In Situ Observations of Kelvin-Helmholtz Waves along a Frontal Inversion

Abstract

An unusual set of observations of amplifying Kelvin-Helmholtz waves along a New England coastal front inversion is presented and compared to linear theory. The waves were observed by instrumented aircraft traversing the coastal front at various levels and measuring temperature and horizontal and vertical wind components at one-second intervals. The most prominent group of waves had amplitudes in the neighborhood of 100 m, comparable to the total depth of the inversion, and wavelengths near 1.7 km. Spectral analysis of data from an aircraft pass located just above the inversion indicates that Kelvin-Helmholtz billows of 1?2-km wavelength were present over a distance of at least 25 km. Numerical solution of the Taylor-Goldstein equation for observed and specified basic-state vertical profiles confirms that instability of the Kelvin-Helmholtz type is the dominant mode of instability. The wavelength of the most unstable mode is found to be strongly sensitive to the relative thicknesses of the shear and inversion layers. The observed waves illustrate a mechanism by which dammed cold air is eroded and accelerated toward the mountains, particularly as the ambient onshore wind increases or the cold air warms.