Observations of Stably Stratified Flow through a Microscale Gap

• Published in: Journal of the atmospheric sciences Vol. 78; no. 1; pp. 189 - 208
• Main Authors: Vassallo, Daniel, Krishnamurthy, Raghavendra, Menke, Robert, Fernando, Harindra J. S.
• Format: Journal Article
• Language: English
• Published: Boston American Meteorological Society 01.01.2021
• Subjects: Aspect ratio; Atmospheric variability; Brunt-vaisala frequency; Climatology; Design of experiments; Doppler lidar; Doppler sonar; Environmental conditions; Experimental design; Field investigations; Height; Lee waves; Lidar; Mountains; Remote sensors; Ridges; Sensor arrays; Stratified flow; Topography
• ISSN: 0022-4928; 1520-0469
• DOI: 10.1175/JAS-D-20-0087.1

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 U 0 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 / U 0 and various gap aspect ratios. Modified forms of G were proposed to account for real-world atmospheric variability, and the results are discussed in terms of a gap-averaged value G c . The nature of gap 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-jetting regime [ G c > O (1)] were identified. The measurements were in broad agreement with previous numerical and theoretical studies on a single ridge with a gap or double-ridge topography, although details vary. This is the first-ever detailed field study reported on microscale [ O (100) m] gap flows, and it provides useful data and insights for future theoretical and numerical studies.