Upstream Blocking and Airflow Over Mountains

Abstract

)obstacle of height h, then naive energy arguments (and common sense) indicate that if Nh/U is sufficiently large, fluid near the ground would be blocked on the upstream side and not flow over the obstacle. Casual observations and folklore have long indicated that this phenomenon is common near mountain ranges in the atmosphere. However, the nature and mechanics of how it occurs have only recently become clear. It is now known that upstream blocking in large-Reynolds­ number flows propagates as a wave phenomenon, generated by nonlinear effects over the topography. These waves may be linear or nonlinear depending on circumstances, and they propagate primarily as columnar motions, meaning that they permanently alter the density and horizontal velocity profiles as they pass through the fluid ahead of the obstacle. Blocking occurs when these changes reach sufficient amplitude. Since they alter the upstream conditions, the understanding of these upstream dis­ turbances caused by the obstacle is a prerequisite for calculating the steady­ state flow over an obstacle, regardless of the other details of the flow. These effects generally depend on the topography being approximately two-dimensional (2D) with sufficiently large height. They are common in geophysical situations such as fjords, estuaries, and in the atmosphere. Since blocking is primarily a two-dimensional stratified phenomenon, in this review we exclude the effects of rotation and are concerned with