical simulations will be carried out to determine the criteria for flow stagnation and wave breaking in directional shear flows over 3D mountains. This will allow us to judge whether the deduction based on linear theory of, that wave breaking will always occur in these flows, is accurate. Finally, we will make an attempt to find solutions to Long’s equation when the wind profile varies with height, using the method of multiple scales and a weakly nonlinear approximation. This will yield values of the surface drag affected both by shear and nonlinearity. We also intend to study the absorbing / reflecting/ transmitting behaviour of critical levels (where the flow velocity is zero) in these weakly nonlinear conditions.