Intermittent and Elliptical Inertial Oscillations in the Atmospheric Boundary Layer

Abstract

As a convective boundary layer over land decays in the late afternoon, the atmosphere responds to the release of turbulent stresses. For many years, this response has been presumed to take the form of an inertial oscillation, a horizontal circulation with a frequency equal to the local Coriolis frequency, though published documentation of inertial oscillations in the atmosphere has been rare. In fact, documentation of inertial oscillations has been more associated with frontal passages than with the evening transition of the atmospheric boundary layer. A month of boundary layer wind profiler data from the Cooperative Atmosphere?Surface Exchange Study-1999 field program is analyzed here with the Hilbert?Huang transform (HHT), which allows analysis of intermittent, nonstationary, and amplitude-varying wave events. Inertial motions are found in this dataset, but neither the onset times of these inertial motions nor the preferred levels of occurrence are consistent with the evening-transition hypothesis. Rather, significant correlations of inertial motions with frontal passages are observed. The elliptical nature of the observed inertial motions is consistent with amplification by deformation frontogenesis. The HHT is first demonstrated with a 5-day time series of temperature data to illustrate how the technique allows simultaneous identification of the stationary diurnal temperature cycle, as well as intermittent and nonstationary cooling events like frontal passages and density currents. The age of one density current is calculated from its dispersion characteristics, verifying that the density current in question results from rapid cooling near sunset.