Observations of Stably Stratified Flow through a Microscale Gap

This paper reports the findings of a comprehensive field investigation on flow through a mountain gap subject to a range of stably stratified environmental conditions. This study was embedded within the Perdigão field campaign, which was conducted in a region of parallel double-ridge topography with ridge-normal wind climatology. One of the ridges has a well-defined gap (col) at the top, and an array of in situ and remote sensors, including a novel triple Doppler lidar system, was deployed around it. The experimental design was mostly guided by previous numerical and theoretical studies conducted with an idealized configuration where a flow (with characteristic velocity U₀ and buoyancy frequency N) approaches normal to a mountain of height h with a gap at its crest, for which the governing parameters are the dimensionless mountain height G = ^Nh/U₀  and various gap aspect ratios. Modified formsof G were proposed to account for real-world atmospheric variability, andthe results are discussed in terms of a gap-averaged value G_c. The nature ofgap flow was highly dependent on G_c, wherein a nearly neutral flow regime (G_c< 1), a transitional mountain wave regime (G_c ∼ O(1)), and a gap-jettingregime (G_c > O(1)) were identified. The measurements were in broad agreement with previous numerical and theoretical studies on a single ridge with agap or double ridge topography, although details vary. This is the first ever detailed field study reported on microscale (O(100 m))gap flows, and providesuseful data and insights for future theoretical and numerical studies.