Observed generation of an atmospheric gravity wave by shear instability in the mean flow of the planetary boundary layer

Abstract

Observations of a single boundary-layer event — the generation of an atmospheric gravity wave by an unstable shear flow at Haswell, Colorado on November 12, 1971 — are briefly described and discussed. The observations were made using: (a) an acoustic echo sounder, (b) anemometers mounted at two fixed levels on a 150-m tower, (c) an anemometer and a thermometer mounted on a movable carriage on the tower, and (d) a microbarograph array, including one microbarograph mounted atop the tower. The wave phase velocity (−3.5–4.0 m s⁻¹) was found to equal the wind velocity in the middle of the shear flow, as assumed by other authors. The wave-associated vertical fluxes of momentum and energy measured just above the wave critical layer were estimated to be −5 dyn cm⁻² and −800 erg cm⁻² s⁻¹, respectively. These are large values. The annual average vertical flux of momentum at temperate and high latitudes is −0.25 dyn cm⁻², while the average kinetic energy dissipation rate in a unit column of atmosphere is −5 × 10³ erg cm⁻² s⁻¹. If the region of wave generation was itself propagating horizontally, its propagation velocity was large compared with the horizontal phase speed of the small-scale waves generated. Wave generation appeared to occur over an area large compared with the size of the microbarograph array (i.e., ≫ 2 km).